Although the function of check valves is to protect pumps under emergency conditions, they can be the cause for damage to pump or pipeline system. Problems like waterhammer and valve slamming due to the valve closure often can be attributed to a wrong valve selection. Despite research efforts and many experiments still no standards exist that include the dynamic behaviour of check valves. Check valves are usually installed at the downstream side of pumps to allow flow in one direction and prevent flow in reverse direction. The ideal check valve has no resistance to flow in normal flow direction and infinite resistance in reverse flow direction. In practice however, check valves do have resistance in normal flow direction and do not close at zero flow, but after flow reversal. The amount of reverse flow depends on the valve type, size and configuration (spring strength, counter weight, etc). The reduction of the reverse flow to zero (valve closed) is accompanied with pressure surges (waterhammer) and valve slamming. The fact that some manufacturers suggest that their check valves do not induce pressure surges, because they close prior to flow reversal or because they close silent, is misleading.
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