Maintenance of Instruments & Systems

Transcription

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

2 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 v

3 Table of Contents 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 vi

4 Table of Contents 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 vii

5 Table of Contents 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 st 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 viii

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

7 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 history and speculates about future advances of instrumentation and control (I&C) system maintenance; it also covers some of the fundamental principles, vocabulary, 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-ofthe-art computer-controlled system? Should a new instrument technician be expected to maintain pneumatic instrumentation? This volume documents experiences 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 hardware is still operating effectively today a tribute to the worldwide manufacturers 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, recorders, 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 operators to adjust valves in response to visual instrument readings. 1

8 Introduction 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 instrument mechanic was specialized to maintain the pneumatic control systems. By the mid 1950s, electronic analog instrumentation had been formally introduced 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 electronic 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-todate operating information. Now the instrument mechanic needed to know electronics and electricity in addition to pneumatics. Larger plants formed Electrical and Instrument (E&I), Instrument and Electronic (I&E), or Electrical and Control (E&C) groups; some formed an Instrument and Control (I&C) Group and had both instrument mechanics 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 computer 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 hardware 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 functions 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. Instrumentation vendors were training the instrument mechanics and electricians to maintain their equipment. 2

9 History of Instrumentation and Control Maintenance 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 control 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. Chemical 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 instrumentation. 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 systems (DDCS), now called the distributed control system (DCS). Multiple minicomputers, 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. Serial 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 controllers 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 microprocessor 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 industry 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 s DCS operator interfaces were refined in the 1980s (see Figure 1-2). Intelligent CRT stations utilized multiple-display formats to condense and organize extensive 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 diagnostics to help operators and maintenance personnel determine if there were any problems. Distributed control systems had many on-line and off-line diagnostics, including process and input alarms, reportable events, error messages, and hardware and software failure reporting. 3

10 Introduction XYZ COMPANY OPERATIONS MANAGER STORES MECHANICAL ELECTRICAL OPERATORS FACILITY ENGINEERING PROCESS ENGINEERING WAREHOUSE MILLWRIGHT ELECTRICAL SHIFT 1 MECHANICAL PROCESS SHIP/REC PIPEFITTER I&C SHIFT 2 ELECTRICAL CHEMICAL BUYERS LABORERS ELECTRONICS SHIFT 3 I&C QUALITY Figure 1-1. Typical 1970s I & C Group Organization Chart. 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 obsolete 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 superimposed on the 4-20 ma signal for ranging and calibrating field devices. The International 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 developed across a wide range of vertical industries, where the focus was to keep the assembly lines and processes running rather than preserving assets. Today, manufacturers are focused on the long-term benefits of factory floor support practices 4

11 History of Instrumentation and Control Maintenance Figure 1-2. Multiple-Display Distributed Control System. that incorporate methods and procedures which ensure production lines are operational 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 periods are contracted to the system manufacturer, and contract maintenance is responsible for calibration. Knowledge of the complete plant, including operations and systems, are learned by all crews and supervision. Each crew member specializes 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 operated 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 maintenance programs are becoming the norm. Many of the instrumentation tasks are completed by contract personnel. Work in the plant is becoming multi-disciplined. 5

12 Introduction PskdjkjdidP PLANT MANAGER MAINTENANCE MANAGER ENVIRONMENTAL AND HEALTH (CHEMIST) PLANT ENGINEER PRODUCTION MANAGER 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 Figure 1-3. Typical Gas Fired Electrical Generating Plant Organization Chart. 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 upgrading older installations. They should ensure that the system is ergonomically easy to repair and well documented. Training should be done before a new system arrives 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 6

13 Need for Instrumentation and Control Maintenance and Engineering 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 maintenance personnel. The types of equipment in control systems cover many disciplines: 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 mechanical advantage, as well as to have mechanical dexterity and aptitude, logical thought, computer literacy, process knowledge, and the ability to work with others in different disciplines. Because of the many different knowledge factors, the individual crafts (electrician, 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 responsibilities 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, electronic, 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 today, since most have never been instrument mechanics or technicians. In fact, many of them know very little about pneumatics, electronics, or computers. Today, 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 advice on the control system? Instrumentation and control system engineers or maintenance engineers with an I&C background. Instrumentation and control system engineers assist the mechanics and technicians and keep the supervisors and 7

14 Introduction Don t neglect the knowledge and experience gained in the past. managers informed. They need to be a part of the design and start-up of the control 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 experience of the past and what that may teach about the present. We learn from our past experiences. Being involved in the problems we encountered and the solutions that were found yesterday helps us make better decisions 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 control systems. New problems are encountered in newer systems, but past application experience will help solve the new problems. 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 maintenance should provide less down time, and the down time that occurs can be scheduled. We can become pro-active instead of reactive. 8