Relief valve (PSV) sizing — API 520
Required orifice area and the API 526 letter for gas and vapour (critical and subcritical), saturated steam and liquid relief valves.
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API 520 sizing for gas, steam, liquid and two-phase screening, inlet pressure loss and back pressure checks, and a datasheet-ready report — any units, visible formulas.
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How this calculator works
The required effective discharge area by the sizing equations of API Standard 520 Part I, then the next larger standard orifice letter (D to T) and effective area from API Standard 526:
- Gas or vapour, critical flow: A = W / (C·Kd·P1·Kb·Kc) · √(T·Z/M), with C from the ratio of specific heats k. Critical flow applies when the back pressure is below P1·(2/(k+1))k/(k−1).
- Gas, subcritical flow: A = 17.9·W / (F2·Kd·Kc) · √(T·Z / (M·P1·(P1−P2))).
- Saturated steam: A = 190.5·W / (P1·Kd·Kb·Kc·KN), with the Napier correction KN above 103.4 bar a.
- Liquid (certified valves): A = 11.78·Q / (Kd·Kw·Kc·Kv) · √(G / (P1−P2)), with the viscosity correction Kv found by iteration.
Here A is in mm², W in kg/h, Q in L/min, pressures in kPa (absolute for gas and steam, gauge differential for liquid) and T in K. The SI results were checked against the same equations in their US-unit form (in², lb/h, psia, °R, gpm) to within 0.11 %, and the coefficient C against the API 520 table (C = 356 at k = 1.40 in US units).
Example: 10,000 kg/h of a hydrocarbon vapour (M = 44.1, Z = 0.88, k = 1.13) relieving at 60 °C from a valve set at 10 bar g with 10 % overpressure: P1 = 12.01 bar a, critical flow, required area 879 mm² — a K orifice (1,186 mm²), 74 % used.
Important: this is a screening and checking tool. The relieving rate comes from the overpressure scenario (API 521), and the final selection must use the valve maker's certified Kd and back-pressure data and be checked by a competent engineer. Superheated steam: use the gas equation with steam's Z and k. References: API Standard 520 Part I, Sizing, Selection, and Installation of Pressure-relieving Devices; API Standard 526, Flanged Steel Pressure-relief Valves; API Standard 521.