Section 54.01-40 - External pressure (modifies UG-28)(a) The exemption from external pressure consideration provided by the note under UG-28(f) does not apply.(b) Vessels which may at times be subjected to partial vacuum due to nature of the contents, temperature, unloading operations, or other facet of employment must either have vacuum breaker protection or be designed for not less than one-half atmosphere of external pressure.CGFR 70-143, 35 FR 19906, Dec. 30, 1970