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Training Report
The Maharaja Sayajirao University of Baroda
Faculty of Technology and Engineering
Vadodara Manufacturing Division
Gujarat Olefins Plant
(Instrumentation Department)
A Report by:
Maulin R. Amin
(Electronics Engineering)
Table of Contents:
Certificate 01
Preface 02
Acknowledgement 03
Reliance Industries Limited (Vadodara Manufacturing Division)
1) Company Profile 04
2) Commitments 05
3) Vadodara 09
4) Plants at VMD 09
Gujarat Olefins Plant : Units and NCP Process
1) Introduction 13
2) Introduction to NCP 13
1) Product 13
2) By-Products 13
3) Numbering of equipment 14
4) Characteristics of Naphtha 15
5) Principle of Cracking 15
6) Mechanism of Cracking 16
7) Variable of Cracking Operation 17
8) Factors affecting heater operation 18
9) Function of dilution steam in cracking 18
10) Effects of partial pressure during cracking 19
11) Functions of DMDS in feed stock
Naphtha Cracker Plant
1) Modes of Operation 20
2) Process Flow Diagram 21
3) Instrumentation and Piping Diagram 22
Instrumentation in NCP
1) Introduction 23
2) Functions and Advantages 23
3) Terminology and Symbols in Control system engineering 23
4) Control and Field Instrumentation Documentation 35
P a g e | 1
CERTIFICATE MAULIN AMIN
Certificate
This is to certify that Mr. Maulin Riteshbhai Amin, student of Faculty of
Technology and Engineering, The Maharaja Sayajirao University of Baroda has
successfully completed his training at Gujarat Olefins Plant at Reliance Industries
Limited, Vadodara Manufacturing Division towards the completion of the period
from 21st
May 2013 to 22nd
June 2013 under my supervision and guidance with
utmost satisfaction.
It indeed gives us pleasure to highlight that Mr. Maulin R Amin worked
hard and with deep sincerity throughout his training duration. I appreciate his
sincere efforts and I am sure that the experience gained during the course of this
training will enable him to take up more challenging tasks in future.
Date:
Mr. Navin Chandra Patel
Sr. General Manager
Gujarat Olefins Plant
Reliance Industries Limited
Vadodara Manufacturing Division
Vadodara, Gujarat
Mr. B.P. Shah
HR Manager
Reliance Industries Limited
Vadodara Manufacturing Division
Vadodara, Gujarat
P a g e | 2
PREFACE MAULIN AMIN
Preface
India had definitely become a strong country as far as the petrochemical industry is
concerned. India has undoubtedly strengthened its position in the international market by
roping in foreign investment and trying to establish its strong base abroad. In the present area,
it has become necessary to compete on the global stage.
It is equally important to have practical as well as theoretical knowledge. Theoretical
knowledge can be easily acquired from books and publications, but has to pass through an on-
site training phase for practical knowledge.
Having said this, one has to observe and study the equipment used for the process, its
construction, start up, shut down procedures, operating problems, its solution, emergencies,
etc. The theories and usual practices cited in books and literature differ up to an appreciable
extent from the industrial practices.
Another attractive feature is to learn industrial management and discipline, which is
equally important in life.
This is only possible through industrial training.
P a g e | 3
ACKNOWLEDGEMENT MAULIN AMIN
Acknowledgement
It’s a pleasure to thank all the authorities and personnel, who directly or indirectly are
involved in successful completion of my in-plant training.
I bestow my gratitude to Mr. B.P. Shah for granting us the permission to obtain training
at Reliance Industries Limited, Vadodara Manufacturing Division.
I am very thankful to Mr Navin Chandra Patel for encouraging me at each step of our
training.
I am thankful to Mr Mahendra Upadhyay my industry mentor, for continuously guiding
and supporting me throughout the training.
Mr. Rahul Ravi and Mr. Adersh V, my training mentor for their consistent guidance
and support throughout my training and for constantly ensuring that the training is opening up
new aspects of Instrumentation industry for my learning. They not only solved my difficulties,
but also shared their immense experience from their service in this industry.
I finally would like to thank each and every employee of Gujarat Olefins Plant for all
the support and help they provided during the course of training.
My training would have been incomplete without their support and expertise.
RELIANCE INDUSTRIES LIMITED MAULIN AMIN
About
Reliance Industries Limited
(Vadodara Manufacturing Division)
P a g e | 4
RELIANCE INDUSTRIES LIMITED MAULIN AMIN
1)Company Profile:
Reliance Industries Limited (RIL) is world’s leading and India’s no. one private limited
company. RIL group is highly diversified group and is in to multiproduct business like oil/gas
exploration, retail of petro/consumer products and manufacturing of petrochemical/refining
and textile products and also in to infrastructure and transportation sector.
RIL-VMD was earlier a part of Indian Petrochemicals Corporation Limited (IPCL) with the
management controlled by government of India. In 2002, due to divestment of equity, the
management control went in the hands of Reliance Petro Invest Co. of RIL group. On
September 5th
2007, merging of IPCL with RIL was legally concluded.
RIL – VMD’s multi product manufacturing portfolio includes polymers, synthetic rubber,
synthetic fibre and fibre intermediates, solvents and industrial chemicals. It has several
distinctions to its credit. Accredited earlier for Best Performance Award among petrochemical
companies worldwide (CI London), FICCI awards, ICMA awards, National Energy Awards
and several awards from National Safety Council, USA and British Safety Council, UK. In fact
it has integrated management system in place comprising of ISO 9001, ISO 14001 and OSHAS
18001 certification for all plants and departments of site.
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RELIANCE INDUSTRIES LIMITED MAULIN AMIN
2)Commitments:
P a g e | 6
RELIANCE INDUSTRIES LIMITED MAULIN AMIN
P a g e | 7
RELIANCE INDUSTRIES LIMITED MAULIN AMIN
P a g e | 8
RELIANCE INDUSTRIES LIMITED MAULIN AMIN
P a g e | 9
VADODARA MANUFACTURING DIVISION MAULIN AMIN
Vadodara:
Vadodara Manufacturing Division located in Vadodara, Gujarat. It comprises of a Naptha
cracker and 15 downstream plants for the manufacture of polymers, fibres, fibre intermediates
and chemicals:
Plants at the Vadodara Manufacturing Division
Name of Plant Commissioned Year
Naphtha Cracker 1979
LDPE 1979
Mono Ethylene Glycol/Ethylene Oxide 1979
Butadiene Extraction 1979
Polybutadiene Rubber Plant 1 1979
Polybutadiene Rubber Plant II 1996
Benzene Extraction 1979
LAB 1979
Acrylonitrile Plant 1979
Acrylic Fibre Monocomponent 1979
Acrylates 1983
VCM 1984
PVC 1984
Polypropylene Copolymer Pant 1988
Acrylic Fibre Bi-component Plant 1989
Polypropylene Plant 1996
1. Gujarat Olefins Plant (GOP)
Raw material: Naptha
Products:
 Ethylene
 Propylene
 Cu Stream
 Pyrolysis gasoline
 Carbon black feedstock
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VADODARA MANUFACTURING DIVISION MAULIN AMIN
2. Gujarat Aromatic Plant (GAP)*
Xylenes
Raw material: Naptha
Products:
 Orthoxylene
 Mixed Xylene
 Para Xylene
 Solvent CIX
 Hepton
Dimethyl terephthalate plant
Raw material: Para Xylene
Products:
 Methanol
 DMT
 Methyl Benzoate
 Dimethyl Isophthalate
3. Integrated Offsite Plant (IOP)
Various utility system associated with the process units and offsite facilities. Under IOP
projects are as under:
 Fire water system
 Service water system
 Drinking water and semitate water system
 D.M. water system
 Process water system
 Cooling water system
 Steam water system
 Compressed air system
4. Ethylene Glycol Plant (EG)
Raw material: Ethylene
Products:
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VADODARA MANUFACTURING DIVISION MAULIN AMIN
 Ethylene Glycol
 Ethylene Oxide
 Di ethylene glycol
 Tri ethylene glycol
 Poly ethylene glycol
5. Acrylonitrile Plant (ACN)
Raw material: Propylene and Ammonia
Products:
 A.C.N.
 Acetonitrile
 Hydro cynic acid
6. Propylene Plant (P.P.)
Raw material: Propylene
Products:
 Poly propylene
 Atactic polymer
7. Gas Turbine Power Plant (GTTP)
It is a gas turbine plant which produces electricity of 72MWH. It is the last plant
commissioned at Baroda complex.
8. Vinyl chloride/ Polyvinyl chloride (VC/PVC)
Raw material: Ethylene and Chlorine
Products:
 Vinyl chloride
 Poly vinyl chloride
9. Acrylic Fibres (AF)*
Raw materials: Acrylonitrite and Methyl acrylate
Products: Acrylic fibres
10. Low density poly ethylene (LDPE)
Raw material: Ethylene
Products: Low Density Poly Ethylene
11. Linear Alkyl Benzene (LAB)*
Raw material: Benzene and Superior kerosene
Products:
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VADODARA MANUFACTURING DIVISION MAULIN AMIN
 Linear alkyl benzene
 Poly alkyl benzene
 Heavy n-paraffin
 Centon
 L.R.2030
12. Poly Propylene co-polymer Plant (PPCP)
Raw material: Propylene
Products:
 Poly propylene co-polymer
 Atactic co-polymer
13. Poly Butadiene Rubber (PBR)
Raw material: Butadiene
Products: Poly Butadiene Rubber
14. Acrylates Plant (ACR)*
Raw material:
 Acrylonitrile
 Sulphuric Acid
 Alcohols
 Ammonia
Products:
 Ammonium sulphates
 Ethyl acrylates
 Butyl acrylates
 2-Ethyl hexyl acrylates
 Methyl acrylates
15. Petrol Resins Plant (PR)
Raw material: Pyrolysis gasoline
Products: Petroleum resins
*Plants not in Operation
About Gujarat Olefins Plant
Units and NCP Process
P a g e | 13
GUJARAT OLEFINS PLANT MAULIN AMIN
1)Introduction:
GOP is the mother plant of RIL, VMD. Here, naphtha is cracked to produce feed stock for
other plants at RIL.
The main products of GOP are ethylene, propylene, gasoline and C4 raffinate.
 Ethylene is supplied to LDPE and EG plants.
 Propylene formed has two grades, based on the level of purity.
o Polymer grade (PG), 99% pure is supplied to PP4.
o Chemical grade (CG), 95% pure is treated for the removal of impurities.
 Mixed C4 products are used in the formation of rubber at PBR I and PBR II plants.
The GOP consists of the following units:
 Naphtha Cracker Plant (NCP)
 Benzene Butadiene Hydrogenation (BBH)
 Pyrolysis Gasoline Hydrogenation (PGH)
 Benzene Extraction
 Butadiene Extraction
 Feed purification unit
 Off sites
2)Introduciton to Naphtha Cracker Plant:
The naphtha cracker plant, designated as Unit 21 of GOP is designed by M/s ABB Lummus
Global and its detailed engineering is done by M/s EIL. It was commissioned in March 1978.
It was designed at for nameplate capacity of 130 KTA.
1) Product:
 Ethylene
2) By-products:
 Propylene (PG, CG)
 Mix C4
 Pyrolysis Gasoline
 CBFS
 Fuel Gas (HP, MP, LP methane)
P a g e | 14
GUJARAT OLEFINS PLANT MAULIN AMIN
 Hydrogen
The NCP is divided into three zones to facilitate easier operation and understanding. The three
zones are:
 AB zone: cracking and quenching, high temperature fractionation
 C zone: compression and refrigeration
 D Zone: chilling, cold fractionation and recovery
3) Numbering of Equipment:
At RIL, there is a numbering system that assigns a number each to equipment of the plant. For
example: 21-11-333
It stands for:
NCP unit code is 21
Equipment Number Section
100-199 Cracking
200-299 Heat fractionation and compression
300-399 Chilling
400-499 Propylene Refrigeration
500-599 Ethylene Refrigeration
Code Equipment
11 Heat Exchange
12 Vessel
13 Column
14 Reactor
15 Pumps
16 Furnace/ Heater
17 Blower/ Compressor
18 Filter/De-super-heaters/Miscellaneous Equipment
P a g e | 15
GUJARAT OLEFINS PLANT MAULIN AMIN
4) Characteristics of Naphtha:
Naphtha is the raw material for NCP. Naphtha is a colourless, volatile and flammable liquid
mixture of hydrocarbons, having specific gravity of 0.69. Based on the boiling points, naphtha
is of two types: light and heavy. Light naphtha (boiling range -35℃ to 135℃) is thermally
cracked to obtain olefins.
According to PIONA analysis, the average composition of naphtha is as follows:
Component Specification (Wt %) Maximum (Wt %) Minimum (Wt %)
Paraffin’s 75 74.26 79.99
Naphthenes 18 14.13 19.08
Aromatics 6.5 5.28 7.23
Olefins 0.5 0.18 0.61
Sulphur (ppm) 170 105 308
Specific Gravity 0.6824 0.678 0.692
C5 and C6 are also recycled for the naphtha feed. Naphtha is obtained from crude oil refining
done in RIL Jamnagar Plant. It is transported to Dahej via shipping and then from there to
storage drums in the Vadodara Plant via pipelines. Provisions have also been made to import
Naphtha from Kandla Dahej.
5) Principle of Cracking:
Cracking is the process where heavy hydrocarbons are broken down into simpler hydrocarbons,
thermally or by using catalysts.
Naphtha is cracked thermally by using the process of steam cracking.
Naphtha contains a large number of hydrocarbons and during steam cracking a large number
of chemical reactions take place, most of them are based on free radicals. Thus the actual
reaction that take place are complex and difficult to model.
However, pyrolysis of ethane provides a simple illustration to understand the phenomenon of
free radical mechanism.
1. Initiation: Ethane molecule splits homolytically into two methyl radicals
𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶3 → 2𝐶𝐶𝐶𝐶3
P a g e | 16
GUJARAT OLEFINS PLANT MAULIN AMIN
2. Hydrogen Abstraction: Methyl radical removes hydrogen radical from another ethane
molecule to give an ethyl radical.
𝐶𝐶𝐶𝐶3
* + 𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶3 → 𝐶𝐶𝐶𝐶4 + 𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶2
*
3. Radical Decomposition: Ethyl radical decomposes to give ethylene molecule and
hydrogen radical.
𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶2
* → 𝐶𝐶𝐶𝐶2 − 𝐶𝐶𝐻𝐻2 + 𝐻𝐻∗
4. Hydrogenation: The hydrogen radical attacks ethane molecule to give a hydrogen
molecule and new ethyl radical.
𝐻𝐻* + 𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶3 → 𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶2
* + 𝐻𝐻2
Reaction (4) is followed by reaction (3) and thus, they constitute a chain mechanism.
The net effect can be represented by the equation
𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶3 → 𝐶𝐶𝐶𝐶2 − 𝐶𝐶𝐶𝐶2 + 𝐻𝐻2
Reactions (2) and (3) are called transition reactions.
5. Termination: The chain cycle will terminate when two free radicals react with each
other to produce products that are not free radicals. When the chain is interrupted, it
becomes necessary to generate new radicals via reactions (1), (2) and (3) to start a new
chain.
Apart from primary reactions discussed above, secondary reactions occur too.
6) Mechanism of cracking:
The reactions taking place can be broadly classified into two categories:
1. Primary Reaction:
When naphtha along with the steam is heated to such a temperature that the heavier
naphtha molecules break down into smaller molecules, these are known as primary
reactions.
2. Secondary Reaction:
The cracking operation comprises of reactions other than the primary rections, these are
called secondary reactions.
They are as follows:
a. Dehydrogenation – gives olefins
b. Dehydrocyclization – gives aromatics
P a g e | 17
GUJARAT OLEFINS PLANT MAULIN AMIN
c. Condensation – Two or more, smaller fragments combine to form large stable
structures. It gives gas oil, fuel oil and tars.
d. Hydrogenation – Gives paraffin, di-olefins and acetylene are obtained from
olefins.
e. Reactions involving further pyrolysis of olefins. It results into formation of
olefins, di-olefins and acetylene.
7) Principles and governing variables of cracking operations:
The overall cracking reactions are endothermic. High temperature and low partial pressure of
hydrocarbons favour the reactions.
 Residence time:
It is defined as the length of time for which the naphtha feed is in cracking furnace at
or above its cracking temperature. This variable is the prime factor in deciding the yirld
pattern of the cracking furnace. Other things being equal, a short residence time gives
a higher yield of ethylene due to the suppressions of secondary reactions.
The residence time is usually kept as 0.5 seconds.
 Severity:
The severity of the operation is dependent on the following:
Coil Outlet Temperature (COT): Higher the COT, more severe is the cracking.
Residence time of naphtha cracking: With the feed rate and stream rate, the residence
time also gets fixed.
Pressure in the cracking coils: Lower the pressure, higher the severity of cracking for a
furnace of definite design and dimension. The minimum pressure available at the charge
gas of the first stage suction drum governs the coil pressure.
Thus the only process variable which controls the severity of cracking is COT.
 Selectivity:
Hydrocarbon undergoing pyrolysis is the most complex, mixture of molecules and free
radicals, which reacts with one another in multiple ways simultaneously.
Based on established theories and supported experimental data, the production of
olefins and di-olefins had been found to be favoured by two ways:
a. Short Residence Time
b. Lower Hydrocarbon Partial Pressure
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GUJARAT OLEFINS PLANT MAULIN AMIN
For liquid feed stock the methane to ethylene ratio found in the heater effluent was used as a
good overall indicator of pyrolysis heater selectivity.
Low Methane to Ethylene Ratio corresponds to a high total yield of ethylene, butadiene and
butylene.
Steam cracking is done as it has the advantage of lowering the partial pressure of the
hydrocarbons in the feed and reducing the deposit of coke.
The following reaction variables have been suggested as the optimum conditions for naptha
cracking:
Temperature 760℃ 𝑡𝑡𝑡𝑡 860 ℃
Total Pressure 1 atmosphere
Hydrocarbon/ Dilution Steam 2:1 to 1:1
Residence time 0.5 sec
DMDS
For naphtha: 100ppm
For ethane: 150ppm
8) Factors affecting heater operation:
1. Feed Rate:
Increasing the feed rate will decrease the residence time. However, it will require
greater high heat duty.
2. Dilution Steam Rate:
Increasing the dilution steam rate will decrease the residence time and decrease the
hydrocarbon partial pressure also resulting in better selectivity and more valuable
products. But, more dilution steam will increase operation cost.
3. Coil Outlet Temperature:
High temperature gives higher conversion and higher yield of ethylene, however too
high temperature may give too high fouling rate and can also decrease the yield of
propylene and butadiene. Varying the outlet temperature can vary the ratio of ethylene
to propylene, thus will be varied according to the market demand at particular time.
9) Functions of dilution steam in cracking:
 It reduces the hydrocarbon partial pressure and thereby encourages higher selectivity
of desired olefin products.
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GUJARAT OLEFINS PLANT MAULIN AMIN
 It reduces the partial pressure of the high boiling aromatic hydrocarbons in the zone of
high conversion, lessening the tendency to form coke within the cracking coils and
deposit on the walls of the TLE (Transfer Line Exchanger)
 It has sufficient adding effect on the tube metal to significantly diminish the catalytic
effect of iron and nickel which otherwise would promote the carbon forming reaction.
10)Effects of partial pressure during cracking:
The partial pressure of hydrocarbon affects the chemical equilibrium arid the reaction rates and
thus influences the product distribution.
Optimum partial pressure is P = 0.5 – 10.6 kg/cm2
g.
Optimum total pressure, P = 1 atm.
11)Functions of DMDS in feed stock:
Coke formation in pyrolysis furnace and TLE presents serious operation problems. When
unsaturated hydrocarbons diffuse through the gas film boundary layer to the high temperature
tube well, they undergo dehydrogenation reactions leading ultimately to coke formation.
The sulphur on the feedstock or the recomposed DMDS (which is injected along with the feed
stock) forms a sulphide film in the active tube metal, temporarily poisoning the catalytic effect
of Nickel and Iron and thus reduces coke formation.
P a g e | 19
About Naphtha Cracker Plant
 Modes of Operation: Heater
 Process Flow Diagram: Cracking
 Piping and Instrumentation Diagram:
Naphtha Cracking Heater
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NAPHTHA CRACKER PLANT MAULIN AMIN
Modes of Operation:
The NCP Heater has three operating modes:
1. HSS
2. Cracking
3. Decoke
1. HSS (High Steam Standby):
In this mode, the plant is fed with steam to allow it to reach the temperature required for
cracking, before being fed with naphtha. Cracking works flawlessly if performed at the
desired temperature. So, the heater temperature is allowed to rise until it reaches 820℃.
The plant is operated in this mode for about 24 hours, i.e. until the cracking temperature is
reached.
2. Cracking:
In this mode, the cracking of naphtha takes place. It a process related more to the vast
fortitude of Chemical Engineering and Science. Instrumentation is incorporated here, just
to control the process.
3. Decoke:
Coke formation might lead to serious trouble. So the place is operated in this mode, to rid
the plant from coke formation with in. It’s an approximate 48 hours procedure.
P a g e | 21
NAPHTHA CRACKER PLANT MAULIN AMIN
PROCESS
FLOW
DIAGRAM
P a g e | 22
NAPHTHA CRACKER PLANT MAULIN AMIN
Piping and
Instrumentat-
ion Diagram
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About Instrumentation in
Naphtha Cracker Plant
 Introduction to Instrumentation
 Function and Advantages
 Terminology and Symbols in
Control Engineering
 Control and Field Instrumentation
Documentation
 4-20 mA Current Loop
 Transmitters
 Instrument Connections
 Letatwin LM 380A
 Tricon System
 DCS
P a g e | 23
1) Introduction:
Measurement is the process of determining the amount, degree or capacity by comparison with
the accepted standards of the system units being used.
Instrumentation is a technology of measurement which serves sciences, engineering, medicine
and etc.
Instrument is a device for determining the value or magnitude of a quantity or variable.
Electronic instrument is based on electrical or electronic principles for its measurement
functions.
2) Functions and Advantages:
The 3 basic functions of instrumentation:-
1. Indicating – visualize the process/operation
2. Recording – observe and save the measurement reading
3. Controlling – to control measurement and process
Advantages of electronic measurement
1. Results high sensitivity rating – the use of amplifier
2. Increase the input impedance – thus lower loading effects
3. Ability to monitor remote signal
3) Terminology and Symbols in Control system Engineering:
Planning, design and start-up of process control systems require clear and unambiguous
communication between all parts involved. To ensure this, we need a clear definition of the
terms used - as far as the documentation is concerned - standardized graphical symbols. These
symbols help us represent control systems or measurement and control tasks as well as their
device-related solution in a simple and clear manner.
3.1 Terminology in Control Engineering:
To maintain a physical quantity, such as pressure, flow or temperature at a desired level during
a technical process, this quantity can be controlled either by means of open loop control or
closed loop control.
P a g e | 24
3.1.1 Open loop control
In an open loop control system, one or more input variables of a system act on a process
variable. The actual value of the process variable is not being checked, with the result that
possible deviations - e.g. caused by disturbances are not compensated for in the open loop
control process. Thus, the characteristic feature of open loop control is an open action flow.
The task of the operator illustrated in Fig. 1 is to adjust the pressure (p2) in a pipeline by means
of a control valve. For this purpose, he utilizes an assignment specification that determines a
certain control signal (y) issued by the remote adjuster for each set point (w). Since this method
of control does not consider possible fluctuations in the flow, it is recommended to use open
loop control only in systems where disturbances do not affect the controlled variable in an
undesired way.
3.1.2 Closed loop control
In a closed loop control system, the variable to be controlled (controlled variable x) is
continuously measured and then compared with a predetermined value (reference variable w).
If there is a difference between these two variables (error e or system deviation xw), adjustments
are being made until the measured difference is eliminated and the controlled variable equals
the reference variable. Hence, the characteristic feature of closed loop control is a closed action
flow.
p1
y
p2
variable pcontrolsFig. 1: Operator the process 2 via remote adjuster
Assignment:
wa y=> a p=> 2a
wb => yb => p2b
etc.
P a g e | 25
The operator depicted in Fig. 2 monitors the pressure p2 in the pipeline to which different
consumers are connected. When the consumption increases, the pressure in the pipeline
decreases. The operator recognizes the pressure drop and changes the control pressure of the
pneumatic control valve until the desired pressure p2 is indicated again. Through continuous
monitoring of the pressure indicator and immediate reaction, the operator ensures that the
pressure is maintained at the desired level. The visual feedback of the process variable p2 from
the pressure indicator to the operator characterizes the closed action flow.
3.1.3 Process
A process comprises the totality of actions effecting each other in a system in which matter,
energy, or information are converted, transported or stored. Adequate setting of boundaries
helps determine sub-processes or complex processes.
Examples:
Generation of electricity in a power plant
Distribution of energy in a building
Production of pig iron in a blast furnace
Transportation of goods
p1 p2
Fig. 2: Operator controls the process variable p2 an a closed loop
P a g e | 26
3.1.4 Control loop
The components of a control loop each have different tasks and are distinguished as follows:
The components of the final control equipment are part of the controlling system as well as
part of the controlled system.
The distinction made above results directly from the distribution of tasks. The actuator
processes and amplifies the output signal of the controller, whereas the final control element -
as part of the controlled system - manipulates the mass and energy flow.
3.2 Symbols in Control Engineering
3.2.1 Signal flow diagrams
A signal flow diagram is the symbolic representation of the functional interactions in a system.
The essential components of control systems are represented by means of block diagrams. If
required, the task represented by a block symbol can be further described by adding a written
text.
However, block diagrams are not suitable for very detailed representations. The symbols
described below are better suited to represent functional details clearly.
P a g e | 27
3.2.2 Blocks and lines of action
The functional relationship between an output signal and an input signal is symbolized by a
rectangle (block). Input and output signals are represented by lines and their direction of action
(input or output) is indicated by arrows.
Example: Root-extracting a quantity (Fig. 3)
(E.g. flow rate measurement via differential pressure sensors)
Example: Representing dynamic behaviour (Fig. 4)
(E.g. liquid level in a tank with constant supply)
Example: Summing point (Fig. 5)
The output signal is the algebraic sum of the input signals. This is symbolized by the summing
point. Any number of inputs can be connected to one summing point which is represented by
a circle. Depending on their sign, the inputs are added or subtracted.
xe xa
Fig. 3: Root-extracting a differential pressure signal
xe = differential pressure xa = root-extracted differential pressure
xe xa
Fig. 4: Development of a liquid level over time
xe = inflow xa = liquid level
P a g e | 28
Example: Branch point (Fig. 6)
A branch point is represented by a point. Here, a line of action splits up into two or more lines
of action. The signal transmitted remains unchanged.
Example: Signal flow diagram of open loop and closed loop control
The block diagram symbols described above help illustrate the difference between open loop
and closed loop control processes clearly.
In the open action flow of open loop control (Fig. 7), the operator positions the remote adjuster
only with regard to the reference variable w. Adjustment is carried out according to an
assignment specification (e.g. a table: set point w1 = remote adjuster position v1; w2 = v2; etc.)
determined earlier.
x1
x2
x1 = x2 = x3
x3
Fig. 6: Branch point
xe1
xe2 +
+
_
xe3
xa
xa = xe1 + xe2 – xe3
Fig. 5: Summing point
P a g e | 29
In the closed action flow of closed loop control (Fig. 8), the controlled variable x is measured
and fed back to the controller, in this case man. The controller determines whether this variable
assumes the desired value of the reference variable w. When x and w differ from each other,
the remote adjuster is being adjusted until both variables are equal.
3.2.3 Device-related representation
Using the symbols and terminology defined above, Fig. 9 shows the typical action diagram of
a closed loop control system
xw +
_
Fig. 8: Block diagram of manual closed loop control
man
remote
adjuster
control
valve
system
xw
controlFig. 7: Block diagram of manual open loop
man
remote
adjuster system
control
valve
z
ew +
–
yr xy
r
Fig. 9: Block diagram of a control loop
controlling
element
measuring
equipment
controller
final
control
element
system
actuator
P a g e | 30
Whenever the technical solution of a process control system shall be pointed out, it is
recommended to use graphical symbols in the signal flow diagram (Fig. 10). As this
representation method concentrates on the devices used to perform certain tasks in a process
control system, it is referred to as solution-related representation. Such graphical
representations make up an essential part of the documentation when it comes to planning,
assembling, testing, start-up and maintenance.
1 Sensor (temp.) 2 Transmitter
3 Signal converter 4 Controller
5 Pneumatic linear valve 6 Heat exchanger
Each unit has its own graphical symbol that is usually standardized. Equipment consisting of
various units is often represented by several lined-up symbols.
control of a heat exchanger systemFig. 10: Graphical symbols for describing temperature
1
23
4
5 6
P a g e | 31
It is recommended to always use standardized graphical symbols. In case a standardized
symbol does not exist, you may use your own.
PI
controlcontrollers,forsymbolsGraphical11:Fig. valves and software-based functions
hand-operated
actuator
motor-driven
actuator
diaphragm
actuator
valve with
diaphragm
actuator
motor-driven
valvebutterfly
valve
controllercontroller
symbol)former(
withvalve
diaphragm actuator
attachedand
positioner
PIcontroller
root-extracting
element,
software-based
countersoftware
with limit switch
functions performed by
software are marked
with a flag
P a g e | 32
3.3Control Systems and Structures
Depending on the job to be done, many different structures of control can be used. The main
criterion of difference is the way the reference variable w is generated for a certain control
loop. In setting the controller, it is also important to know whether the reference variable is
principally subject to changes or disturbance variables need to be compensated for.
 To attain good disturbance reaction, the controller must restore the original equilibrium
as soon as possible (Fig. 13).
 To attain good reference action, the controlled variable must reach a newly determined
equilibrium fast and accurately (Fig. 14).
P
F
F
T
Pt 100 DIN
P
P L
L
I
symbols for sensors, transmitters, adjusters and indicatorsFig. 12: Graphical
level
sensor
temperature
sensor
pressure
sensor
adjusterindicatoranalog
sensorflow
transmitterpressure
electricwith
outputstandardized
signal
transmittercurrent
pneumaticwith
outputstandardized
signal
i/p converter,
electr. into pneum.
standardized
signal
P a g e | 33
3.3.1 Fixed set point control
In fixed set point control, the reference variable w is set to a fixed value. Fixed set point
controllers are used to eliminate disturbances and are therefore designed to show good
disturbance reaction.
x
z
t
t
Fig. 13: Disturbance reaction
w
t
x
t
Fig. 14: Reference action
Fig. 15: Temperature control by means of fixed set point control
w
x
P a g e | 34
The temperature control system in Fig. 15 will serve as an example for fixed set point control.
The temperature of the medium flowing out of the tank is to be kept at a constant level by
controlling the heating circuit. This will provide satisfactory results as long as high fluctuations
in pressure caused by disturbances do not occur in the heating circuit.
3.3.2 Follow-up control
In contrast to fixed set point control, the reference variable in follow-up control systems does
not remain constant but changes over time. Usually, the reference variable is predetermined by
the plant operator or by external equipment. A reference variable that changes fast requires a
control loop with good reference action. If, additionally, considerable disturbances need to be
eliminated, the disturbance reaction must also be taken into account when designing the
controller.
3.3.3 Cascade control
Cascade control systems require a minimum of two controllers, these are the master or primary
and the follower or secondary controller. The characteristic feature of this control system is
that the output variable of the master controller is the reference variable for the follower
controller.
Employing cascade control, the temperature control of the heat exchanger (Fig. 16) provides
good results also when several consumers are connected to the heating circuit. The fluctuations
in pressure and flow are compensated for by the secondary flow controller (w2, x2) which acts
as final control element to be positioned by the primary temperature controller.
by means of cascadeFig. control16: Temperature control
w1=wsoll x2
x1 w2
q
P a g e | 35
In our example the outer (primary) control loop (w1, x1) must be designed to have good
disturbance reaction, whereas the inner –secondary– control loop requires good reference
action.
3.3.4 Ratio control
Ratio control is a special type of follow-up control and is used to maintain a fixed ratio between
two quantities. This requires an arithmetic element (V). Its input variable is the measured value
of the process variable 1 and its output variable manipulates the process variable 2 in the control
loop. Fig 17 illustrates a mixer in which the flow rate q2 of one material is controlled in
proportion to the flow rate q1 of another material.
4) Control and Field Instrumentation Documentation:
To successfully work with (and design) control systems, it is essential to understand the
documents that are typically used to illustrate process control and associated field
instrumentation. The documentation of process control and associated field instrumentation is
normally created by the engineering firm that designs and constructs the plant. The company
that commissioned the plant may have an internal documentation standard the engineering firm
will be required to follow.
For an older installation, the plant documentation may only exist as a series of paper
documents. Today the documentation created for a new or upgraded plant is produced
controlFig. 17: Ratio
V
q2
q1
q2 Vq= 1
w
x
P a g e | 36
electronically using automated design tools and software. The tools and software selected by
the plant or engineering firm for initial plant design or upgrade will influence the
documentation format and how documentation is maintained at the plant site. Also, the
selection of the control system determines to what extent the system is self-documenting.
Self-documenting – the automatic creation of documents that follow defined conventions for
naming and structure.
If the documentation generated by the control system does not follow standards that have been
established for process control and instrumentation, then it may be necessary to manually create
this documentation.
Control System - A component, or system of components functioning as a unit, which is
activated either manually or automatically to establish or maintain process performance within
specification limits.
In general, four types of drawings that are commonly used to document process control and
associated field instrumentation.
4.1 Plot Plan
It is often helpful to look at the plot plan to get an overview of how a plant is physically
organized. By examining the plot plan, it is possible to get an idea of where a piece of
equipment is located in the plant. A typical plot plan is shown in Figure 4-1.
As will become clear in the following chapters, understanding the physical layout of the plant
and the distances between pieces of equipment can often provide insight into the expected
transport delay associated with material or product flow between pieces of equipment. For
example, how long does it take a liquid, gas, or solid material flow to get from one point in the
process to another?
Transport Delay – Time required for a liquid, gas or solid material flow to move from one
point to another through the process.
Physically, if the major pieces of process equipment are laid out far apart, then the transport
delay can be significant and in some cases, impact control performance. Also, the physical
layout of a plant will impact the length of wiring runs and communication distance from the
control system to the field devices; thus, it is a good idea to use the plot plan to get a sense of
the plant layout and a feel for the location of process equipment and process areas.
P a g e | 37
4.2 Process Flow Diagram
To meet market demands, a company may commission an engineering firm to build a new plant
or to modify an existing plant to manufacture a product that meets certain specifications and
that can be manufactured at a specific cost. Given these basic objectives, a process engineer
will select the type of chemical or mechanical processing that best meets the planned
production, quality, and efficiency targets. For example, if the equipment is to be used to make
more than one product then the process engineer may recommend a batch process. For
example, a batch reactor may be used to manufacture various grades of a lubrication additive.
Once these basic decisions are made, the process engineer selects the equipment that will most
cost-effectively meet the company’s objectives. Based on the production rate, the process
engineer selects the size of the processing equipment and determines the necessary connections
between the pieces of equipment. The process engineer then documents the design in a process
flow diagram (PFD). The process flow diagram typically identifies the major pieces of
equipment, the flow-paths through the process, and the design operating conditions—that is,
the flow rates, pressures, and temperatures at normal operating conditions and the target
production rate.
Figure 4-1. Plot Plan
DRAINAGE DITCH
OFFICE
BUILDING
POND
NO. 1 POND
NO. 2
TRUCK
LOADING/UNLOADING
AREAS
WATER
TREATMENT
PUMP
STATION
N
SCALE (FEET)
100500
POWER
HOUSE
CONTROL
ROOM
PACKAGING
/ SHIPPING
T1
T2 T3
S1 S2 S3
Company Name )(
PLOT PLAN
PLANT ( )
SHEET DRAWING NUMBER REV
1 OF 1 DA200023 1
P a g e | 38
Process flow diagram – Drawing that shows the general process flow between major pieces
of equipment of a plant and the expected operating conditions at the target production rate.
Since the purpose of the process flow diagram is to document the basic process design and
assumptions, such as the operating pressure and temperature of a reactor at normal production
rates, it does not include many details concerning piping and field instrumentation. In some
cases, however, the process engineer may include in the PFD an overview of key measurements
and control loops that are needed to achieve and maintain the design operating conditions.
Figure 4-2
P a g e | 39
Control Loop - One segment of a process control system.
During the design process, the process engineer will typically use high fidelity process
simulation tools to verify and refine the process design. The values for operating pressures,
temperatures, and flows that are included in the PFD may have been determined using these
design tools. An example of a process flow diagram is shown in Figure 4-2. In this example,
the design conditions are included in the lower portion of the drawing.
4.3 Piping and Instrumentation Diagram
The instrumentation department of an engineering firm is responsible for the selection of field
devices that best matches the process design requirements. This includes the selection of the
transmitters that fit the operating conditions, the type and sizing of valves, and other
implementation details. An instrumentation engineer selects field devices that are designed to
work under the normal operating conditions specified in the process flow diagram. Tag
numbers are assigned to the field devices so they may be easily identified when ordering and
shipping, as well as installing in the plant.
Tag number – Unique identifier that is assigned to a field device.
The decisions that are made concerning field instrumentation, the assignment of device tags,
and piping details are documented using a piping and instrumentation diagram (P&ID). A
piping and instrumentation diagram is similar to a process flow diagram in that it includes an
illustration of the major equipment. However, the P&ID includes much more detail about the
piping associated with the process, to include manually operated blocking valves. It shows the
field instrumentation that will be wired into the control system, as well as local pressure,
temperature, or level gauges that may be viewed in the field but are not brought into the control
system.
As mentioned earlier, the engineering company that is creating the P&ID normally has
standards that they follow in the creation of this document. In some cases, the drawing includes
an overview of the closed loop and manual control, calculations, and measurements that will
be implemented in the control system.
Closed loop control - Automatic regulation of a process inputs based on a measurement of
process output.
P a g e | 40
Manual control – Plant operator adjustment of a process input.
However, details on the implementation of these functions within the control system are not
shown on the P&ID. Even so, the P&ID contains a significant amount of information and in
printed form normally consists of many D size drawings (22 x 34 inches; 559 x 864 mm) or
the European equivalent C1 (648 x 917mm). The drawings that make up the P&ID are normally
organized by process area, with one or more sheets dedicated to the equipment,
instrumentation, and piping for one process area.
Piping and Instrumentation Diagram – Drawing that shows the instrumentation and piping
details for plant equipment.
The P&ID acts as a directory to all field instrumentation and control that will be installed on a
process and thus is a key document to the control engineer. Since the instrument tag (tag
number) assigned to field devices is shown on this document, the instrument tag associated
with, for example, a measurement device or actuator of interest may be quickly found. Also,
based on the instrument tags, it is possible to quickly identify the instrumentation and control
associated with a piece of equipment. For example, a plant operator may report to Maintenance
that a valve on a piece of equipment is not functioning correctly. By going to the P&ID the
maintenance person can quickly identify the tag assigned to the valve and also learn how the
valve is used in the control of the process. Thus, the P&ID plays an important role in the design,
installation and day to day maintenance of the control system. It is a key piece of information
in terms of understanding what is currently being used in the plant for process control. An
example of a P&ID is shown in Figure 4-3.
When you are doing a survey of an existing plant, obtaining a copy of the plant P&IDs is a
good starting point for getting familiar with the process and instrumentation. Unfortunately,
the presentation of process control on the P&ID is not standardized and varies with the
engineering firm that creates the plant design. In some cases, process control is illustrated at
the top of the drawing and its use of field instrumentation is indicated by arrows on the drawing
that point from the field instrumentation to the control representation. Another common
approach is to show control in the main body of the drawing with lines connected to the field
instrumentation. Using either approach complicates the drawing and its maintenance since
process control design may change with plant operational requirements.
For this reason, the P&ID may only show the field instrumentation, with other documentation
referenced that explains the control and calculations done by the control system. For example,
P a g e | 41
when the process involves working with hazardous chemicals, then a controller functional
description (CFD) may be required for process safety management (PSM). Standards have been
established by OSHA for controller functional descriptions. [3]
4.4 Loop Diagram
The piping and instrumentation diagram identifies, but does not describe in detail, the field
instrumentation that is used by the process control system, as well as field devices such as
manual blocking valves that are needed in plant operations. Many of the installation details
associated with field instrumentation, such as the field devices, measurement elements, wiring,
Figure 4-3
P a g e | 42
junction block termination, and other installation details are documented using a loop diagram.
ISA has defined the ISA-5.4 standard for Instrument Loop Diagrams. [4] This standard does
not mandate the style and content of instrument loop diagrams, but rather it is a consensus
concerning their generation. A loop diagram, also commonly known as a loop sheet, is created
for each field device that has been given a unique tag number. The loop diagrams for a process
area are normally bound into a book and are used to install and support checkout of newly
installed field devices. After plant commissioning, the loop Diagrams provide the wiring details
that a maintenance person needs to find and troubleshoot wiring to the control system.
Loop Diagram – Drawing that shows field device installation details including wiring and the
junction box (if one is used) that connects the field device to the control system.
The loop diagram is a critical piece of documentation associated with the installation of the
control system. As has been mentioned, the engineering company that is designing a process
normally has standards that they follow in the creation of a loop diagram. These standards may
be documented by the creation of a master template that illustrates how field devices and
nomenclature are used on the drawing.
The loop diagram typically contains a significant amount of detail. For example, if a junction
box is used as an intermediate wiring point, the loop diagram will contain information on the
wiring junctions from the field device to the control system. An example of a loop diagram is
shown in Figure 4-4.
As is illustrated in this example, junction box connections are shown on the line that shows the
division between the field and the rack room. The loop diagram shows the termination numbers
used in the junction box and the field device and for wiring to the control system input and
output cards. Also, the Display and Schematic portions of the loop diagram provide information
on how the field input and output are used in the control system.
Figure 4-4 shows installation details for a two-wire level transmitter that is powered through
the control system analog input card. Also, connections are shown between the control system
analog output card and an I/P transducer and pneumatic valve actuator. Details such as the 20
psi air supply to the I/P and the 60 psi air supply to the actuator are shown on this drawing.
Based on information provided by the loop diagram, we know that the I/P will be calibrated to
provide a 3–15 psi signal to the valve actuator. In addition, specific details are provided on the
level measurement installation. Since the installation shows sensing lines to the top and bottom
P a g e | 43
of the tank, it becomes clear that the tank is pressurized and that level will be sensed based on
the differential pressure.
In this particular installation, the instrumentation engineer has included purge water to keep
the sensing line from becoming plugged by material in the tank. Even fine details such as the
manual valve to regulate the flow of purge water are included in the loop diagram to guide the
installation and maintenance of the measurement device.
In this example, the loop diagram shows the installation of a rotameter. A rotameter consists
of a movable float inserted in a vertical tube and may be used to provide an inexpensive
mechanical means of measuring volumetric flow rate in the field.
As this example illustrates, the loop diagram provides information that is critical to the
installation, checkout, and maintenance of field devices. By examining the loop diagram, it is
possible to learn details that may not be obvious when you are touring the plant site. For
example, as was previously presented in Chapter 3 on measurement, there are various ways to
measure temperature. In the case of a temperature measurement, the loop diagram will provide
information on the temperature transmitter, as well as the measurement element that is used.
Figure 4-5 shows the loop diagram for a temperature measurement in which a three-wire RTD
element is used for the temperature measurement. Details such as the grounding of the shield
for the element wire and for the twisted pair going from the transmitter to the control system
are noted on the loop diagram.
The process of creating and reviewing loop diagrams is made easier by the fact that many
components used to represent measurement and control in similar applications are often
repeated. For example, the manner in which the control valve, actuator, and associated I/P
transducer were represented in the loop diagram example for a level application is duplicated
in other loop diagrams that depict a regulating (control) valve. This is illustrated in the loop
diagram shown in Figure 4-6 for a pressure application in which a regulating valve is used in
the control of pressure. Also in this example, the pressure measurement is made with a two-
wire transmitter. As will be noted by comparing Figure 4-4 and Figure 4-6, the wiring for the
pressure transmitter is similar to that used for the level transmitter.
In some cases, the operator uses a process measurement as an indicator or as an input to a
calculation that is done in the control system. A loop diagram may be developed for these types
of measurement that details the device installation and wiring to the control system. In such
cases, the loop diagram will contain no definitions of control. Figure 4-7 shows a flow
P a g e | 44
measurement made by measuring the differential pressure across an orifice plate. The hole in
the orifice plate is sized to give a specific pressure drop at the maximum flow rate that the
process is designed to support. As noted in the lower portion of the loop diagram, the orifice
plate is sized to provide a differential pressure of 500 inches H2O at a flow rate of 750 gpm.
Figure 4-4
P a g e | 45
Also noted in the loop diagram is the control system is expected to take the square root of this
differential pressure to obtain an indication of flow rate.
Figure 4-5
P a g e | 46
Figure 4-6
P a g e | 47
4.5 Tagging Conventions
The tagging conventions that were shown in examples of a P&ID and loop diagrams may be
confusing to someone who has not worked with these documents. The naming convention
Figure 4-7
P a g e | 48
illustrated in the P&ID and loop diagram examples are fairly well standardized in North
America. To a certain extent, similar conventions are used in Europe and Asia to document
process instrumentation and control.
TYPICAL TAG NUMBER
TIC 103 - Instrument Identification or Tag Number
T 103 - Loop Identifier
103 - Loop Number
TIC - Function Identification
T - First-letter
IC - Succeeding-Letters
EXPANDED TAG NUMBER
10-PAH-5A - Tag Number
10 - Optional Prefix A - Optional Suffix
Note: Hyphens are optional as separators
The letters that make up the first few characters of a typical tag number (the “leading letters”)
are used to identify the function performed by the field device or by the control system.
Following these leading letters is a number. The number that appears on the tag is known as
the loop number. The loop number is used to uniquely identify one or more field devices that
are used to perform a specific function. This combination of function letter and loop number
allows a field device in a process area to be precisely identified. Knowing the device tag
number is required when filling out a work order to or in discussing a field measurement with
an operator or instrument technician. The tag number assigned to a field device is normally
stamped on a tag that is attached to the device.
All the devices that are used together to perform a specific function are normally assigned the
same loop number. For example, the flow transmitter and regulating valve used to measure and
regulate the flow of a process stream may be assigned loop number 101.
The loop number normally has only three digits. Consequently, the number of field devices
that can be uniquely identified using the standard tag number convention is very limited. For
this reason, an expanded tag number convention, is used in the process industry. The expanded
tag number convention allows a number to be inserted in front of the function, and that number
is usually the process area number. A plant is divided into process areas that are assigned a
number. The combination of the area number, the function letters, and the loop number is
unique within a plant.
P a g e | 49
In some cases, multiple field devices may be used to perform a similar function. For example,
in some boiler applications the temperature of each tube in the superheater section of a boiler
may have individual temperature measurements. Rather than assigning a loop number to each
measurement, the expanded tag number convention assigns a common loop number to all these
measurements. When this is done, one or more characters may be added after the loop number
to uniquely identify each measurement. For the boiler example, loop number 105 might be
assigned to all the superheater temperature measurements and the individual measurements
may be identified by adding an A after the loop number for the first measurement, a B for the
second measurement, and a C for the third measurement.
This option to add letters after the loop number allows unique tag numbers to be created for
each measurement, even when a large number of similar measurements is made in a process
area. A hyphen may be optionally used in the tag number to separate the area number or
characters added after the loop number. However, in general, the use of a hyphen in the tag
number is not recommended since in many control systems, the length of a tag number is
limited to a maximum number of characters (e.g., 12 or 16 characters).
The identification letters used to specify the function of a field device are organized in a specific
manner. The meaning of a letter varies depending on whether it is the first letter or a succeeding
letter. A table of the identification letters defined by the standard is shown in Figure 4-8. If the
first letter of a tag number is an A, this indicates that the primary function of the device is
analysis; if the first letter is an F, then the primary function is flow. When the first letter is H,
this indicates that a manual or Hand function is to be performed. By reviewing this table of
identification letters, the use of the letters in a tag number can be easily determined.
In some cases, a letter can only be used as a succeeding letter. For example, the letter D would
never be used as a first character, but it may be used as a succeeding letter to indicate
“differential.” Thus, the combination PD is valid and would indicate the function of the device
is “pressure, differential,” that is, differential pressure. The combination HIC would be used to
indicate hand indicator controller, that is, manual control. Indication and control based on an
analytic measurement would be identified as AIC. The letter combination FIC is quite common
and used to indicate a “flow indicating control” function. A control valve used in pressure
control would be identified using the letters PV, pressure valve. A temperature measurement
used only for indication would be identified as TI, “temperature indication.” A position
transmitter would be identified using the letters ZT. The use of tag numbers in a process control
P a g e | 50
system is quite straightforward and must be understood to work with or create documentation
for a process control system.
Figure 4-8. ISA-5.1 Identification Letters
First Letters Succeeding Letters
Measured/Initiating
Variable
Variable
Modifier
Readout/Passive
Function
Output/Active
Function
Function
Modifier
A Analysis Alarm
B Burner, Combustion User’s Choice User’s Choice User’s Choice
C User’s Choice Control Close
D User’s Choice Difference, Differential Deviation
E Voltage Sensor, Primary
Element
F Flow, Flow Rate Ratio
G User’s Choice Glass, Gauge, Viewing
Device
H Hand High
I Current Indicate
J Power Scan
K Time, Schedule Time Rate of Change Control Station
L Level Light Low
M User’s Choice Middle, Intermediate
N User’s Choice User’s Choice User’s Choice User’s Choice
O User’s Choice Orifice, Restriction Open
P Pressure Point (Test Connection)
Q Quantity Integrate, Totalize Integrate, Totalize
R Radiation Record Run
S Speed, Frequency Safety Switch Stop
T Temperature Transmit
U Multivariable Multifunction Multifunction
V Vibration, Mechanical
Analysis
Valve, Damper, Louver
W ForceWeight, Well, Probe
X Unclassified X-axis Accessory Devices,
Unclassified
Unclassified Unclassified
Y Event, State, Presence Y-axis Auxiliary Devices,
Z Position, Dimension Z-axis, Safety
Instrumented System
Driver, Actuator, Unclassified
final control element
P a g e | 51
4.6 Line and Function Symbols
Different types of lines are used in process flow diagrams, piping and instrumentation
diagrams, and loop diagrams to indicate the type of connection between field devices and the
control system. The ISA-5.1 standard defines the instrument line symbols that are commonly
used in control system documentation. As illustrated in Figure 4-9, a solid line is used to
represent a physical connection to the process. Two slashes shown as points along a line are
used to indicate a pneumatic signal. One of the most common ways to indicate an electric signal
is a dashed line as defined in ISA-5.1. Communication links between devices and functions of
a distributed control system are indicated by small bubbles along the line as illustrated in Figure
4-9.
The previous examples of the process flow diagram, piping and instrumentation diagram, and
loop diagram contained one or more circle symbols. In these drawings, a circle is used to
indicate a discrete instrumentation or control function. A horizontal line drawn through the
middle of the circle indicates the function may be accessed by the plant operator. There are
many functions, such as those
performed by an I/P transducer or
valve positioner, that are typically
not directly accessible by the
operator. Also, some field devices
for measurement and actuation
may only be accessed by control
or calculation functions in the
control system and thus would not
be shown in the documentation as
being directly accessed by the
operator. However, the associated control or calculation function that is accessed by the
operator would include a horizontal line. As illustrated in Figure 4-10, one of the conventions
Figure 4-9
Figure 4-10
P a g e | 52
advocated in ISA-5.1 is to include a square around the circle if the associated function is
accessed by an operator through a video display of a distributed control system (DCS).
However, in practice, this convention is often not followed.
It is common practice to illustrate the valve body, as well as the valve actuator and positioner
function in control system documentation. The ISA-5.1 standard addresses the representation
of a valve body. Most types of valves are addressed by this standard. However, the engineering
firm that is designing a process plant may have adopted some variation of what is shown in
ISA-5.1. In such cases, it is common practice for the engineering firm to provide a drawing
that explains the symbol functions included in their documentation. Also, in some cases, a
general valve representation is used rather than different representations for a rotary valve or
sliding stem valve. [5] Generally, a damper will be shown rather than the general valve symbol
to indicate the regulation of air or gas flow to a boiler or a similar process such as a kiln or
heater. An excerpt from ISA-5.1 Valve Body and Damper Symbols are illustrated in Figure 4-
11.
Since the type of actuator used with a valve body may impact the operation and failure mode
of the valve, the type of actuator is normally indicated in control system documentation. The
representation of common types of actuators as defined by ISA-5.1 is shown in Figure 4-12. A
complete representation of the valve is provided by combining the valve representation with
the actuator representation. When a positioner is used with a valve, the diaphragm
representation may be combined with a representation of the actuator and the valve body.
A special actuator symbol is defined by the standard for motorized actuators. Motorized
actuators are used in some industry segments because upon loss of power, the last valve
position is maintained. Also, better resolution may be achieved using a motorized actuator for
Figure 4-11 Figure 4-12
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a specific application, such as the basis weight valve that is used to regulate the thick stock
flow to a paper machine in paper manufacturing. [6] Solenoid actuators that are used to
automate valves used in on-off service are shown using a special symbol.
Installation details are provided in the loop diagram documentation. Some of details may also
be provided in the P&ID. For example, the P&ID and loop diagram may show the orifice that
must be installed to measure flow using a differential pressure transmitter. Hand-operated
valves that are used to block flow during start-up or maintenance are shown since they impact
process operation if not properly set up. Also, the installation of inline instrumentation, such
as a magnetic flow meter, is commonly shown in a unique manner in the control system
documentation. In addition, measurement elements such as an RTD or a thermocouple are
shown since they may be physically installed some distance from the field transmitter or the
control system. To provide a consistent means of documenting the physical installation, the
ISA-5.1 standard includes symbols for many of these common installation details and field
devices. A sample of some of these symbols is shown in Figure 4-13.
4.7 Equipment Representation
A representation of major pieces of process equipment is normally included in control system
documentation. This allows the field instrumentation installation to be shown in relationship to
the process equipment. Example process equipment representations are illustrated in Figure 4-
14.
A general vessel representation may be appropriate for vessels, agitators, heat exchangers, and
pumps that do not play an important role in the control system. For example, an agitator on a
tank may not directly impact the control associated with the tank level. A special representation
is provided for a reactor. A jacketed vessel symbol may be used when a vessel is heated or
Figure 4-13
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cooled by circulating liquid through an outside shell. Such a design is commonly used in the
batch industry, and permits the vessel’s contents to be heated or cooled without coming in
contact with the circulating liquid. A symbol is also provided for flat-bottomed and coneshaped
storage tanks that are open to the atmosphere. The examples include a representation of a heat
exchanger, which is used to heat or cool a liquid stream. A symbol is defined for an agitator
that may be used to ensure good mixing of liquids in a vessel. Also, a pump symbol is shown
in these examples.
5) 4-20mA Current Loop
5.1 Why Use a Current Loop?
The 4-20mA current loop shown in Figure 5-1 is a common method of transmitting sensor
information in many industrial process-monitoring applications. A sensor is a device used to
measure physical parameters such as temperature, pressure, speed, liquid flow rates, etc.
Transmitting sensor information via a current loop is particularly useful when the information
has to be sent to a remote location over long distances (1000 feet, or more). The loop’s
operation is straightforward: a sensor’s output voltage is first converted to a proportional
current, with 4mA normally representing the sensor’s zero-level output, and 20mA
representing the sensor’s full-scale output. Then, a receiver at the remote end converts the 4-
20mA current back into a voltage which in turn can be further processed by a computer or
display module.
However, transmitting a sensor’s output as a voltage over long distances has several drawbacks.
Unless very high input-impedance devices are used, transmitting voltages over long distances
produces correspondingly lower voltages at the receiving end due to wiring and interconnect
Figure 4-14
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resistances. However, high-impedance instruments can be sensitive to noise pickup since the
lengthy signal-carrying wires often run in close proximity to other electrically noisy system
wiring. Shielded wires can be used to minimize noise pickup, but their high cost may be
prohibitive when long distances are involved.
Sending a current over long distances produces voltage losses proportional to the wiring’s
length. However, these voltage losses— also known as “loop drops”—do not reduce the 4-
20mA current as long as the transmitter and loop supply can compensate for these drops. The
magnitude of the current in the loop is not affected by voltage drops in the system wiring since
all of the current (i.e., electrons) originating at the negative (-) terminal of the loop power
supply has to return back to its positive (+) terminal—fortunately, electrons cannot easily jump
out of wires!
5.2 Current Loop Components
A typical 4-20mA current-loop circuit is made up of four individual elements: a
sensor/transducer; a voltage-to-current converter (commonly referred to as a transmitter and/or
signal conditioner); a loop power supply; and a receiver/monitor. In loop powered applications,
all four elements are connected in a closed, series circuit, loop configuration (see Figure 5-1).
Sensors provide an output voltage whose value represents the physical parameter being
measured. (For example, a thermocouple is a type of sensor which provides a very low-level
output voltage that is proportional to its ambient temperature.) The transmitter amplifies and
conditions the sensor’s output, and then converts this voltage to a proportional 4-20mA dc-
current that circulates within the closed series-loop. The receiver/monitor, normally a
subsection of a panel meter or data acquisition system, converts the 4-20mA current back into
a voltage which can be further processed and/or displayed.
The loop power-supply generally provides all operating power to the transmitter and receiver,
and any other loop components that require a well-regulated dc voltage. In loop-powered
Figure 5-1
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applications, the power supply’s internal elements also furnish a path for closing the series
loop. +24V is still the most widely used power supply voltage in 4-20mA process monitoring
applications. This is due to the fact that +24V is also used to power many other instruments
and electromechanical components commonly found in industrial environments. Lower supply
voltages, such as such as +12V, are also popular since they are used in computer-based systems.
5.3 Loop Drops
One of a process monitor’s most important specifications—be it a loop-powered or locally
powered device—is the total resistance (or “burden”) it presents to the transmitter’s output
driver. Most transmitter’s data sheets specify the maximum loop resistance the transmitter can
drive while still providing a full-scale 20mA output (the worst-case level with regards to loop
burden).
Ohm’s Law states that the voltage drop developed across a current-carrying resistor can be
found by multiplying the resistor’s value by the current passing through it. Stated in
mathematical terms:
E = I x R where E is the voltage drop in volts, I is the current through the resistor in amperes,
and R is the resistor’s value in Ohms (the ‘Ω’ symbol is commonly used to represent Ohms).
The sum of the voltage drops around a series loop has to be equal to the supply voltage. For
example, when a loop-powered application is powered from a 24V power source, the sum of
all the voltage drops around the series loop has to also equal 24V. Every component through
which the 4-20mA loop current passes develops a maximum voltage drop equal to that
component’s resistance
multiplied by 0.020
Amperes (20mA). For
example, referring to Figure
2 the DMS-20PC-4/20S’s
250Ω resistance yields a
maximum loop drop of:
250x 0.020A = 5.0V
Figure 5-2
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5.4 Transmitter Ratings
With the above loop-drop theory in mind, and assuming a +24V loop-powered application in
which the transmitter’s minimum operating voltage is 8V, and the process monitor drops only
4V, a logical question which arises is what happens to the “extra” 12V? The extra 12V has to
be dropped entirely by the transmitter since most process monitors have purely resistive inputs
combined with Zener diodes that limit their maximum voltage drop.
Transmitters usually state both minimum and maximum operating voltages. The minimum
voltage is that which is required to ensure proper transmitter operation, while the maximum
voltage is determined by its maximum rated power-dissipation, as well as by its
semiconductors’ breakdown ratings. A transmitter’s power dissipation can be determined by
multiplying its loop drop by the highest anticipated output current, usually, but not always,
20mA. For example, if a transmitter drops 30V at an over range output level of 30mA, its power
dissipation is:
30V x 0.030A = 0.9 watts
5.5 Wiring Resistance
Because copper wires exhibit a dc-resistance directly proportional to their length and gauge
(diameter), this application note would not be complete without discussing the important topic
of wiring—specifically the effects wiring resistance has on overall system performance.
Applications in which two or more loop-monitoring devices are connected over very long, 2-
way wiring distances (1000-2000 feet) normally use +24V supplies because many transmitters
require a minimum 8V-supply for proper operation. When this 8-volt minimum is added to the
typical 3-4 volts dropped by each process monitor and the 2-4 volts dropped in the system
wiring and interconnects, the required minimum supply voltage can easily exceed 16V. The
following worked-out example will illustrate these important concepts.
The voltage drop developed along a given length of wire is found by multiplying the wire’s
total resistance by the current passing through it. The wire’s total resistance is found by looking
up its resistance (usually expressed in Ohms per 1000 feet) in a wire specifications table.
Referring to Figure 3 if a transmitter’s output is delivered to a remote process monitor using
2000 feet (660 meters) of 26-guage, solid copper wire having a resistance of 40.8Ω per 1000
feet, the one-way voltage dropped by the wire when the transmitter’s output is 20mA is equal
to:
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E = 0.020 Amperes x [2000 feet x (40.8Ω /1000 feet)]
E = 0.020A x 81.6Ω = 1.63V
However, the current must travel 2000 feet down to the process monitor and another 2000 feet
back to the transmitter’s “+” output terminal, for a total of 4000 feet. As noted above, 26-gauge
wire has a resistance of 40.8Ω per 1000 feet, yielding a total loop resistance (R) equal to 4000
feet x (40.8Ω /1000 feet) = 163.2Ω. The total voltage dropped over the 4000 feet of wiring is
therefore: E = 0.020A x 163.2Ω
E = 3.27V.
Looking down the loop towards the remote process monitor, the transmitter sees the sum of the
3.27V wire drop and the 5.0V process-monitor drop, for a total loop-drop of 8.27V. If the
transmitter itself requires a minimum of 8V (this is also considered a voltage drop) for proper
operation, the lowest power supply voltage required for the system shown in Figure 5-3 is
16.3V.
5.6 In depth of 4 to 20 mA analog current signals
The most popular form of signal transmission used in modern industrial instrumentation
systems (as of this writing) is the 4 to 20 milliamp DC standard. This is an analog signal
standard, meaning that the electric current is used to proportionately represent measurements
or command signals.
Typically, a 4 milliamp current value represents 0% of scale, a 20 milliamp current value
represents 100% of scale, and any current value in between 4 and 20 milliamps represents a
commensurate percentage in between 0% and 100%. The following table shows the
corresponding current and percentage values for each 25% increment between 0% and 100%.
Figure 5-3
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Every instrument technician tasked with maintaining 4-20 mA instruments commits these
values to memory, because they are referenced so often:
Current Value % of Scale
4mA 0%
8mA 25%
12mA 50%
16mA 75%
20mA 100%
For example, if we were to calibrate a 4-20 mA temperature transmitter for a measurement
range of 50 to 250 degrees C, we could relate the current and measured temperature values on
a graph like this:
This is not unlike the pneumatic instrument signal standard or 3 to 15 pounds per square inch
(PSI), where a varying air pressure signal represents some process measurement in an analog
(proportional) fashion. Both signal standards are referred to as live zero because their ranges
begin with a non-zero value (3 PSI in the case of the 3-15 PSI standard, and 4 milliamps in the
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case of the 4-20 mA standard). This “live” zero provides a simple means of discriminating
between a legitimate 0% signal value and a failed signal (e.g. leaking tube or severed cable).
DC current signals are also used in control systems to command the positioning of a final
control element, such as a control valve or a variable-speed motor drive (VSD). In these cases,
the milliamp value does not directly represent a process measurement, but rather how the
degree to which the final control element influences the process. Typically (but not always!),
4 milliamps commands a closed (shut) control valve or a stopped motor, while 20 milliamps
commands a wide-open valve or a motor running at full speed.
Thus, most industrial control systems use at least two different 4-20 mA signals: one to
represent the process variable (PV) and one to represent the command signal to the final control
element (the “manipulated variable” or MV):
The relationship between these two signals depends entirely on the response of the controller.
There is no reason to ever expect the PV and MV current signals to be equal to each other, for
they represent entirely different variables. In fact, if the controller is reverse-acting, it is entirely
normal for the two current signals to be inversely related: as the PV signal increases going to
a reverse-acting controller, the output signal will decrease. If the controller is placed into
“manual” mode by a human operator, the output signal will have no automatic relation to the
PV signal at all, instead being entirely determined by the operator’s whim.
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5.7 Figure below illustrates three basic transmitter current loop types:
 Type 2 is a 2-wire transmitter energized by the loop current where the loop source
voltage (compliance) is included in the
receiver. The transmitter floats and signal
ground is in the receiver.
 Type 3 is a 3-wire transmitter energized
by a supply voltage at the transmitter. The
transmitter sources the loop current.
Transmitter common is connected to receiver
common
 Type 4 is a 4-wire transmitter energized
by a supply voltage at the transmitter. The
transmitter sources the loop current to a
floating receiver load.
If a transmitter has field inputs, which
provide signals referenced to field grounds potential ground loops exist. This potentially will
cause signal gradation. It is necessary to understand that all 4-20 mA transmitters may not
necessarily be identical in their ability to provide current into different loads. For example, a
typical 4-20mA transmitter module could not drive a 100k-ohm load. This would require a
compliance source of 2000 volts (20mAx100kΩ). The class standard ensures that modules of
identical classes are interchangeable with respect to their drive capabilities. It is noteworthy to
mention here that one should always completely examine all module specifications before
replacing units.
5.8 4-wire (“self-powered”) transmitter current loops
DC electric current signals may also be used to communicate process measurement information
from transmitters to controllers, indicators, recorders, alarms, and other input devices. Recall
that the purpose of a transmitter is to sense some physical variable (e.g. pressure, temperature,
flow) and then report that quantity in the form of a signal, in this case a 4 to 20 milliamp DC
current proportional to that measured quantity. The simplest form of 4-20 mA measurement
loop is one where the transmitter has two terminals for the 4-20 mA signal wires to connect,
and two more terminals where a power source connects. These transmitters are called “4-wire”
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or self-powered. The current signal from the transmitter connects to the process variable input
terminals of the controller to complete the loop:
Typically, process controllers are not equipped to directly accept milliamp input signals, but
rather voltage signals. For this reason we must connect a precision resistor across the input
terminals to convert the 4-20 mA signal into a standardized analog voltage signal that the
controller can understand. A voltage signal range of 1 to 5 volts is standard, although some
models of controller use different voltage ranges and therefore require different precision
resistor values. If the voltage range is 1-5 volts and the current range is 4-20 mA, the precision
resistor value must be 250 ohms.
Since this is a digital controller, the input voltage at the controller terminals is interpreted by
an analog-to-digital converter (ADC) circuit, which converts the measured voltage into a digital
number that the controller’s microprocessor can work with.
5.9 2-wire (“loop-powered”) transmitter current loops
It is possible to convey electrical power and communicate analog information over the same
two wires using 4 to 20 milliamps DC, if we design the transmitter to be loop-powered. A loop-
powered transmitter connects to a process controller in the following manner:
Here, the transmitter is not really a current source in the sense that a 4-wire transmitter is.
Instead, a 2-wire transmitter’s circuitry is designed to act as a current regulator, limiting current
in the series loop to a value representing the process measurement, while relying on a remote
source of power to motivate current to flow. Please note the direction of the arrow in the
transmitter’s dependent current source symbol, and how it relates to the voltage polarity marks.
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Refer back to the
illustration of a 4-wire
transmitter circuit for
comparison. The
current “source” in
this loop-powered
transmitter actually
behaves as an
electrical load, while
the current source in the 4-wire transmitter functioned as a true electrical source.
A loop-powered transmitter gets its operating power from the minimum terminal voltage and
current available at its two terminals. With the typical source voltage being 24 volts DC, and
the maximum voltage dropped across the controller’s 250 ohm resistor being 5 volts DC, the
transmitter should always have at least 19 volts available at its terminals. Given the lower end
of the 4-20 mA signal range, the transmitter should always have at least 4 mA of current to run
on. Thus, the transmitter will always have a certain minimum amount of electrical power
available on which to operate, while regulating current to signal the process measurement.
Internally, the electronic hardware of a 2-wire transmitter circuitry resembles the following
(simplified) diagram. Note that everything shown within the shaded rectangle is represented
by the “2-wire transmitter” circle in the previous diagram:
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All sensing, scaling, and output conditioning circuitry inside the transmitter must be designed
to run on less than 4 mA of DC current, and at a modest terminal voltage. In order to create
loop currents exceeding 4 mA – as the transmitter must do in order to span the entire 4 to 20
milliamp signal range – the transmitter circuitry uses a transistor to shunt (bypass) extra current
from one terminal to the other as needed to make the total current indicative of the process
measurement. For example, if the transmitter’s internal operating current is only 3.8 mA, and
it must regulate loop current at a value of 16 mA to represent a condition of 75% process
measurement, the transistor will bypass 12.2 mA of current.
The very low amount of electrical power available at a 2-wire transmitter’s terminals limits its
functionality. If the transmitter requires more electrical power than can be delivered with 4
milliamps and 19 volts (minimum each), the only solution is to go with a 4-wire transmitter
where the power conductors are separate from the signal conductors. An example of a process
transmitter that must be 4-wire is a chemical analyser such as a chromatograph, requiring
enough power to operate an electrical heater, solenoid valves, and an on-board computer to
process the sensor data. There is simply no way to operate a machine as complex and power-
draining as a 2010-era chromatograph on 4 milliamps and 19 volts!
Early current-based industrial transmitters were not capable of operating on such low levels of
electrical power, and so used a different current signal standard: 10 to 50 milliamps DC. Loop
power supplies for these transmitters ranged upwards of 90 volts to provide enough power for
the transmitter. Safety concerns made the 10-50 mA standard unsuitable for some industrial
installations, and modern microelectronic circuitry with its reduced power consumption made
the 4-20 mA standard practical for nearly all types of process transmitters.
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6) Instrumentation Terms:
Industrial measurement and control systems have their own unique terms and standards. Here
are some common instrumentation terms and their definitions:
Process:
The physical system we are attempting to control or measure. Examples: water filtration
system, molten metal casting system, steam boiler, oil refinery unit, power generation unit.
Process Variable, or PV:
The specific quantity we are measuring in a process. Examples: pressure, level, temperature,
flow, electrical conductivity, pH, position, speed, vibration.
Set point, or SP:
The value at which we desire the process variable to be maintained at. In other words, the
“target” value of the process variable.
Primary Sensing Element, or PSE:
A device that directly senses the process variable and translates that sensed quantity into an
analog representation (electrical voltage, current, resistance; mechanical force, motion, etc.).
Examples: thermocouple, thermistor, bourdon tube, microphone, potentiometer,
electrochemical cell, accelerometer.
Transducer:
A device that converts one standardized instrumentation signal into another standardized
instrumentation signal, and/or performs some sort of processing on that signal. Often referred
to as a converter and sometimes as a “relay.” Examples: I/P converter (converts 4-20 mA
electric signal into 3-15 PSI pneumatic signal), P/I converter (converts 3-15 PSI pneumatic
signal into 4-20 mA electric signal), square root extractor (calculates the square root of the
input signal).
Note: in general science parlance, a “transducer” is any device that converts one form of energy
into another, such as a microphone or a thermocouple. In industrial instrumentation, however,
we generally use “primary sensing element” to describe this concept and reserve the word
“transducer” to specifically refer to a conversion device for standardized instrumentation
signals.
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Transmitter:
A device that translates the signal produced by a primary sensing element (PSE) into a
standardized instrumentation signal such as 3-15 PSI air pressure, 4-20 mA DC electric current,
Fieldbus digital signal packet, etc., which may then be conveyed to an indicating device, a
controlling device, or both.
Lower- and Upper-range values, abbreviated LRV and URV, respectively:
The values of process measurement deemed to be 0% and 100% of a transmitter’s calibrated
range. For example, if a temperature transmitter is calibrated to measure a range of temperature
starting at 300 degrees Celsius and ending at 500 degrees Celsius, 300 degrees would be the
LRV and 500 degrees would be the URV.
Zero and Span:
Alternative descriptions to LRV and URV for the 0% and 100% points of an instrument’s
calibrated range. “Zero” refers to the beginning-point of an instrument’s range (equivalent to
LRV), while “span” refers to the width of its range (URV − LRV). For example, if a
temperature transmitter is calibrated to measure a range of temperature starting at 300 degrees
Celsius and ending at 500 degrees Celsius, its zero would be 300 degrees and its span would
be 200 degrees.
Controller:
A device that receives a process variable (PV) signal from a primary sensing element (PSE) or
transmitter, compares that signal to the desired value for that process variable (called the set
point), and calculates an appropriate output signal value to be sent to a final control element
(FCE) such as an electric motor or control valve.
Final Control Element, or FCE:
A device that receives the signal from a controller to directly influence the process. Examples:
variable-speed electric motor, control valve, electric heater.
Manipulated Variable, or MV:
Another term to describe the output signal generated by a controller. This is the signal
commanding (“manipulating”) the final control element to influence the process.
Automatic mode:
When the controller generates an output signal based on the relationship of process variable
(PV) to the set point (SP).
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Manual mode:
When the controller’s decision-making ability is bypassed to let a human operator directly
determine the output signal sent to the final control element.
7) Instrumentation Transmitters:
6.1 Introduction
6.1.1 What are transmitters?
A transmitter is a device which converts the reading from a primary sensor or transducer into
a standard signal and transmits that signal to a monitor or controller. The methods of
successfully transmitting the data to the control room are listed below
6.1.2 Types of signals used by transmitters:
There are three kinds of signals that are present in the process industry to transmit the reading
of a process variable from the instrument to the centralized control system. These are,
1. Pneumatic signals
2. Analog signals
3. Digital signals
Pneumatic signals:
These are the signals produced by changing the air pressure in the signal pipe in proportion to
the measured change in a process variable. The pneumatic signal range which is the common
industrial standard is 3-15 psig. The 3 corresponds to the lower range value (LRV) and the 15
corresponds to the upper range value (URV). It is still a very commonly used signal type.
However, since the invention of electronic instruments in the 1960s, the lower costs involved
in running electrical signal wire through a plant as opposed to running pressurized air tubes has
made pneumatic signal technology less popular
Figure 6-1: Pneumatic type Pressure Transmitter
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Analog signals:
It is an electrical signal whose current’s or voltage’s magnitude represents some physical
measurement or control quantity. An instrument is often classified as being “analog” simply
by virtue of using an analog signal standard to communicate information, even if the internal
construction and design of the instrument may be mostly digital in nature.
The most common standard for transmitting an analog signal is the 4-20 mA current signals.
With this signal, a transmitter sends a small current through a set of wires. The signal generated
is a kind of a gauge in which 4 mA represents the lowest possible measurement, or zero, and
20 mA represents the highest possible measurement.
Example: Consider a process that must be maintained at 100 °C. An RTD temperature
sensor and transmitter are installed in the process vessel, and the transmitter is set to produce
a 4 mA signal when the process temperature is at 95 °C and a 20 mA signal when the process
temperature is at 105 °C. The transmitter will transmit a 12 mA signal when the temperature
is at the 100 °C set point. As the sensor’s resistance property changes in response to changes
in temperature, the transmitter outputs a 4–20 mA signal that is proportional to the
temperature changes. The signal transmitted can be converted to a temperature reading or
an input to a control device.
Why is this analogue signal conditioning required?
Traditionally, data acquisition systems have acquired analog data in the form of
temperatures, accelerations, strains, positions, etc. This type of data has always required
analog signal conditioning in order for the data system to accept it as an input source. For
example, the full-scale output of a transducer may be in the range of 0-20mVDC where the
input range to the data system is 0-5Vdc. In this case, it must be noted that voltage
amplification is required [3]
.
Amplifiers are considered to be the most common piece of signal conditioning equipment
because of their wide range of uses, such as amplification, attenuation, DC-shifting,
impedance matching, isolation, and others. The instrumentation amplifier amplifies the
difference between two signals.
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Wheatstone bridge:
Another very important device in instrumentation systems, the Wheatstone bridges are
normally associated with strain gages but are also used as bridge completion networks for
resistive transducers such as Resistive Temperature Devices (RTDs). The Wheatstone
bridge consists of four resistances, R1-R4 and a voltage source V for exciting the bridge,
see figure 3. The transducer is placed in one arm of the bridge with a steady-state resistance
equal to the other three resistances. Therefore, only when the transducer’s steady-state
resistance changes, there is an output of the bridge.
Figure 3: Wheatstone bridge
Digital Signals:
The most recent addition to process signal control technology are the digital signals. Digital
signals are discrete levels or values that are combined in specific ways to represent process
variables and also carry other information, such as diagnostic information. The
methodology used to combine the digital signals is referred to as protocol. Digital signal
conditioning can be considered as changing one form of digital data to another form. An
example would be the serial-to parallel or parallel-to-serial conversion. Some even consider
the analog-to-digital conversion as digital signal conditioning.
Digital multiplexing can also be considered as digital signal conditioning, one type of digital
data is transformed into another type. Another form of digital signal conditioning related to
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instrumentation systems is digital filtering. There are two forms of digital filters. They are the
finite impulse response (FIR) filters and the infinite impulse response (IIR) filters.
6.1.3 Telemetry:
Telemetering is the reproduction, at a convenient location, of measurements makes at
remote point. It is the method of getting information from one point to the other.
The telemetry system can be defined as everything required to converting baseband data to
radio frequency (RF) data and back again at some different location. This includes
modulation of the information signal to an RF carrier, transmission, acquiring, receiving,
and demodulation of the signal back into its original baseband form.
Telemetry system components from the test vehicle side include the information signal, any
required encryption or pre-modulation filtering, a telemetry transmitter, a power splitter if
required, and vehicle antenna(s). Telemetry system components on the receiving side include
reception antenna(s), pre-amps and splitters, telemetry receiver, demodulator, decryptor and
bit synchronizer, if required.
In general, a telemetering system consists of:
1. A measuring instrument which may measure flow, liquid level, pressure, temperature
or any other variable.
2. A conversion element that converts the measured variable into a proportional air
pressure and electrical quantity.
3. The pressure lines or connecting wires which may carry the transmitted variable from
the transmitter to the receiver.
4. A receiver which indicates the size of the transmitted variable and may also record or
control the measured variable.
6.1.4 Transmission channel:
As soon as the data leaves the test vehicle in the form of a modulated RF carrier through a
telemetry antenna, it will experience anomalies associated with the transmission medium. This
medium is normally air. Most transmission antennas on test vehicles are Omni directional,
which means the transmitted signal is sent in all directions. When more than one path, or ray,
makes it into the receive antenna feed, multipath effects may occur. Most of the time, there is
one dominant ray received due to the narrow beam width of the receive antenna. When one or
more of these reflected paths, caused by terrain variations between the test vehicle and
receiving antenna, are within the beam width of the antenna, it can either add constructively or
destructively to the direct ray.
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6.1.5 Receiving station:
The role of the receiving station is to receive the transmitted signal and to recreate the
original data on the ground.
6.2 Transmitters in Process Industries:
The types of transmitters used in Process Industries include:
1. Pressure transmitters
2. Temperature transmitters
3. Flow transmitters
4. Level transmitters
5. Analytic (O2 [oxygen], CO [carbon monoxide], and pH) transmitters.
8) Continuous pressure measurement:
In many ways, pressure is the primary variable for a wide range of process measurements.
Many types of industrial measurements are actually inferred from pressure, such as:
• Flow (measuring the pressure dropped across a restriction)
• Liquid level (measuring the pressure created by a vertical liquid column)
• Liquid density (measuring the pressure difference across a fixed-height liquid column)
• Weight (hydraulic load cell)
Even temperature may be inferred from pressure measurement, as in the case of a fluid-filled
chamber where fluid pressure and fluid temperature are directly related. As such, pressure is a
very important quantity to measure, and measure accurately.
8.1 Electrical pressure elements:
Several different technologies exist for the conversion of fluid pressure into an electrical signal
response. These technologies form the basis of electronic pressure transmitters: devices
designed to measure fluid pressure and transmit that information via electrical signals such as
the 4-20 mA analog standard, or in digital form such as HART or FOUNDATION Fieldbus.
8.1.1 Piezo-resistive (strain gauge) sensors
Piezoresistive means “pressure-sensitive resistance,” or a resistance that changes value with
applied pressure. The strain gauge is a classic example of a piezoresistive element, a typical
strain gauge element shown here on the tip of finger:
P a g e | 72
INSTRUMENTATION IN NAPHTHA CRACKER PLANT MAULIN AMIN
In order to be practical, a strain gauge must be glued (bonded) on to a larger specimen capable
of withstanding an applied force (stress):
As the test specimen is stretched or compressed by the application of force, the conductors of
the strain gauge are similarly deformed. Electrical resistance of any conductor is proportional
to the ratio of length over cross-sectional area (R ∝
𝑙𝑙
𝐴𝐴
), which means that tensile deformation
(stretching) will increase electrical resistance by simultaneously increasing length and
decreasing cross-sectional area while compressive deformation (squishing) will decrease
electrical resistance by simultaneously decreasing length and increasing cross-sectional area.
Attaching a strain gauge to a diaphragm results in a device that changes resistance with applied
pressure. Pressure forces the diaphragm to deform, which in turn causes the strain gauge to
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Full

  • 1. Training Report The Maharaja Sayajirao University of Baroda Faculty of Technology and Engineering Vadodara Manufacturing Division Gujarat Olefins Plant (Instrumentation Department) A Report by: Maulin R. Amin (Electronics Engineering)
  • 2. Table of Contents: Certificate 01 Preface 02 Acknowledgement 03 Reliance Industries Limited (Vadodara Manufacturing Division) 1) Company Profile 04 2) Commitments 05 3) Vadodara 09 4) Plants at VMD 09 Gujarat Olefins Plant : Units and NCP Process 1) Introduction 13 2) Introduction to NCP 13 1) Product 13 2) By-Products 13 3) Numbering of equipment 14 4) Characteristics of Naphtha 15 5) Principle of Cracking 15 6) Mechanism of Cracking 16 7) Variable of Cracking Operation 17 8) Factors affecting heater operation 18 9) Function of dilution steam in cracking 18 10) Effects of partial pressure during cracking 19 11) Functions of DMDS in feed stock Naphtha Cracker Plant 1) Modes of Operation 20 2) Process Flow Diagram 21 3) Instrumentation and Piping Diagram 22 Instrumentation in NCP 1) Introduction 23 2) Functions and Advantages 23 3) Terminology and Symbols in Control system engineering 23 4) Control and Field Instrumentation Documentation 35
  • 3. P a g e | 1 CERTIFICATE MAULIN AMIN Certificate This is to certify that Mr. Maulin Riteshbhai Amin, student of Faculty of Technology and Engineering, The Maharaja Sayajirao University of Baroda has successfully completed his training at Gujarat Olefins Plant at Reliance Industries Limited, Vadodara Manufacturing Division towards the completion of the period from 21st May 2013 to 22nd June 2013 under my supervision and guidance with utmost satisfaction. It indeed gives us pleasure to highlight that Mr. Maulin R Amin worked hard and with deep sincerity throughout his training duration. I appreciate his sincere efforts and I am sure that the experience gained during the course of this training will enable him to take up more challenging tasks in future. Date: Mr. Navin Chandra Patel Sr. General Manager Gujarat Olefins Plant Reliance Industries Limited Vadodara Manufacturing Division Vadodara, Gujarat Mr. B.P. Shah HR Manager Reliance Industries Limited Vadodara Manufacturing Division Vadodara, Gujarat
  • 4. P a g e | 2 PREFACE MAULIN AMIN Preface India had definitely become a strong country as far as the petrochemical industry is concerned. India has undoubtedly strengthened its position in the international market by roping in foreign investment and trying to establish its strong base abroad. In the present area, it has become necessary to compete on the global stage. It is equally important to have practical as well as theoretical knowledge. Theoretical knowledge can be easily acquired from books and publications, but has to pass through an on- site training phase for practical knowledge. Having said this, one has to observe and study the equipment used for the process, its construction, start up, shut down procedures, operating problems, its solution, emergencies, etc. The theories and usual practices cited in books and literature differ up to an appreciable extent from the industrial practices. Another attractive feature is to learn industrial management and discipline, which is equally important in life. This is only possible through industrial training.
  • 5. P a g e | 3 ACKNOWLEDGEMENT MAULIN AMIN Acknowledgement It’s a pleasure to thank all the authorities and personnel, who directly or indirectly are involved in successful completion of my in-plant training. I bestow my gratitude to Mr. B.P. Shah for granting us the permission to obtain training at Reliance Industries Limited, Vadodara Manufacturing Division. I am very thankful to Mr Navin Chandra Patel for encouraging me at each step of our training. I am thankful to Mr Mahendra Upadhyay my industry mentor, for continuously guiding and supporting me throughout the training. Mr. Rahul Ravi and Mr. Adersh V, my training mentor for their consistent guidance and support throughout my training and for constantly ensuring that the training is opening up new aspects of Instrumentation industry for my learning. They not only solved my difficulties, but also shared their immense experience from their service in this industry. I finally would like to thank each and every employee of Gujarat Olefins Plant for all the support and help they provided during the course of training. My training would have been incomplete without their support and expertise.
  • 6. RELIANCE INDUSTRIES LIMITED MAULIN AMIN About Reliance Industries Limited (Vadodara Manufacturing Division)
  • 7. P a g e | 4 RELIANCE INDUSTRIES LIMITED MAULIN AMIN 1)Company Profile: Reliance Industries Limited (RIL) is world’s leading and India’s no. one private limited company. RIL group is highly diversified group and is in to multiproduct business like oil/gas exploration, retail of petro/consumer products and manufacturing of petrochemical/refining and textile products and also in to infrastructure and transportation sector. RIL-VMD was earlier a part of Indian Petrochemicals Corporation Limited (IPCL) with the management controlled by government of India. In 2002, due to divestment of equity, the management control went in the hands of Reliance Petro Invest Co. of RIL group. On September 5th 2007, merging of IPCL with RIL was legally concluded. RIL – VMD’s multi product manufacturing portfolio includes polymers, synthetic rubber, synthetic fibre and fibre intermediates, solvents and industrial chemicals. It has several distinctions to its credit. Accredited earlier for Best Performance Award among petrochemical companies worldwide (CI London), FICCI awards, ICMA awards, National Energy Awards and several awards from National Safety Council, USA and British Safety Council, UK. In fact it has integrated management system in place comprising of ISO 9001, ISO 14001 and OSHAS 18001 certification for all plants and departments of site.
  • 8. P a g e | 5 RELIANCE INDUSTRIES LIMITED MAULIN AMIN 2)Commitments:
  • 9. P a g e | 6 RELIANCE INDUSTRIES LIMITED MAULIN AMIN
  • 10. P a g e | 7 RELIANCE INDUSTRIES LIMITED MAULIN AMIN
  • 11. P a g e | 8 RELIANCE INDUSTRIES LIMITED MAULIN AMIN
  • 12. P a g e | 9 VADODARA MANUFACTURING DIVISION MAULIN AMIN Vadodara: Vadodara Manufacturing Division located in Vadodara, Gujarat. It comprises of a Naptha cracker and 15 downstream plants for the manufacture of polymers, fibres, fibre intermediates and chemicals: Plants at the Vadodara Manufacturing Division Name of Plant Commissioned Year Naphtha Cracker 1979 LDPE 1979 Mono Ethylene Glycol/Ethylene Oxide 1979 Butadiene Extraction 1979 Polybutadiene Rubber Plant 1 1979 Polybutadiene Rubber Plant II 1996 Benzene Extraction 1979 LAB 1979 Acrylonitrile Plant 1979 Acrylic Fibre Monocomponent 1979 Acrylates 1983 VCM 1984 PVC 1984 Polypropylene Copolymer Pant 1988 Acrylic Fibre Bi-component Plant 1989 Polypropylene Plant 1996 1. Gujarat Olefins Plant (GOP) Raw material: Naptha Products:  Ethylene  Propylene  Cu Stream  Pyrolysis gasoline  Carbon black feedstock
  • 13. P a g e | 10 VADODARA MANUFACTURING DIVISION MAULIN AMIN 2. Gujarat Aromatic Plant (GAP)* Xylenes Raw material: Naptha Products:  Orthoxylene  Mixed Xylene  Para Xylene  Solvent CIX  Hepton Dimethyl terephthalate plant Raw material: Para Xylene Products:  Methanol  DMT  Methyl Benzoate  Dimethyl Isophthalate 3. Integrated Offsite Plant (IOP) Various utility system associated with the process units and offsite facilities. Under IOP projects are as under:  Fire water system  Service water system  Drinking water and semitate water system  D.M. water system  Process water system  Cooling water system  Steam water system  Compressed air system 4. Ethylene Glycol Plant (EG) Raw material: Ethylene Products:
  • 14. P a g e | 11 VADODARA MANUFACTURING DIVISION MAULIN AMIN  Ethylene Glycol  Ethylene Oxide  Di ethylene glycol  Tri ethylene glycol  Poly ethylene glycol 5. Acrylonitrile Plant (ACN) Raw material: Propylene and Ammonia Products:  A.C.N.  Acetonitrile  Hydro cynic acid 6. Propylene Plant (P.P.) Raw material: Propylene Products:  Poly propylene  Atactic polymer 7. Gas Turbine Power Plant (GTTP) It is a gas turbine plant which produces electricity of 72MWH. It is the last plant commissioned at Baroda complex. 8. Vinyl chloride/ Polyvinyl chloride (VC/PVC) Raw material: Ethylene and Chlorine Products:  Vinyl chloride  Poly vinyl chloride 9. Acrylic Fibres (AF)* Raw materials: Acrylonitrite and Methyl acrylate Products: Acrylic fibres 10. Low density poly ethylene (LDPE) Raw material: Ethylene Products: Low Density Poly Ethylene 11. Linear Alkyl Benzene (LAB)* Raw material: Benzene and Superior kerosene Products:
  • 15. P a g e | 12 VADODARA MANUFACTURING DIVISION MAULIN AMIN  Linear alkyl benzene  Poly alkyl benzene  Heavy n-paraffin  Centon  L.R.2030 12. Poly Propylene co-polymer Plant (PPCP) Raw material: Propylene Products:  Poly propylene co-polymer  Atactic co-polymer 13. Poly Butadiene Rubber (PBR) Raw material: Butadiene Products: Poly Butadiene Rubber 14. Acrylates Plant (ACR)* Raw material:  Acrylonitrile  Sulphuric Acid  Alcohols  Ammonia Products:  Ammonium sulphates  Ethyl acrylates  Butyl acrylates  2-Ethyl hexyl acrylates  Methyl acrylates 15. Petrol Resins Plant (PR) Raw material: Pyrolysis gasoline Products: Petroleum resins *Plants not in Operation
  • 16. About Gujarat Olefins Plant Units and NCP Process
  • 17. P a g e | 13 GUJARAT OLEFINS PLANT MAULIN AMIN 1)Introduction: GOP is the mother plant of RIL, VMD. Here, naphtha is cracked to produce feed stock for other plants at RIL. The main products of GOP are ethylene, propylene, gasoline and C4 raffinate.  Ethylene is supplied to LDPE and EG plants.  Propylene formed has two grades, based on the level of purity. o Polymer grade (PG), 99% pure is supplied to PP4. o Chemical grade (CG), 95% pure is treated for the removal of impurities.  Mixed C4 products are used in the formation of rubber at PBR I and PBR II plants. The GOP consists of the following units:  Naphtha Cracker Plant (NCP)  Benzene Butadiene Hydrogenation (BBH)  Pyrolysis Gasoline Hydrogenation (PGH)  Benzene Extraction  Butadiene Extraction  Feed purification unit  Off sites 2)Introduciton to Naphtha Cracker Plant: The naphtha cracker plant, designated as Unit 21 of GOP is designed by M/s ABB Lummus Global and its detailed engineering is done by M/s EIL. It was commissioned in March 1978. It was designed at for nameplate capacity of 130 KTA. 1) Product:  Ethylene 2) By-products:  Propylene (PG, CG)  Mix C4  Pyrolysis Gasoline  CBFS  Fuel Gas (HP, MP, LP methane)
  • 18. P a g e | 14 GUJARAT OLEFINS PLANT MAULIN AMIN  Hydrogen The NCP is divided into three zones to facilitate easier operation and understanding. The three zones are:  AB zone: cracking and quenching, high temperature fractionation  C zone: compression and refrigeration  D Zone: chilling, cold fractionation and recovery 3) Numbering of Equipment: At RIL, there is a numbering system that assigns a number each to equipment of the plant. For example: 21-11-333 It stands for: NCP unit code is 21 Equipment Number Section 100-199 Cracking 200-299 Heat fractionation and compression 300-399 Chilling 400-499 Propylene Refrigeration 500-599 Ethylene Refrigeration Code Equipment 11 Heat Exchange 12 Vessel 13 Column 14 Reactor 15 Pumps 16 Furnace/ Heater 17 Blower/ Compressor 18 Filter/De-super-heaters/Miscellaneous Equipment
  • 19. P a g e | 15 GUJARAT OLEFINS PLANT MAULIN AMIN 4) Characteristics of Naphtha: Naphtha is the raw material for NCP. Naphtha is a colourless, volatile and flammable liquid mixture of hydrocarbons, having specific gravity of 0.69. Based on the boiling points, naphtha is of two types: light and heavy. Light naphtha (boiling range -35℃ to 135℃) is thermally cracked to obtain olefins. According to PIONA analysis, the average composition of naphtha is as follows: Component Specification (Wt %) Maximum (Wt %) Minimum (Wt %) Paraffin’s 75 74.26 79.99 Naphthenes 18 14.13 19.08 Aromatics 6.5 5.28 7.23 Olefins 0.5 0.18 0.61 Sulphur (ppm) 170 105 308 Specific Gravity 0.6824 0.678 0.692 C5 and C6 are also recycled for the naphtha feed. Naphtha is obtained from crude oil refining done in RIL Jamnagar Plant. It is transported to Dahej via shipping and then from there to storage drums in the Vadodara Plant via pipelines. Provisions have also been made to import Naphtha from Kandla Dahej. 5) Principle of Cracking: Cracking is the process where heavy hydrocarbons are broken down into simpler hydrocarbons, thermally or by using catalysts. Naphtha is cracked thermally by using the process of steam cracking. Naphtha contains a large number of hydrocarbons and during steam cracking a large number of chemical reactions take place, most of them are based on free radicals. Thus the actual reaction that take place are complex and difficult to model. However, pyrolysis of ethane provides a simple illustration to understand the phenomenon of free radical mechanism. 1. Initiation: Ethane molecule splits homolytically into two methyl radicals 𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶3 → 2𝐶𝐶𝐶𝐶3
  • 20. P a g e | 16 GUJARAT OLEFINS PLANT MAULIN AMIN 2. Hydrogen Abstraction: Methyl radical removes hydrogen radical from another ethane molecule to give an ethyl radical. 𝐶𝐶𝐶𝐶3 * + 𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶3 → 𝐶𝐶𝐶𝐶4 + 𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶2 * 3. Radical Decomposition: Ethyl radical decomposes to give ethylene molecule and hydrogen radical. 𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶2 * → 𝐶𝐶𝐶𝐶2 − 𝐶𝐶𝐻𝐻2 + 𝐻𝐻∗ 4. Hydrogenation: The hydrogen radical attacks ethane molecule to give a hydrogen molecule and new ethyl radical. 𝐻𝐻* + 𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶3 → 𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶2 * + 𝐻𝐻2 Reaction (4) is followed by reaction (3) and thus, they constitute a chain mechanism. The net effect can be represented by the equation 𝐶𝐶𝐶𝐶3 𝐶𝐶𝐶𝐶3 → 𝐶𝐶𝐶𝐶2 − 𝐶𝐶𝐶𝐶2 + 𝐻𝐻2 Reactions (2) and (3) are called transition reactions. 5. Termination: The chain cycle will terminate when two free radicals react with each other to produce products that are not free radicals. When the chain is interrupted, it becomes necessary to generate new radicals via reactions (1), (2) and (3) to start a new chain. Apart from primary reactions discussed above, secondary reactions occur too. 6) Mechanism of cracking: The reactions taking place can be broadly classified into two categories: 1. Primary Reaction: When naphtha along with the steam is heated to such a temperature that the heavier naphtha molecules break down into smaller molecules, these are known as primary reactions. 2. Secondary Reaction: The cracking operation comprises of reactions other than the primary rections, these are called secondary reactions. They are as follows: a. Dehydrogenation – gives olefins b. Dehydrocyclization – gives aromatics
  • 21. P a g e | 17 GUJARAT OLEFINS PLANT MAULIN AMIN c. Condensation – Two or more, smaller fragments combine to form large stable structures. It gives gas oil, fuel oil and tars. d. Hydrogenation – Gives paraffin, di-olefins and acetylene are obtained from olefins. e. Reactions involving further pyrolysis of olefins. It results into formation of olefins, di-olefins and acetylene. 7) Principles and governing variables of cracking operations: The overall cracking reactions are endothermic. High temperature and low partial pressure of hydrocarbons favour the reactions.  Residence time: It is defined as the length of time for which the naphtha feed is in cracking furnace at or above its cracking temperature. This variable is the prime factor in deciding the yirld pattern of the cracking furnace. Other things being equal, a short residence time gives a higher yield of ethylene due to the suppressions of secondary reactions. The residence time is usually kept as 0.5 seconds.  Severity: The severity of the operation is dependent on the following: Coil Outlet Temperature (COT): Higher the COT, more severe is the cracking. Residence time of naphtha cracking: With the feed rate and stream rate, the residence time also gets fixed. Pressure in the cracking coils: Lower the pressure, higher the severity of cracking for a furnace of definite design and dimension. The minimum pressure available at the charge gas of the first stage suction drum governs the coil pressure. Thus the only process variable which controls the severity of cracking is COT.  Selectivity: Hydrocarbon undergoing pyrolysis is the most complex, mixture of molecules and free radicals, which reacts with one another in multiple ways simultaneously. Based on established theories and supported experimental data, the production of olefins and di-olefins had been found to be favoured by two ways: a. Short Residence Time b. Lower Hydrocarbon Partial Pressure
  • 22. P a g e | 18 GUJARAT OLEFINS PLANT MAULIN AMIN For liquid feed stock the methane to ethylene ratio found in the heater effluent was used as a good overall indicator of pyrolysis heater selectivity. Low Methane to Ethylene Ratio corresponds to a high total yield of ethylene, butadiene and butylene. Steam cracking is done as it has the advantage of lowering the partial pressure of the hydrocarbons in the feed and reducing the deposit of coke. The following reaction variables have been suggested as the optimum conditions for naptha cracking: Temperature 760℃ 𝑡𝑡𝑡𝑡 860 ℃ Total Pressure 1 atmosphere Hydrocarbon/ Dilution Steam 2:1 to 1:1 Residence time 0.5 sec DMDS For naphtha: 100ppm For ethane: 150ppm 8) Factors affecting heater operation: 1. Feed Rate: Increasing the feed rate will decrease the residence time. However, it will require greater high heat duty. 2. Dilution Steam Rate: Increasing the dilution steam rate will decrease the residence time and decrease the hydrocarbon partial pressure also resulting in better selectivity and more valuable products. But, more dilution steam will increase operation cost. 3. Coil Outlet Temperature: High temperature gives higher conversion and higher yield of ethylene, however too high temperature may give too high fouling rate and can also decrease the yield of propylene and butadiene. Varying the outlet temperature can vary the ratio of ethylene to propylene, thus will be varied according to the market demand at particular time. 9) Functions of dilution steam in cracking:  It reduces the hydrocarbon partial pressure and thereby encourages higher selectivity of desired olefin products.
  • 23. P a g e | 19 GUJARAT OLEFINS PLANT MAULIN AMIN  It reduces the partial pressure of the high boiling aromatic hydrocarbons in the zone of high conversion, lessening the tendency to form coke within the cracking coils and deposit on the walls of the TLE (Transfer Line Exchanger)  It has sufficient adding effect on the tube metal to significantly diminish the catalytic effect of iron and nickel which otherwise would promote the carbon forming reaction. 10)Effects of partial pressure during cracking: The partial pressure of hydrocarbon affects the chemical equilibrium arid the reaction rates and thus influences the product distribution. Optimum partial pressure is P = 0.5 – 10.6 kg/cm2 g. Optimum total pressure, P = 1 atm. 11)Functions of DMDS in feed stock: Coke formation in pyrolysis furnace and TLE presents serious operation problems. When unsaturated hydrocarbons diffuse through the gas film boundary layer to the high temperature tube well, they undergo dehydrogenation reactions leading ultimately to coke formation. The sulphur on the feedstock or the recomposed DMDS (which is injected along with the feed stock) forms a sulphide film in the active tube metal, temporarily poisoning the catalytic effect of Nickel and Iron and thus reduces coke formation.
  • 24. P a g e | 19 About Naphtha Cracker Plant  Modes of Operation: Heater  Process Flow Diagram: Cracking  Piping and Instrumentation Diagram: Naphtha Cracking Heater
  • 25. P a g e | 20 NAPHTHA CRACKER PLANT MAULIN AMIN Modes of Operation: The NCP Heater has three operating modes: 1. HSS 2. Cracking 3. Decoke 1. HSS (High Steam Standby): In this mode, the plant is fed with steam to allow it to reach the temperature required for cracking, before being fed with naphtha. Cracking works flawlessly if performed at the desired temperature. So, the heater temperature is allowed to rise until it reaches 820℃. The plant is operated in this mode for about 24 hours, i.e. until the cracking temperature is reached. 2. Cracking: In this mode, the cracking of naphtha takes place. It a process related more to the vast fortitude of Chemical Engineering and Science. Instrumentation is incorporated here, just to control the process. 3. Decoke: Coke formation might lead to serious trouble. So the place is operated in this mode, to rid the plant from coke formation with in. It’s an approximate 48 hours procedure.
  • 26. P a g e | 21 NAPHTHA CRACKER PLANT MAULIN AMIN PROCESS FLOW DIAGRAM
  • 27. P a g e | 22 NAPHTHA CRACKER PLANT MAULIN AMIN Piping and Instrumentat- ion Diagram
  • 28. P a g e | 22 About Instrumentation in Naphtha Cracker Plant  Introduction to Instrumentation  Function and Advantages  Terminology and Symbols in Control Engineering  Control and Field Instrumentation Documentation  4-20 mA Current Loop  Transmitters  Instrument Connections  Letatwin LM 380A  Tricon System  DCS
  • 29. P a g e | 23 1) Introduction: Measurement is the process of determining the amount, degree or capacity by comparison with the accepted standards of the system units being used. Instrumentation is a technology of measurement which serves sciences, engineering, medicine and etc. Instrument is a device for determining the value or magnitude of a quantity or variable. Electronic instrument is based on electrical or electronic principles for its measurement functions. 2) Functions and Advantages: The 3 basic functions of instrumentation:- 1. Indicating – visualize the process/operation 2. Recording – observe and save the measurement reading 3. Controlling – to control measurement and process Advantages of electronic measurement 1. Results high sensitivity rating – the use of amplifier 2. Increase the input impedance – thus lower loading effects 3. Ability to monitor remote signal 3) Terminology and Symbols in Control system Engineering: Planning, design and start-up of process control systems require clear and unambiguous communication between all parts involved. To ensure this, we need a clear definition of the terms used - as far as the documentation is concerned - standardized graphical symbols. These symbols help us represent control systems or measurement and control tasks as well as their device-related solution in a simple and clear manner. 3.1 Terminology in Control Engineering: To maintain a physical quantity, such as pressure, flow or temperature at a desired level during a technical process, this quantity can be controlled either by means of open loop control or closed loop control.
  • 30. P a g e | 24 3.1.1 Open loop control In an open loop control system, one or more input variables of a system act on a process variable. The actual value of the process variable is not being checked, with the result that possible deviations - e.g. caused by disturbances are not compensated for in the open loop control process. Thus, the characteristic feature of open loop control is an open action flow. The task of the operator illustrated in Fig. 1 is to adjust the pressure (p2) in a pipeline by means of a control valve. For this purpose, he utilizes an assignment specification that determines a certain control signal (y) issued by the remote adjuster for each set point (w). Since this method of control does not consider possible fluctuations in the flow, it is recommended to use open loop control only in systems where disturbances do not affect the controlled variable in an undesired way. 3.1.2 Closed loop control In a closed loop control system, the variable to be controlled (controlled variable x) is continuously measured and then compared with a predetermined value (reference variable w). If there is a difference between these two variables (error e or system deviation xw), adjustments are being made until the measured difference is eliminated and the controlled variable equals the reference variable. Hence, the characteristic feature of closed loop control is a closed action flow. p1 y p2 variable pcontrolsFig. 1: Operator the process 2 via remote adjuster Assignment: wa y=> a p=> 2a wb => yb => p2b etc.
  • 31. P a g e | 25 The operator depicted in Fig. 2 monitors the pressure p2 in the pipeline to which different consumers are connected. When the consumption increases, the pressure in the pipeline decreases. The operator recognizes the pressure drop and changes the control pressure of the pneumatic control valve until the desired pressure p2 is indicated again. Through continuous monitoring of the pressure indicator and immediate reaction, the operator ensures that the pressure is maintained at the desired level. The visual feedback of the process variable p2 from the pressure indicator to the operator characterizes the closed action flow. 3.1.3 Process A process comprises the totality of actions effecting each other in a system in which matter, energy, or information are converted, transported or stored. Adequate setting of boundaries helps determine sub-processes or complex processes. Examples: Generation of electricity in a power plant Distribution of energy in a building Production of pig iron in a blast furnace Transportation of goods p1 p2 Fig. 2: Operator controls the process variable p2 an a closed loop
  • 32. P a g e | 26 3.1.4 Control loop The components of a control loop each have different tasks and are distinguished as follows: The components of the final control equipment are part of the controlling system as well as part of the controlled system. The distinction made above results directly from the distribution of tasks. The actuator processes and amplifies the output signal of the controller, whereas the final control element - as part of the controlled system - manipulates the mass and energy flow. 3.2 Symbols in Control Engineering 3.2.1 Signal flow diagrams A signal flow diagram is the symbolic representation of the functional interactions in a system. The essential components of control systems are represented by means of block diagrams. If required, the task represented by a block symbol can be further described by adding a written text. However, block diagrams are not suitable for very detailed representations. The symbols described below are better suited to represent functional details clearly.
  • 33. P a g e | 27 3.2.2 Blocks and lines of action The functional relationship between an output signal and an input signal is symbolized by a rectangle (block). Input and output signals are represented by lines and their direction of action (input or output) is indicated by arrows. Example: Root-extracting a quantity (Fig. 3) (E.g. flow rate measurement via differential pressure sensors) Example: Representing dynamic behaviour (Fig. 4) (E.g. liquid level in a tank with constant supply) Example: Summing point (Fig. 5) The output signal is the algebraic sum of the input signals. This is symbolized by the summing point. Any number of inputs can be connected to one summing point which is represented by a circle. Depending on their sign, the inputs are added or subtracted. xe xa Fig. 3: Root-extracting a differential pressure signal xe = differential pressure xa = root-extracted differential pressure xe xa Fig. 4: Development of a liquid level over time xe = inflow xa = liquid level
  • 34. P a g e | 28 Example: Branch point (Fig. 6) A branch point is represented by a point. Here, a line of action splits up into two or more lines of action. The signal transmitted remains unchanged. Example: Signal flow diagram of open loop and closed loop control The block diagram symbols described above help illustrate the difference between open loop and closed loop control processes clearly. In the open action flow of open loop control (Fig. 7), the operator positions the remote adjuster only with regard to the reference variable w. Adjustment is carried out according to an assignment specification (e.g. a table: set point w1 = remote adjuster position v1; w2 = v2; etc.) determined earlier. x1 x2 x1 = x2 = x3 x3 Fig. 6: Branch point xe1 xe2 + + _ xe3 xa xa = xe1 + xe2 – xe3 Fig. 5: Summing point
  • 35. P a g e | 29 In the closed action flow of closed loop control (Fig. 8), the controlled variable x is measured and fed back to the controller, in this case man. The controller determines whether this variable assumes the desired value of the reference variable w. When x and w differ from each other, the remote adjuster is being adjusted until both variables are equal. 3.2.3 Device-related representation Using the symbols and terminology defined above, Fig. 9 shows the typical action diagram of a closed loop control system xw + _ Fig. 8: Block diagram of manual closed loop control man remote adjuster control valve system xw controlFig. 7: Block diagram of manual open loop man remote adjuster system control valve z ew + – yr xy r Fig. 9: Block diagram of a control loop controlling element measuring equipment controller final control element system actuator
  • 36. P a g e | 30 Whenever the technical solution of a process control system shall be pointed out, it is recommended to use graphical symbols in the signal flow diagram (Fig. 10). As this representation method concentrates on the devices used to perform certain tasks in a process control system, it is referred to as solution-related representation. Such graphical representations make up an essential part of the documentation when it comes to planning, assembling, testing, start-up and maintenance. 1 Sensor (temp.) 2 Transmitter 3 Signal converter 4 Controller 5 Pneumatic linear valve 6 Heat exchanger Each unit has its own graphical symbol that is usually standardized. Equipment consisting of various units is often represented by several lined-up symbols. control of a heat exchanger systemFig. 10: Graphical symbols for describing temperature 1 23 4 5 6
  • 37. P a g e | 31 It is recommended to always use standardized graphical symbols. In case a standardized symbol does not exist, you may use your own. PI controlcontrollers,forsymbolsGraphical11:Fig. valves and software-based functions hand-operated actuator motor-driven actuator diaphragm actuator valve with diaphragm actuator motor-driven valvebutterfly valve controllercontroller symbol)former( withvalve diaphragm actuator attachedand positioner PIcontroller root-extracting element, software-based countersoftware with limit switch functions performed by software are marked with a flag
  • 38. P a g e | 32 3.3Control Systems and Structures Depending on the job to be done, many different structures of control can be used. The main criterion of difference is the way the reference variable w is generated for a certain control loop. In setting the controller, it is also important to know whether the reference variable is principally subject to changes or disturbance variables need to be compensated for.  To attain good disturbance reaction, the controller must restore the original equilibrium as soon as possible (Fig. 13).  To attain good reference action, the controlled variable must reach a newly determined equilibrium fast and accurately (Fig. 14). P F F T Pt 100 DIN P P L L I symbols for sensors, transmitters, adjusters and indicatorsFig. 12: Graphical level sensor temperature sensor pressure sensor adjusterindicatoranalog sensorflow transmitterpressure electricwith outputstandardized signal transmittercurrent pneumaticwith outputstandardized signal i/p converter, electr. into pneum. standardized signal
  • 39. P a g e | 33 3.3.1 Fixed set point control In fixed set point control, the reference variable w is set to a fixed value. Fixed set point controllers are used to eliminate disturbances and are therefore designed to show good disturbance reaction. x z t t Fig. 13: Disturbance reaction w t x t Fig. 14: Reference action Fig. 15: Temperature control by means of fixed set point control w x
  • 40. P a g e | 34 The temperature control system in Fig. 15 will serve as an example for fixed set point control. The temperature of the medium flowing out of the tank is to be kept at a constant level by controlling the heating circuit. This will provide satisfactory results as long as high fluctuations in pressure caused by disturbances do not occur in the heating circuit. 3.3.2 Follow-up control In contrast to fixed set point control, the reference variable in follow-up control systems does not remain constant but changes over time. Usually, the reference variable is predetermined by the plant operator or by external equipment. A reference variable that changes fast requires a control loop with good reference action. If, additionally, considerable disturbances need to be eliminated, the disturbance reaction must also be taken into account when designing the controller. 3.3.3 Cascade control Cascade control systems require a minimum of two controllers, these are the master or primary and the follower or secondary controller. The characteristic feature of this control system is that the output variable of the master controller is the reference variable for the follower controller. Employing cascade control, the temperature control of the heat exchanger (Fig. 16) provides good results also when several consumers are connected to the heating circuit. The fluctuations in pressure and flow are compensated for by the secondary flow controller (w2, x2) which acts as final control element to be positioned by the primary temperature controller. by means of cascadeFig. control16: Temperature control w1=wsoll x2 x1 w2 q
  • 41. P a g e | 35 In our example the outer (primary) control loop (w1, x1) must be designed to have good disturbance reaction, whereas the inner –secondary– control loop requires good reference action. 3.3.4 Ratio control Ratio control is a special type of follow-up control and is used to maintain a fixed ratio between two quantities. This requires an arithmetic element (V). Its input variable is the measured value of the process variable 1 and its output variable manipulates the process variable 2 in the control loop. Fig 17 illustrates a mixer in which the flow rate q2 of one material is controlled in proportion to the flow rate q1 of another material. 4) Control and Field Instrumentation Documentation: To successfully work with (and design) control systems, it is essential to understand the documents that are typically used to illustrate process control and associated field instrumentation. The documentation of process control and associated field instrumentation is normally created by the engineering firm that designs and constructs the plant. The company that commissioned the plant may have an internal documentation standard the engineering firm will be required to follow. For an older installation, the plant documentation may only exist as a series of paper documents. Today the documentation created for a new or upgraded plant is produced controlFig. 17: Ratio V q2 q1 q2 Vq= 1 w x
  • 42. P a g e | 36 electronically using automated design tools and software. The tools and software selected by the plant or engineering firm for initial plant design or upgrade will influence the documentation format and how documentation is maintained at the plant site. Also, the selection of the control system determines to what extent the system is self-documenting. Self-documenting – the automatic creation of documents that follow defined conventions for naming and structure. If the documentation generated by the control system does not follow standards that have been established for process control and instrumentation, then it may be necessary to manually create this documentation. Control System - A component, or system of components functioning as a unit, which is activated either manually or automatically to establish or maintain process performance within specification limits. In general, four types of drawings that are commonly used to document process control and associated field instrumentation. 4.1 Plot Plan It is often helpful to look at the plot plan to get an overview of how a plant is physically organized. By examining the plot plan, it is possible to get an idea of where a piece of equipment is located in the plant. A typical plot plan is shown in Figure 4-1. As will become clear in the following chapters, understanding the physical layout of the plant and the distances between pieces of equipment can often provide insight into the expected transport delay associated with material or product flow between pieces of equipment. For example, how long does it take a liquid, gas, or solid material flow to get from one point in the process to another? Transport Delay – Time required for a liquid, gas or solid material flow to move from one point to another through the process. Physically, if the major pieces of process equipment are laid out far apart, then the transport delay can be significant and in some cases, impact control performance. Also, the physical layout of a plant will impact the length of wiring runs and communication distance from the control system to the field devices; thus, it is a good idea to use the plot plan to get a sense of the plant layout and a feel for the location of process equipment and process areas.
  • 43. P a g e | 37 4.2 Process Flow Diagram To meet market demands, a company may commission an engineering firm to build a new plant or to modify an existing plant to manufacture a product that meets certain specifications and that can be manufactured at a specific cost. Given these basic objectives, a process engineer will select the type of chemical or mechanical processing that best meets the planned production, quality, and efficiency targets. For example, if the equipment is to be used to make more than one product then the process engineer may recommend a batch process. For example, a batch reactor may be used to manufacture various grades of a lubrication additive. Once these basic decisions are made, the process engineer selects the equipment that will most cost-effectively meet the company’s objectives. Based on the production rate, the process engineer selects the size of the processing equipment and determines the necessary connections between the pieces of equipment. The process engineer then documents the design in a process flow diagram (PFD). The process flow diagram typically identifies the major pieces of equipment, the flow-paths through the process, and the design operating conditions—that is, the flow rates, pressures, and temperatures at normal operating conditions and the target production rate. Figure 4-1. Plot Plan DRAINAGE DITCH OFFICE BUILDING POND NO. 1 POND NO. 2 TRUCK LOADING/UNLOADING AREAS WATER TREATMENT PUMP STATION N SCALE (FEET) 100500 POWER HOUSE CONTROL ROOM PACKAGING / SHIPPING T1 T2 T3 S1 S2 S3 Company Name )( PLOT PLAN PLANT ( ) SHEET DRAWING NUMBER REV 1 OF 1 DA200023 1
  • 44. P a g e | 38 Process flow diagram – Drawing that shows the general process flow between major pieces of equipment of a plant and the expected operating conditions at the target production rate. Since the purpose of the process flow diagram is to document the basic process design and assumptions, such as the operating pressure and temperature of a reactor at normal production rates, it does not include many details concerning piping and field instrumentation. In some cases, however, the process engineer may include in the PFD an overview of key measurements and control loops that are needed to achieve and maintain the design operating conditions. Figure 4-2
  • 45. P a g e | 39 Control Loop - One segment of a process control system. During the design process, the process engineer will typically use high fidelity process simulation tools to verify and refine the process design. The values for operating pressures, temperatures, and flows that are included in the PFD may have been determined using these design tools. An example of a process flow diagram is shown in Figure 4-2. In this example, the design conditions are included in the lower portion of the drawing. 4.3 Piping and Instrumentation Diagram The instrumentation department of an engineering firm is responsible for the selection of field devices that best matches the process design requirements. This includes the selection of the transmitters that fit the operating conditions, the type and sizing of valves, and other implementation details. An instrumentation engineer selects field devices that are designed to work under the normal operating conditions specified in the process flow diagram. Tag numbers are assigned to the field devices so they may be easily identified when ordering and shipping, as well as installing in the plant. Tag number – Unique identifier that is assigned to a field device. The decisions that are made concerning field instrumentation, the assignment of device tags, and piping details are documented using a piping and instrumentation diagram (P&ID). A piping and instrumentation diagram is similar to a process flow diagram in that it includes an illustration of the major equipment. However, the P&ID includes much more detail about the piping associated with the process, to include manually operated blocking valves. It shows the field instrumentation that will be wired into the control system, as well as local pressure, temperature, or level gauges that may be viewed in the field but are not brought into the control system. As mentioned earlier, the engineering company that is creating the P&ID normally has standards that they follow in the creation of this document. In some cases, the drawing includes an overview of the closed loop and manual control, calculations, and measurements that will be implemented in the control system. Closed loop control - Automatic regulation of a process inputs based on a measurement of process output.
  • 46. P a g e | 40 Manual control – Plant operator adjustment of a process input. However, details on the implementation of these functions within the control system are not shown on the P&ID. Even so, the P&ID contains a significant amount of information and in printed form normally consists of many D size drawings (22 x 34 inches; 559 x 864 mm) or the European equivalent C1 (648 x 917mm). The drawings that make up the P&ID are normally organized by process area, with one or more sheets dedicated to the equipment, instrumentation, and piping for one process area. Piping and Instrumentation Diagram – Drawing that shows the instrumentation and piping details for plant equipment. The P&ID acts as a directory to all field instrumentation and control that will be installed on a process and thus is a key document to the control engineer. Since the instrument tag (tag number) assigned to field devices is shown on this document, the instrument tag associated with, for example, a measurement device or actuator of interest may be quickly found. Also, based on the instrument tags, it is possible to quickly identify the instrumentation and control associated with a piece of equipment. For example, a plant operator may report to Maintenance that a valve on a piece of equipment is not functioning correctly. By going to the P&ID the maintenance person can quickly identify the tag assigned to the valve and also learn how the valve is used in the control of the process. Thus, the P&ID plays an important role in the design, installation and day to day maintenance of the control system. It is a key piece of information in terms of understanding what is currently being used in the plant for process control. An example of a P&ID is shown in Figure 4-3. When you are doing a survey of an existing plant, obtaining a copy of the plant P&IDs is a good starting point for getting familiar with the process and instrumentation. Unfortunately, the presentation of process control on the P&ID is not standardized and varies with the engineering firm that creates the plant design. In some cases, process control is illustrated at the top of the drawing and its use of field instrumentation is indicated by arrows on the drawing that point from the field instrumentation to the control representation. Another common approach is to show control in the main body of the drawing with lines connected to the field instrumentation. Using either approach complicates the drawing and its maintenance since process control design may change with plant operational requirements. For this reason, the P&ID may only show the field instrumentation, with other documentation referenced that explains the control and calculations done by the control system. For example,
  • 47. P a g e | 41 when the process involves working with hazardous chemicals, then a controller functional description (CFD) may be required for process safety management (PSM). Standards have been established by OSHA for controller functional descriptions. [3] 4.4 Loop Diagram The piping and instrumentation diagram identifies, but does not describe in detail, the field instrumentation that is used by the process control system, as well as field devices such as manual blocking valves that are needed in plant operations. Many of the installation details associated with field instrumentation, such as the field devices, measurement elements, wiring, Figure 4-3
  • 48. P a g e | 42 junction block termination, and other installation details are documented using a loop diagram. ISA has defined the ISA-5.4 standard for Instrument Loop Diagrams. [4] This standard does not mandate the style and content of instrument loop diagrams, but rather it is a consensus concerning their generation. A loop diagram, also commonly known as a loop sheet, is created for each field device that has been given a unique tag number. The loop diagrams for a process area are normally bound into a book and are used to install and support checkout of newly installed field devices. After plant commissioning, the loop Diagrams provide the wiring details that a maintenance person needs to find and troubleshoot wiring to the control system. Loop Diagram – Drawing that shows field device installation details including wiring and the junction box (if one is used) that connects the field device to the control system. The loop diagram is a critical piece of documentation associated with the installation of the control system. As has been mentioned, the engineering company that is designing a process normally has standards that they follow in the creation of a loop diagram. These standards may be documented by the creation of a master template that illustrates how field devices and nomenclature are used on the drawing. The loop diagram typically contains a significant amount of detail. For example, if a junction box is used as an intermediate wiring point, the loop diagram will contain information on the wiring junctions from the field device to the control system. An example of a loop diagram is shown in Figure 4-4. As is illustrated in this example, junction box connections are shown on the line that shows the division between the field and the rack room. The loop diagram shows the termination numbers used in the junction box and the field device and for wiring to the control system input and output cards. Also, the Display and Schematic portions of the loop diagram provide information on how the field input and output are used in the control system. Figure 4-4 shows installation details for a two-wire level transmitter that is powered through the control system analog input card. Also, connections are shown between the control system analog output card and an I/P transducer and pneumatic valve actuator. Details such as the 20 psi air supply to the I/P and the 60 psi air supply to the actuator are shown on this drawing. Based on information provided by the loop diagram, we know that the I/P will be calibrated to provide a 3–15 psi signal to the valve actuator. In addition, specific details are provided on the level measurement installation. Since the installation shows sensing lines to the top and bottom
  • 49. P a g e | 43 of the tank, it becomes clear that the tank is pressurized and that level will be sensed based on the differential pressure. In this particular installation, the instrumentation engineer has included purge water to keep the sensing line from becoming plugged by material in the tank. Even fine details such as the manual valve to regulate the flow of purge water are included in the loop diagram to guide the installation and maintenance of the measurement device. In this example, the loop diagram shows the installation of a rotameter. A rotameter consists of a movable float inserted in a vertical tube and may be used to provide an inexpensive mechanical means of measuring volumetric flow rate in the field. As this example illustrates, the loop diagram provides information that is critical to the installation, checkout, and maintenance of field devices. By examining the loop diagram, it is possible to learn details that may not be obvious when you are touring the plant site. For example, as was previously presented in Chapter 3 on measurement, there are various ways to measure temperature. In the case of a temperature measurement, the loop diagram will provide information on the temperature transmitter, as well as the measurement element that is used. Figure 4-5 shows the loop diagram for a temperature measurement in which a three-wire RTD element is used for the temperature measurement. Details such as the grounding of the shield for the element wire and for the twisted pair going from the transmitter to the control system are noted on the loop diagram. The process of creating and reviewing loop diagrams is made easier by the fact that many components used to represent measurement and control in similar applications are often repeated. For example, the manner in which the control valve, actuator, and associated I/P transducer were represented in the loop diagram example for a level application is duplicated in other loop diagrams that depict a regulating (control) valve. This is illustrated in the loop diagram shown in Figure 4-6 for a pressure application in which a regulating valve is used in the control of pressure. Also in this example, the pressure measurement is made with a two- wire transmitter. As will be noted by comparing Figure 4-4 and Figure 4-6, the wiring for the pressure transmitter is similar to that used for the level transmitter. In some cases, the operator uses a process measurement as an indicator or as an input to a calculation that is done in the control system. A loop diagram may be developed for these types of measurement that details the device installation and wiring to the control system. In such cases, the loop diagram will contain no definitions of control. Figure 4-7 shows a flow
  • 50. P a g e | 44 measurement made by measuring the differential pressure across an orifice plate. The hole in the orifice plate is sized to give a specific pressure drop at the maximum flow rate that the process is designed to support. As noted in the lower portion of the loop diagram, the orifice plate is sized to provide a differential pressure of 500 inches H2O at a flow rate of 750 gpm. Figure 4-4
  • 51. P a g e | 45 Also noted in the loop diagram is the control system is expected to take the square root of this differential pressure to obtain an indication of flow rate. Figure 4-5
  • 52. P a g e | 46 Figure 4-6
  • 53. P a g e | 47 4.5 Tagging Conventions The tagging conventions that were shown in examples of a P&ID and loop diagrams may be confusing to someone who has not worked with these documents. The naming convention Figure 4-7
  • 54. P a g e | 48 illustrated in the P&ID and loop diagram examples are fairly well standardized in North America. To a certain extent, similar conventions are used in Europe and Asia to document process instrumentation and control. TYPICAL TAG NUMBER TIC 103 - Instrument Identification or Tag Number T 103 - Loop Identifier 103 - Loop Number TIC - Function Identification T - First-letter IC - Succeeding-Letters EXPANDED TAG NUMBER 10-PAH-5A - Tag Number 10 - Optional Prefix A - Optional Suffix Note: Hyphens are optional as separators The letters that make up the first few characters of a typical tag number (the “leading letters”) are used to identify the function performed by the field device or by the control system. Following these leading letters is a number. The number that appears on the tag is known as the loop number. The loop number is used to uniquely identify one or more field devices that are used to perform a specific function. This combination of function letter and loop number allows a field device in a process area to be precisely identified. Knowing the device tag number is required when filling out a work order to or in discussing a field measurement with an operator or instrument technician. The tag number assigned to a field device is normally stamped on a tag that is attached to the device. All the devices that are used together to perform a specific function are normally assigned the same loop number. For example, the flow transmitter and regulating valve used to measure and regulate the flow of a process stream may be assigned loop number 101. The loop number normally has only three digits. Consequently, the number of field devices that can be uniquely identified using the standard tag number convention is very limited. For this reason, an expanded tag number convention, is used in the process industry. The expanded tag number convention allows a number to be inserted in front of the function, and that number is usually the process area number. A plant is divided into process areas that are assigned a number. The combination of the area number, the function letters, and the loop number is unique within a plant.
  • 55. P a g e | 49 In some cases, multiple field devices may be used to perform a similar function. For example, in some boiler applications the temperature of each tube in the superheater section of a boiler may have individual temperature measurements. Rather than assigning a loop number to each measurement, the expanded tag number convention assigns a common loop number to all these measurements. When this is done, one or more characters may be added after the loop number to uniquely identify each measurement. For the boiler example, loop number 105 might be assigned to all the superheater temperature measurements and the individual measurements may be identified by adding an A after the loop number for the first measurement, a B for the second measurement, and a C for the third measurement. This option to add letters after the loop number allows unique tag numbers to be created for each measurement, even when a large number of similar measurements is made in a process area. A hyphen may be optionally used in the tag number to separate the area number or characters added after the loop number. However, in general, the use of a hyphen in the tag number is not recommended since in many control systems, the length of a tag number is limited to a maximum number of characters (e.g., 12 or 16 characters). The identification letters used to specify the function of a field device are organized in a specific manner. The meaning of a letter varies depending on whether it is the first letter or a succeeding letter. A table of the identification letters defined by the standard is shown in Figure 4-8. If the first letter of a tag number is an A, this indicates that the primary function of the device is analysis; if the first letter is an F, then the primary function is flow. When the first letter is H, this indicates that a manual or Hand function is to be performed. By reviewing this table of identification letters, the use of the letters in a tag number can be easily determined. In some cases, a letter can only be used as a succeeding letter. For example, the letter D would never be used as a first character, but it may be used as a succeeding letter to indicate “differential.” Thus, the combination PD is valid and would indicate the function of the device is “pressure, differential,” that is, differential pressure. The combination HIC would be used to indicate hand indicator controller, that is, manual control. Indication and control based on an analytic measurement would be identified as AIC. The letter combination FIC is quite common and used to indicate a “flow indicating control” function. A control valve used in pressure control would be identified using the letters PV, pressure valve. A temperature measurement used only for indication would be identified as TI, “temperature indication.” A position transmitter would be identified using the letters ZT. The use of tag numbers in a process control
  • 56. P a g e | 50 system is quite straightforward and must be understood to work with or create documentation for a process control system. Figure 4-8. ISA-5.1 Identification Letters First Letters Succeeding Letters Measured/Initiating Variable Variable Modifier Readout/Passive Function Output/Active Function Function Modifier A Analysis Alarm B Burner, Combustion User’s Choice User’s Choice User’s Choice C User’s Choice Control Close D User’s Choice Difference, Differential Deviation E Voltage Sensor, Primary Element F Flow, Flow Rate Ratio G User’s Choice Glass, Gauge, Viewing Device H Hand High I Current Indicate J Power Scan K Time, Schedule Time Rate of Change Control Station L Level Light Low M User’s Choice Middle, Intermediate N User’s Choice User’s Choice User’s Choice User’s Choice O User’s Choice Orifice, Restriction Open P Pressure Point (Test Connection) Q Quantity Integrate, Totalize Integrate, Totalize R Radiation Record Run S Speed, Frequency Safety Switch Stop T Temperature Transmit U Multivariable Multifunction Multifunction V Vibration, Mechanical Analysis Valve, Damper, Louver W ForceWeight, Well, Probe X Unclassified X-axis Accessory Devices, Unclassified Unclassified Unclassified Y Event, State, Presence Y-axis Auxiliary Devices, Z Position, Dimension Z-axis, Safety Instrumented System Driver, Actuator, Unclassified final control element
  • 57. P a g e | 51 4.6 Line and Function Symbols Different types of lines are used in process flow diagrams, piping and instrumentation diagrams, and loop diagrams to indicate the type of connection between field devices and the control system. The ISA-5.1 standard defines the instrument line symbols that are commonly used in control system documentation. As illustrated in Figure 4-9, a solid line is used to represent a physical connection to the process. Two slashes shown as points along a line are used to indicate a pneumatic signal. One of the most common ways to indicate an electric signal is a dashed line as defined in ISA-5.1. Communication links between devices and functions of a distributed control system are indicated by small bubbles along the line as illustrated in Figure 4-9. The previous examples of the process flow diagram, piping and instrumentation diagram, and loop diagram contained one or more circle symbols. In these drawings, a circle is used to indicate a discrete instrumentation or control function. A horizontal line drawn through the middle of the circle indicates the function may be accessed by the plant operator. There are many functions, such as those performed by an I/P transducer or valve positioner, that are typically not directly accessible by the operator. Also, some field devices for measurement and actuation may only be accessed by control or calculation functions in the control system and thus would not be shown in the documentation as being directly accessed by the operator. However, the associated control or calculation function that is accessed by the operator would include a horizontal line. As illustrated in Figure 4-10, one of the conventions Figure 4-9 Figure 4-10
  • 58. P a g e | 52 advocated in ISA-5.1 is to include a square around the circle if the associated function is accessed by an operator through a video display of a distributed control system (DCS). However, in practice, this convention is often not followed. It is common practice to illustrate the valve body, as well as the valve actuator and positioner function in control system documentation. The ISA-5.1 standard addresses the representation of a valve body. Most types of valves are addressed by this standard. However, the engineering firm that is designing a process plant may have adopted some variation of what is shown in ISA-5.1. In such cases, it is common practice for the engineering firm to provide a drawing that explains the symbol functions included in their documentation. Also, in some cases, a general valve representation is used rather than different representations for a rotary valve or sliding stem valve. [5] Generally, a damper will be shown rather than the general valve symbol to indicate the regulation of air or gas flow to a boiler or a similar process such as a kiln or heater. An excerpt from ISA-5.1 Valve Body and Damper Symbols are illustrated in Figure 4- 11. Since the type of actuator used with a valve body may impact the operation and failure mode of the valve, the type of actuator is normally indicated in control system documentation. The representation of common types of actuators as defined by ISA-5.1 is shown in Figure 4-12. A complete representation of the valve is provided by combining the valve representation with the actuator representation. When a positioner is used with a valve, the diaphragm representation may be combined with a representation of the actuator and the valve body. A special actuator symbol is defined by the standard for motorized actuators. Motorized actuators are used in some industry segments because upon loss of power, the last valve position is maintained. Also, better resolution may be achieved using a motorized actuator for Figure 4-11 Figure 4-12
  • 59. P a g e | 53 a specific application, such as the basis weight valve that is used to regulate the thick stock flow to a paper machine in paper manufacturing. [6] Solenoid actuators that are used to automate valves used in on-off service are shown using a special symbol. Installation details are provided in the loop diagram documentation. Some of details may also be provided in the P&ID. For example, the P&ID and loop diagram may show the orifice that must be installed to measure flow using a differential pressure transmitter. Hand-operated valves that are used to block flow during start-up or maintenance are shown since they impact process operation if not properly set up. Also, the installation of inline instrumentation, such as a magnetic flow meter, is commonly shown in a unique manner in the control system documentation. In addition, measurement elements such as an RTD or a thermocouple are shown since they may be physically installed some distance from the field transmitter or the control system. To provide a consistent means of documenting the physical installation, the ISA-5.1 standard includes symbols for many of these common installation details and field devices. A sample of some of these symbols is shown in Figure 4-13. 4.7 Equipment Representation A representation of major pieces of process equipment is normally included in control system documentation. This allows the field instrumentation installation to be shown in relationship to the process equipment. Example process equipment representations are illustrated in Figure 4- 14. A general vessel representation may be appropriate for vessels, agitators, heat exchangers, and pumps that do not play an important role in the control system. For example, an agitator on a tank may not directly impact the control associated with the tank level. A special representation is provided for a reactor. A jacketed vessel symbol may be used when a vessel is heated or Figure 4-13
  • 60. P a g e | 54 cooled by circulating liquid through an outside shell. Such a design is commonly used in the batch industry, and permits the vessel’s contents to be heated or cooled without coming in contact with the circulating liquid. A symbol is also provided for flat-bottomed and coneshaped storage tanks that are open to the atmosphere. The examples include a representation of a heat exchanger, which is used to heat or cool a liquid stream. A symbol is defined for an agitator that may be used to ensure good mixing of liquids in a vessel. Also, a pump symbol is shown in these examples. 5) 4-20mA Current Loop 5.1 Why Use a Current Loop? The 4-20mA current loop shown in Figure 5-1 is a common method of transmitting sensor information in many industrial process-monitoring applications. A sensor is a device used to measure physical parameters such as temperature, pressure, speed, liquid flow rates, etc. Transmitting sensor information via a current loop is particularly useful when the information has to be sent to a remote location over long distances (1000 feet, or more). The loop’s operation is straightforward: a sensor’s output voltage is first converted to a proportional current, with 4mA normally representing the sensor’s zero-level output, and 20mA representing the sensor’s full-scale output. Then, a receiver at the remote end converts the 4- 20mA current back into a voltage which in turn can be further processed by a computer or display module. However, transmitting a sensor’s output as a voltage over long distances has several drawbacks. Unless very high input-impedance devices are used, transmitting voltages over long distances produces correspondingly lower voltages at the receiving end due to wiring and interconnect Figure 4-14
  • 61. P a g e | 55 resistances. However, high-impedance instruments can be sensitive to noise pickup since the lengthy signal-carrying wires often run in close proximity to other electrically noisy system wiring. Shielded wires can be used to minimize noise pickup, but their high cost may be prohibitive when long distances are involved. Sending a current over long distances produces voltage losses proportional to the wiring’s length. However, these voltage losses— also known as “loop drops”—do not reduce the 4- 20mA current as long as the transmitter and loop supply can compensate for these drops. The magnitude of the current in the loop is not affected by voltage drops in the system wiring since all of the current (i.e., electrons) originating at the negative (-) terminal of the loop power supply has to return back to its positive (+) terminal—fortunately, electrons cannot easily jump out of wires! 5.2 Current Loop Components A typical 4-20mA current-loop circuit is made up of four individual elements: a sensor/transducer; a voltage-to-current converter (commonly referred to as a transmitter and/or signal conditioner); a loop power supply; and a receiver/monitor. In loop powered applications, all four elements are connected in a closed, series circuit, loop configuration (see Figure 5-1). Sensors provide an output voltage whose value represents the physical parameter being measured. (For example, a thermocouple is a type of sensor which provides a very low-level output voltage that is proportional to its ambient temperature.) The transmitter amplifies and conditions the sensor’s output, and then converts this voltage to a proportional 4-20mA dc- current that circulates within the closed series-loop. The receiver/monitor, normally a subsection of a panel meter or data acquisition system, converts the 4-20mA current back into a voltage which can be further processed and/or displayed. The loop power-supply generally provides all operating power to the transmitter and receiver, and any other loop components that require a well-regulated dc voltage. In loop-powered Figure 5-1
  • 62. P a g e | 56 applications, the power supply’s internal elements also furnish a path for closing the series loop. +24V is still the most widely used power supply voltage in 4-20mA process monitoring applications. This is due to the fact that +24V is also used to power many other instruments and electromechanical components commonly found in industrial environments. Lower supply voltages, such as such as +12V, are also popular since they are used in computer-based systems. 5.3 Loop Drops One of a process monitor’s most important specifications—be it a loop-powered or locally powered device—is the total resistance (or “burden”) it presents to the transmitter’s output driver. Most transmitter’s data sheets specify the maximum loop resistance the transmitter can drive while still providing a full-scale 20mA output (the worst-case level with regards to loop burden). Ohm’s Law states that the voltage drop developed across a current-carrying resistor can be found by multiplying the resistor’s value by the current passing through it. Stated in mathematical terms: E = I x R where E is the voltage drop in volts, I is the current through the resistor in amperes, and R is the resistor’s value in Ohms (the ‘Ω’ symbol is commonly used to represent Ohms). The sum of the voltage drops around a series loop has to be equal to the supply voltage. For example, when a loop-powered application is powered from a 24V power source, the sum of all the voltage drops around the series loop has to also equal 24V. Every component through which the 4-20mA loop current passes develops a maximum voltage drop equal to that component’s resistance multiplied by 0.020 Amperes (20mA). For example, referring to Figure 2 the DMS-20PC-4/20S’s 250Ω resistance yields a maximum loop drop of: 250x 0.020A = 5.0V Figure 5-2
  • 63. P a g e | 57 5.4 Transmitter Ratings With the above loop-drop theory in mind, and assuming a +24V loop-powered application in which the transmitter’s minimum operating voltage is 8V, and the process monitor drops only 4V, a logical question which arises is what happens to the “extra” 12V? The extra 12V has to be dropped entirely by the transmitter since most process monitors have purely resistive inputs combined with Zener diodes that limit their maximum voltage drop. Transmitters usually state both minimum and maximum operating voltages. The minimum voltage is that which is required to ensure proper transmitter operation, while the maximum voltage is determined by its maximum rated power-dissipation, as well as by its semiconductors’ breakdown ratings. A transmitter’s power dissipation can be determined by multiplying its loop drop by the highest anticipated output current, usually, but not always, 20mA. For example, if a transmitter drops 30V at an over range output level of 30mA, its power dissipation is: 30V x 0.030A = 0.9 watts 5.5 Wiring Resistance Because copper wires exhibit a dc-resistance directly proportional to their length and gauge (diameter), this application note would not be complete without discussing the important topic of wiring—specifically the effects wiring resistance has on overall system performance. Applications in which two or more loop-monitoring devices are connected over very long, 2- way wiring distances (1000-2000 feet) normally use +24V supplies because many transmitters require a minimum 8V-supply for proper operation. When this 8-volt minimum is added to the typical 3-4 volts dropped by each process monitor and the 2-4 volts dropped in the system wiring and interconnects, the required minimum supply voltage can easily exceed 16V. The following worked-out example will illustrate these important concepts. The voltage drop developed along a given length of wire is found by multiplying the wire’s total resistance by the current passing through it. The wire’s total resistance is found by looking up its resistance (usually expressed in Ohms per 1000 feet) in a wire specifications table. Referring to Figure 3 if a transmitter’s output is delivered to a remote process monitor using 2000 feet (660 meters) of 26-guage, solid copper wire having a resistance of 40.8Ω per 1000 feet, the one-way voltage dropped by the wire when the transmitter’s output is 20mA is equal to:
  • 64. P a g e | 58 E = 0.020 Amperes x [2000 feet x (40.8Ω /1000 feet)] E = 0.020A x 81.6Ω = 1.63V However, the current must travel 2000 feet down to the process monitor and another 2000 feet back to the transmitter’s “+” output terminal, for a total of 4000 feet. As noted above, 26-gauge wire has a resistance of 40.8Ω per 1000 feet, yielding a total loop resistance (R) equal to 4000 feet x (40.8Ω /1000 feet) = 163.2Ω. The total voltage dropped over the 4000 feet of wiring is therefore: E = 0.020A x 163.2Ω E = 3.27V. Looking down the loop towards the remote process monitor, the transmitter sees the sum of the 3.27V wire drop and the 5.0V process-monitor drop, for a total loop-drop of 8.27V. If the transmitter itself requires a minimum of 8V (this is also considered a voltage drop) for proper operation, the lowest power supply voltage required for the system shown in Figure 5-3 is 16.3V. 5.6 In depth of 4 to 20 mA analog current signals The most popular form of signal transmission used in modern industrial instrumentation systems (as of this writing) is the 4 to 20 milliamp DC standard. This is an analog signal standard, meaning that the electric current is used to proportionately represent measurements or command signals. Typically, a 4 milliamp current value represents 0% of scale, a 20 milliamp current value represents 100% of scale, and any current value in between 4 and 20 milliamps represents a commensurate percentage in between 0% and 100%. The following table shows the corresponding current and percentage values for each 25% increment between 0% and 100%. Figure 5-3
  • 65. P a g e | 59 Every instrument technician tasked with maintaining 4-20 mA instruments commits these values to memory, because they are referenced so often: Current Value % of Scale 4mA 0% 8mA 25% 12mA 50% 16mA 75% 20mA 100% For example, if we were to calibrate a 4-20 mA temperature transmitter for a measurement range of 50 to 250 degrees C, we could relate the current and measured temperature values on a graph like this: This is not unlike the pneumatic instrument signal standard or 3 to 15 pounds per square inch (PSI), where a varying air pressure signal represents some process measurement in an analog (proportional) fashion. Both signal standards are referred to as live zero because their ranges begin with a non-zero value (3 PSI in the case of the 3-15 PSI standard, and 4 milliamps in the
  • 66. P a g e | 60 case of the 4-20 mA standard). This “live” zero provides a simple means of discriminating between a legitimate 0% signal value and a failed signal (e.g. leaking tube or severed cable). DC current signals are also used in control systems to command the positioning of a final control element, such as a control valve or a variable-speed motor drive (VSD). In these cases, the milliamp value does not directly represent a process measurement, but rather how the degree to which the final control element influences the process. Typically (but not always!), 4 milliamps commands a closed (shut) control valve or a stopped motor, while 20 milliamps commands a wide-open valve or a motor running at full speed. Thus, most industrial control systems use at least two different 4-20 mA signals: one to represent the process variable (PV) and one to represent the command signal to the final control element (the “manipulated variable” or MV): The relationship between these two signals depends entirely on the response of the controller. There is no reason to ever expect the PV and MV current signals to be equal to each other, for they represent entirely different variables. In fact, if the controller is reverse-acting, it is entirely normal for the two current signals to be inversely related: as the PV signal increases going to a reverse-acting controller, the output signal will decrease. If the controller is placed into “manual” mode by a human operator, the output signal will have no automatic relation to the PV signal at all, instead being entirely determined by the operator’s whim.
  • 67. P a g e | 61 5.7 Figure below illustrates three basic transmitter current loop types:  Type 2 is a 2-wire transmitter energized by the loop current where the loop source voltage (compliance) is included in the receiver. The transmitter floats and signal ground is in the receiver.  Type 3 is a 3-wire transmitter energized by a supply voltage at the transmitter. The transmitter sources the loop current. Transmitter common is connected to receiver common  Type 4 is a 4-wire transmitter energized by a supply voltage at the transmitter. The transmitter sources the loop current to a floating receiver load. If a transmitter has field inputs, which provide signals referenced to field grounds potential ground loops exist. This potentially will cause signal gradation. It is necessary to understand that all 4-20 mA transmitters may not necessarily be identical in their ability to provide current into different loads. For example, a typical 4-20mA transmitter module could not drive a 100k-ohm load. This would require a compliance source of 2000 volts (20mAx100kΩ). The class standard ensures that modules of identical classes are interchangeable with respect to their drive capabilities. It is noteworthy to mention here that one should always completely examine all module specifications before replacing units. 5.8 4-wire (“self-powered”) transmitter current loops DC electric current signals may also be used to communicate process measurement information from transmitters to controllers, indicators, recorders, alarms, and other input devices. Recall that the purpose of a transmitter is to sense some physical variable (e.g. pressure, temperature, flow) and then report that quantity in the form of a signal, in this case a 4 to 20 milliamp DC current proportional to that measured quantity. The simplest form of 4-20 mA measurement loop is one where the transmitter has two terminals for the 4-20 mA signal wires to connect, and two more terminals where a power source connects. These transmitters are called “4-wire”
  • 68. P a g e | 62 or self-powered. The current signal from the transmitter connects to the process variable input terminals of the controller to complete the loop: Typically, process controllers are not equipped to directly accept milliamp input signals, but rather voltage signals. For this reason we must connect a precision resistor across the input terminals to convert the 4-20 mA signal into a standardized analog voltage signal that the controller can understand. A voltage signal range of 1 to 5 volts is standard, although some models of controller use different voltage ranges and therefore require different precision resistor values. If the voltage range is 1-5 volts and the current range is 4-20 mA, the precision resistor value must be 250 ohms. Since this is a digital controller, the input voltage at the controller terminals is interpreted by an analog-to-digital converter (ADC) circuit, which converts the measured voltage into a digital number that the controller’s microprocessor can work with. 5.9 2-wire (“loop-powered”) transmitter current loops It is possible to convey electrical power and communicate analog information over the same two wires using 4 to 20 milliamps DC, if we design the transmitter to be loop-powered. A loop- powered transmitter connects to a process controller in the following manner: Here, the transmitter is not really a current source in the sense that a 4-wire transmitter is. Instead, a 2-wire transmitter’s circuitry is designed to act as a current regulator, limiting current in the series loop to a value representing the process measurement, while relying on a remote source of power to motivate current to flow. Please note the direction of the arrow in the transmitter’s dependent current source symbol, and how it relates to the voltage polarity marks.
  • 69. P a g e | 63 Refer back to the illustration of a 4-wire transmitter circuit for comparison. The current “source” in this loop-powered transmitter actually behaves as an electrical load, while the current source in the 4-wire transmitter functioned as a true electrical source. A loop-powered transmitter gets its operating power from the minimum terminal voltage and current available at its two terminals. With the typical source voltage being 24 volts DC, and the maximum voltage dropped across the controller’s 250 ohm resistor being 5 volts DC, the transmitter should always have at least 19 volts available at its terminals. Given the lower end of the 4-20 mA signal range, the transmitter should always have at least 4 mA of current to run on. Thus, the transmitter will always have a certain minimum amount of electrical power available on which to operate, while regulating current to signal the process measurement. Internally, the electronic hardware of a 2-wire transmitter circuitry resembles the following (simplified) diagram. Note that everything shown within the shaded rectangle is represented by the “2-wire transmitter” circle in the previous diagram:
  • 70. P a g e | 64 All sensing, scaling, and output conditioning circuitry inside the transmitter must be designed to run on less than 4 mA of DC current, and at a modest terminal voltage. In order to create loop currents exceeding 4 mA – as the transmitter must do in order to span the entire 4 to 20 milliamp signal range – the transmitter circuitry uses a transistor to shunt (bypass) extra current from one terminal to the other as needed to make the total current indicative of the process measurement. For example, if the transmitter’s internal operating current is only 3.8 mA, and it must regulate loop current at a value of 16 mA to represent a condition of 75% process measurement, the transistor will bypass 12.2 mA of current. The very low amount of electrical power available at a 2-wire transmitter’s terminals limits its functionality. If the transmitter requires more electrical power than can be delivered with 4 milliamps and 19 volts (minimum each), the only solution is to go with a 4-wire transmitter where the power conductors are separate from the signal conductors. An example of a process transmitter that must be 4-wire is a chemical analyser such as a chromatograph, requiring enough power to operate an electrical heater, solenoid valves, and an on-board computer to process the sensor data. There is simply no way to operate a machine as complex and power- draining as a 2010-era chromatograph on 4 milliamps and 19 volts! Early current-based industrial transmitters were not capable of operating on such low levels of electrical power, and so used a different current signal standard: 10 to 50 milliamps DC. Loop power supplies for these transmitters ranged upwards of 90 volts to provide enough power for the transmitter. Safety concerns made the 10-50 mA standard unsuitable for some industrial installations, and modern microelectronic circuitry with its reduced power consumption made the 4-20 mA standard practical for nearly all types of process transmitters.
  • 71. P a g e | 65 6) Instrumentation Terms: Industrial measurement and control systems have their own unique terms and standards. Here are some common instrumentation terms and their definitions: Process: The physical system we are attempting to control or measure. Examples: water filtration system, molten metal casting system, steam boiler, oil refinery unit, power generation unit. Process Variable, or PV: The specific quantity we are measuring in a process. Examples: pressure, level, temperature, flow, electrical conductivity, pH, position, speed, vibration. Set point, or SP: The value at which we desire the process variable to be maintained at. In other words, the “target” value of the process variable. Primary Sensing Element, or PSE: A device that directly senses the process variable and translates that sensed quantity into an analog representation (electrical voltage, current, resistance; mechanical force, motion, etc.). Examples: thermocouple, thermistor, bourdon tube, microphone, potentiometer, electrochemical cell, accelerometer. Transducer: A device that converts one standardized instrumentation signal into another standardized instrumentation signal, and/or performs some sort of processing on that signal. Often referred to as a converter and sometimes as a “relay.” Examples: I/P converter (converts 4-20 mA electric signal into 3-15 PSI pneumatic signal), P/I converter (converts 3-15 PSI pneumatic signal into 4-20 mA electric signal), square root extractor (calculates the square root of the input signal). Note: in general science parlance, a “transducer” is any device that converts one form of energy into another, such as a microphone or a thermocouple. In industrial instrumentation, however, we generally use “primary sensing element” to describe this concept and reserve the word “transducer” to specifically refer to a conversion device for standardized instrumentation signals.
  • 72. P a g e | 66 INSTRUMENTATION IN NAPHTHA CRACKER PLANT MAULIN AMIN Transmitter: A device that translates the signal produced by a primary sensing element (PSE) into a standardized instrumentation signal such as 3-15 PSI air pressure, 4-20 mA DC electric current, Fieldbus digital signal packet, etc., which may then be conveyed to an indicating device, a controlling device, or both. Lower- and Upper-range values, abbreviated LRV and URV, respectively: The values of process measurement deemed to be 0% and 100% of a transmitter’s calibrated range. For example, if a temperature transmitter is calibrated to measure a range of temperature starting at 300 degrees Celsius and ending at 500 degrees Celsius, 300 degrees would be the LRV and 500 degrees would be the URV. Zero and Span: Alternative descriptions to LRV and URV for the 0% and 100% points of an instrument’s calibrated range. “Zero” refers to the beginning-point of an instrument’s range (equivalent to LRV), while “span” refers to the width of its range (URV − LRV). For example, if a temperature transmitter is calibrated to measure a range of temperature starting at 300 degrees Celsius and ending at 500 degrees Celsius, its zero would be 300 degrees and its span would be 200 degrees. Controller: A device that receives a process variable (PV) signal from a primary sensing element (PSE) or transmitter, compares that signal to the desired value for that process variable (called the set point), and calculates an appropriate output signal value to be sent to a final control element (FCE) such as an electric motor or control valve. Final Control Element, or FCE: A device that receives the signal from a controller to directly influence the process. Examples: variable-speed electric motor, control valve, electric heater. Manipulated Variable, or MV: Another term to describe the output signal generated by a controller. This is the signal commanding (“manipulating”) the final control element to influence the process. Automatic mode: When the controller generates an output signal based on the relationship of process variable (PV) to the set point (SP).
  • 73. P a g e | 67 INSTRUMENTATION IN NAPHTHA CRACKER PLANT MAULIN AMIN Manual mode: When the controller’s decision-making ability is bypassed to let a human operator directly determine the output signal sent to the final control element. 7) Instrumentation Transmitters: 6.1 Introduction 6.1.1 What are transmitters? A transmitter is a device which converts the reading from a primary sensor or transducer into a standard signal and transmits that signal to a monitor or controller. The methods of successfully transmitting the data to the control room are listed below 6.1.2 Types of signals used by transmitters: There are three kinds of signals that are present in the process industry to transmit the reading of a process variable from the instrument to the centralized control system. These are, 1. Pneumatic signals 2. Analog signals 3. Digital signals Pneumatic signals: These are the signals produced by changing the air pressure in the signal pipe in proportion to the measured change in a process variable. The pneumatic signal range which is the common industrial standard is 3-15 psig. The 3 corresponds to the lower range value (LRV) and the 15 corresponds to the upper range value (URV). It is still a very commonly used signal type. However, since the invention of electronic instruments in the 1960s, the lower costs involved in running electrical signal wire through a plant as opposed to running pressurized air tubes has made pneumatic signal technology less popular Figure 6-1: Pneumatic type Pressure Transmitter
  • 74. P a g e | 68 INSTRUMENTATION IN NAPHTHA CRACKER PLANT MAULIN AMIN Analog signals: It is an electrical signal whose current’s or voltage’s magnitude represents some physical measurement or control quantity. An instrument is often classified as being “analog” simply by virtue of using an analog signal standard to communicate information, even if the internal construction and design of the instrument may be mostly digital in nature. The most common standard for transmitting an analog signal is the 4-20 mA current signals. With this signal, a transmitter sends a small current through a set of wires. The signal generated is a kind of a gauge in which 4 mA represents the lowest possible measurement, or zero, and 20 mA represents the highest possible measurement. Example: Consider a process that must be maintained at 100 °C. An RTD temperature sensor and transmitter are installed in the process vessel, and the transmitter is set to produce a 4 mA signal when the process temperature is at 95 °C and a 20 mA signal when the process temperature is at 105 °C. The transmitter will transmit a 12 mA signal when the temperature is at the 100 °C set point. As the sensor’s resistance property changes in response to changes in temperature, the transmitter outputs a 4–20 mA signal that is proportional to the temperature changes. The signal transmitted can be converted to a temperature reading or an input to a control device. Why is this analogue signal conditioning required? Traditionally, data acquisition systems have acquired analog data in the form of temperatures, accelerations, strains, positions, etc. This type of data has always required analog signal conditioning in order for the data system to accept it as an input source. For example, the full-scale output of a transducer may be in the range of 0-20mVDC where the input range to the data system is 0-5Vdc. In this case, it must be noted that voltage amplification is required [3] . Amplifiers are considered to be the most common piece of signal conditioning equipment because of their wide range of uses, such as amplification, attenuation, DC-shifting, impedance matching, isolation, and others. The instrumentation amplifier amplifies the difference between two signals.
  • 75. P a g e | 69 INSTRUMENTATION IN NAPHTHA CRACKER PLANT MAULIN AMIN Wheatstone bridge: Another very important device in instrumentation systems, the Wheatstone bridges are normally associated with strain gages but are also used as bridge completion networks for resistive transducers such as Resistive Temperature Devices (RTDs). The Wheatstone bridge consists of four resistances, R1-R4 and a voltage source V for exciting the bridge, see figure 3. The transducer is placed in one arm of the bridge with a steady-state resistance equal to the other three resistances. Therefore, only when the transducer’s steady-state resistance changes, there is an output of the bridge. Figure 3: Wheatstone bridge Digital Signals: The most recent addition to process signal control technology are the digital signals. Digital signals are discrete levels or values that are combined in specific ways to represent process variables and also carry other information, such as diagnostic information. The methodology used to combine the digital signals is referred to as protocol. Digital signal conditioning can be considered as changing one form of digital data to another form. An example would be the serial-to parallel or parallel-to-serial conversion. Some even consider the analog-to-digital conversion as digital signal conditioning. Digital multiplexing can also be considered as digital signal conditioning, one type of digital data is transformed into another type. Another form of digital signal conditioning related to
  • 76. P a g e | 70 INSTRUMENTATION IN NAPHTHA CRACKER PLANT MAULIN AMIN instrumentation systems is digital filtering. There are two forms of digital filters. They are the finite impulse response (FIR) filters and the infinite impulse response (IIR) filters. 6.1.3 Telemetry: Telemetering is the reproduction, at a convenient location, of measurements makes at remote point. It is the method of getting information from one point to the other. The telemetry system can be defined as everything required to converting baseband data to radio frequency (RF) data and back again at some different location. This includes modulation of the information signal to an RF carrier, transmission, acquiring, receiving, and demodulation of the signal back into its original baseband form. Telemetry system components from the test vehicle side include the information signal, any required encryption or pre-modulation filtering, a telemetry transmitter, a power splitter if required, and vehicle antenna(s). Telemetry system components on the receiving side include reception antenna(s), pre-amps and splitters, telemetry receiver, demodulator, decryptor and bit synchronizer, if required. In general, a telemetering system consists of: 1. A measuring instrument which may measure flow, liquid level, pressure, temperature or any other variable. 2. A conversion element that converts the measured variable into a proportional air pressure and electrical quantity. 3. The pressure lines or connecting wires which may carry the transmitted variable from the transmitter to the receiver. 4. A receiver which indicates the size of the transmitted variable and may also record or control the measured variable. 6.1.4 Transmission channel: As soon as the data leaves the test vehicle in the form of a modulated RF carrier through a telemetry antenna, it will experience anomalies associated with the transmission medium. This medium is normally air. Most transmission antennas on test vehicles are Omni directional, which means the transmitted signal is sent in all directions. When more than one path, or ray, makes it into the receive antenna feed, multipath effects may occur. Most of the time, there is one dominant ray received due to the narrow beam width of the receive antenna. When one or more of these reflected paths, caused by terrain variations between the test vehicle and receiving antenna, are within the beam width of the antenna, it can either add constructively or destructively to the direct ray.
  • 77. P a g e | 71 INSTRUMENTATION IN NAPHTHA CRACKER PLANT MAULIN AMIN 6.1.5 Receiving station: The role of the receiving station is to receive the transmitted signal and to recreate the original data on the ground. 6.2 Transmitters in Process Industries: The types of transmitters used in Process Industries include: 1. Pressure transmitters 2. Temperature transmitters 3. Flow transmitters 4. Level transmitters 5. Analytic (O2 [oxygen], CO [carbon monoxide], and pH) transmitters. 8) Continuous pressure measurement: In many ways, pressure is the primary variable for a wide range of process measurements. Many types of industrial measurements are actually inferred from pressure, such as: • Flow (measuring the pressure dropped across a restriction) • Liquid level (measuring the pressure created by a vertical liquid column) • Liquid density (measuring the pressure difference across a fixed-height liquid column) • Weight (hydraulic load cell) Even temperature may be inferred from pressure measurement, as in the case of a fluid-filled chamber where fluid pressure and fluid temperature are directly related. As such, pressure is a very important quantity to measure, and measure accurately. 8.1 Electrical pressure elements: Several different technologies exist for the conversion of fluid pressure into an electrical signal response. These technologies form the basis of electronic pressure transmitters: devices designed to measure fluid pressure and transmit that information via electrical signals such as the 4-20 mA analog standard, or in digital form such as HART or FOUNDATION Fieldbus. 8.1.1 Piezo-resistive (strain gauge) sensors Piezoresistive means “pressure-sensitive resistance,” or a resistance that changes value with applied pressure. The strain gauge is a classic example of a piezoresistive element, a typical strain gauge element shown here on the tip of finger:
  • 78. P a g e | 72 INSTRUMENTATION IN NAPHTHA CRACKER PLANT MAULIN AMIN In order to be practical, a strain gauge must be glued (bonded) on to a larger specimen capable of withstanding an applied force (stress): As the test specimen is stretched or compressed by the application of force, the conductors of the strain gauge are similarly deformed. Electrical resistance of any conductor is proportional to the ratio of length over cross-sectional area (R ∝ 𝑙𝑙 𝐴𝐴 ), which means that tensile deformation (stretching) will increase electrical resistance by simultaneously increasing length and decreasing cross-sectional area while compressive deformation (squishing) will decrease electrical resistance by simultaneously decreasing length and increasing cross-sectional area. Attaching a strain gauge to a diaphragm results in a device that changes resistance with applied pressure. Pressure forces the diaphragm to deform, which in turn causes the strain gauge to