Prooftesting critical instruments

Critical instruments assigned a Class 1 include those necessary to avoid a failure which may cause the perils listed above or instruments which fail to inform of upset conditions which may result in perils. Testing of these instrument systems may be mandated by regulatory agencies, in-house technical safety review committees, HAZOP studies, or designated as critical by operations supervisors. All of these shutdown systems and alarms must be prooftested in accordance with a proper schedule. [8]... 

Serious Consequences—Class 2. Equipment or the critical instruments serving equipment whose failure could possibly cause, or fail to warn of upset conditions, uncontrolled releases of dangerous materials, situations that could result in accidental fires and explosions. Furthermore these failures could result in serious conditions involving environmental releases, property or production losses, or other non-life-threatening situations. These particular pieces of equipment, the safety shutdown systems and the alarms that serve this equipment are given a slightly lower priority. However, they are also inspected, tested, or prooftested on a regular schedule, but may be allowed to have some leniency in compliance. 

Figure 10-25 An instrument mechanic prooftests a critical instrument panel.

Serious Consequences—Class 2. Safety Critical instruments whose failure could either cause, or fail to inform of, serious conditions involving environmental releases, property or production losses, or other non-life-threatening situations. These instruments are given a slightly lower priority, but are also prooftested on a regular schedule. 

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 prooftested on a well-defined schedule. 

Class 2 Safety Critical instruments include alarms or trips on refrigeration systems, rectifiers, cooling towers, kettles, and stills. [71 Normal Consequences—Class 3. Instrument systems that are used to alert the chemical pracess operator of a nonhazardous abnormal condition that might otherwise be undetected. The failure to react to one of these alarms may create an off-spec ificalion product such as a low-temperature alarm on certain distillation columns. These systems are not included in the prooftest program. 

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]... 

The Safety Instrumented Systems (SIS) and critical alarms assigned a Class 1 include those that have been mandated as such by state or federal agencies an in-house technical safety review committee HAZOP studies and specific alarms deemed critical by operations supervisors. All of these Safety Instrumented Systems and alarms are on a regular prooftesting schedule. 

Six independent groups of technical personnel provide prooftesting services. Prooftesting is performed by electricians, critical-metering mechanics, refrigeration mechanics, analyzer-repair technicians, instrument-maintenance personnel, and chemical process operators. 

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