ASME B31.1 Hydrostatic Test Pressure Calculator (¶137.4.5, 90% of Yield) — Sample Report

1 · Design inputs

Cold reheat line from the high-pressure turbine exhaust to the reheater: NPS 24 Sch 40 seamless carbon steel pipe (24 in OD, 0.688 in nominal wall), design 650 psig at 700°F, hydrostatic test with water at ambient temperature. ASSUMED inputs for this example: yield strength Sy = 35,000 psi at test temperature, y = 0.4, 12.5% mill under-tolerance. Read Sy for your own material from BPVC Section II, Part D, Table Y-1.

2 · Method

Required test pressures, B31.1 ¶137.4.5 and ¶137.5.5 (identical wording in B31.1-2026 and B31.1-2024):

P_T,hydro = 1.5·P

1.2·P ≤ P_T,pneumatic ≤ 1.5·P

Stress limit during test, ¶102.3.3(c) in B31.1-2026 (¶102.3.3(b) in B31.1-2024), hoop direction:

t_test = t_nominal·(1 − mill tolerance)

σ_hoop = P_T·( D_o − 2·y·t_test ) / ( 2·t_test ) ≤ 0.9·S_y

The hydrostatic and pneumatic factors are entered, not built in, so the result states the basis it was run on. The stress check needs a word of explanation, because the Code gives the limit and leaves the stress formula to the engineer. The calculator uses the ¶104.1.2 straight-pipe equation solved for stress, a conservative basis with two deliberate choices. The wall is the least the mill may have delivered, nominal × (1 − under-tolerance), because that is the wall the test has to be safe on. The additional thickness A is not deducted: on the day of the test the corrosion allowance is still metal. The Code gives the 90% limit and no stress formula, so this form, on the minimum wall with no allowance deducted, is the stated and deliberately conservative basis of the check. Setting y = 0 turns the expression into the outside-diameter form P·D/2t, which is the conservative bound if your procedure calls for it. S_y is the yield strength at test temperature, from BPVC Section II, Part D, Table Y-1 for the material; the calculator has no default for it and reports pressures only until one is entered. It also returns the pressure at which the hoop stress reaches the limit, which is the number to compare with a test pressure raised for static head.

3 · Calculation

1. Hydrostatic test pressure, ¶137.4.5: P_T = 1.5 × 650 = 975 psig. No S_T/S term enters, however hot the design temperature.
2. Pneumatic alternative, ¶137.5.5: not less than 1.2 × 650 = 780 psig and not more than 1.5 × 650 = 975 psig.
3. Wall the test has to be safe on: t_test = 0.688 × (1 − 0.125) = 0.602 in. The corrosion allowance is still in place and is not deducted.
4. Hoop stress at the hydrostatic test: D_o − 2·y·t_test = 24 − 2 × 0.4 × 0.602 = 23.5184 in; σ_hoop = 975 × 23.5184 / (2 × 0.602) = 22,930.4 / 1.204 = 19,045 psi.
5. Limit, ¶102.3.3(c): 0.9 × 35,000 = 31,500 psi. Ratio 19,045 / 31,500 = 0.605, so the hoop check passes.
6. Pressure at which the wall reaches the limit: 31,500 × 1.204 / 23.5184 = 37,926 / 23.5184 = 1,612.6 psig.

4 · Results PASS

5 · Verification statement

The result values above are locked as fixture leak-test-pressure.json, case “B31.1 ¶137.4.5 flat 1.5·P with the ¶102.3.3 hoop check — cold reheat NPS 24 Sch 40, P=650, Sy=35000 at test temperature”, tolerance 0.001, in the calc-core continuous verification gate (see https://pipingtoolset.com/trust/validation.html). Allowable-stress and factor data are user-supplied inputs; citations: ASME B31.1-2026, Power Piping — ¶137.4.5, required hydrostatic test pressure: not less than 1.5 times design pressure at any point, not over the maximum allowable test pressure of any non-isolated component nor the test-stress limit; 10-minute hold, then reduction to design pressure for the leak examination. Same wording in B31.1-2024. · ASME B31.1-2026 ¶102.3.3(c), During Test — hoop stress, and the sum of longitudinal stresses from test pressure and live and dead loads at test, each limited to 90% of yield strength (0.2% offset) at test temperature. This is ¶102.3.3(b) in B31.1-2024; ¶137.1.4 and ¶137.4.5 of both editions print the cross-reference as 102.3.3(b). · ASME B31.1-2026 ¶137.5.5 — pneumatic test pressure not less than 1.2 nor more than 1.5 times design pressure. ¶137.3.1 — boiler external piping tested to BPVC Section I, PG-99. ¶137.3.2 — hydrostatic test by default for nonboiler external piping. ¶137.2.6 — relief device at 1⅓ times test pressure recommended against thermal expansion of the test medium. · ASME B31.1-2026 ¶104.1.2(a), eq. (3) (eq. (7) in B31.1-2024) — the straight-pipe relation the hoop stress form is taken from. Yield strength at test temperature: ASME BPVC Section II, Part D, Table Y-1 — typed from your own copy.