Frequencies, prooftest

It takes years to develop and fine-tune a prooftest program. Time is required to identify all of the instrument loops that need to be included, and more time is needed to systematically collect or develop data sheets containing basic operational information for each device or loop. During the development stage, test methods need to be defined, and the appropriate personnel responsible for testing must be identified and trained. Finally, test frequencies must be decided. 

Assigning prooftest frequencies for complex, safety-instrumentation loops requires sound engineering judgment for simple systems. For more complex, interlock systems, the frequency is a function of the tolerable hazard rates. For example, DuPont Sabine River Works (Orange, Texas) reported it had 35,000 instruments in service. Every safety interlock is... 

Test-frequency changes to shorten or lengthen the test cycle at PPG are based upon evaluations of prooftest results, performance of the components in the loop, or changes in equipment or service. When the desired test pass rate (100-percent pass rate for Class 1 loops) is not met, causes of failure are evaluated. If warranted, a corrective action (such as a design change, test frequency change or request for further study) is initiated. 

Effect of coating-to-fiber adhesion on prooftest failure frequency. (Reproduced with permission from Reference 10. Copyright 1985 IEEE). 

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