Critical instrument systems loops

Once a critical instrument loop is identified, a procedure for testing the entire loop must be written. The test procedure will influence the design of the new system, since, if possible, the test should be an actual performance test. For example, if a high temperature should close a valve, the ideal test would consist of raising the temperature to see if the value closes. Efforts should be made to avoid test procedures which require temporary wiring disconnects, valve closures, and so on, which might not be returned properly to operating condition. 

As described above, performance testing is an important consideration in the design of a critical instrument loop. Components of the system must be selected for ease of testing, as well as for their ruggedness and reliability. 

In computer systems, program logic is an important component of the critical instrument loop. Security precautions must be taken to prevent inadvertent changes in programming. In addition, manual overrides are discouraged. For example, if it is critical to shut down a pump during an emergency situation, a manual on switch may not be desirable. 

Class 1 safety instrumentation loops include alarms and trips on storage tanks containing flammable or toxic liquids, devices to control high temperature and high pressure on exothermic-reaction vessels, and control mechanisms for low-flow, high-temperature fluids on fired heaters. Other Class 1 instruments include alarms that warn of flame failure on fired heaters, and vapor detectors for emergency valve isolation and sprinkler-system activation. All of these alarms, shutdown valves, and other critical instruments are regularly proof-tested to a well-defined schedule. 

Instruments are provided to monitor the key process variables during plant operation. They may be incorporated in automatic control loops, or used for the manual monitoring of the process operation. They may also be part of an automatic computer data logging system. Instruments monitoring critical process variables will be fitted with automatic alarms to alert the operators to critical and hazardous situations. 

In this chapter only the first step in the specification of the control systems for a process will be considered the preparation of a preliminary scheme of instrumentation and control, developed from the process flow-sheet. This can be drawn up by the process designer based on his experience with similar plant and his critical assessment of the process requirements. Many of the control loops will be conventional and a detailed analysis of the system behaviour will not be needed, nor justified. Judgement, based on experience, must be used to decide which systems are critical and need detailed analysis and design. 

As applicable, quality-related critical data should be identified in the loop/ instrument schedule and system input/output (I/O) listings. 

When PPG Lake Charles first initiated its prooftest program, efforts to classify which safety devices were truly critical were not defined specific enough. Hence, the original program allowed too many instruments into the test system which created a top-heavy burden. To prevent this from happening, the following information should be developed for critical loops [8]... 

Some of these features may be identified at an early stage as potential problem areas, which you may find difficult to address with off-the-shelf control systems and, in particular, sensors. Response time is obviously a critical feature of control and instrumentation in PI, where one may be reducing reaction times, for example, close to their minimum. A comparison of response times of different types of loop element is given in Table 12.2. 

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