Category: Education
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Chapter 33 Instrumentation cyber-security
As digital technology finds greater application in industrial measurement and control systems, these systems become subject to digital vulnerabilities. Cyber-security, which used to be strictly limited to information technology (IT) systems such as those used in office and research environments (e.g. desktop computers, printers, internet routers), is now a pressing concern for industrial measurement and…
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32.6 Safety Instrumented Functions and Systems
A Safety Instrumented Function, or SIF, is one or more components designed to execute a specific safety-related task in the event of a specific dangerous condition. The over-temperature shutdown switch inside a clothes dryer or an electric water heater is a simple, domestic example of an SIF, shutting off the source of energy to the appliance in…
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32.5 Overpressure protection devices
Fluid pressure exerts force on any surface area it contacts, as described by the formula F = PA. One practical consequence of this fact is that process vessels and pipelines may catastrophically burst if subjected to excessive fluid pressure. If subjected to excessive vacuum, some vessels and piping may implode (collapse in on themselves). Not only do these…
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32.4 High-reliability control systems
As discussed at the beginning of this chapter, instrumentation safety may be broadly divided into two categories: the safety hazards posed by malfunctioning instruments, and special instrument systems designed to reduce safety hazards of industrial processes. This section regards the first category. All methods of reliability improvement incur some extra cost on the operation, whether…
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32.3 Practical measures of reliability
In reliability engineering, it is important to be able to quantity the reliability (or conversely, the probability of failure) for common components, and for systems comprised of those components. As such, special terms and mathematical models have been developed to describe probability as it applies to component and system reliability. 32.3.1 Failure rate and MTBF Perhaps…
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32.2 Concepts and laws of probability
While the term “probability” may evoke images of imprecision, probability is in fact an exact mathematical concept: the ratio a specific outcome to total possible outcomes where 1 (100%) represents certainty and 0 (0%) represents impossibility. A probability value between 1 and 0 describes an outcome that occurs some of the time but not all of the…
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Chapter 32 Industrial Process Safety and Instrumentation
This chapter discusses instrumentation issues related to industrial process safety. Instrumentation safety may be broadly divided into two categories: how instruments themselves may pose a safety hazard (electrical signals possibly igniting hazardous atmospheres), and how instruments and control systems may be configured to detect unsafe process conditions and automatically shut an unsafe process down. In…
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31.8 Techniques for analyzing control strategies
Control strategies such as cascade, ratio, feedforward, and those containing limit and selector functions can be quite daunting to analyze, especially for students new to the subject. As a teacher, I have seen first-hand where students tend to get confused on these topics, and have seen how certain problem-solving techniques work well to overcome these…
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31.7 Limit, Selector, and Override controls
Another category of control strategies involves the use of signal relays or function blocks with the ability to switch between different signal values, or re-direct signals to new pathways. Such functions are useful when we need a control system to choose between multiple signals of differing value in order to make the best control decisions.…
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31.6 Feedforward with dynamic compensation
As we have seen, feedforward control is a way to improve the stability of a feedback control system in the face of changing loads. Rather than rely on feedback to make corrective changes to a process only after some load change has driven the process variable away from setpoint, feedforward systems monitor the relevant load(s) and use…
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31.5 Feedforward control
“Feedforward” is a rather under-used control strategy capable of managing a great many types of process problems. It is based on the principle of preemptive load counter-action: that if all significant loads on a process variable are monitored, and their effects on that process variable are well-understood, a control system programmed to take appropriate action based on…
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31.4 Relation control
A control strategy similar to ratio control is relation control. This is similar to ratio control in that a “wild” variable determines the setpoint for a captive variable, but with relation control the mathematical relationship between the wild and captive variables is one of addition (or subtraction) rather than multiplication (or division). In other words, a relation…
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31.3 Ratio control
Most people reading this book have likely had the experience of adjusting water temperature using two hand valves as they took a shower: one valve controlling the flow of hot water and the other valve controlling the flow of cold water. In order to adjust water temperature, the proportion of one valve opening to the other must…
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31.2 Cascade control
A simple control system drawn in block diagram form looks like this: Information from the measuring device (e.g. transmitter) goes to the controller, then to the final control device (e.g. control valve), influencing the process which is sensed again by the measuring device. The controller’s task is to inject the proper amount of negative feedback…
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Chapter 31 Basic Process Control Strategies and Control System Configurations
In a simple control system, a process variable (PV) is measured and compared with a setpoint value (SP). A manipulated variable (MV, or output) signal is generated by the controller and sent to a final control element, which then influences the process variable to achieve stable control. The algorithm by which the controller develops its…