Distributed Control Systems (DCS) - Maintenance Considerations

Modern DCS Applications, Selection &
Troubleshooting
XRS Consulting Engineers and Project
Managers 1
Section 1 XRS Consulting Engineers and Project
Managers 1
Managers 1
Appendix XRS Consulting Engineers and Project
Managers 1
Section 17
Section 17
Maintenance Considerations

Troubleshooting
Managers 4
Managers 4
Managers 4
Managers 4
Section 17
Prior to 1962, by the military specifications that were
in use at the time, the concept of availability was
calculated to be equal to the mean time before
failure, divided by the sum of the mean time before
failure plus the Mean Time To Repair (MTTR).
This concept, in process control, is the calculation of
what is in the path of the control signal from the
input through to the output that could contribute to
the failure of that loop.
MEAN TIME BETWEEN FAILURES
This document is a partial preview. Full document download can be found on Flevy:
http://flevy.com/browse/document/distributed-control-systems-dcs-maintenance-considerations-3070

Troubleshooting
Managers 7
Managers 7
Managers 7
Managers 7
Section 17
Ratio: Time system is operating correctly to total hours of scheduled operation.
● MTBF – Mean Time Between Failures
= MTTF + MTTR
Not always
symmetrical
Stated or published period of time for which user may except device or
system to operate before failure occurs; computed arithmetical mean of time,
… not a tested value! Value will drop by about 50% for every 10oC increases;
All other ambient factors will have similar impact.
● MTTF – Mean Time To Failure; calculated measure of overall reliability.
● MTTFD – Mean Time To Failure (to) Dangerous condition.
● MTTFS - Mean Time To Failure (to) Safe condition.
● MTTR - Mean Time To Repair; average time required to perform corrective
maintenance on failed device or system.
Availability = MTTF/(MTTF + MTTR)

Troubleshooting
Managers 10
Managers 10
Managers 10
Managers 10
Section 17
Many buyers of distributed control systems who ask for
this MTBF number do not realize that the intention of
this calculation has been to ensure the signal path
integrity.
They frequently request the MTBF for all the hardware
in a system.

Troubleshooting
Managers 13
Managers 13
Managers 13
Managers 13
Section 17
No two systems are ever alike, even those from the
same vendor.
There is no way an MTBF can have meaning in many of
those attempts to look for system security and
availability.

Troubleshooting
Managers 16
Managers 16
Managers 16
Managers 16
Section 17
Spare Parts

Troubleshooting
Managers 19
Managers 19
Managers 19
Managers 19
Section 17
The modules or cards themselves are usually far too
complex for the average service department to make
detailed repairs.
Sometimes it’s easy enough to replace chips, but if the
problem goes further it is far easier to replace the
entire module with a replacement spare and send the
damaged module back to the vendor.
Of course, more things to stock would be fuses and
lamps.
SPARE PARTS

Troubleshooting
Managers 22
Managers 22
Managers 22
Managers 22
Section 17
TYPES OF FAILURES
Physical Failures
Physical failures are almost always permanent and are
usually caused by a component or module which has
worn out, burned out, or has physically broken
because of vibration or shock.
If all the physical components in a system are
working, and the system still fails to perform its
function, the failure is classified as “functional.”

Troubleshooting
Managers 25
Managers 25
Managers 25
Managers 25
Section 17
Types of Faults

Troubleshooting
Managers 28
Managers 28
Managers 28
Managers 28
Section 17
Manufacturing Faults
Manufacturing faults occur when some step in the process of
manufacturing a component or module or system is done
incorrectly.
An example of this kind of fault is when a resistor in a circuit
has a corroded wire that no longer conducts properly. Now,
this resistor was coated to protect against corrosion, but
closer examination shows that the coating was not applied
correctly when the resistor was made.
Small voids in the coating occur because the coating machine
was not cleaned on schedule.
TYPES OF FAULTS

Troubleshooting
Managers 31
Managers 31
Managers 31
Managers 31
Section 17
Moisture & Corrosion
Humidity and corrosive chemicals in the atmosphere can
cause serious failures in electronics.
The first, and by far the most effective, layer of
protection is the use of conformal coating on all PC
boards.
Some vendors dislike this approach because the coating
can retain the heat. If the coating material is selected
properly, it can be a conductor of heat, helping it to
dissipate.
Components that cannot be coated, such as connectors,
use gold-plated contacts that are lubricated with a
special anti-fretting compound.
TYPES OF FAULTS

Troubleshooting
Managers 34
Managers 34
Managers 34
Managers 34
Section 17
Diagnostics

Troubleshooting
Managers 37
Managers 37
Managers 37
Managers 37
Section 17
The results of such diagnostic checks are usually
displayed on video screens so they can be used to
identify the trouble.
By providing continuous online monitoring of system
performance, self-diagnostics minimizes the
component Mean Time To Repair (MTTR) through the
prompt and timely reporting of component failures.
DIAGNOSTICS

Troubleshooting
Managers 40
Managers 40
Managers 40
Managers 40
Section 17

Troubleshooting
Managers 43
Managers 43
Managers 43
Managers 43
Section 17
PROM Check Sum Test — The contents of a PROM is
banked into RAM, and the check sum is computed.
The result can be compared with the value check sum
result, which can be burned into PROM.
Dead Man (Watchdog) Timers (DMTs) — Timers that
must be reset at specified intervals to determine that
the programs are still executing. Some dead man
timers will alarm if the reset is too often, indicating a
program loop around the reset command.
DIAGNOSTICS
Processor Cards

Troubleshooting
Managers 46
Managers 46
Managers 46
Managers 46
Section 17
Memory Cards
If there is a large database common to many processor
cards, then in many systems it is often stored on a
single memory card.
The Random Access Memory (RAM) of this card should
be either nonvolatile or battery backed up.
If battery backup is used, the capacity (the ability of
the battery to sustain current over time) of the
battery should be known to the system, and low
capacity should be alarmed.
DIAGNOSTICS

Troubleshooting
Managers 49
Managers 49
Managers 49
Managers 49
Section 17
Communications
Communication Between Cards on a Back-plane
Since back-plane noise might be responsible for an error in
data, a second attempt can be made when a fault is
detected.
If successive attempts fail, transfer to backup is called for.
The number of attempts allowed before the transfer to
backup depends upon the amount of time available to
complete the communication and the likely duration of
transient noise.
DIAGNOSTICS

Troubleshooting
Managers 52
Managers 52
Managers 52
Managers 52
Section 17
Communications
Communication between devices is a likely source of
short disruptions.
As the distances between communicating devices
increase, the likelihood of disruptions caused by the
environment may increase.
Extra precautions must be taken to make such
communications secure and to provide a fault-tolerant
system.
DIAGNOSTICS

1
Flevy (www.flevy.com) is the marketplace
for premium documents. These
documents can range from Business
Frameworks to Financial Models to
PowerPoint Templates.
Flevy was founded under the principle that
companies waste a lot of time and money
recreating the same foundational business
documents. Our vision is for Flevy to
become a comprehensive knowledge base
of business documents. All organizations,
from startups to large enterprises, can use
Flevy— whether it's to jumpstart projects, to
find reference or comparison materials, or
just to learn.
Contact Us
Please contact us with any questions you may have
about our company.
• General Inquiries
support@flevy.com
• Media/PR
press@flevy.com
• Billing
billing@flevy.com

Distributed Control Systems (DCS) - Maintenance Considerations

Recommandé

Recommandé

Contenu connexe

Plus de Flevy.com Best Practices

Plus de Flevy.com Best Practices (20)

Dernier

Dernier (20)

Distributed Control Systems (DCS) - Maintenance Considerations