P = [10S(D2 - d2)] / [7(D2 + d2)]
Where:
P = Maximum marked test pressure permitted;
S = U / 3.0
Where:
U = Tensile strength of that specimen which shows the lower tensile strength of the two specimens taken from the tank and tested as prescribed in § 179.500-7(b) .
3 = Factor of safety.
(D2 - d2/(D2 + d2) = The smaller value obtained for this factor by the operations specified in § 179.500-4(c) .
d = Maximum inside diameter (inches) for the location under consideration; to be determined by direct measurement to an accuracy of 0.05 inch.
t = Minimum thickness of wall for the location under consideration; to be determined by direct measurement to an accuracy of 0.001 inch.
Take D = d + 2t.
Calculate the value of (D2-d2)/(D2 + d2)
49 C.F.R. §§179.500-4