Choosing the right check valve to prevent waterhammer damage.
Waterhammer destroys pipes, pumps, valves and reputations. It’s often caused by pump power failures and valves that slam shut.
The right kind of check valve can protect the pipeline and other equipment from waterhammer. The wrong kind causes it.
Check valves are generally installed on pump delivery manifolds to prevent return flow. Their performance doesn’t matter much during normal operation because all the work is done by the pump control valve. But if the power fails or if there is no pump control valve they become critical and can either cause or reduce waterhammer.
Check valves that are forced closed by reverse flow may shut too fast and cause waterhammer. A valve that closes under it’s own force during the short period between forward and reverse flow after the pump stops is better at preventing waterhammer.
Here’s how some of the most regularly used check valves affect waterhammer:
1. Swing check valves.
There are two problems with it’s most basic form as a wafer pattern check valve.
– It depends on reverse flow to close. This means rapid closure and severe waterhammer.
– It depends on the forward flow to keep it open. When the flow velocity is low the opening force is low. This can result in unstable operation.
The design has been improved over the years by adding an external shaft, tilting the disc and using double doors.
An external shaft allows a counterweight to be fitted to speed up closure and increase the closing force. It’s also often used to fit a hydraulic damper to soften the last moments of closure.
The tilting disc arrangement balances the forces on the disc to allow for more compact designs in larger sizes. The offset shaft means that the disc provides some of the closing force so you need a smaller counterweight.
Double door check valves are normally spring actuated so they close faster. This is aided by the lower inertia of the doors.
2. Diaphragm check valves.
These valves can reduce waterhammer due to the spring force in the rubber diaphragm which causes the valve to close before return flow occurs and the dampening effect of the rubber diaphragm after closure has taken place.
3. Silent (piston) check valves.
This valve consists of a piston with a spring behind it to increase closing speed. This is probably the best known design to reduce waterhammer becuse its spring can strong enough to ensure the required closure speed and it has a short stroke length to ensure very fast and “slam-less” closure.
4. Nozzle check valves.
This is probably the best check valve. It combines the benefit of a very fast closing valve with a nozzle flow path which ensures a very low pressure drop in the valve.
Check valves play a vital role in waterhammer reduction. That’s not limited to installing them on the pump discharge. The can also be used:
– On a pump by-pass to allow water from the pump suction into the delivery line to fill the low pressure void created in a pump power failure.
– In long pipelines with many gradient changes. Strategically placed check valves could reduce waterhammer caused by column separation and also separate the “momentum” in the pipeline into smaller parts.
A fuller version of this article with pictures is available for download here: SOME CONSIDERATIONS IN SURGE PROTECTION EQUIPMENT