Inlet piping to PRVs

Pressure relief device (PRD) is a safety device to protect equipment or piping from damage due to overpressure or vacuum, but in this chapter, we wiU discuss FRD to provcfit overpressure only. Commonly used pressure relief devices are spring-loaded and pilot-operated pressure relief valve (PRV), rupture disk, or pin-actuated pressure relief device. For equipment, it is installed cither directly on the equipment or at its inlet/outlet piping. When the equipment or piping pressure reaches the pressure relief de vice set pressure, it will open to relieve fluid inside the equipment or piping to avoid overpressure. The relief fluid can be vapor, liquid, or two phase mixture. The relieved fluid is either sent to floic system to be disposed or relieved to atmosphere. 

For existing PRV, following guideline can be used to check the adequacy of PRV inlet piping. For non-fiie case, allow 4% Ps, and for fire case, allow 5% Ps for PRV inlet line pressure loss. 

The inlet pressure of relief fluid to PRV inlet piping is the system pressiue at relief condition. This pressure for non-fire case is 110% Ps, and for fire case, it is 121% Ps. 

The isolation valve at the inlet of the PRV should have a low pressure drop. This is to avoid an inlet piping pressure drop exceeding 3% of the PRV set pressure. 

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