An instrument air failure may cause the failure of all air-actuated instrumentation. Discharges to the disposal system can result from overpressure of process equipment or depressuring directly to the disposal system.
Determination of Credibility
Instrument air may be lost due to the shutdown of the compressors used to maintain system pressure, plugging of filters or driers, or due to a line rupture within the system. The presence of multiple air compressors, air receivers, or make-up from other plant air supplies decreases the likelihood of a total loss of instrument air due to compressor shutdown. However, due to the possibility of a line rupture within the system and the difficulty in predicting the impact of such a rupture, consideration is generally given to a total loss of instrument air.
Determination of Impact on the Collection and Disposal System
The immediate result of an instrument air failure will be failure of air-actuated control and shutdown valves, which will fail closed, in place, or open. Particular consideration should be given to control valves or depressuring valves that fail open and discharge directly to the collection and disposal system. Note that a depressuring or shutdown valve may have an independent air supply (e.g., an air bottle) to prevent failure-mode operation of the valve if air were otherwise lost.