This document provides a history of instrumentation and control maintenance from the 1930s to the present. It describes how maintenance evolved from pipe fitters maintaining early pneumatic systems to the development of specialized instrumentation and control technicians. With advances in technology, the required skills of maintenance personnel expanded to include electronics, computers, and various control systems. Modern plants utilize modular design, contract maintenance, and skeleton crews enabled by advanced automation. Distributed control systems are now standard, and maintenance focuses on ensuring production and preserving capital assets through effective management practices.

1. Maintenance of
Instruments & Systems
2nd Edition
Lawrence D. Goettsche, Editor
Practical Guides
for Measurement and Control

2. v
Table of Contents
About the Editor and Contributors xi
Chapter 1 Introduction 1
Overview 1
History of Instrumentation and Control Maintenance 1
Need for Instrumentation and Control Maintenance and Engineering 6
Chapter 2 Fundamental Principles 9
Overview 9
Electronic Field Instrumentation 9
Why Maintain? 10
Maintenance vs. Troubleshooting 19
Calibration and Reasons to Calibrate 20
Troubleshooting 21
Basic Troubleshooting Techniques 22
Designed with Maintenance in Mind 25
Chapter 3 Diagrams, Symbols, and Specifications 31
Overview 31
Process (Piping) & Instrumentation Diagram 31
Instrument Loop Diagrams 32
Logic Diagrams 39
Highway Drawings 49
Specifications 51
Instrument Symbols 54
Instrument Symbols 58
Chapter 4 Maintenance Personnel 73
Overview 73
Multi-Disciplined 74
Continuous Training 74
Training of Maintenance Workers 74
Multicraft/Multiskilled, Multi-Disciplined 78
Knowledge Factors 80
Skills 85
Job Titles and Descriptions 88
Credentialing 91
Certification 94

3. Table of Contents
vi
Chapter 5 Maintenance Management and Engineering 97
Overview 97
The Need for Maintenance Management 98
Maintenance Philosophy 98
Maintenance Management Organization 99
Basic Requirements for a Maintenance Department 100
Planning and Scheduling 102
Work Order System 102
MTTF, MTTR, and Availability 104
Training Maintenance Workers 107
Preparing Functional Specifications 109
Computerized Maintenance Management Systems 110
Office/Shop Layout 115
Centralized/Decentralized Shops 118
Chapter 6 Pressure and Flow Instruments 121
Overview 121
Pressure Transmitters 121
Differential Pressure Technology 132
Level Transmitters 138
Flow Transmitters 143
Magnetic Flowmeters 146
Mass Flowmeters 151
Turbine Flowmeters 156
Open Channel Flowmeters 158
Vortex Shedding Flowmeter 161
Vortex Shedding Meters 161
Positive Displacement Flowmeters 162
Positive Displacement Meters 164
Target Flowmeters 164
Thermal Mass Flowmeters 166
Ultrasonic Flowmeters 167
Variable Area Flowmeters 168
Insertion (Sampling) Flowmeters 170
Chapter 7 Maintenance Engineering 171
Overview 171
Engineering Assistance 173
Maintenance Involvement in New Projects 174
Successful Maintenance 177
The High Maintenance System 178
Documentation Control 179
Alternative Methods of Maintenance 180
Service/Contract Maintenance 180
In-House Maintenance versus Contract Maintenance 181
New Systems Installations and Checkout 184
Preventive Maintenance 185
Power, Grounding, and Isolation Requirements 186
Instrument Air Requirements 196
Communication Requirements 197
Heating, Ventilating, Cooling, and Air Conditioning Systems 198

4. Table of Contents
vii
Chapter 8 Temperature Devices 201
Overview 201
Thermocouples 206
Resistance Temperature Devices 213
Thermistors 217
Integrated Circuit Temperature Transducer 218
Infrared Temperature Transducers 218
Optical Fiber Thermometry 220
Thermometers 220
Chapter 9 Panel and Transmitting Instruments 233
Overview 233
Panel and Behind-Panel Instruments 233
Panel Meters 241
Discrete Switches 241
Potentiometers 242
Recorders 242
Transducers 242
Smart Transmitters 244
Chapter 10 Analytical Instruments 259
Overview 259
Field Analytical Instrument Systems 259
Field Analytical Instruments 260
Organization 262
Personnel 262
Maintenance Approaches 263
Service Factor 263
Maintenance Work Load 264
Spare Parts 265
Vendor Support 265
Application Unique Issues 265
Installation Issues 266
Chapter 11 Primary Elements and Final Control Devices 267
Overview 267
Temperature 267
Primary Elements 273
Primary Element Location 276
Control Valves 277
Troubleshooting Guide 283
Chapter 12 Pneumatic Instruments 287
Overview 287
Instrument Air Requirements 287
Pneumatic Field Instruments 288

5. Table of Contents
viii
Chapter 13 Calibration 299
Overview 299
Field Calibration 300
Calibrating in Hazardous Locations 313
In-Shop Calibration 324
Other Aspects of Calibration 328
Chapter 14 Tuning 337
Overview 337
Loop Classification by Control Function 337
Control Algorithms 339
Loop Tuning 347
Flow Loops 351
Chapter 15 Distributed Control Systems 353
Overview 353
Distributed Control System Maintenance 353
Maintenance Goals and Objectives 353
Programmable Logic Controllers 368
Chapter 16 Software and Network Maintenance 373
Overview 373
Computer Operating Environment 374
21st Century Maintenance Technology 383
Chapter 17 Safety 389
Overview 389
Electrical Hazards 390
Hazardous Areas 392
Contamination 398
Pressures and Vacuums 399
High Voltage 400
Moving and Rotating Machinery 401
High and Low Temperatures 401
Gases and Chemicals 402
Heights and Confined Spaces 403
Program Changes, Software Control 404
Process Considerations 406
Communication 406
Cryogenic Considerations 406
Nuclear Plants 409
Ergonomics 412
Acknowledgment 413
Standards and Recommended Practices 413
Chapter 18 Fiber Optics 417
Overview 417
Construction 418
Classification 418
Sensing Modes 418
Advantages 419

6. Table of Contents
ix
Disadvantages 419
Applications 420
Analog Input/Output Modules 423
Sensors 423
Appendix A Glossary of Terms 427
Appendix B Bibliography 441
Index 447

7. 1
1
Introduction
Overview
The Maintenance volume is key to the Practical Guides Series and certainly a
key to the profitability of companies through ensuring that the control system is
maintained so the plant can produce its products. This volume includes some his-
tory and speculates about future advances of instrumentation and control (I&C)
system maintenance; it also covers some of the fundamental principles, vocabu-
lary, symbolism, standards, and safety. It suggests the necessary basic knowledge
required of I&C technicians and the interaction of maintenance in the retrofitting
and start-up of control systems.
History of Instrumentation and Control Maintenance
From pneumatic instrumentation to computer-controlled systems — what a
change! Is a seasoned instrument mechanic expected to troubleshoot a state-of-
the-art computer-controlled system? Should a new instrument technician be ex-
pected to maintain pneumatic instrumentation? This volume documents expe-
riences in the older types of systems as well as in the newer, state-of-the-art
systems.
1930s
Distributed control is not new. In 1938, when Chemical Processing published
its first issue, mechanisms for control were indeed distributed throughout the
plant. Process control consisted of operator adjustments to hand valves that were
based on direct readings of local gages. Control room instrumentation has taken
some dramatic turns along the way — from large-scale pneumatic recorders to
miniature analog electronic controllers to microprocessor-based digital systems.
Chemical and petroleum plants were among the first to use control systems
for their processes. Pneumatic instrumentation became the leader in automatic
control because of its safety. Pipe fitters were asked to perform maintenance on
these early pneumatic instruments. In many cases, outmoded control room hard-
ware is still operating effectively today — a tribute to the worldwide manufactur-
ers of process control instrumentation.
In the late 1930s and early 1940s, operators relied on local instrument gages
to monitor production processes. Control panels that did exist were located in the
field near process sensing points. Typically, only a handful of indicators, record-
ers, and controllers were mounted on a local panel. Often, the process fluids were
piped directly into control panels.
Where fill fluids were needed, mercury was commonly used. Control panels
served as a convenient means for improving control coordination by allowing op-
erators to adjust valves in response to visual instrument readings.

8. Introduction
2
1940s
In the 1940s the use of pneumatic proportional controllers was increasing, so
the early pipe fitters had to understand more of the theory of process and control.
New words such as integral, derivative, sensors, and final control elements were
added to their vocabularies.
By the late 1940s, a trend toward the concentration of controls in centralized
locations had begun.
1950s
In the 1950s, operating unit control rooms were built to centralize operations
and to accommodate operators assigned to monitor control boards on a full-time
basis. With the growing number and complexity of the indicators, recorders, and
controllers and the “need” to operate the plant remotely from these panels, the in-
strument mechanic was specialized to maintain the pneumatic control systems.
By the mid 1950s, electronic analog instrumentation had been formally intro-
duced but did not win industry acceptance until the late 1950s and early 1960s.
With the exception of chemical and petroleum plants, most new plants used elec-
tronic analog instrumentation because of the greater cost of tubing work between
pneumatic transmitters and controllers and the expensive pneumatic auxiliaries,
such as air compressors, filters, and dryers.
Increasing plant complexity necessitated increasing amounts of accurate, up-to-
date operating information.
Now the instrument mechanic needed to know electronics and electricity in
addition to pneumatics. Larger plants formed Electrical and Instrument (E&I), In-
strument and Electronic (I&E), or Electrical and Control (E&C) groups; some
formed an Instrument and Control (I&C) Group and had both instrument mechan-
ics and instrument technicians. The knowledge required by I&C mechanics and
technicians meant training was necessary, so vendors provided training on the
equipment they sold.
1960s
Digital computers began to appear in control rooms in the 1960s. The com-
puter’s initial role was essentially that of a data logging device from which paper
printouts could be obtained. However, the concept of direct digital control (DDC)
gained notoriety in the 1960s.
1970s
By the mid 1970s, the drawbacks to DDC had become apparent. The central
computer approach depended on the availability of a single large computer.
Highly trained computer personnel were needed to maintain the computer hard-
ware and to deal with the high-level software languages.
Single-loop analog control continued to flourish during the early 1970s.
Thousands of electronic signal wires crisscrossed central control rooms, adding
complexity to the pursuit of improved coordination. Recognizing multiple func-
tions inherent in panel instruments, split architecture systems were introduced.
Analog display stations were segregated from rack-mounted printed circuit cards
in the quest for functional modularity.
I&C groups flourished, everyone was retrofitting and updating plants, and
new plants provided more and more instrumentation requirements. Instrumenta-
tion vendors were training the instrument mechanics and electricians to maintain
their equipment.

9. History of Instrumentation and Control Maintenance
3
Standards for instrumentation were being developed, and manufacturers
started listening to ISA when developing their new instruments.
A marriage between single-loop electronic analog control and pneumatic con-
trol developed because of the need for powerful control valve actuators.
The simplicity and accuracy of electronic controllers, recorders, and indicators
made them the choice for instrument panels.
Current-to-pneumatic converters and pneumatic-to-current converters linked
electronic instruments to pneumatic instruments and sensors and actuators. Chem-
ical plants used pneumatic instruments in the hazardous areas along with signal
wires to transmit the signals to central control rooms in safe areas.
Most plants built after the mid 1970s used electronic rather than pneumatic in-
strumentation. Pneumatic valves, however, are still used almost exclusively for
throttling control and even on-off control. About the same time in this period
Honeywell® and Yokogawa® introduced the first distributed digital control sys-
tems (DDCS), now called the distributed control system (DCS). Multiple mini-
computers, geographically and functionally distributed, performed monitoring
and control tasks that had been previously handled by the central DDC computer.
Each microprocessor-based controller was shared by up to eight control loops. Se-
rial bit communication over coaxial cable linked individual system devices.
As these distributed control systems became the standard for newer chemical
and petroleum plants and the older single-loop pneumatic and electronic control-
lers were replaced, the I&C groups were trained on the new DCS. This was the
first introduction of computers to the I&C technicians, and DCS manufacturers
designed their systems to be configured and maintained by I&C groups — not
highly trained computer personnel. As a technological breakthrough, the micro-
processor accelerated advances in control system design. At the operator interface
level, distributed control contributed to an unforeseen development. For the first
time, CRT display consoles gained acceptance as the primary operator interface,
and conventional single-loop analog stations were reduced to an emergency
backup role at many early distributed control system installation sites. Long,
floor-to-ceiling panelboards were replaced with low-profile CRT workstation
consoles. Keyboards, CRTs and printers served as modern tools for seated control
room operators.
By the end of the 1970s, control system innovations had advanced beyond in-
dustry’s capacity to keep pace. Most plant sites contained an assortment of control
technologies that spanned three decades. Instrumentation and control specialists
(mechanics, technicians, and engineers) were commonplace in industry. Special
I&C groups were established, as shown in the organizational chart of Figure 1-1.
1980s
DCS operator interfaces were refined in the 1980s (see Figure 1-2). Intelligent
CRT stations utilized multiple-display formats to condense and organize exten-
sive operating information. Hierarchical arrangements of plant-, area-, group-, and
loop-level displays simplified on-screen database presentation. Real-time color
graphics added further comprehensive overviews of unit operations.
Most microprocessor-based control systems had a vast array of alarms and di-
agnostics to help operators and maintenance personnel determine if there were
any problems. Distributed control systems had many on-line and off-line diagnos-
tics, including process and input alarms, reportable events, error messages, and
hardware and software failure reporting.

10. Introduction
4
1990s
Trends for the 1990s were computer-integrated manufacturing (CIM) and
management information systems (MIS). These interfaced the real-time devices
(field devices at the machinery/process level) through distributed controllers to
multiple-station coordination, then on to scheduling, production, and management
information to the plant level for overall planning, execution, and control. Further
development of artificial intelligence and expert systems gave advanced control
new meaning.
With the introduction of computers and databases, maintenance management
systems (MMS) helped maintenance and management personnel determine repair
frequency and spare parts availability and made decisions on when to replace ob-
solete equipment.
Distributed control systems (DCS), programmable logic controllers (PLC),
computer control systems (CCS), supervisory control and data acquisition
(SCADA) and smart field devices were the norm. A digital signal was superim-
posed on the 4-20 mA signal for ranging and calibrating field devices. The Interna-
tional Organization for Standardization (ISO) Open Systems Interconnection (OSI)
model and interconnection of devices made by different manufactures has opened
systems architecture, replacing proprietary communications among devices.
2000s
Historically, factory floor maintenance methods and practices have been de-
veloped across a wide range of vertical industries, where the focus was to keep the
assembly lines and processes running rather than preserving assets. Today, manu-
facturers are focused on the long-term benefits of factory floor support practices
Figure 1-1. Typical 1970s I & C Group Organization Chart.
PROCESS
ENGINEERING
FACILITY
ENGINEERING
MECHANICAL ELECTRICALSTORES
MILLWRIGHTWAREHOUSE
PIPEFITTERSHIP/REC
LABORERSBUYERS
SHIFT 2I&C
SHIFT 3ELECTRONICS
OPERATORS
SHIFT 1ELECTRICAL PROCESSMECHANICAL
CHEMICALELECTRICAL
QUALITYI&C
OPERATIONS
MANAGER
XYZ COMPANY

11. History of Instrumentation and Control Maintenance
5
that incorporate methods and procedures which ensure production lines are opera-
tional and preserve capital assets.
Skids and modular systems became the norm in the design of new plants. New
gas electrical generating plants have been built from start to operational within a
two year period. These plants are designed to be operated with a skeleton crew of
25 to 30 personnel, including operators, maintenance crew, and supervisors.
A crew of three operate and maintain in 12 hour shifts. Major overhaul peri-
ods are contracted to the system manufacturer, and contract maintenance is re-
sponsible for calibration. Knowledge of the complete plant, including operations
and systems, are learned by all crews and supervision. Each crew member special-
izes in two or three systems.
A newer gas fired electrical generating plant organization chart is shown in
Figure 1-3 which differentiates between maintenance and production. Because
modern automation systems are installed, three units can be maintained and oper-
ated with 30 employees. Old coal-fired plants needed up to 200 people to operate
them.
With the concept of skeleton crews to operate the plant, contractor type main-
tenance programs are becoming the norm. Many of the instrumentation tasks are
completed by contract personnel. Work in the plant is becoming multi-disciplined.
Figure 1-2. Multiple-Display Distributed Control System.

12. Introduction
6
Need for Instrumentation and Control Maintenance
and Engineering
“Maintenance of instrumentation and process control systems from simple
gages to complex distributed control systems is essential for the continuation of
our industry.” Statements such as this have been repeated thousands of times by
company presidents, manufacturing directors, and production superintendents.
Maintenance personnel should be involved with new installations and upgrad-
ing older installations. They should ensure that the system is ergonomically easy
to repair and well documented. Training should be done before a new system ar-
rives so the maintenance department can help in installing and checking it out.
Equipment manufacturers provide engineering and start-up assistance. So the
majority of the new opportunities to work in the I&C field is through original
equipment manufacturers or service contract employees.
Because of the equipment’s complexity, assistance is needed from the original
equipment manufacturer. Configuration of control systems and instruments
should be done by those very familiar with the system requirements and system/
instrument capabilities.
Instrumentation tells us the process parameters in which we are operating. A
simple gage tells the temperature or pressure; the more complex instrumentation
Figure 1-3. Typical Gas Fired Electrical Generating Plant Organization Chart.
PskdjkjdidP
MAINTENANCE
MANAGER
PLANT
ENGINEER
PRODUCTION
MANAGER
ENVIRONMENTAL
AND HEALTH
(CHEMIST)
WELDER
ELECTRICIAN
I&C
MACHINIST
(M-F 8 hrs)
SHIFT
SUPERVISORS
WATER/LAB TECH.
AUX. OPERATOR
(Outside)
CONTROL
OPERATOR
12 hr shift
Rotating 24/7
PLANT
MANAGER

13. Need for Instrumentation and Control Maintenance and Engineering
7
tells much more about the process. Proper operation of all equipment is required
to make a quality product and to do it safely.
The technological advances of the past few years and the trends for more
technical and specialized equipment require better trained and educated mainte-
nance personnel. The types of equipment in control systems cover many disci-
plines: mechanical, electrical, electronic, computer science, chemical, and
environmental, among others.
The instrumentation and control field is more than electronics — it is a systems
experience. It is necessary to know the physics of heat, light, noise, and mechani-
cal advantage, as well as to have mechanical dexterity and aptitude, logical
thought, computer literacy, process knowledge, and the ability to work with others in dif-
ferent disciplines.
Because of the many different knowledge factors, the individual crafts (elec-
trician, mechanic, pipe fitter, etc.) have to work together, and finger pointing will
sometimes occur. Electrical engineers, mechanical engineers, chemical engineers,
and process engineers must understand each other and determine where their re-
sponsibilities start and stop.
The field has grown with the application of computers, artificial intelligence,
self-tuning, computer-integrated manufacturing (CIM), and so on. Larger
companies train pipe fitters to be instrument mechanics in pneumatic plants and
electricians to be instrument technicians in electronic plants. Knowledge of the
process is needed to design new systems; therefore, all engineering disciplines get
involved with the instrumentation and control system. Those who were fortunate
to get involved in early instrumentation and control systems have become the I&C
maintenance personnel and the control systems engineers of today.
The complexity of control loops and systems requires specialists. The systems
concept requires more varied knowledge and the overall concept of control rather
than component troubleshooting and replacement.
When the control system doesn't work, the plant doesn't produce. The control
system design can determine the profitability of a company. If it is maintainable
and the mechanics, technicians, and engineers are trained, the production output
of the plant will be high.
Corrective, preventive, and operational maintenance must be performed by
qualified and experienced I&C maintenance personnel.
Because of the complexity of existing control systems that utilize many fields
of expertise, several maintenance backgrounds are also required. This group is
now required to maintain, troubleshoot, and calibrate pneumatic, electrical, elec-
tronic, and computerized instruments and systems. The systems approach, which
looks at the whole picture to gain an understanding of the process, is the special
attribute of I&C maintenance personnel.
When assistance is needed, I&C personnel must have someone to go to for
help. In the past, maintenance supervisors had a broad knowledge of most of the
equipment and could make decisions on how to repair, when to repair, and so on.
A few years ago, many supervisors were instrument mechanics, but contemporary
maintenance supervisors are managers who know very little about the operation
and maintenance of the wide variety of instruments and control systems used to-
day, since most have never been instrument mechanics or technicians. In fact,
many of them know very little about pneumatics, electronics, or computers. To-
day, knowledge of the process, knowledge of the overall system, and knowledge
of the expertise of their employees is far more important than knowledge of how
to repair an individual instrument.
Who should the maintenance supervisors and managers go to for expert ad-
vice on the control system? Instrumentation and control system engineers or
maintenance engineers with an I&C background. Instrumentation and control sys-
tem engineers assist the mechanics and technicians and keep the supervisors and

14. Introduction
8
managers informed. They need to be a part of the design and start-up of the con-
trol systems.
Much money is being spent for training, fault tolerant systems, redundancy,
and new techniques. One simple but essential area that may be neglected is the ex-
perience of the past and what that may teach about the present.
We learn from our past experiences. Being involved in the problems we en-
countered and the solutions that were found yesterday helps us make better deci-
sions today. The learning technology that produces greater retention levels uses
the most senses, such as hearing, seeing, and feeling. The applications of older
systems should be used as the basis for designing newer and generally faster con-
trol systems. New problems are encountered in newer systems, but past applica-
tion experience will help solve the new problems.
Don’t neglect the knowledge
and experience gained in the
past.
Good maintenance saves money. With the equipment working properly, the
process quality and production will be high. When equipment fails, production
normally stops, and many production personnel cannot do their jobs. With good
maintenance management, spare parts are available quickly to reduce the mean
time to repair (MTTR). When the equipment is repaired properly, the mean time
between failures (MTBF) is extended. The proper frequencies of preventive main-
tenance should provide less down time, and the down time that occurs can be
scheduled. We can become pro-active instead of reactive.