Minimum thickness of shells and heads.

(a) For the purposes of this section, mild steel is steel with a guaranteed minimum tensile strength of 37 decanewtons per square millimeter (53,650 p.s.i.) and a guaranteed elongation of 27 percent or greater.

(b) Except as otherwise provided in this subchapter, the shell and heads of each portable tank constructed of reference mild steel:

(1) With a maximum cross-sectional dimension of 1.8 m (5.9 feet) or less, shall be at least 5 mm (0.197 inches) thick; or,

(2) With a maximum cross-sectional dimension exceeding 1.8 m (5.9 feet), shall be at least 6.35mm (0.250 inches) thick.

(c) The minimum thickness of the shell and heads of each portable tank constructed of a steel other than the reference mild steel, shall be obtained from the following formula:

Formula for metric units

e1 = (10e0) / (Rm1 A1)1/3

Formula for nonmetric units

e1 = (112.3e0) / (Rm1 A1)1/3

where: e0 = Required thickness of the reference steel from § 178.270-5(b)—millimeters (inches); e1 = Equivalent thickness of the steel used—millimeters (inches); Rm1 = Specified minimum tensile strength of the steel used—decanewtons per square millimeter (p.s.i.); and A1 = Specified minimum percentage elongation of the steel used—percent times 100 (i.e., if 20% use 20.0).

(d) When other than the standard minimum thickness for the reference mild steel is specified for a tank in this subchapter, the specified minimum shell and head thickness must be at least equal to the larger of the thicknesses calculated from the formula given in § 178.270-5(c) and the following formula:

Formula for metric units

e1 = (10e0 d1) / 1.8(Rm1 A1)1/3

Formula for nonmetric units

e1 = (112.3e0 d1) / 5.9(Rm1 A1)1/3

where: e1 = Equivalent thickness of the steel used—millimeters (inches); e0 = The specified minimum shell and head thickness of the reference mild steel specified in the IM Tank Table—millimeters (inches); d1 = Actual outside diameter of the tank—m (feet); Rm1 = Specified minimum tensile strength of the steel used—decanewtons per square millimeter (p.s.i.); and A1 = Specified minimum percentage elongation of the steel used—percent times 100 (i.e., if 20% use 20.0).

Note: For paragraphs (c) and (d) of this section the actual values for the tensile strength and percent elongation for the steel, as determined through tests on specimens from the group of plates to be used in the fabrication of the tank, may be substituted for the specified minimum values in the calculation prescribed in this paragraph (See § 178.270-3 of this part). Test records or certification of test results by the material producer or tank manufacturer must be retained by the tank manufacturer for a period not less than 15 years and must be made available to the Department or the owner of the tank.

[Amdt. 178-65, 46 FR 9896, Jan. 29, 1981, as amended by Amdt. 178-97, 56 FR 66284, Dec. 20, 1991; 57 FR 45465, Oct. 1, 1992; Amdt. 178-99, 58 FR 51534, Oct. 1, 1993; 66 FR 45387, 45389, Aug. 28, 2001]