-4 Thickness of wall.

(a)Minimum thickness of wall of each finished tank shall be such that at a pressure equal to 7/10 of the marked test pressure of the tank, the calculated fiber stress in psi at inner wall of tank multiplied by 3.0 will not exceed the tensile strength of any specimen taken from the tank and tested as prescribed in § 179.500-7(b). Minimum wall thickness shall be 1/4 inch.#

(b)Calculations to determine the maximum marked test pressure permitted to be marked on the tank shall be made by the formula:#

P = [10S(D 2 − d 2)] / [7(D 2 + d 2)]#

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. (D 2 − d 2/(D 2 + d 2) = The smaller value obtained for this factor by the operations specified in § 179.500-4(c).#

(c)Measure at one end, in a plane perpendicular to the longitudinal axis of the tank and at least 18 inches from that end before necking-down:#

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 (D 2−d 2)/(D 2 + d 2)#

(1)Make similar measurements and calculation for a corresponding location at the other end of the tank.#

(2)Use the smaller result obtained, from the foregoing, in making calculations prescribed in paragraph (b) of this section.#