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Purpose of motor relief valves

Q: We have a two-stage, electric-motor-driven, rotary screw compressor at our facility. There are relief valves installed after the low-pressure compressor element and after the high-pressure compressor element. What is the purpose of the relief valves, or what would cause over-pressure to occur? Are they sized based on the compressor capacity?

A: Without having a P&ID of the compressor system, it is difficult to identify any possible source that could dead end the compressors in question. It is common practice to protect positive displacement compressors with discharge relief valves for any potential blockage of the compressor system. Sizing is normally based on the compressor’s capacity.