Relief Valve Sizing Calculator (Gas, API 520)
Size a pressure relief valve for gas or vapour in critical flow with the API 520 Part I equation. The workbook calculates the relieving pressure, coefficient C and required area, checks for critical flow and selects the API 526 orifice letter.
Download
- Calculator sheet with a worked example
- How to Use sheet with steps
- 47 built in formulas, no macros
- Yellow input cells, grey results
- Calculation sign off block
- Excel, Google Sheets and LibreOffice
Worked Example
These are the results of the example already filled in the workbook. Replace the yellow cells with your own data.
API 526 orifices
| Letter | Area (mm²) | Area (in²) | % used | Selection |
|---|---|---|---|---|
| D | 71.0 | 0.110 | 1,487 | |
| E | 126.5 | 0.196 | 834 | |
| F | 198.1 | 0.307 | 533 | |
| G | 324.5 | 0.503 | 325 | |
| H | 506.5 | 0.785 | 208 | |
| J | 830.3 | 1.287 | 127 | |
| K | 1,186.0 | 1.838 | 89 | SELECT |
| L | 1,841.0 | 2.854 | 57 | |
| M | 2,323.0 | 3.601 | 45 | |
| N | 2,800.0 | 4.340 | 38 | |
| P | 4,116.0 | 6.380 | 26 | |
| Q | 7,129.0 | 11.050 | 15 |
… and 2 more rows in the download.
About this Calculator
Relief valves are the last line of protection against overpressure. Sizing them correctly needs the governing relief case, the right overpressure and the API 520 equation. This workbook gives a quick, documented area calculation and orifice selection that can be checked against the vendor sizing.
Basis: API 520 Part I gas sizing equation (SI units) and API 526 effective orifice areas. For estimates and checks; final sizing by the relief study and valve vendor.
How to use
- Enter the governing relief flow and conditions from the relief study.
- Set the allowable overpressure for the relief case and the total back pressure.
- Enter Kd (0.975 for preliminary gas sizing) and Kb and Kc where they apply.
- Read the required area; the first API 526 orifice large enough is marked SELECT.
- If the critical flow check shows CHECK, the flow is subcritical and the subcritical equation must be used.
Formulas used
- P1 = set pressure × (1 + overpressure) + atmospheric.
- C = 0.03948 √(k (2/(k + 1))^((k + 1)/(k − 1))).
- A = W ÷ (C Kd P1 Kb Kc) × √(T Z ÷ M).
- Critical if P2 ÷ P1 ≤ (2/(k + 1))^(k/(k − 1)).
Related tools
Frequently Asked Questions
What is API 526?
The API standard for flanged steel relief valves, which defines orifice letters D to T with fixed effective areas.
Which Kd should I use?
For preliminary gas sizing 0.975 is common. Final sizing uses the certified coefficient of the actual valve.
Does this cover liquid relief?
No. Liquid relief uses a different API 520 equation with viscosity correction; use the online liquid relief valve calculator.
Learn more on the blog
Instrumentation and control standards
A practical guide on instrumentationblog.in that explains the engineering behind this workbook.
Read Full Article