PSV Reaction Force

Open-discharge relief-valve tailpipe reaction force per API 520 Part II

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Engineering Tips

Computes the reaction force at the open (atmospheric) discharge of a relief-valve tailpipe for gas/vapour at choked flow — the load the tailpipe support must react, opposite to the jet.

API 520 Part II:

• F_m = (W/366)·√(k·T/((k+1)·M))

• F_p = A·P — pressure force

• F = F_m + F_p — steady-state

• F_design = F·DLF — support design

Recommended values

Flow W: Use the rated relieving capacity (the governing relief scenario), not the set-pressure flow.
Pressure P: Static discharge pressure from the choked-flow analysis — above atmospheric for sonic discharge.
DLF: API suggests 1.1–2.0. Use 2.0 for a conservative pop-action support design.

Limitations

Open discharge: Atmospheric open discharge only. Closed / vent-header discharge needs a full transient (gas-hammer) analysis.
Combine loads: The reaction acts along the tailpipe axis and combines with weight and thermal loads at the support.

About the standards

API 520 Part II (7th ed.): Installation of pressure-relieving devices — para 4.4 reaction force.
API 520 Part I / API 521: orifice sizing and relief-load determination that feed this check.

PSV Reaction Force

Input Parameters

Relieving Fluid

Discharge Geometry & Pressure