The principle of this Air Cushion Valves is to allow large quantities of air in the pumping main during separation, entrap the air, compressed it with the returning air column and expel the air under controlled pressure so as to dissipate the energy of the returning water column. The valve has a spring loaded air inlet port, an outlet normally closed by float, a spring loaded outlet poppet valve and an adjustable middle valve control orifice.

When there is sudden stoppage of pump due to power failure, partial vacuum is created in the main. With deferential pressure, the spring-loaded port opens and admits outside air into the main. At atmospheric pressure the inlet valve closes under spring pressure. The entrapped air is compressed by returning water column till the poppet valve opens with float is dropped position, the air is expelled through poppet valve and controlled orifice under predetermined pressure thus dissipating the energy of the returning water column.