Wall Thickness

Formula

t = P × D / (2 × (S × E × W + P × Y))

tm = t + c

T = tm / (1 − mill undertolerance)

t = T × (1 − mill undertolerance) − c

P = 2 × S × E × W × t / (D − 2 × Y × t)

ASME B31.3 Equation (3a) for straight pipe under internal pressure. The second pair is that equation rearranged for pressure, after the corrosion allowance and the mill undertolerance. P is internal design gauge pressure. D is outside diameter. T is the nominal wall you enter when you ask for allowable pressure. S is the allowable stress you enter. E here is the quality factor, not the modulus. W is the weld joint strength reduction factor. Y is the coefficient from the code table. c is the corrosion and mechanical allowance you add.

The W field starts at 1.0. That means no weld reduction. In the creep range the factor can be lower. Check ASME B31.3 paragraph 302.3.5 before you keep 1.0.

The Y field starts at 0.4. That is the Table 304.1.1-1 value for ferritic steel when t is less than D/6 and the temperature is at or below 482°C (900°F). It is not the value for austenitic stainless or for every other metal, and it is not the value when t is at least D/6. Check the table.

The mill field starts at 12.5%. That is the usual pipe-wall undertolerance: the nominal wall may be 12.5% under the ordered thickness. Confirm it on the pipe specification.

Equation (3a) is not a completed pipe specification.