<Li> Low and vacuum pressure safety valve (LVPSV): An automatic system that relieves by the static pressure of a gas . The relieving pressure is small, negative or positive, and near the atmospheric pressure . </Li> <Li> Pressure vacuum release valve (PVRV): A combination of a vacuum pressure and a relief valve in one housing . Used on storage tanks for liquids to prevent implosion or over pressure . </Li> <Li> Snap acting: The opposite of modulating, refers to a valve that "pops" open . It snaps into full lift in milliseconds . Usually accomplished with a skirt on the disc so that the fluid passing the seat suddenly affects a larger area and creates more lifting force . </Li> <Li> Modulating: Opens in proportion to the overpressure . </Li>

Difference between pressure relief valve and vacuum relief valve