Relief Valve Sizing Calculator for Gas and Vapour (API 520)
Size a pressure relief valve for gas or vapour in critical flow using the API 520 Part I equation, then pick the API 526 orifice letter. Use it for checks and estimates; final sizing belongs with the valve vendor and your relief study.
- P1 = set × (1 + overpressure) + atmosphere = 1,201.3 kPa a
- C = 0.03948 × √(k(2/(k+1))^((k+1)/(k−1))) = 0.02703
- A = W ÷ (C × Kd × P1 × Kb × Kc) × √(TZ/M) = 1,055 mm²
API 526 standard orifices
| Letter | Area mm² | Area in² |
|---|---|---|
| D | 70.97 | 0.11 |
| E | 126.5 | 0.196 |
| F | 198.1 | 0.307 |
| G | 324.5 | 0.503 |
| H | 506.5 | 0.785 |
| J | 830.3 | 1.29 |
| K ✓ | 1186 | 1.84 |
| L | 1841 | 2.85 |
| M | 2323 | 3.6 |
| N | 2800 | 4.34 |
| P | 4116 | 6.38 |
| Q | 7129 | 11 |
| R | 10323 | 16 |
| T | 16774 | 26 |
API 520 Gas Sizing (Critical Flow, SI)
C = 0.03948 × √[k × (2/(k+1))(k+1)/(k−1)]
A is the effective discharge area in mm², W the flow in kg/h, P1 the upstream relieving pressure in kPa absolute (set pressure plus allowable overpressure plus atmospheric), T the relieving temperature in K, Z the compressibility, M the molecular weight, Kd the effective discharge coefficient (0.975 for a preliminary gas size), Kb the back pressure correction and Kc 0.9 with a rupture disc upstream, else 1.0. Orifice letters and areas are from API 526.
Worked example
10,000 kg/h of air (M 28.96, k 1.4) at 50 °C, set at 10 barg with 10 % overpressure: P1 = 1201.3 kPa a, C = 0.02703 and A ≈ 1,055 mm², so a K orifice (1186 mm²) is selected.
Frequently Asked Questions
What is API 520?
The API standard for sizing, selection and installation of pressure relieving devices. Part I covers sizing and selection.
What is an API 526 orifice letter?
A standard effective orifice size, from D (71 mm²) to T (16,774 mm²), used to select flanged steel relief valves.
Can I use this for final design?
Use it for estimates and checks. Final sizing needs the governing relief case, certified Kd from the manufacturer and a reviewed relief study.
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