Check valves on pump installations.
Check valves are generally installed on pump delivery manifolds to prevent return flow when the pump stops. They may also be used as a foot valve on the suction side to ensure that the line is full when the pump starts.
The check valve’s performance is not a significant factor during normal flow – if the installation includes a pump control valve. (The control valve does all the work.) But, it becomes critical during a sudden pump stop (eg on electrical failure) or if there is no control valve fitted.
Check valves can cause water hammer during a sudden pump stop. The worst offenders are designs where the reverse flow forces the valve closed. This creates effectively instantaneous closure under high velocity. Check valves that close under their own force during the short period between forward and reverse flow are better.
Check valves on the suction side of pumps also need careful consideration. Criteria for selection are:
- The valve needs to close fast enough to prevent water hammer.
- The valve needs to close at least as fast as the check valve on the delivery side. Slower closing allows a void / vacuum to develop in the suction and delivery manifolds. This may result in unsteady start-up even with a pump control valve installed.
- Low pressure drop. This is essential to ensure the NPSH of the pump is protected.
- Positive seating. The check valve on the pump suction is to ensure that the suction line is full on pump start-up. If the valve does not seal properly this cannot be achieved.
Other uses for check valves.
Check valves play a vital role in water hammer reduction. They can also be used as follows:
- On a pump by-pass to allow water from the pump suction to the delivery line to fill the low pressure void created during a pump trip-out.
- In long pipelines with many gradient changes, strategically placed check valves could reduce water hammer caused by column separation and separate the momentum in the pipeline into smaller parts.
Types of check valves.
I’ve listed the most common types of check valves with a brief description of each below.
Swing check valves.
In its most basic form – as a wafer pattern swing check valve – this valve depends on reverse flow to close, resulting in severe water-hammer.
Another disadvantage is that the valve can become unstable. This happens because the opening force from the flow decreases as the valve opens. The weight of the door counteracts this opening force and tries to close the valve.
Over the years the swing check valve has been re-designed into different formats i.e.
- With an external shaft and counterweight to speed up closure and provide greater closing force. A hydraulic damper may be fitted to the shaft to soften the final closure.
- Tilting disc arrangement. This design balances the forces on the disc to allow for more compact designs in larger valves. The offset shaft also ensures that the disc itself provides closing force, which can reduce required counterweight.
- Double-door check valves. These valves are normally spring actuated to increase speed of operation. This is aided by the lower inertia of the doors. Although this valve has become very popular due to its low price, it suffers from high pressure loss and disc flutter which can cause early failure of the spring due to work hardening
Diaphragm check valves.
This valve has a good reputation for reducing water hammer. That’s because:
- the rubber diaphragm causes the valve to close before return flow occurs
- and the dampening effect of the rubber diaphragm after closure has taken place.
Early failure of the diaphragm due to extrusion through the centre-plate holes can be a problem.
Piston check valves (silent check valves).
This valve consists of a piston with a spring behind to provide the required closing speed and ensure slam-free operation.
This is one of the best known designs for reducing water hammer, but it suffers from high pressure drop and size limits.
Nozzle check valves.
This is the “ultimate” check valve. It combines the benefit of very fast closing and a nozzle flow path for very low pressure drop. The inner body also protects the spring from turbulence and clogging. Multiple shafts are used for stability in larger valves (400mm upwards).