Except as allowed under Sec. 154.703, cargo tanks must:
(a) Have their safety relief valves set at a pressure equal to or greater than the vapor pressure of the cargo at 45 [deg]C (113 [deg]F) but not greater than the MARVS under Sec. 154.405; or
(b) Be refrigerated by a system meeting Sec. 154.702, and each refrigerated incompatible cargo refrigerated by a separate system.