Another Approach—Prooftesting at a Louisiana Plant

At PPG Industries in Lake Charles, Louisiana, numerous instrument loops provide critical safety, alarm, and shutdown functions. These protective instruments are located on reactors, oil heaters, incinerators, cracking furnaces, compressors, steam-heated vaporizers, kettles, distillation columns, boilers, turbines, and other critical equipment. Process analyzers and flammable vapor detectors also enhance the overall process safety environment. [8] 

Instrument loops serving equipment described above can function in either an on-line or a standby manner both types can fail. Failure of an on-line loop, such as a failure of level control valve, becomes known rather quickly when the operation deviates either gradually or drastically from the normal. Depending on the type of failure, this may place a demand on the standby loop. 

After all, failure of a standby instrument loop, such as a alarm or safety interlock, will not become evident until a potential hazard is detected. Potential defects developing in these loops must be discovered by periodic prooftesting. 

A prooftest program cannot be left to someone s memory. It must follow a well-structured format to accomplish the essential steps, regardless of the myriad of other activities and distractions that tend to absorb all the supervisors and mechanics time. 

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] 

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