Mechanical and Industrial Measurements Process Instrumentation and Control

The subject of Mechanical and Industrial Measurements (Process Instrumentation & Control) is gaining lot of popularity because of growing complexity of process industries and need to control the plants precisely, efficiently and safely from a central control room. The subject has become interesting with the advent of microprocessors and all devices becoming intelligent. With the increase in plant sizes and the complexity of the processes, more stringent conditions are being imposed on the specifications of instruments. As a result, the designs of instruments become obsolete within a very short period of time (5-8 years) and the instrument manufacturers have to accept this challenge. The trend today is for the miniaturisation of instruments coupled with increased sensitivity and accuracy. For reasons of interchangeability, it has become the universal practice to use current based instruments having output of 4 to 20 mA (unified system), i.e. whatever be the parameter being sensed, output of transducer will be same. In this system all secondary instruments become identical (only their calibration scale being different) giving very good appearance to the control room and these have so many other inherent advantages. The present day control room has no dedicated indicators and recorders. All displays are available on CRTs. Sensors and transmitters have become smart with self diagnostic capability and these have capability to transmit control room as radio signal.

Digital revolution has made rapid strides in all fields including instruments and control systems which have undergone revolutionary changes. The look of control rooms has undergone sea change. Modern control rooms make use of large video screen with few CRTS and keybounds.

Reliable Instrumentation and Control Systems are very vital for safe and efficient operation of any process. Scaling up of the size of units and growing complexity in the process have put severe demands of reliability and availability on instrumentation and control systems. Simultaneously there have been rapid advances in the field of electronics more so in the recent past, resulting in much improved and reliable hardware which have brought about sea change in the field of instrumentation and control.

For better supervision and overall coordination by a few operators, centralized control room concept is preferred in preference to staggered control centres. With centralized control room concept and increased unit size, number of control and measurement devices increased which led to adoption of computer based Data Acquisition System (DAS), DAS was meant to provide plant information in a concise and logical manner to the operator on CRT and also perform other functions e.g. data logging, alarm display, sequence of events recording, operator guidance messages, performance calculations, etc.

Advent of microprocessors in the field of electronics has resulted in revolutionary changes in the field of control and instrumentation. Variety of new products called smart or intelligent devices incorporating microprocessors including transmitters, indicators, recorders, analytical instruments, programmable logic controllers, advanced closed loop controllers etc. have appeared. Distributed digital control (DDC) Systems, which use unified hardware for different sub-systems of instrumentation and control (viz. sequence and interlock controls, closed loop control based on advanced control strategies, data acquisition system, etc.) are now available. The control room operator is equipped with important tools like alarm management, diagnostic and decision support systems, etc. enabling him to operate the plant safely, increasing its availability, efficiency, safety, reliability, timely maintenance, etc. DDC based systems have many advantages such as flexibility of modification, drift free operation, reduction in variety of equipment because of unified hardware, less cable costs with geographical distribution concept, expandability, self-diagnostics etc. It is possible to run data base throughout the plant, thus minimising cost of dedicated cables from plant to control room.

Since numericals give more insight to students in understanding basic philosophy, many numerical and solved examples have been added in almost all the chapters.

Due to the rapid developments taking place in the field of measurements and instrumentation, the task of student and the fresher in this field has become more complicated, because first the student is required to know the basic fundamentals and then keep pace with what is going on today. It is hoped that this book will be instrumental in achieving this objective.

Considering the trend for DDC systems in almost all industrial sectors, the Chapter on Distributed Digital Control Systems has been enlarged to include features of most of the systems available in the country with reputed collaborations from all over the world. The literature of all the leading suppliers viz. BHEL, ILK, ESPL, Keltron Controls, UPTRON, Siemens, Tata Honeywell, Bells Control, Bailey, Taylor, Keonics, Fisher, Rosemount etc. has been consulted and matter reporduced from their leaflets and operating instructions.

Author would also like to acknowledge the assistance taken from proceedings of various seminars/symposia held in the country and abroad on Modern Trends in Instrumentation & Control practices.

I am highly thankful to all the instrument manufacturers both in the country and abroad who have been kind enough to send me the technical literature on their products and whose literature has been consulted in writing this book. I am thankful to the Director General, Bureau of Indian Standards for permitting me to reproduce the extracts from the Standard No. IS : 2952 (Part I)—1964. My sincere thanks also go to my wife Smt. Sunita Jain for her assistance in the preparation of this Edition.

Any comments/suggestions for the improvement of the book from the readers shall be highly welcome.

I. Standards Applicable for Various Instrument/control Devices
II. Equivalents of Degrees Celsius and Fahrenheit
III. System of Units
IV. Units & Conversions
V. Chemical Elements
VI. Major Constants
VII. Engineering Conversion Factors