Characteristics of spring-loaded pressure relief valves

If vapor pressure relief valve is used in liquid pressure relief service, it may take 25% overpressure to achieve full lift of disk (full PRV open position). For ASME code certified liquid PRV, it is required that PRV full open at 10% overpressure. 

The different behavior of vapor and liquid relief for spring-loaded PRV is because of the different nature of vapor and liquid. Vapor is light, will expand, and move fast, but liquid is heavy, will not expand, and move slowly. 

Some spring-loaded PRV may have soft seat (nonmetalHc) at its orifice to prevent disk and nozzle (metallic) damage. This may pose some temperature limttation on the relief service, 

Unlike spring-loaded PRV, the open or close of pilot-operated PRV is not affected by its backpressure. This means it can be used to protect equipment with higher pressure in relief valve discharge manifolds. 

Pilot vent is either vent to atmosphere or to the PRV outlet line. Pilot valve in actual is similar to a ring-loaded PRV. If hs backpressure varies, a bellow type pilot valve should be used. 

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