Free Engineering Tool
Safety Valve Sizing Calculator
Size pressure safety valves per ISO 4126 / API 520. Calculate required orifice area and select standard orifice letter for gas, steam, or liquid service.
Results
Gas / Vapor Sizing (API 520)
Standard API Orifice Designations (API 526)
| Letter | Area (mm²) | Area (in²) | Typical Inlet × Outlet |
|---|---|---|---|
| D | 71.0 | 0.110 | 1″ × 2″ |
| E | 126.5 | 0.196 | 1″ × 2″ |
| F | 198.1 | 0.307 | 1½” × 2½” |
| G | 324.5 | 0.503 | 1½” × 3″ |
| H | 506.5 | 0.785 | 2″ × 3″ |
| J | 830.3 | 1.287 | 3″ × 4″ |
| K | 1186 | 1.838 | 3″ × 6″ |
| L | 1841 | 2.853 | 4″ × 6″ |
| M | 2323 | 3.600 | 4″ × 6″ |
| N | 2800 | 4.340 | 4″ × 6″ |
| P | 4116 | 6.380 | 6″ × 8″ |
| Q | 7129 | 11.05 | 6″ × 10″ |
| R | 10323 | 16.00 | 8″ × 10″ |
| T | 16774 | 26.00 | 8″ × 12″ |
⚠️ Note: This calculator provides preliminary sizing. Final valve selection must be verified by the valve manufacturer and comply with applicable codes (ASME VIII, PED, etc.).
A safety valve (or pressure relief valve, PSV) automatically releases fluid when pressure exceeds a set point, protecting equipment from overpressure. Required by ASME Section VIII, PED, and ISO 4126.
Per API 520: A = W / (C × Kd × P₁ × Kb × √(M/(T×Z))), where W = mass flow, C = gas constant factor dependent on k, Kd = discharge coefficient (~0.975), P₁ = relieving pressure absolute, Kb = back pressure factor, M = molecular weight, T = temperature (K), Z = compressibility.
API 526 defines standard orifice sizes: D (71 mm²) through T (16,774 mm²). Each letter corresponds to a fixed orifice area and standardized valve body sizes.
Set pressure = pressure at which valve begins to open. Relieving pressure = set pressure + allowable overpressure (typically 10%). Sizing uses relieving pressure (absolute) as P₁.
Yes. Constant back pressure reduces effective differential. The correction factor Kb adjusts capacity. Balanced bellows or pilot-operated valves are less affected by back pressure than conventional valves.
The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.
Professional vibration and process monitoring instruments for industrial maintenance.
Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
