Introduction to Plant Automation and Controls

Raymond F. Gardner

• Editore: CRC Press
• Anno: 2020
• ISBN:
  9781000204360
• Edizione: 1
• Argomento: Tecnología e ingeniería
• Categoria: Automatización en ingeniería

Introduction to Plant Automation and Controls addresses all aspects of modern central plant control systems, including instrumentation, control theory, plant systems, VFDs, PLCs, and supervisory systems. Design concepts and operational behavior of various plants are linked to their control philosophies in a manner that helps new or experienced engineers understand the process behind controls, installation, programming, and troubleshooting of automated systems.

This groundbreaking book ties modern electronic-based automation and control systems to the special needs of plants and equipment. It applies practical plant operating experience, electronic-equipment design, and plant engineering to bring a unique approach to aspects of plant controls including security, programming languages, and digital theory. The multidimensional content, supported with 500 illustrations, ties together all aspects of plant controls into a single-source reference of otherwise difficult-to-find information.

The increasing complexity of plant control systems requires engineers who can relate plant operations and behaviors to their control requirements. This book is ideal for readers with limited electrical and electronic experience, particularly those looking for a multidisciplinary approach for obtaining a practical understanding of control systems related to the best operating practices of large or small plants. It is an invaluable resource for becoming an expert in this field or as a single-source reference for plant control systems.

Author

Raymond F. Gardner is a professor of engineering at the U.S. Merchant Marine Academy at Kings Point, New York, and has been a practicing engineer for more than 40 years.

Chapter 1

Measurement fundamentals and instrumentation

Instrumentation is used to measure equipment performance, indicate plant operating parameters, and record trends, and instrumentation forms the starting point for automatic control systems. Since many plant processes cannot be directly observed, instrumentation provides the means for visualizing what the system is doing and how the system is behaving. Instrumentation is used for quantifying operating levels and performance, verifying efficiency, troubleshooting, and for comparing manufacturer’s data or plant heat-balance parameters for making informed operational decisions. Instrumentation is key to proper plant operation. Typical parameters that are measured in plants include:

•Pressure

•Temperature

•Liquid level

•Flow rates

•RPM

•Viscosity

•Acceleration

•Smoke opacity

•Stack-gas oxygen content

•Electrical parameters, etc.

INSTRUMENTATION DEFINITIONS

Several terms are used to define instrumentation performance, including sensitivity, accuracy, precision/repeatability, and resolution. They are defined as follows:

•Sensitivity: is the change in output reading versus the change in input measurement. For instance, if an electronic pressure gage reads between 0-100psi and uses 4-20mA as the signal, the sensitivity is (20-4mA)/(100-0psi) = 0.16mA/psi.

•Accuracy: is the comparison of the measured value versus the true value. For instance, if many people step on a scale and their weights always read 5 pounds too low, the scale is accurate to within 5 pounds.

•Precision: is somewhat synonymous with “repeatability.”

If the same parameter is measured over and over, the output reading should be the same every time, but it is probable that there will be some variation in the reading. For instance, if a person were to weigh himself on a scale many times, and each scale reading was within ± 1 lb of all other readings, then the scale is precise to within 1 lb.

It is possible that an instrument is precise, but not accurate. If the true weight is 150 lb, but every reading exactly indicates 145 lb, it is interpreted that the scale is very precise, but also, that it is inaccurate. In other words, the scale is precisely wrong by 5 lb every time.

•Resolution: is the smallest change in input that yields an interpretable change in output. As an example, an analog pressure-gage scale may have index marks for each psi, implying that the resolution is 1 psi, assuming that the pointer is frictionless and moves smoothly as pressure changes. However, if during operation, gradual pressure changes cause no movement until the pointer suddenly jumps two index marks to overcome friction, its resolution would be 2 psi.

•Turndown: is often expressed as a ratio. Turndown is the lowest reading before the gage becomes unacceptably inaccurate.

For instance, the full-scale reading of a flow meter may be 1,000 gpm, but the lowest reading on the meter scale might be 100 gpm. This meter has a 10:1 turndown ratio.

GAGE ACCURACY

When procuring instruments, the manufacturer presents accuracy in terms relative to its “% Full Scale” value, i.e., the worst accuracy is compared to the gage’s highest reading. For example, if a pressure gage has a full-scale reading of 1,000 psig and a rated accuracy of ±1% F.S., it is expected to be accurate to within 1,000psig×0.01=±10 psig, high or low (see Figure 1.1). Now assume that this gage is installed on a system whose normal pressure is 100 psig. In this case, it is possible that the true pressure can fall anywhere between 90 psig to 110 psig and still be within specifications. Therefore, even though this gage has ±1% full-scale accuracy, in this application, its accuracy is really within ±10% when used in a 100-psig system:

±10 psig deviation100 psig systempressure×100=±10% accuracy

Figure 1.1A 1,000-psig pressure gage having an advertised full-scale accura...