The 8900 Series manway pressure and pressure/vacuum relief vents provide emergency pressure relief beyond that furnished by the normal pressure vent for atmospheric and low-pressure tanks, as well as convenient access for tank cleaning, inspection, and repair. 

Emergency relief vents provide relief from excessive internal pressure which may be caused by an abnormal condition; such as ruptured internal heating coils, an external fire, a failure of the tank blanketing system, a sudden cooling of the tank or a break in the tank wall.