Minimum thickness of shell and heads for portable tanks
Pipeline and Hazardous Materials Safety Administration · Jan 11, 1993 · Reference No. 93-0318
Interprets: 49 CFR § 178.270-5, 49 CFR § 178.270-14
Reggie’s quick take
Someone asked how to calculate minimum shell and head thickness for a stainless steel intermodal (1M) portable tank, 7.67 feet in diameter, where standard equivalent metal thickness is specified. The agency said the reference mild steel value (eo) is 6.35 mm (0.250 inch), because tanks over 1.8 meters (5.9 feet) in maximum cross-sectional dimension must be at least that thick under § 178.270-5(b)(2). For this tank, only the formula in § 178.270-5(c) applies; the § 178.270-5(d) formula is used only when a thickness other than the standard minimum is specified. The agency also said the tank's marking under § 178.270-14(b)(11) should show no less than 6.35 mm.
Written by AI (Claude) on Oct 22, 2026. Robots make mistakes and leave things out, so the letter below is what counts. Not legal advice.
This is in response to your letter of August 17, 1992, concerning the minimum thickness of shell and heads for intermodal (1M) portable tanks. You inquire whether the minimum thickness formulas, in 49 CFR 178.270-5, apply to a stainless steel 1M portable tank with a diameter of 7.67 feet when standard equivalent metal thickness is specified. I apologize for the delay in responding to your letter and hope it has not caused you any inconvenience.
I will address your questions in the order presented.
Question. What value of "required thickness of reference mild steel (eo)" is to be used in the calculation of "equivalent thickness of steel used (el)"?
Answer. As specified in § l78.270-5(b)(2), the required thickness of shell and heads for a portable tank with a maximum cross-sectional dimension exceeding 1.8 meters (5.9 feet) shall be at least 6.35mm (0.250 inches) thick. Therefore, the required value of eo to be used in the calculation of el is 6.35mm or 0.250 inch, depending on whether the calculation is made in metric or nonmetric units.
Question. For this example, are we required to calculate minimum wall according to 'both equations §§ 178.270-5(c) and l78.270-5(d)?
Answer. No, You would calculate the required shell and head thickness using the formula contained in § 178.270-5(c). The formula contained in § 178.270-5(d) is only used when other than the standard minimum thickness is specified for a tank .
Question. If we are required for this example to calculate only per §178.270-5(c), should the marking on the tank as required by § 178.270-l4(b)(11) be no less than 6.35mm? .
Answer. Yes.
I trust this information is helpful.
A letter of interpretation applies the regulations to the facts the requester described. It isn’t a regulation and isn’t legal advice. The agency’s copy.