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Process Safety Guide:
GBHE-PSG-008
Pressure Relief Systems
Causes of Relief Situations Vol.2 of 6
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Process Safety Guides: Causes of Relief Situations
INDEX
VOL. I BACKGROUND TO RELIEF SYSTEM DESIGN
(This includes principles of pressure relief and use of this Guide, alternatives,
statutory and mandatory requirements, and reporting).
VOL. II CAUSES OF RELIEF SITUATIONS
VOL. III CALCULATION OF REQUIRED RELIEF RATE
VOL. IV SELECTION, SIZING, AND INSTALLATION OF PRESSURE
RELIEF DEVICES
(This includes the pressure setting in relation to the design pressure of the
protected equipment).
VOL. V DISCHARGE AND DISPOSAL SYSTEM DESIGN
VOL. VI REFERENCE SECTIONS
DOCUMENTS REFERRED TO IN THIS PROCESS GUIDE
It is emphasized that this document is only a Guide, describing good practice at
the date of issue, and is not itself mandatory (although some mandatory
instructions are quoted). When used in this Guide, the words "must", "shall", and
"should" have no legal force and are not mandatory, except where they are part
of a quoted mandatory instruction from another source.
The word" must" has not been used, except when part of a quotation.
"Shall" is a strong recommendation of GBHE based upon experience or upon the
position adopted by recognized authorities, and the engineer may quote
compliance with this guide only when that recommendation has been followed.
"Should" is a recommendation based upon the judgment of experienced people
but recognizes that some discretion may be appropriate.
Note:
This Guide includes references to and quotations from external and British
Standards. The reader should always check if the Standards have been updated
since the last issue of this Guide.

Volume 2: Causes of Relief Situations
SECTION 1: IDENTIFICATION OF EVENTS LEADING TO OVERPRESSURE /
UNDERPRESSURE
1 INTRODUCTION
1.1 General Statement
1.2 Hazard and Operability Studies and Hazard Analysis
2 METHOD OF IDENTIFICATION
2.1 Potential Causes of System Overpressure
2.1.1 Principles and Terminology
2.1.2 Plant or Process Conditions
2.1.3 Prime Events
2.2 Procedure
2.2.1 Tabulation of Potentially Hazardous Events
2.2.2 Stepwise Procedure for Each System
3 DEFINITION OF A PROCESS EQUIPMENT SYSTEM (PES)
SECTION 2 : EXTERNAL FIRE
4 INTRODUCTION
5 FACTORS TO CONSIDER
5.1 Nature of Material Inside Equipment
5.2 Extent of Fire Zone
5.3 Type of Fire
5.4 Loss of Structural Strength
5.5 Radiation from Adjacent Fires
5.7 Thermochemical and/or Decomposition Effects
5.8 Small Vessels
6 PITFALLS WITH FIRE

SECTION 3: PROCESS ABNORMALITIES
7 INTRODUCTION
8 SYSTEM BLOCKED-IN (CONDITION 1)
8.1 General Considerations
8.2 Mal-operation
8.3 Process Aberration
8.4 Pitfalls with Blocked-in Equipment
9 RESTRICTED OUTLET (CONDITION 2)
9.1 Mal-operation
9.1.2 Outlets Closed or Restricted
9.1.2 Outlets too Small
9.2.1 General Considerations
9.2.2 Causes of Blockage
9.3 Pitfalls with Restricted Outlet Condition
10 RESTRICTED INLET (CONDITION 3)
10.1 Equipment at Risk
10.2 Events Leading to Underpressure (Vacuum)
10.3 Pitfalls with Restricted Inlet
11 CHEMICAL REACTION (CONDITION 4)
11.1 Normal and Abnormal Chemical Process
11.2 Runaway Reaction
11.3 Chemical Reaction in Relation to Prime Events
11.3.1 Fire
11.3.2 Mal-operation
11.3.3 Process Aberration
11.3.4 Equipment Failures
11.3.5 Service Failures
11.4 Underpressure Relief
SECTION 4: EQUIPMENT AND SERVICES FAILURES
12 INTRODUCTION

13 EQUIPMENT FAILURE
13.1 System Blocked-in or Restricted Outlet (Conditions 1 and 2)
13.1.1 Factors to Consider
13.1.2 Failures Involving Inter-stream Leakage (Heat Exchangers and Similar Items)
13.1.3 Fired Heaters
13.1.4 Control System Failure
13.1.5 Machines (and Ejectors)
13.2 Restricted Inlet (Condition 3)
13.2.2 Failures Involving Inter-stream Leakage
13.3 Chemical Reaction (Condition 4)
13.4 Pitfalls with Equipment Failure
14 SERVICES FAILURE
14.2 Principal Services
14.2.1 Cooling Water
14.2.2 Electrical Power
14.2.3 Instrument Air
14.2.4 Steam: HP, IP, LP
14.2.5 Fuel Gas and Fuel Oil
14.3 Secondary Services
14.3.1 Heat Transfer Fluids (hot oil, "Thermex", hot water)
14.3.2 Hydraulic Oil
14.3.3 Inert Gas and Nitrogen
14.3.4 Refrigerant
14.3.5 Water (other than cooling water)
14.4 Pitfalls with Service Failures
SECTION 5 : AMBIENT CHANGES
15 INTRODUCTION
16 ENVIRONMENTAL CHANGES
16.1 Atmospheric Conditions
16.2 Plant Environment
17 EXPANSION/CONTRACTION OF VAPOR INVENTORY
17.1 Low-Pressure Storage Tanks
17.2 Low Temperature Atmospheric Pressure Storages and Equipment
17.3 Sealed Containers (IBC)
18 HYDRAULIC EXPANSION - PIPELINES AND VESSELS

TABLE
1 IDENTIFICATION OF EVENTS LEADING TO OVER / UNDERPRESSURE

SECTION 1.0 IDENTIFICATION OF EVENTS LEADING TO
OVERPRESSURE/UNDERPRESSURE
1 INTRODUCTION
1.1 General Statement
In every pressure relief study the first and often most difficult step is that of
recognizing every potential cause of overpressure in the process
equipment system being studied. This Guide does not, however, include
any consideration of dust, gas/air or vapor phase explosions where the
speed of reaction is such that the equipment will be vented directly to the
atmosphere. See also Part C, Section 5 of this Guide.
This Part B is a guide to the qualitative identification of common causes of
overpressure in process equipment. It cannot be exhaustive; the process
engineer and relief systems team should look for any credible situation in
addition to those given in this Part which could lead to a need for pressure
relief (a relief situation).
Note 1:
OVERPRESSURE: Following common usage, the term "overpressure" or
"system overpressure" is used in this Guide to refer to any excess of
pressure in a system over its design pressure. It should be distinguished
from the more specific term "relief valve overpressure" defined in Vol. 1
of this Guide in relation to the set pressure of a relief valve.
Note 2:
UNDERPRESSURE: The corresponding term "underpressure" (system
under pressure) is used for any situation where the absolute pressure is
reduced below that at which the equipment might be damaged or collapse
(an under pressure relief situation).
To create a relief situation, there must be an energy source. Pressure
relief may be needed when either:
(a) T he normal mass balance is disturbed;
or
(b) The energy balance, is disturbed.
An example of each of these situations is given below:

(1) Outlets from vessel are blocked or restricted while inlets remain
open to source of fluid at a pressure higher than design pressure.
(2) Reaction goes out of control (runaway) leading to energy release
too great for normal equipment to handle.
Sections 1 through 5 are intended to lead the user through the process of
identifying all situations that need to be considered, at an early stage in
the development of engineering line diagrams. It is recognized that the
individual user needs an introduction to the recommended method though
he may or may not require detailed technical advice, dependent upon his
own level of experience.
ALL USERS ARE STRONGLY RECOMMENDED TO READ THIS
SECTION 1.
1.2 Hazard and Operability Studies and Hazard Analysis
It is essential that engineers and/or teams responsible for pressure relief
systems (sometimes referred to as Relief and Blowdown Teams) identify
possible causes of system overpressure and hence the main relief system
equipment early in the project so that design and ordering may proceed.
Every attempt should be made to ensure that every significant risk is
considered. Further examination in the form of detailed Hazard and
Operability Studies provides a check that no causes have been missed.
Quantitative Hazard Analysis can be used to assist in deciding for what
combinations of hazards a relief system needs to be designed or whether
a particular event is sufficiently remote to be discounted.
In high risk situations, Hazard Analysis may be essential to determine the
level of integrity required of the relief system itself.

2 METHOD OF IDENTIFICATION
2.1 Potential Causes of System Overpressure
2.1.1 Principles and Terminology
When reading this Section, refer to Table 1.
Inadvertent pressure rise (or fall) leading to the need for pressure (or
vacuum) relief is caused by certain EVENTS occurring while the plant or
process is in a susceptible CONDITION. An EVENT (e.g. thermal energy
input) may arise from:
(a) A fault or defect (e.g. control system failure).
(b) Normal process mass and heat flows.
(c) Changes in ambient conditions.
(d) External fire.
Faults or defects may arise from human error, design limitation, equipment
failure or external factors. Many faults can occur without a potential hazard
arising because the conditions are not appropriate at the time.
Of a number of possible approaches, the GBHE Process Engineering
(Relief Systems Team) recommends the method given in this Guide for
the identification of inadvertent causes of pressure rise or fall. The plant
items are first grouped together into one or more process equipment
system(s) for the purpose of the pressure relief study. All events likely to
cause the pressure to rise or fall are identified by considering the plant
and/or process to be in each of four PLANT/PROCESS CONDITIONS in
turn. To help the user to carry out this procedure a number of common
faults, defects etc. have been assembled in Table 1 under the column
headings of four PRIME EVENTS.
Note:
To avoid duplication in the Table, some events are listed in a box only
under the CONDITION under which they would be most likely to occur.
They may, however, be relevant to other CONDITIONS and so it is
imperative to consider the contents of all other boxes in the same
column. The user should also consult the corresponding Sections of Part
B of this Guide.

Sometimes a condition in which the system becomes prone to
overpressure generation is not a normal condition of operation but is itself
caused by a fault. For example, a blocked-in condition may be caused by
solidification of process material as a result of a heating failure.
Overpressure may then be generated either by normal or abnormal
events.
2.1.2 Plant or Process Conditions
The four conditions tabulated and to be considered in turn are:
CONDITION 1 : SYSTEM BLOCKED-IN
A process equipment system having both inlets and outlets closed but
which can still be subject to energy input especially by heat transfer. The
closing of blocking valves may be intentional or accidental so CONDITION
1 may be normal or occur as a consequence of an operational fault.
CONDITION 2 : RESTRICTED OUTLET
The situation where, because of restriction of flow through the outlets, the
maximum rate of discharge from the system at its design pressure is less
than either the potential inflow from all sources or the rate of expansion of
the contents. The outlet(s) may be closed, restricted unintentionally or too
small.
CONDITION 3 : RESTRICTED INLET
The situation where the potential inflow to the system is less than either
the maximum outflow or the rate of contraction of the contents because of
restriction of inlets. This condition is only a hazard for equipment not
designed for operation under vacuum. The inlet(s) may be closed,
restricted unintentionally or too small.
CONDITION 4 : CHEMICAL REACTION
The potentially hazardous situation when a chemical process (usually, but
not necessarily, exothermic) is capable of causing the pressure to rise if
outflow of fluid or removal of heat from the system is restricted;
alternatively a chemical reaction may also cause the pressure to fall when
inflows are restricted.
In some cases the chemical reaction itself may go out of control if the
operating conditions deviate from normal. Either the basic chemical
reaction rate then accelerates uncontrollably or undesirable side reactions
are initiated which rapidly predominate. In either case heat may be

evolved at a rate that cannot be dissipated, or gas/vapor may be
generated at a rate that cannot be discharged via the normal routes
available. When chemical absorption of gas occurs, the process
has potential for creating an underpressure relief situation. Thus the
involvement of chemical reactions with potential for causing overpressure
(or underpressure) is defined as a PROCESS CONDITION; an abnormal
reaction is an EVENT which can be triggered by various faults.
CONDITION 4 can occur in combination with either 1, 2 or 3.
Note:
Clearly overpressure can arise under CONDITIONS 1, 2 or 4 and
underpressure can arise under
CONDITIONS 1, 3 or 4.
2.1.3 Prime Events
The four PRIME EVENTS tabulated and whose effect has to be examined
for each one of the PLANT/PROCESS CONDITIONS are:
PRIME EVENT A : EXTERNAL FIRE
A plant fire is treated as a PRIME EVENT rather than as an effect of some
other EVENT which started it because:
(a) Direct quantitative relief requirements follow from considering the
heating effect of fire.
(b) Secondary effects are sometimes caused by fire.
(c) The initial cause of the fire does not affect the relief systems study.
(d) It is established practice in pressure relief studies.
PRIME EVENT B : PROCESS ABNORMALITY
Abnormalities include both mal-operation and process aberration. Mal-
operation means that the process is inadvertently operated in an abnormal
way. Process aberration is an abnormality which may occur within the
process itself as a result of factors outside normal control - such as
deviations in chemical or physical state of materials.

PRIME EVENT C : EQUIPMENT AND SERVICES FAILURE
Equipment failure (or breakdown) of mechanical systems, machines or
control systems.
Services failure is that of ordinary services (steam, water, electrical power,
etc.) supplied from outside the process unit. It may be partial or total.
PRIME EVENT D : AMBIENT CHANGES
Usually this means a change in the atmospheric conditions (temperature,
rainfall, wind, etc.) that can cause problems due to pressure changes in
low-pressure storage tanks and in pipelines. Other environmental changes
have to be included (e.g. heat radiation from nearby installations) in some
plant studies.
2.2 Procedure
2.2.1 Tabulation of Potentially Hazardous Events
Table 1 is a tabulation of possible events caused by faults and other
factors in design and operation that constitute the PRIME EVENTS as
defined in 2.1.3. All relevant listed items plus any others that may be
suggested should be considered by the team or engineer responsible for
relief and blowdown studies.
The Table serves as a check list to be used in conjunction with the more
detailed advice in Sections 2 through 5 to which cross reference is made.
These Sections are intended to lead the user to recognize possible
hazards other than those given in the Table.
It is not only important to identify all the potential causes of a relief
situation but also to establish whether each source will affect the design of
the relief system; hence it is necessary to calculate the required relief rate
for each source as it is identified and consider whether:
(a) The various sources are either independent and their effect simply
additive or whether they are subject to more complex interactions.
(The effect can be either less or more than simply additive).
(b) Any lesser sources, either alone or in combination, are significant
after designing the relief system for the major causes.

2.2.2 Stepwise Procedure for Each System
The general procedure for identification of causes of overpressure and
calculation of the total required relief rates is as follows. Recommended
methods for performing the calculation of individual rates are covered in
Part C of this Guide.
(a) For each process equipment system mark a boundary on the line
diagram for the system such that all equipment within the boundary
can be effectively served by one pressure relief device or
combination of devices.
(b) Consider the process equipment system to be in each of the four
PLANT/PROCESS CONDITIONS that are relevant, in turn.
Whatever the CONDITIONS, think carefully about the events listed
under all other CONDITIONS in Table 1 to identify every event
that could lead to a relief situation.
(c) With the help of Sections 2 to 5, try to recognize any other
significant event which has not already been considered.
(d) Check whether the effect of a fault could pass either forwards or
backwards through the train of plant items and cause a relief
situation at a remote location.
(e) Consider whether the effect of any significant cause can be easily
reduced in magnitude or eliminated, by changes either in design or
operating procedures.
(f) Determine the total required relief rate as follows:
(1) calculate the required relief rate for each individual event;
Note:
With large projects, because of long delivery times for main
plant items and relief devices, it is sometimes necessary to
estimate relief rates before detailed equipment data are
available. It will be necessary to check these calculations
later, when sufficient data are available, and make any
necessary modification to the relief system design.

(2) select the highest individual or highest credible combination
of relief rates (determined by hazard analysis if necessary);
(3) if the total required relief rate is excessive, consider in detail
any way by which it may be reduced.
Note 1:
It may be necessary to do this after considering the
combined relief requirements of a number of process
equipment systems. For instance, the combination may
affect the design of a flare system more than the size of the
relief device.
Note 2:
The effect of some events cannot be precisely defined for
the purpose of calculating relief rate and may seem to be
unlikely occurrences. Such events may only be ignored if
they can be made acceptably remote (checked by hazard
analysis if necessary). Otherwise, the best possible
calculation should be made and an appropriate factor of
safety applied.
(g) With large plants, to facilitate the subsequent design of common disposal
systems and also as an aid in reporting, it is recommended that a code
number be allocated to each fault or defect identified as a potential cause
of a relief situation.
The code number can be used to rapidly identify anyone fault that could
affect more than one process equipment system at the same time.
Note:
For a large plant the required relief rate resulting from services failure (for
instance electrical power) is often the sum of a number of relief rates
produced by coincident effects in more than one process equipment
system. All these rates contribute to a combined relief discharge to flare,
scrubber or other disposal system.

3 DEFINITION OF A PROCESS EQUIPMENT SYSTEM (PES)
(For Relief and Blowdown Studies)
Items or groups of items of equipment which will be connected by
pipework to an appropriate pressure relief device, and not containing any
means of internal isolation, are considered to be one system for the
purpose of protection from overpressure or underpressure. If any item or
part of the system can be isolated by mechanical means or by an
abnormal process condition, such as accidental blockage (ice, etc.) then
that system must be subdivided into separate systems each of which may
require protection against overpressure or underpressure. See Part D of
this Guide.
Note:
It is important to ensure that at all times the interconnecting pipe work and
ducts within items of equipment are capable of passing the relevant fluids
at the required relief rate to the relief system.
Isolatable sections of pipework within the plant often do not require
pressure relief. (This normally refers to fire or thermal relief). However,
long pipelines, some large diameter pipes and pipes containing liquid with
high vapor pressure at ambient temperature may require protection and
should be treated as a process equipment system. See Section 5.
Sometimes, Administrative Safety Procedures can be proposed by the
Relief and Blowdown Team or the Project Team with regard to isolation
practices. Isolation valves between two systems can be locked open in
certain circumstances to allow the two systems to be regarded as one for
pressure relief purposes. Any such proposals should be agreed by the
appropriate Works authority. It should be remembered that isolation valves
even when fully open might impair the safe operation of a relief system.
Note:
The preceding statement does not refer to the isolation of relief devices
from the equipment they protect - a practice which is only permitted in a
very limited number of circumstances and which shall be approved by the
appropriate authority. See Part A.
When defining a PES the isolation of equipment by means of slip-plates,
blanks, or bobbin pieces need not be taken into consideration provided
that the operation is part of the normal maintenance procedures and
equipment is drained/vented until its return to on-line duty.

However, the precise siting of slip-plates and blanks is important in
ensuring that pipework and items of equipment still in service do not
become isolated from their normal relief systems. Thus, the position of any
isolating device should be clearly identified on the engineering line
diagram when the PES is defined.
It is essential to take into consideration any specification breaks in
pipework when defining each PES.

TABLE 1: IDENTIFICATION OF EVENTS LEADING TO OVER /
UNDERPRESSURE

SECTION 2:
EXTERNAL FIRE
4 INTRODUCTION
At an early stage in a project, any areas in the plant in which a significant fire could
occur for a prolonged period need to be identified as a fire zone. Any vessel located
in a fire zone could be subject to energy input from a fire either by direct contact with
the flame or by radiation from a nearby fire. This may be sufficient to generate
excessive pressure in the vessel so that pressure relief ("fire relief" being the
generally accepted term) will be necessary. Alternatively, although a fire is possible,
the amount of available fuel may be insufficient to raise the temperature and hence
the vapor pressure of the vessel contents to the extent that makes pressure relief
necessary.
When assessing the need to provide either fire relief, vapor depressring facilities or
any other protection of process equipment against overpressure, all the
consequences of failure of the equipment (by rupture or other major breakdown)
should be borne in mind, for example:
(a) Release of flammable material increasing the scale of the fire.
(b) Discharge of toxic substances to the atmosphere.
(c) The cost of damage to the plant (repairs and outage).
(d) Risk to personnel and plant from fragmenting vessel.
(e) Vapor explosion as a result of external ignition of material released as a
result of the failure.
When fire is the main hazard, in addition to a pressure relief system, it is usually
prudent to provide protective equipment such as:
(1) Use of fire resistant lagging or other materials resistant to fire to protect
appropriate areas.
(2) Vapor depressurizing systems to reduce the system pressure below the relief
set pressure - especially when there is a risk of weakening the metal by
overheating (See 5.4).
(3) Liquid release systems discharging to an appropriately protected dump tank -
particularly for toxic or flammable materials.
Even when fire is a lesser hazard, consideration should still be given to such
measures.

When considering external fire the plant condition is always considered to be
"Blocked-in" except when it is physically impossible to block it in. In all cases, decide
whether fire should be considered together with other PRIME EVENTS (e.g.
equipment failure).
5 FACTORS TO CONSIDER
5.1 Nature of Material Inside Equipment
Examine the properties of the process materials that can be present at any time and
establish the effect of overheating. The commonest pressure hazard due to fire
arises with vessels containing:
(a) Volatile liquids whose vapor pressure might exceed the design
pressure when heated by fire.
(b) Materials that can undergo abnormal chemical reaction with
evolution of gaseous products when overheated in a fire. See Part
C, Section 5 of this Guide.
5.2 Extent of Fire Zone
A very broad guideline is that a fire hazard exists in equipment which:
(a) Contains more than about 2 t of flammable liquid;
and/or
(b) Is continuously fed at a rate of about 2 t/h or more of flammable
liquid, gas or vapor unless it is safe to assume that the flow can be
reliably stopped in the event of a fire. See Process Safety Guide
No.2.
The space surrounding a fire hazard constitutes a "fire zone". Subject to
accepted limitations based on experience, the zone should be taken as
either 12 m by 12 m square or 6 m laterally from the boundary of the
equipment constituting the fire hazard unless special provision is made to
limit the spread of fire. Suitable fire resistant walls and screens or the plant
drainage layout may be considered as limiting the extent of the fire zone.

Note:
Engineers responsible for specifying fire zones should satisfy themselves
that these guidelines are appropriate to the particular situation in question
because local environmental factors often affect the demarcation.
5.3 Type of Fire
Experimental and other research work has shown that the heat input can
vary with the type of fire surrounding the vessel. Vessels completely
enveloped by flames are subject to much greater heat fluxes than those in
situations where the heat transfer is reduced by air currents, smoke, etc.
See Part C, Section 1 of this Guide.
More specifically, at heights of more than 15 m above the fire source, heat
transfer is considered to be drastically reduced by smoke, eddies and
wind currents. Normally fire relief need not therefore be considered for
vessels and other equipment that are more than 15 m above the
source of the fire except for the following:
(a) Equipment in buildings or other locations where a "chimney" effect
can be created.
(b) Any vessel where the heat input can be increased by conductive
heat transfer (usually via heavy structural steelwork).
(c) Equipment in locations where some induced airflow could increase
the height of the fire (e.g. fire near an up draught air cooler).
5.4 Loss of Structural Strength
A process vessel that is heated above its design temperature may fail at a
pressure well below its design pressure. Hence, a relief valve set at
design pressure cannot be relied upon to protect a vessel subjected to fire
for a prolonged period. This risk is greatest to parts of the vessel not
internally wetted by liquid.
Vessels sited in a fire zone may therefore need to be provided with
remotely operated independent systems to guard against such failure by
discharging some or all of the vessel contents in order to lower the
pressure in the vessel to a safe value below the design pressure and
usually less than the relief set pressure.

This can be achieved by discharging vapor ("vapor depressuring") or
liquid ("liquid dumping"). Refer to Process Safety Guide No.2 for more
detailed guidance.
Localized flame impingement may also lead to failure at pressures below
design pressure and should, if possible, be prevented by lagging, screens,
etc. Otherwise provide a depressuring system.
In cases where supercritical temperature will be reached before the relief
pressure, loss of evaporative cooling by boiling inside the equipment will
allow the metal to overheat and lose its strength much sooner than when it
is in contact with liquid. This is, therefore, another possible case for
providing depressuring facilities.
Radiation may affect equipment in a neighboring area not designated as a
fire zone. Fires may spread to other areas including non-fire zones if the
plant layout and arrangements for drainage are not designed with this
hazard in mind.
5.6 Reduction of Heat Absorbed by Equipment
Measures taken to reduce heat absorption can significantly reduce the
size and number of protective devices required for fire relief. Such
measures include the use of fire resistant lagging and sloping the ground
beneath the plant. The reduction that may be allowed depends on the
specific actions and the prevailing situation. See Part C, Section 1 of this
Guide.
Sometimes it is possible to design out completely the need for fire relief by
such measures as:
(a) Use of very thick fire resistant insulation.
(b) Use of fire resistant concrete protection for plate-fin exchangers.
(c) Separation of air coolers by concrete floors.
(d) Burying pipes underground.

5.7 Thermochemical and/or Decomposition Effects
Materials on hot surfaces may well undergo side reactions which generate
much heat simultaneously with that from the external fire. Consideration
shall therefore be given to all possible side reactions and the temperature
at which such effects would begin.
5.8 Small Vessels
Some vessels may be sufficiently small for rupture in a fire to be regarded
as a minor risk so that pressure relief need not be provided. The following
hazards should be examined before making any decision not to provide
pressure relief:
(a) Pressure energy (depending on pressure at failure and volume of
vessel).
(b) Nature of contents likely to be released.
(c) Damage likely to be caused by fragmentation, blast, etc.
In the case of pressure energy, guidance is given in SI 2169 - "The
Pressure Systems and Transportable Gas Containers Regulations 1989"
for exemption from registration. Exemption may be granted for equipment
which meets both the following criterion:
The product P x V is less than 0.25 bar g m3
where: P = design pressure (bar g)
V = vessel volume (m3
)
These regulations come into force in July 1994.
The existing regulations granted exemptions if:
P x V is less than 1 bar g m3
and design pressure less than 7 bar g.

These remain valid until July 1994.
It is strongly recommended that the criterion of P x V < 0.25 bar g is used
for all new designs and modifications.
Provided that the vessel is not greatly overdesigned (i.e. it would only
rupture at many times the design pressure) the same criteria may be used
when considering the possible elimination of pressure relief devices on a
small vessel.
Caution shall be exercised, however, when using these criteria to justify
that there is no need for pressure relief where:
(i) very high -pressure can be generated in a fire (e.g. cylinders
containing LPG);
(ii) brittle fracture can be a possibility;
(iii) any potential cause of metal failure such as corrosion can be
recognized.
6 PITFALLS WITH FIRE
During stand by, start up or shut down the fluid in the equipment may be
totally different from that during normal operation. The relief system shall
be designed to cope with the worst case identified by a study of all
possible normal and abnormal operating conditions.
Often, many items subject to different events are included in the process
equipment system for which fire relief will be provided and this means that
careful attention should be given to the following aspects:
(a) Fire relief should not be assumed to be the limiting case for
pressure relief. The required relief rate for each event should first
be evaluated.
(b) High pressure losses can be built-up within the process equipment
system itself if each item and all interconnecting pipework are not
adequately designed so that material can flow without restriction to
the relief outlet. This is a very important aspect in the case of fire
relief. Typical examples of the effect of restrictions are:

(1) condensers that can become flooded;
(2) overhead pipework connected to a reflux drum on ground level that
can be liquid-logged.
British Standards govern the provision of fire relief on certain vessels such
as air receivers, refrigeration units and steam boilers or receivers. See
Part A of this Guide.
SECTION 3:
PROCESS ABNORMALITIES
(Mal-operation and Process Aberration)
7 INTRODUCTION
Whenever intervention by operators is used in the operation of a process
plant or any other equipment, mistakes can and do occur; any such event
is called "mal-operation". If there is a potential for pressure rise such mal-
operation would be serious unless its effects are considered and provided
for at the design stage. Similarly, pressure rise may be caused by
malfunction of the process as a result of:
(a) Unusual operating conditions.
(b) Inconsistent raw materials.
(c) Unknown features inherent in the process.
Any such event is called II process aberration II •
All credible events of either type that could occur should be examined.
The study is applied to continuous process plant while on line, starting up
or shutting down and during shutdown periods; it is also applied to batch
operations at all phases of the operating cycle. These events should be
considered for each of the previously given PLANT OR PROCESS
CONDITIONS that is possible in the given situation. See 2.1 & Table 1.
The aim is to identify any combination of factors that can lead to a relief
situation. Hence, both those events that can directly cause overpressure
(or underpressure) in the existing CONDITION of the plant or process and
those that create a new CONDITION in which the system is prone to
overpressure (or underpressure) are included in this Section.

8 SYSTEM BLOCKED-IN (CONDITION 1)
This condition can arise with any equipment which can be isolated or shut
down whilst containing liquids or gases. With continuous processes, the
blocked-in condition usually arises from mal-operation but with many
batch processes the equipment is deliberately isolated and blocked-in
during normal operation. When blocked-in, the contents can be subject to
energy input from:
(a) A heating service.
(b) Stored up thermal energy.
(c) Sources of mechanical energy.
(d) Chemical reaction.
The following typical situations are some that can occur when a plant is
shut down or put on standby:
(1) Vessels isolated while containing liquid - as occurs during many
batch processes. If partially filled, consider the vapor pressure; if
completely filled, see (2).
(2) Pipelines and vessels isolated while full of liquid. They may require
relief protection from hydraulic expansion; consideration should be
given to the need for relief whenever liquid can be trapped between
isolation valves. See Section 5 and Part C, Section 8 of this Guide.
(3) Machinery (pumps especially) left full of liquid when shut down.
With very large pumps or those handling cryogenic liquids the
pump casing may need to be fitted with a pressure relief device;
thermal relief via the seal may be adequate in some cases but
not always acceptable. See Section 5 and Part C, Section 8 of this
Guide.

8.2 Mal-operation
After a blocked-in CONDITION has been created a number of EVENTS
can subject the equipment to energy input; any such event which is a mal-
operation may cause overpressure. Note the following examples:
(a) Heat exchange equipment continuing to transfer heat into the
system after the heat sink has been isolated.
(b) Electric heaters, if not switched off, may raise the temperature
sufficiently to weaken metal walls as well as heat the contents. This
could create a need for relief at a pressure lower than the design
pressure.
(c) An agitator or pumped circulation system left running after a vessel
has been isolated can be a source of energy input that would raise
the temperature sufficiently to cause appreciable hydraulic
expansion, initiate abnormal chemical reactions or significantly
increase vapor pressure.
(d) Circulation pumps running on total recycle (kickback) can similarly
heat the contents of the process equipment system with the same
effects.
Note:
Shaft seals may be damaged by a pressure which is within the safe
working pressure for the pump body and other parts of the
equipment.
(e) Chemical reactions may be:
(1) operated intentionally under blocked-in conditions, See 11.3;
(2) accidentally blocked-in by mal-operation (e.g. wrong closing
of valves);
(3) blocked-in by malfunction (e.g. blocking of lines by
inadvertent solidification).

Blocking of lines and equipment is covered under RESTRICTED OUTLET
and also RESTRICTED INLET. Apart from abnormal chemical reactions
there are other instances of unusual behavior that can cause a pressure
rise in either the blocked-in or restricted outlet condition. See Clauses 9
and 10.
The following is one such case that should be examined when
appropriate.
Roll-over is a phenomenon originally experienced with many large
liquefied natural gas (LNG) storage tanks due to the formation of an
unstable system of liquid layers of various densities and temperatures.
Such instability may be caused in several ways, particularly by:
(a) Feeding of a higher density liquid on to (or near to) the surface of a
lower density liquid.
(b) Feeding of a lower density liquid to the bottom of a vessel
containing a higher density liquid. This cause of unstable layering is
less likely than (a) because of the upward displacement resulting
from the inflow.
(c) Evaporation of more volatile components from the upper layers,
following heat input from the environment so that these layers
become more dense than the lower layers.
(d) Heat input to lower layers (e.g. by base heating) that can increase
the temperature and decrease the density to a value less than that
of upper layers.
The result may be that an unstable condition is set up in which the
hydrostatic head temporarily prevents the boiling of the lower layers. If this
unstable condition is disturbed, the higher density layers begin to sink and
lower density layers to rise; once this happens, mixing of the layers can
rapidly accelerate.
The subsequent attainment of new equilibrium conditions may be
accompanied by a violent boil-off of LNG.

This is due to material containing more volatile components suddenly
achieving a higher temperature by mixing and/or material rapidly reaching
a zone where the hydrostatic head is lower. Such boiling is capable of
generating considerable overpressure.
Similar phenomena have occurred in atmospheric storage tanks and
should be considered wherever liquids of different densities and boiling
points can be layered and stored for long periods of time.
Every effort should be made to design out the likelihood of roll-over. There
are two principal approaches available:
(1) Design to ensure adequate mixing at all times (e.g. by provision of
jet mixing facilities).
(2) Assume that layering will occur and where there is a risk of
subsequent mixing being caused by thermal effects, minimize the
risk by use of effective insulation.
8.4 Pitfalls with Blocked-in Equipment
(a) Heat exchangers have two sides and need to be treated as two
vessels one of which is an energy source to the other.
(b) A relief situation can be generated in heated pipes irrespective of
ambient conditions. Estimation of maximum achievable
temperature followed by a consideration of resultant pressure
effects is required for all heated lines (including steam-traced,
jacketed and electrically heated pipes).
(c) Internal rupture can occur in canned motor pumps and certain other
items having a weak internal wall despite the fact that the outer
casing may be adequately designed. Take the maximum differential
pressure into account.
(d) The layering of two immiscible liquids may be a potential cause of a
relief situation if boiling would occur when the layers are mixed.
See 9.1.2.3.

9 RESTRICTED OUTLET (CONDITION 2)
9.1 Mal-operation
9.1.1 Outlets Closed or Restricted
Equipment items often suffer the total loss of one or more outlet paths due
to closed valves and mechanical equipment. This condition may be either
normal to the process or result from a mal-operation or failure.
Examples of such mal-operations are:
(a) Inadvertent closure of valve(s), or valve(s) incorrectly remaining
closed during or after start up.
(b) Incorrect closure of more than one valve at the same time during
changeover operations of spare equipment or during batch
operations.
Note:
Non-return valves on inlets operating the way they are intended ad
in same way as a blockage and should not be disregarded. See
Part C, Section 3 of this Guide.
(c) Sudden stoppage of machinery, for instance a compressor. This is
frequently accompanied by backflow from the downstream high
pressure source through the bypass or through the machine itself.
(d) Incorrect setting of a controller causing partial or total control valve
closure. There is a high risk of these events during start up and
shutdown operations.
Most EVENTS which lead to pressure rise under BLOCKED-IN
CONDITIONS also apply to RESTRICTED OUTLET and will not be
repeated here. See 8.2.
Note:
Pressure surge (over or under pressure) may be caused by any of the
above mal-operations and sometimes by normal operation, in which case
special design considerations may be invoked. See Part C, Section 9 of
this Guide.

9.1.2 Outlets too Small
There are also process mal-operations which generate or admit to the
equipment so much vapor or liquid that although it is on-line with its
normal outlets functioning, fluid accumulates so that the pressure rises
above normal. The events listed below can also occur while the outlets of
the vessel are closed, for instance during start up, stand by or shutdown.
9.1.2.1 Gas Breakthrough
Liquid is frequently transferred under gas pressure into another vessel
(often a low-pressure storage tank) through a regulating valve or other
device. The valve may fail to close when all the liquid has been transferred
and gas will pass through the line so that the low-pressure vessel can
be subjected to the higher (blowing) pressure. If its design pressure is less
than the blowing pressure it will require relief protection.
Deliberate line blowing is often used to empty pipelines completely after
movement of material into a storage tank - especially when the material
solidifies easily. Always consider the risk of over pressuring the receiving
vessel.
9.1.2.2 Liquid Overfilling
Liquid overfilling of storage tanks is often possible, and an overflow large
enough to accommodate the "pump in" rate of liquid feed is needed.
Whenever such an overflow is not practicable, the pressure relief system
shall be sized to pass this flow.
Liquid overfilling of process vessels is less often recognized as a potential
demand on the relief system. However, repeated operating experience
has shown that it is quite possible to fill even the largest distillation
columns with liquid and for it to take many hours for operating teams to
establish what is happening.
Liquid overfilling is a particular problem during start-up, when instruments
are being brought into commission and when the plant is far from steady
state. Accidental misrouting of streams and the unavailability (or mis-
calibration) of instruments may result in incomplete or misleading
diagnostic data which can take hours or even days to resolve.

The consequences of a major ingress of liquid into a relief system which is
not designed for it can be very serious: the pipework and its supports may
not be designed for the weight; the pipework may be damaged by
slugging; the liquid knock-out and pump-out systems may be
overwhelmed; the relief system capacity may be reduced to the extent that
other demands cannot be handled. In the worst case, a loss of
containment may occur, followed by a possible fire or toxic release.
For the design of the relief system, it should be assumed that ANY
process vessel, regardless of size, is capable of being flooded with liquid.
This requirement should only be relaxed following a hazard analysis of the
installation, or if an assessment of the inventories in the system shows
that overfilling is not a credible event. For potential relief cases with liquid
flowrates, designing out the relief case is strongly recommended. This
requires a combination of suitable instrumentation and operator action.
To "design out" liquid overfilling as a relief case, at least an independent,
diverse, extra-high level alarm will be required and on critical applications
an independent third instrument may be needed. The alarms shall be
allied to suitable operator training so that immediate action can be taken.
Consideration should be given to hard-wiring important alarms (as
opposed to routing them solely through a Distributed Control System) so
that the alarms remain effective if the DCS loses power.
There are a number of pitfalls in considering liquid overfilling of process
vessels:
(a) The belief that a particular vessel is too big to overfill is misguided.
Experienced operating teams have overfilled distillation columns of
over 500 rn3
volume.
(b) Many level instruments rely on an assumed liquid density to convert
the measured parameter into an equivalent liquid level. During
actual operation, and particularly during start-up, a range of
densities may be experienced by the instrument. If the actual
density differs from the calibration density, the level the instrument
indicates will not be the true level. This can result in alarms and
trips coming in at the wrong level (if at all).
(c) A realistic view should be taken of the actions an operator can take
to prevent overfilling. In particular, allowance should be made for
the increased response time of an operator when under stress as
well as the practical limitations on what the operator can do.

Sufficient instrumentation should be available to help the operator
to diagnose the problem and effective action should be possible
(e.g., by using an alternative disposal route or by stopping a feed
stream).
(d) Any instrumentation provided should be located to give a realistic
amount of time for action. On some distillation columns, the extra-
high level alarm is located half way up the column to indicate that a
very serious problem is developing which needs immediate
attention. Instrumentation for instrumentation's sake is not good
enough.
(e) Normal disposal routes may not be available, especially if loss of
the normal disposal route is the cause of the problem. The
proposed disposal route needs to be able to handle not only the
rate, but also the quantity of material. Problems can be particularly
acute during start-up of new equipment when construction debris
can result in the rapid blockage of pump suction strainers. If the
duty is hot, these may take several hours to cool before cleaning
can commence. A similar blockage on the spare pump, during this
time, will result in a loss of pump-out capability.
(f) Large vapor duty designed to protect reboilers on a distillation
column can pass considerable quantities of liquid if the column
overfills. While this may occur at any time, there is an increased
risk of it happening during a start-up or shut-down. The designer is
cautioned to pay particular attention to the composition of the liquid
which may be discharged, as this may be very different from the
normal flowsheet.
(g) In general, care needs to be taken over the composition of the
liquid discharged by the safety valve: during start-up, compositions
may be very different from design and this can cause problems in
the disposal system. Unexpected corrosive materials, materials
which cause chilling as they flash or materials which may undergo
chemical reactions or phase changes may enter the disposal
system with unforeseen results.

9.1.2.3 Contact Between Immiscible Liquids
Hot heavy "oil" added to water or vice versa can cause violent boil-off with
the release of large volumes of steam as a result of heat transfer to the
water layer if the oil is above 100 o
C.
A similar effect may occur as a result of contact between any two
immiscible or partially miscible liquids even if the resultant temperature is
below the boiling point of both liquids. If the sum of the two vapor
pressures is greater than the design pressure, relief will be required for
vessels where such liquids are mixed. (Ref 5).
9.1.2.4 Low-Pressure Storage Tanks
Low-pressure storage tanks are very susceptible to process mal-operation
as they are frequently operated intentionally in the closed outlet condition.
Such tanks should be fitted with a pressure/vacuum relief system which
can be expected to operate frequently unless the vessel is also provided
with a gas blanket control system. Their low design pressures (normally
less than 100 mbar g positive pressure and less than 10 mbar g negative
pressure) make them vulnerable to overpressure or underpressure by
more events than are other vessels.
Note:
The mal-operations listed as 9.1.2.1, 9.1.2.2 and 9.1.2.3 tend to occur
most frequently with low-pressure storage tanks; the risk is, however, not
limited to these and should be considered for any type of equipment
where the design pressure might be exceeded.
9.2.1 General Considerations
Some aberrations in the process can cause the total loss or partial
restriction of one or more outlet paths from equipment.
Consider the behavior of the process under normal and abnormal
operating conditions. Question whether blockages or other disturbances,
either total or partial, can upset the mass and energy balance within the
plant.

Side reactions which may be insignificant to the process yield can
sometimes produce small amounts of products that can cause blockages
(e.g. tars).
The presence of gas and liquid together can lead to choke flow velocities
much lower than the sonic velocity of the gas stream alone and so reduce
the effective outflow.
9.2.2 Causes of Blockage
Some examples are listed below:
(a) Freezing: to form a solid blockage or restriction is a danger in many
plants, particularly when an aqueous liquid is processed or steam
inerting of vessels or stacks is used.
(b) Solid Hydrates: are formed by certain process fluids, particularly
hydrocarbons, when contaminated with traces of water. These can
then collect in low points or low velocity areas and build up to form
a solid blockage.
(c) Degradation Accompanied by Polymerization: occurs with many
reactive chemicals with the formation of long-chain solid or jelly-like
polymers. This may occur when they are heated or contaminated or
simply stored for a long period. The addition of stabilizers may
prevent this but if there remains any risk of blockage it should be
considered.
(d) Iron Scale and other Corrosion Products: can also cause
blockages; opportunities for this to happen especially in carbon
steel lines of small diameter should be identified.
(e) Settling of Slurries/Suspensions: can cause blockages in low
velocity zones. Suspensions that are highly non-Newtonian can
become immobile as a result of a relatively small reduction in the
driving pressure.
(f) Sublimation: of many products which are stored hot can be a
problem because blockages may form where the vapor cools near
the exit of a tank or vessel.

(g) Entrainment: of liquid droplets is a frequent cause of blockages or
partial blockages. Entrainment is caused by high gas velocities,
especially where the gas stream impinges directly on to a liquid
surface. Coalescence and deposition is likely to occur in parts of
the system where the velocity is lower and may possibly give rise to
an hydraulic head. If the material can solidify in cooler zones
downstream it is likely to cause a blockage.
(h) Sparging of Gas: through a liquid in a vessel or evolution of gas
within it may cause the liquid to "swell", without disengagement of
the gas, enough for it to enter the gas/vapor off take line; this may
also lead to blockage or restriction of flow.
(i) Foaming: can also be a cause of entrainment and this possibility
should be examined in relation to any tendency of the liquid phase
to form a stable foam.
(k) Dust, Floss and Fine Solid Particles: are often carried over by gas
streams and may cause blockages downstream.
9.3 Pitfalls with Restricted Outlet Condition
(a) Positive displacement machinery is frequently provided by the
manufacturer with its own pressure relief protection against a
closed outlet. Check the capacity of the valve fitted against the
required relief rate taking account of:
(1) the increase in density of gas after compression;
(2) any significant difference between gas density under normal
operation and relief conditions.
(b) Pumping of liquid into a vessel expels gas/vapor. With inflow of
volatile liquids the outflow volume may be significantly more than
the incoming liquid volume due to saturation of the blanketing gas
with vapor. This is a normal operation for storage tanks and creates
a frequent demand on the relief system which should be designed
to discharge to a safe place.

(c) When pressure relief is provided on a system prone to blockages
caused by entrainment and solidification, the risk of blockage of the
relief system itself shall be considered and prevented.
(d) Flame arresters frequently have a tendency to get blocked. In many
countries the design requires statutory approval and some of these
approved designs are difficult to check for freedom from blockage
because of their mode of construction. Thus flame arresters fitted to
relief lines and vents can create more danger than they prevent.
Therefore, unless it is a mandatory requirement, arresters should
only be fitted after careful study of the risk of blockage. If it is
necessary to use a type which cannot be properly inspected,
ensure that an emergency relief path is provided in addition.
(e) Increased pressure drop may build up inside process equipment as
a consequence of liquid accumulation (e.g. by flooding of distillation
columns).
10 RESTRICTED INLET (CONDITION 3)
10.1 Equipment at Risk
In this case, the risk to be considered is the effect of system under
pressure or vacuum. Always examine the vessel design for the lowest
pressure that could be created. Large vessels are obviously more likely to
be unable to withstand reduced pressures since general mechanical
rigidity requirements usually provide ample strength with small vessels. If
the vessel would be damaged by the lowest pressure attainable, a
vacuum relief device will be required. In practice, low-pressure storage
tanks and many other vessels designed for positive pressures up to 5 bar
g. (but not specifically designed for vacuum) will usually require protection
against the effect of sub-atmospheric pressures. In many cases the event
likely to cause a vacuum situation is the reverse of that likely to cause
overpressure, i.e. outflow of mass or energy rather than inflow.

10.2 Events Leading to Underpressure (Vacuum)
The following events are usually the result of mal-operation:
(a) Removal of Liquid: by pumping out or gravity drainage.
Note:
Drainage of low-pressure tanks after hydrostatic test is a special
case of this situation; it is usually covered by special operating
procedures intended to eliminate the risk. However, the possibility
of mal-operation may need to be considered.
(b) Removal of Gas: by connection to a low-pressure system - e.g.
vacuum pump.
(c) Cold Liquid Injection: into any vessel containing hot vapors can
cause a very sudden and rapid condensation; this will lead to a
severe drop in pressure unless the volume of vapor is replaced. A
smaller but significant drop may be caused by gas cooling,
without condensation.
(d) Gradual Cooling: of any plant equipment items containing
condensable vapor for example during a plant shutdown, or the
loss of steam to a reboiler may cause a reduced pressure relief
situation as a result of condensation.
(e) Absorption, either Chemical or Physical: can remove gas from the
system and cause a fall in pressure possibly leading to a low-
pressure relief situation. This might occur as a result of process
aberration - e.g. unexpected reaction or emergency cooling of
some associated vessel.
Examples of this situation are:
(1) rapid pressure reduction due to absorption of vapor when
washing out a column filled with a water-soluble gas - e.g.
ammonia or amine vapor;
(2) slow pressure reduction following oxygen absorption by
rusting or oxidation processes in items out of service and
containing water.

(f) Ambient Temperature Fall: with consequent condensation of vapor
and/or contraction of gas causes an inbreathing requirement. For
further guidance on vacuum relief requirements refer to Part C,
Section 7 of this Guide. See also Section 5.
10.3 Pitfalls with Restricted Inlet
In systems where air ingress could result in formation of an explosive
mixture and there is a risk of ignition, the vacuum breaking medium
supplied to the relief system should be an inert gas. It may be preferable
to design the system to withstand full vacuum.
11 CHEMICAL REACTION (CONDITION 4)
11.1 Normal and Abnormal Chemical Process
Many chemical reactions have a potential for producing gas or vapor
especially when the temperature rises. Excessive pressure will be
generated in the system whenever the volume rate of the gas/vapor
evolution exceeds the (single or two-phase) volume outflow. In a blocked-
in system the pressure may rise simply because of an increase in vapor
pressure with temperature rise caused by any exothermic reaction or it
may be due to evolution of a gas during the normal process - i.e. no
chemical abnormality. The reactions involved may be either the main
process reaction or unwanted side reactions. Some circumstances in
which a need for pressure relief may arise are given below:
(a) Limitation of normal gas/vapor outflow from the system due to
BLOCKED-IN or RESTRICTED OUTLET CONDITIONS.
(b) Heat balance disturbed so that the temperature rises (e.g.
excessive heat input or decreased heat removal via heat
exchangers).
(c) Reaction rate greater than design rate as a result of other abnormal
conditions or materials.
(d) Initiation of side reactions which generate more gas or heat than
the normal process.
(e) Any combination of (a), (b), (c) and/or (d).

11.2 Runaway Reaction
A special case of 11.1 (b) is that of an exothermic reaction that
accelerates until it is out of control - the so called "runaway condition".
This situation can arise with many reaction mixtures where the rate
increases with temperature. Irrespective of the cause of an initial increase
in rate above normal, the reaction will accelerate if the corresponding heat
of reaction is not removed. Extremely high rates of vapor evolution may
then be achieved which demand very large relief system capacities
particularly when a large proportion of the mixture is volatile. Very large
relief capacities are also needed when a two-phase vapor-liquid mixture is
likely to enter the relief system as a result of rapid evolution of vapor. See
11.3 and Part C, Section 5 of this Guide.
Some processes that involve evaporative cooling of exothermic reactions
and are normally overall heat-absorbing processes may self-heat and
accelerate if outflow of vapor from the reactor is restricted.
There is a noteworthy potential for generation of overpressure in the case
of reactions involving materials (particularly polymers) having a tendency
to break down into smaller molecules when overheated. When the
breakdown products are volatile (as is often the case) there is bound to be
a large increase in pressure within any confined system.
Thus the CHEMICAL REACTION CONDITION should be examined in
relation to other CONDITIONS obtaining at the time. Abnormal reactions
can be initiated by numerous mal-operations and process aberrations
(See 11.3).
The assessment of potential for a chemical process to become self-
accelerating should always be based on relevant chemical information
obtained from internal records, published information or new
investigations; the information needed is primarily:
(a) Kinetic and thermodynamic data for the normal reaction.
(b) Possible mechanisms for reactions other than the normal ones and
also kinetic and thermodynamic data for these (either known from
experience or deducible from the chemistry).

When doubt exists, an experimental investigation will be necessary to
determine the effect of any abnormal conditions that can be envisaged.
11.3 Chemical Reaction in Relation to Prime Events
11.3.1 Fire
Overheating by fire may be local or general and cause gross chemical
abnormalities and degradation of materials. See Section 2.
11.3.2 Mal-operation
Many errors can lead to abnormal reaction in both continuous and batch
processes. For general guidance a few of these are given below:
(a) Cooling system: wrong setting of temperature controller or manual
valves on coolant circulation.
(b) Evaporative cooling: restriction of vapor outflow, reflux flow or
vacuum control as a result of a faulty controller or setting.
(c) Vapor/Gas outflow: restriction as in (b) when vapor or gas is
normally removed from the system as part of the process.
(d) Abnormal reactant, catalyst or solvent: wrong material or
concentration due to incorrect charging or rate of feeding of the
reactants. Question whether catalyst can be charged twice and
whether incompatible materials in use close by could be a source of
contamination.
(e) Abnormal or Irregular feed rates: many batch processes where
reactant(s) are added semi-continuously or stepwise are sensitive
to this fault - rapid overpressure generation may result from a surge
in reactant feed.
(f) Agitator or mixing reclrculator switched off: this fault is a common
cause of loss of cooling, abnormal local concentrations and
conditions, phase-separation or layering. See Section 4.

11.3.3 Process Aberration
Many process aberrations are possible and it is essential to identify those
EVENTS that can lead to overpressure. The effect of all feasible
irregularities on the chemical reaction should be examined. Some frequent
causes are given as examples:
(a) Inadequate agitation: an agitator design incapable of handling all
possible forms of the mixture that can be present throughout the
cycle (especially with highly viscous or highly non-Newtonian
material) can lead to the situations given in 11.3.2(f) and also
create an agitated inner core surrounded by static material. Large
concentration and temperature differences may accompany such a
condition. See 11.3.1.
(b) Abnormal concentration or activity: a gross abnormality in the
quality of the reactants, solvent, or catalyst can lead to exceptional
reaction rate or unintended chemical changes.
(c) Abnormal chemical behavior: a local hot spot or abnormal local
temperature can also initiate abnormal reactions, decomposition,
etc. Heating systems should be designed to avoid this possibility.
(d) Inflow of Incompatible reactant: an event that can happen in many
situations such as backflow from associated equipment - e.g.
irrigant from an off-gas scrubber after the scrubber outlet has
become blocked or restricted
(e) Temperature control system: the normal system may not be
capable of removing sufficient heat under some unusual conditions
- e.g. abnormal reactivity (as above) or as a result of fouling of
cooling surfaces.
11.3.4 Equipment Failures
These are covered in Section 4 but some are mentioned here
because so many equipment failures can affect chemical reactions
in such a way as to lead to overpressure. Such failures include:
(See Section 4, sub-clause 13.2)

(a) Metal exposure: in the case of a non-metallic protective lining,
contact with metal following breakdown of the lining material can
initiate abnormal chemical reactions and possibly very high rates.
(b) Control systems: loss of effective cooling is an obvious possibility
for initiating abnormal reactions as mentioned earlier. Many other
control failures can lead to abnormal chemical process behavior.
11.3.5 Service Failures
There are many circumstances when failure of a service can upset the
chemical process, some of which are quoted in Section 4. A service failure
may cause overpressure directly but it may also initiate abnormal chemical
changes or rates of reaction. Every such possibility should be considered
in relation to the sensitivity of the materials and process to changed
conditions. See Section 4, Clause 14.
11.4 Underpressure Relief
Underpressure can occur in many circumstances as a result of chemical
reaction in low-pressure equipment following mal-operation or plant
failures. It is only a problem when coincident with closed or restricted
inlets and for pressure relief purposes it is no different from physical
absorption; this problem was therefore covered earlier. See 10.2 (e).

SECTION 4:
EQUIPMENT AND SERVICES FAILURES
12 INTRODUCTION
Relief situations may result from a variety of failures outside the control of
the designers or operators. Such a failure may:
(a) Directly cause a relief situation (e.g. escape of high pressure fluid
from a burst heat exchanger tube into lower pressure equipment).
(b) Indirectly cause a relief situation (e.g. failure of temperature
controller or sticking of control valve allowing temperature to
increase).
(c) Start a chain of events leading to a relief situation (e.g. steam
supply failure allowing pipeline to cool thus causing solidification of
process materials and hence blockages).
Relief situations can be caused by failure (partial or total) of mechanical or
electrical equipment, instruments and any service supplied from outside
the plant. This Section is, for ease of reading, subdivided into equipment
failure and services failure. The designer should bear in mind that any
such fault may cause other prime events (e.g. overheating due to cooling
water failure can initiate many forms of process abnormality). See Table 1.
13 EQUIPMENT FAILURE
13.1 System Blocked-in or Restricted Outlet (Conditions 1 and 2)
The process equipment system may be normally blocked-in or restricted.
It may also be put into one of these conditions by the loss of one or more
exit paths as a result of mechanical failure (e.g. control valve failure). On
the other hand, a failure can cause a sudden increase in mass inflow
from a high pressure source which overloads normal outlets (e.g. burst
tube).

Pressure Relief Systems Vol 2

Pressure Relief Systems Vol 2

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Pressure Relief Systems Vol 2