PRV Backpressure Derating (API 520 Kw / Kb) Calculator
Determine PRV backpressure capacity derating factors Kw and Kb, built-up vs superimposed limits, and valve type selection (Conventional, Balanced Bellows, or Pilot-Operated) per API 520.
PRV Pressures & Relief Service
Discharge Header Backpressures
Derating Factor & Feasibility
Recommended Tools & Equipment
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Frequently Asked Questions
What is the fundamental difference between superimposed and built-up backpressure?
Superimposed backpressure is the static pressure existing in the discharge header before the PRV opens, originating from other equipment connected to the same flare or vent header. Built-up backpressure is generated entirely by the frictional resistance of the discharge piping as a direct result of fluid flowing after the PRV opens.
Why is a conventional PRV strictly limited to 10% backpressure?
In a conventional valve, the bonnet cavity is vented directly to the valve discharge body. Any backpressure acts on the top of the disc holder over an area roughly equal to the seat orifice, adding a downward closing force. This raises the actual opening set pressure psi-for-psi, meaning a valve with 30 psi backpressure will not pop until vessel pressure reaches 130 psi!
How does a balanced bellows eliminate backpressure set-point shifts?
A balanced bellows surrounds the valve stem and disc holder with a flexible metallic bellows whose effective cross-sectional area matches the nozzle seating area. Because backpressure acts equally on the outside of the bellows and the top of the disc, net backpressure forces cancel out to zero, ensuring the valve pops exactly at its marked set pressure.