The valve is so designed that the disc travels 1/4 inch per inch of nominal pipe size when fully opened, thereby allowing full flow. When pumping operation ceases or reversal of flow starts, the spring causes the valve to close immediately without any noticeable loss of head. The column of water, gass or air against the inlet side of the disc acts as a cushion, equalizes back pressure thereby eliminating all shocks, slam and noise to piping system, pumps or meters.