API 520/521 Standards
Comprehensive guide to relief valve header design per API 521, pressure drop calculations, and backpressure limits.
PSV header sizing determines the correct diameter for relief valve discharge collection pipes to safely route relieving fluids to a flare or disposal system. Proper sizing per API 520/521 ensures backpressure stays within acceptable limits and prevents choked flow conditions.
Conventional relief valves allow up to 10% of set pressure as backpressure. Balanced bellows valves permit up to 30% variable backpressure, while pilot-operated valves can handle up to 50% variable backpressure.
API 521 recommends keeping the Mach number below 0.7 in relief valve discharge headers. Exceeding this limit indicates potential choked flow, which reduces the PSV's rated capacity and can cause dangerous backpressure buildup.
The Kb factor is a backpressure correction factor that reduces the rated capacity of a relief valve when discharge backpressure is present. It accounts for the effect of downstream pressure on valve flow performance per API 520.