Relief Valve Sizing Calculator
Size pressure relief valves per API 520 for gas, liquid, and steam service. Includes standard orifice letter designation.
Gas / Vapor Relief Valve
Results
Required Area
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Orifice Designation
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Standard Area
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Relieving Pressure
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Standard API Orifice Sizes
| Letter | Area (in²) | Status |
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How this was calculated
Gas (API 520): A = W * sqrt(T*Z) / (C * Kd * P1 * Kb * sqrt(M)), where C = 520 * sqrt(k * (2/(k+1))^((k+1)/(k-1))).
Liquid (API 520): A = Q * sqrt(SG) / (38 * Kd * Kw * Kv * sqrt(P1-P2)).
Steam (API 520): A = W / (51.5 * Kd * P1 * KN * KSH).
Relieving pressure: P1 = (Set pressure * (1 + overpressure/100)) + 14.696 psia.
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Book a free strategy call →Relief Valve Sizing per API 520
Pressure relief valves (PRVs) are the last line of defense against overpressure in process equipment. API Standard 520 (Sizing, Selection, and Installation of Pressure-Relieving Devices) provides the industry-standard methodology for determining the required orifice area for pressure relief valves in gas, liquid, and steam service.
For gas and vapor service, the required orifice area depends on the mass flow rate, molecular weight, temperature, and compressibility factor, along with correction factors for discharge coefficient (Kd) and back pressure (Kb). The equation accounts for critical (choked) flow through the orifice.
For liquid service, the sizing equation uses volumetric flow rate and accounts for specific gravity, viscosity (Kv), and back pressure (Kw). The differential pressure across the valve drives the flow.
Once the required area is calculated, the next larger standard API orifice size is selected. Standard orifice designations range from D (0.110 in2) to T (26.0 in2), providing a wide range of capacities.
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