How to use the safety relief valve capacity calculator
A pressure relief valve on a boiler or unfired pressure vessel has to pass enough steam at its set pressure to stop the pressure climbing past the allowable overpressure. This tool applies the steam sizing equation used in ASME BPVC Section VIII UG-131 and API 520 Part I to the standard orifice areas listed in API 526.
The equation is W = 51.5 × A × P × Kd × Ksh × Kn, where A is the effective orifice area in in², P is the relieving pressure in psia, Kd is the steam discharge coefficient of 0.975, Ksh is the superheat correction and Kn is the Napier correction that applies only above 1,500 psia. Relieving pressure is the set pressure plus the allowable overpressure plus 14.7 psi: 10 % for normal overpressure protection and 21 % for the external fire case.
Superheat correction values vary with pressure and total temperature, so only a conservative 0.90 is offered here; check the code table for your actual steam condition. Relief devices protect people as well as equipment, so use these numbers for preliminary checking only and confirm the final selection against the manufacturer certified capacity data and the applicable code authority.
Frequently asked questions
Blowdown sets the pressure at which the valve reseats, so it does not appear in the capacity equation. It does matter for the set point: keep the set pressure at least 10 % above operating pressure so the valve can reseat without chattering.
No. This equation is for steam only. Liquid and vapour or gas service use different sizing equations with their own fluid property, compressibility and back pressure corrections, so apply the relevant API 520 equation instead.