Section 64.65 - Vacuum relief device(a) Each MPT that is designed for an external pressure of less than 7.5 psig must have a vacuum relief device.(b) A vacuum relief device for an MPT must-(1) Open at an external pressure of not less than 3 psig; and(2) Have an opening with a cross-section of 0.44 square inch or more.CGD 84-043, 55 FR 37410, Sept. 11, 1990