PRESSURE/VACUUM RELIEF VALVE SETTING
Pressure/vacuum valves on atmospheric pressure fixed roof tanks are usually set at 1/2 oz. per square inch pressure or vacuum. Test data indicate that an increase of 1 oz. per square inch in the pressure set point over the usual 1/2oz. per square inch reduces breathing losses by approximately 7 percent. However, the test data indicate that each additional increase of 1 oz. per square inch in pressure set point reduces the breathing losses in progressively smaller increments.
The pressure and vacuum setting of a breather valve are dictated by the structural characteristics of the tank and should be within safe operating limits. A certain amount of pressure and vacuum beyond this setting is necessary to overcome pressure drop in order to obtain required flow. Proper size and setting can best be determined by reference to API STD accordance with this publication. The pressure setting for pressure/vacuum valves to be installed on large tanks constructed in accordance with API 12:Specification for Large Welded Production Tanks(1957)usually is limited to 1/2 oz. because roof plates will start to shift when the pressure rises much above 1 oz.