ASME B31.1 Maximum Allowable Pressure Calculator (Power Piping) — Sample Report

1 · Design inputs

Boiler feedwater line downstream of the feed pumps: NPS 10 Sch 120 seamless carbon steel pipe (10.75 in OD, 0.844 in nominal wall) at a 450°F design temperature. A feed pump upgrade asks for a 2,500 psig design pressure. ASSUMED inputs for this example: SE = 20,000 psi, y = 0.4, W = 1.0 (carbon steel), A = 0.0625 in for erosion and corrosion, 12.5% mill under-tolerance. Read SE and y for your own material and temperature from Mandatory Appendix A and Table 104.1.2-1.

2 · Method

Maximum design pressure for a definite minimum wall, B31.1 ¶104.1.2(a) — eq. (5) in B31.1-2026, eq. (9) in B31.1-2024, outside-diameter form:

P = 2·S·E·W·(t_m − A) / ( D_o − 2·y·(t_m − A) )

t_m = t_nominal·(1 − mill tolerance)

Eq. (5) is eq. (3) solved for pressure, so the two always agree: size a wall with the wall thickness calculator, feed the result back in here, and you recover the pressure you started with. The term (t_m − A) is the metal the Code lets you count on for pressure. For pipe bought to a nominal wall, t_m is the least the mill may deliver, nominal × (1 − under-tolerance), which is the same manufacturing tolerance ¶104.1.2 tells you to add when you order. For pipe on hand or in stock the paragraph is more generous: the measured minimum wall at the thinner end may be used, in which case you enter that wall with a mill tolerance of zero. A then comes off in full — corrosion, erosion, thread or groove depth, whatever the service will take. S·E is the Mandatory Appendix A allowable, which already includes the joint efficiency; W is the ¶102.4.7 weld strength reduction factor, 1.0 for seamless pipe, for carbon steel and below the tabulated temperatures; y is the Table 104.1.2-1 coefficient. Units are consistent: psi with inches, or MPa with millimetres.

3 · Calculation

1. Minimum wall the mill may deliver: t_m = 0.844 × (1 − 0.125) = 0.7385 in.
2. Thickness available for pressure: t_m − A = 0.7385 − 0.0625 = 0.676 in.
3. Numerator: 2·SE·W·(t_m − A) = 2 × 20,000 × 1.0 × 0.676 = 27,040 lb/in.
4. Denominator: D_o − 2·y·(t_m − A) = 10.75 − 2 × 0.4 × 0.676 = 10.75 − 0.5408 = 10.2092 in.
5. Maximum design pressure: P = 27,040 / 10.2092 = 2,648.6 psig. The 2,500 psig request is covered with 148.6 psi in hand, a ratio of 1.059.

4 · Results COMPUTED

5 · Verification statement

The result values above are locked as fixture b31-allowable-pressure.json, case “B31.1 ¶104.1.2 eq. (5) — boiler feedwater NPS 10 Sch 120, SE=20000, W=1, y=0.4, A=0.0625, 12.5% mill tol”, 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 — ¶104.1.2(a), eq. (5): design pressure for a definite minimum wall, outside-diameter form (eq. (9) in B31.1-2024; eq. (6) / eq. (10) is the inside-diameter form). The NOTE under the definition of P permits rounding the computed pressure to the next higher unit of 10. · ASME B31.1-2026 ¶104.1.2(a), definitions of Do, d and tm — measured outside diameter and measured minimum wall may be used for pipe on hand or in stock; manufacturing tolerance is taken into account when pipe is ordered by nominal wall. · ASME B31.1-2026 (default; B31.1-2024 selectable) — Mandatory Appendix A (SE), Table 102.4.3-1 (E), Table 104.1.2-1 (y, including General Note (b) for Do/tm below 6), ¶102.4.7 and Table 102.4.7-1 (W). Cited by table and line on the Pro pickers; typed from your governing edition otherwise. · ASME B31.1-2026 ¶102.2.4 — allowance for variation from normal operation (15% or 20% over the allowable stress, by duration).