Hydrotest Pressure Calculator
Hydrostatic and pneumatic test pressure per ASME B31.3 para 345, with test hoop-stress and yield checks, gauge range, hold time, and brittle-fracture temperature.
When to use this calculator
Use when preparing a pressure test package or a field test procedure and you need the test pressure defended against the code rather than assumed. It covers both test types — hydrostatic per para 345.4.2 and pneumatic per para 345.5.4 — and then runs the checks that decide whether the test is actually safe to perform: the Barlow hoop stress the test pressure induces, how close that sits to SMYS, the yield-limited maximum test pressure, the acceptable gauge full-scale range, the minimum hold time, and the minimum metal temperature that keeps the line out of the brittle-fracture region while it is pressurised.
Required inputs
- Test type — hydrostatic or pneumatic
- Design pressure P
- Allowable stress at design temperature and at test temperature
- Optional: outside diameter, wall thickness, and SMYS for the stress checks
- Optional: MDMT and test metal temperature for the brittle-fracture check
Expected outputs
- Test pressure, with the S_T/S ratio and whether it was capped
- Test hoop stress and its percentage of SMYS
- Yield-limited maximum test pressure and the margin against it
- Acceptable test-gauge full-scale range and minimum hold time
- Graded warnings — pass, warning, or fail — against each code limit
Formula overview
SI: pressures in MPa (bar also accepted in the app), D and t in mm, stresses in MPa, temperatures in °C. Keep P and P_T in the same unit.
Hydrostatic — para 345.4.2:
P_T = 1.5 · P · (S_T / S) with S_T/S capped at 6.5
Pneumatic — para 345.5.4:
P_T = 1.1 · P
Test stress checks (Barlow):
σ_hoop(test) = P_T · D / (2t)
P_T,max = SMYS · 2t / D
Test conditions:
gauge full scale = 1.5–4 × P_T para 345.2.3
hold ≥ 10 min before examination para 345.2.2
T_test,min = MDMT + 17 °CWorked example
6" Sch 40 A106 Gr.B line, P = 1.0 MPa (10 bar), tested at ambient so
S_T/S = 1.0. D = 168.3 mm, t = 7.11 mm, SMYS = 240 MPa.
P_T = 1.5 × 1.0 × 1.0 = 1.5 MPa (15 bar)
σ_hoop(test) = 1.5 × 168.3 / (2 × 7.11) = 17.8 MPa
= 7% of SMYS → well inside the 90% warning level
P_T,max = 240 × 2 × 7.11 / 168.3 = 20.3 MPa
→ P_T is 13× below the yield limit
Gauge full scale: 1.5–4 × 15 bar = 22.5–60 bar
Hold at 15 bar for at least 10 min, then examine at design pressure.Common mistakes
- Testing to the calculated pressure without checking the weakest component. The line pipe may take it while a Class 150 flange, a soft-seated valve, or a bellows does not — the test pressure is limited by the lowest-rated item inside the test boundary.
- Forgetting the S_T/S cap. Testing a hot line at ambient can make the stress ratio very large; para 345.4.2(b) caps it at 6.5, and beyond that you must also confirm the test pressure does not yield the pipe.
- Ignoring metal temperature. Pressurising carbon steel near or below its MDMT risks brittle fracture — keep the metal at least 17 °C above MDMT during the test, which in cold weather means warming the water, not just the air.
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