Hydrostatic / Pneumatic Leak Test Pressure Calculator (ASME B31.3)
Reports the minimum hydrostatic test pressure Phydro = 1.5·P·(ST/S) of ASME B31.3 ¶345.4.2 Eq. (24), the applied stress ratio under its 6.5 cap, and the 1.1·P pneumatic value of ¶345.5.4. Test-condition yield margin and component test ceilings are not checked.
Computes the minimum hydrostatic test pressure per ASME B31.3 ¶345.4.2 Eq. (24) — 1.5× design pressure scaled by the ratio of allowable stress at test temperature to allowable stress at design temperature — and the pneumatic test pressure at 1.1× design pressure per ¶345.5.4.
Method
Phydro = 1.5 · P · (ST / S) (ST/S capped, default 6.5)
Ppneumatic = 1.1 · P
where ST is the allowable stress at test (ambient) temperature and S at design temperature. The 1.5 hydro factor, 1.1 pneumatic factor, and the stress-ratio cap are inputs so the same card serves B31.1 (¶137.4.5: 1.5×, no ratio) and other sections.
| Inputs | ||
|---|---|---|
| P | Internal design gauge pressure | psi |
| ST | Allowable stress at test temperature (user-supplied) | psi |
| S | Allowable stress at design temperature (user-supplied) | psi |
| hydro factor | Hydrostatic multiplier (1.5 for B31.3) | — |
| ratio cap | Maximum ST/S ratio (6.5 per Eq. 24 note) | — |
| pneu factor | Pneumatic multiplier (1.1 for B31.3) | — |
| Outputs | ||
| ST/S | Stress ratio applied | — |
| P_hydro | Minimum hydrostatic test pressure | psi |
| P_pneu | Pneumatic test pressure | psi |
Limitations — what this calculator is not
- Computes the Code minimum test pressure; it does not verify that test-condition stresses stay below yield (¶345.4.2(b)) — check the weakest component at test temperature, including static head at low points.
- Component and flange test limits (e.g. B16.5 hydrostatic shell test ceilings) may cap the achievable test pressure — not evaluated here.
- Pneumatic testing carries stored-energy hazards and its own relief requirements (¶345.5.1–.3) — this calculator only supplies the pressure value.
Worked example — fixture-verified
Design pressure 300 psi; allowable stress 20,000 psi at ambient test temperature and 17,500 psi at design temperature; B31.3 factors.
| Given | ||
|---|---|---|
| Design pressure P | 300 | psi |
| ST at test temperature | 20,000 | psi |
| S at design temperature | 17,500 | psi |
| Hydro factor | 1.5 | — |
| Pneumatic factor | 1.1 | — |
Step by step
- Stress ratio: ST/S = 20,000 / 17,500 = 1.14286 (under the 6.5 cap).
- Hydro test: 1.5 · 300 · 1.14286 = 514.3 psi.
- Pneumatic alternative: 1.1 · 300 = 330 psi.
| Result COMPUTED | ||
|---|---|---|
| Stress ratio ST/S | 1.14286 | — |
| Hydrostatic test pressure | 514.3 | psi |
| Pneumatic test pressure | 330 | psi |
The ST/S ratio rewards testing cold: a line designed for elevated temperature tests above the bare 1.5× because the metal is stronger at ambient.
leak-test-pressure.json — case “P=300, ST/S=20000/17500, B31.3 factors” (tolerance 0.001) — in the
calc-core release gate. It re-runs on every commit; a red fixture blocks deployment.
See the validation methodology.Sources & citations
- ASME B31.3, Process Piping — ¶345.4.2 Eq. (24) hydrostatic test; ¶345.5.4 pneumatic test.
- ASME B31.1, Power Piping — ¶137.4.5 (same card with factor 1.5 and ratio 1.0).
Per the source & citation policy, allowable-stress and factor table values are user-supplied — this page and the app cite paragraph numbers and never reproduce ASME table data.
FAQ
Why is the hydro test more than 1.5 × design pressure?
Eq. (24) scales 1.5×P by ST/S so the test at ambient temperature produces a stress at least proportionate to the design-temperature condition. When the design temperature is high, S drops, the ratio exceeds 1.0, and the test pressure rises accordingly (capped at a ratio of 6.5).
When is a pneumatic test used instead?
When water is unacceptable (freeze risk, process contamination, supports not rated for water weight). B31.3 sets it at 1.1×P with additional safeguards in ¶345.5 because of the stored energy in compressed gas.
Related calculators
- ASME B31.3 Pipe Wall Thickness Calculator — Sets the design pressure the 1.5× test scales from
- ASME B31.3 Allowable Working Pressure Calculator — Rate the installed wall before setting the test basis
- ASME B31.1 Power Piping Wall Thickness Calculator — Power piping design pressure for the ¶137.4.5 test