Vent System and Loading Criteria for Avoiding Tank Overpressurization.

The extent of cargo tank overpressure during transfer operations was analyzed, taking into account cargo properties, tank characteristics, loading rate, and vent system design. It was found that typical venting systems in use today have more than adequate capacity for gas venting, but inadequate capacity for liquid overfill. The margins of safety for gas venting appear to be about a factor of three in loading rate and a factor of ten in the effective length to diameter ratio of the vent system. Cargo volatility has a relatively small effect on the potential overpressure hazard. For the case of liquid overfill, the maximum permissible ratio of loading rate cu-ft per sec to vent cross sectional area sq-ft is 9 ftsec. Many cargo transfer operations currently equal or exceed this value. Furthermore, tank failure would be expected less than one minute after the tank becomes liquid full. Author