CPVC Pipe Pressure Rating and Wall Thickness Calculator (ASME B31.3 / B31.1) — Sample Report

1 · Design inputs

A CPVC line under ASME B31.3-2024 Chapter VII: 4.5 in outside diameter, 0.3 in minimum wall from the purchase specification, 100 psig design pressure, and 0.05 in taken off for a groove. The design stress is entered as 1000 psi. That is a round example input chosen to make the arithmetic easy to follow. It is not a table value: read yours from the code table at your design temperature.

2 · Method

t = P · D / (2 · S + P)

t_m = t + c

rating of an entered wall: P = 2 · S · t_a / (D − t_a), t_a = wall − minus tolerance − allowances

The thickness equation is B31.3-2024 eq. (26a). B31.1-2026 prints it as eq. (2), t = D / (2·S_a/P + 1), which is the same value written another way. Neither paragraph adds a design factor for thermoplastic pipe: the hydrostatic design stress is already the design stress, so nothing else gets multiplied in. The rating is that equation solved for P with the thickness the wall has left after everything that does not carry pressure is taken off.

The two codes agree on the answer and disagree on the bookkeeping, and the calculator follows each one's wording. B31.3 ¶A304.1.1 defines c as the mechanical allowances (thread or groove depth) plus corrosion and erosion allowance, then requires that the minimum thickness of the pipe selected, considering the manufacturer's minus tolerance, be not less than t_m. So under B31.3 the tolerance comes off the wall and stays out of t_m. B31.1 ¶N-104.1.1 puts the manufacturer's minus tolerance inside c, so under B31.1 it shows up in t_m and the wall is compared as entered. Enter the tolerance once, in its own field, and leave it at zero when the wall you typed is already the minimum wall of the purchase specification. The pass/fail and the rating come out the same under both codes; the reported t_m does not, and a reviewer checking your sheet against the paragraph will expect the one that matches the code on the line list.

S is always a typed input. Under B31.3 it is the hydrostatic design stress of Table B-1 (Table B-1C in U.S. Customary units); under B31.1 it is the hydrostatic design stress of Table N-102.2.1-1. Both tables list stresses at a few temperatures with recommended temperature limits for each material line. B31.3 ¶A302.3.1(b) permits straight-line interpolation between listed temperatures, and B31.1 ¶N-102.3.1(a) permits it where sufficient data are provided. On the Pro tier a table picker fills S from the selected code's table at the design temperature you enter. It applies that rule. Below the lowest listed temperature, B31.3 Table B-1 Note (3) says to use the tabulated stress at all lower temperatures, so the picker fills the lowest tabulated value and says so on the card; ¶A323.2.2(b) keeps the stress at that value below the row's minimum. It refuses instead of extrapolating above the row's maximum or past the last temperature the row lists.

3 · Calculation

1. Pressure design thickness, eq. (26a): t = 100 × 4.5 / (2 × 1000 + 100) = 450 / 2100 = 0.21429 in.
2. Minimum required thickness, eq. (25): t_m = 0.21429 + 0.05 = 0.26429 in.
3. Wall check, ¶A304.1.1: the minimum wall is 0.3 in, which is not less than 0.26429 in. Pass.
4. Thickness left to carry pressure: 0.3 − 0 − 0.05 = 0.25 in.
5. Rating of that wall: P = 2 × 1000 × 0.25 / (4.5 − 0.25) = 500 / 4.25 = 117.65 psig.
6. Design pressure over rating: 100 / 117.65 = 0.85.

4 · Results PASS

5 · Verification statement

The result values above are locked as fixture cpvc-pressure-design.json, case “B31.3 check with 0.05 in allowance: P 100 psi, tm 0.26429 <= wall 0.3 -> pass, rated 117.65 psi”, tolerance 0.00001, 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 B31.3-2024, Process Piping — Chapter VII, ¶A304.1.1 eq. (25) and ¶A304.1.2(a) eq. (26a), straight thermoplastic pipe under internal pressure. · ASME B31.3-2024 — ¶A302.3.1, hydrostatic design stresses of Table B-1 (Table B-1C) and straight-line interpolation between temperatures; ¶A323.2, temperature limitations; ¶A323.4.2(a), fluid service requirements for thermoplastics. · ASME B31.1-2026, Power Piping — Mandatory Appendix N, ¶N-104.1.1 eq. (1) and ¶N-104.1.2(a)(1) eq. (2), straight thermoplastic pipe under internal pressure. · ASME B31.1-2026 — ¶N-102.3.1(a), hydrostatic design stresses of Table N-102.2.1-1 and interpolation; ¶N-101.2.1 and ¶N-101.3.1, design conditions for nonmetallic piping; ¶N-124.9.2, installation limitations for thermoplastics.