Water hammer (also known as hydraulic shock) is a pressure surge caused when an incompressible fluid is forced to stop suddenly in a closed system.
Since liquids (such as water) are essentially in compressible, the sudden closure of a valve leaves moving liquid nowhere to go and energy, (as pressure) ricochets through the system. This pressure wave stresses local equipment, with effects that build cumulatively over time.
Common water hammer damage includes:
- Valve seat warp or destruction.
- Contorted check valve discs.
- Broken or bent pump shafts.
- Premature flattening of seals and o-rings.
- Broken impellers.
- Shattered or cracked pipework.
Complete system service life is significantly reduced by water hammer, with the speed of system failure dependent on the severity of the water hammer effect. In addition to field instrument damage, a far higher than expected incidence of leaks is experienced, which causes operating system inefficiency.
Water hammer risk conditions
Vibration and noise is a particular threat in processes that operate over differing heights, as high pressures are introduced in such systems, to overcome height differentials. If water hammer occurs in high pressure systems, the effect is inherently more dramatic and deleterious. System designs that disrupt smooth laminar flow are also at higher risk of water hammer – where possible, bends and high pressure changes should be minimised.
Water hammer can occur in any application, with avoidable water hammer risk conditions including:
- Fast opening and closing of valves
- Check valves operating too quickly against flow
- Fast start or shut-down of pumps
- Intermittent power or power faults
- Water column separation
The most important element in water hammer avoidance is appropriate valve selection. Reputable suppliers are able to offer water hammer free valves, so appropriate valve selection at the outset will offset risk conditions.
Solenoid valve selection to avoid water hammer
Choose your solenoid valve carefully. Reliable vendors offer ‘water hammer free’ valves which should be the minimum standard for every industrial, agricultural, or domestic application. These valves use a specific fluidic design in the main body causing the diaphragm to close slowly against the process flow. This gentle closure minimises or eliminates water hammer, by avoiding pressure ricochet.
Additionally, reputable manufacturers offer water hammer free solenoid valves with adjustable open/close time functionality. By adjusting the duration of the open or close time, you can avoid sudden changes in fluid velocity. These valves are ideal for challenging applications, as they can be set to take as long as a minute to close the main valve.
Example valve: Type 6213/6281. This servo-assisted valve utilises a small flow path to close the valve, resulting in pressure equalisation. This soft close technique removes the chance of water hammer.
Pneumatically actuated process valve selection
As with solenoid operated valves, the first step to avoiding water hammer with linear process valves is appropriate product selection. Where water hammer is a concern, it is recommended that an angle-seat valve (also known as ‘y-pattern’ valve), or a globe valve (also known as ‘straight’ valve) be selected. Importantly, valves should be configured to close AGAINST the fluid flow velocity. This action balances the close function against the inertia of the media, and naturally eliminates pressure surge. Both spring-return and double-acting function valves can be configured in this matter.
Example valve, Type 2000: When liquid flows under the seat, in the direction indicated above, it reduces the effect of water hammer.
If you didn’t get to choose your valve and you have an actuated ball or butterfly valve, you will have to approach the problem from a different angle. In this case, you will need to vary the actuation speed, especially that of closing. This is normally done by restricting or throttling the air as it EXITS the actuation chamber. For spring-return applications, it is recommended to use a direct-acting 3/2 pilot valve, whereas double-acting can be achieved with a standard assisted valve. As your goal is to throttle only the exiting air, directional speed controls are recommended (see diagrams for spring-return and double-acting applications below.)
Spring return (left): The unidirectional flow control valve allows for no restriction of air volume when entering the actuator (open). Hence the valve will open quickly. When exhausting air is then channeled through the restrictor (closing), the closing speed is controlled.
Double-acting (right): The unidirectional flow control valves fitted on both parts of the actuator allow the control of both the opening and closing speed of the valve. This is achieved by inlet air volume, (which is not restricted) trying to displace air on the opposing chamber which is channeled through the restrictor. Slowing the displacement of the air in the opposing chamber results in the control of speed.
It is important that the pilot valve not be over-sized. It is extremely common to see over-sized pilot valves in the field, choked down to near-closed. Consider how much air your devices need, and the flow rate of our valves – remember too that not all valve flows were created equal; valves from reputable manufacturers often have double or more the flow of the “same sized” competitor valve.
Electrically operated process valve selection
Electrically operated valves are most commonly hard-coupled, with closing time directly proportional to the design. It is consequently necessary that the speed of operation be considered at the design stage. Generally the higher the torque, the slower the valve – designers may consider over-sizing the torque requirement to deliver a slower operation time. Some vendors also offer a ‘slow’ version, which will work well in eliminating water hammer.
This type of actuator provides higher torque, which allows these valves to close slowly, helping solve the issue of water hammer.
Mitigating water hammer effects in valve systems
Water hammer effects are extremely common, and best resolved by appropriate valve selection and system design. However, there are mitigating actions an engineer can take where an existing valve system is affected by water hammer.
Valve open and close speed
The most effective and achievable solution to water hammer is to vary valve open/close speed, to lessen or eliminate pressure effect. Slowing valve closure speed is particularly important.
Varying speed of solenoid valve closure
As discussed earlier, reputable solenoid valve vendors offer speed-adjustable valve options that can readily be installed in an affected system. The speed variation on these valves is arranged via a mechanical screw included in the design of the valve.
Where the existing valve does not have adjustable speed, it is possible to vary the flow rate of liquid through the valve to change the opening and closing speed. This is achieved through the use of restrictors and pipe steps, however great care is needed when attempting this as unanticipated ramifications may be felt. This type of system-wide adjustment should only be considered by a process engineer with broad understanding of the wider process.
Varying speed of pneumatic process valve closure
The simplest method to slow pneumatic process valve closure is the restriction of pneumatic air venting from the actuation chamber. By throttling air release, the valve’s action is proportionally slowed; the valve closes more slowly against
process flow and pressure effects are minimised. When throttling pneumatic air, only the actuation chamber exit should be varied; throttling air supply causes staggered or stepped operation.
Limitation of velocity change in the system
Another approach to water hammer elimination in existing systems is to minimise flow speed variations throughout the system. Similarly to localised speed change variation, this approach requires the holistic system understanding, to avoid unwanted repercussions.
Methods used to achieve velocity change limitation include:
- Use of variable speed drives (VSDs) to eliminate pressure build-up and spikes during system start and stop; this approach has other power benefits.
- Slow filling of systems that are emptied and filled.
- Use of non-return and check valves to appropriately segregate system zones and avoid pressure build-up.
Water hammer is quite common, and has the capacity to damage system instruments and pipework. It can however be eliminated in almost all cases, with the right selection of valve and consideration of the actuation method. As with any problem, it is better to avoid from the beginning than to try and resolve afterwards – although the methods above have been proven in cases where water hammer has arisen. It is strongly recommended that automated valve vendors be included in design phases from the beginning of any project.
Bürkert products that are available for the above applications:
Water hammer free solenoid valves
Speed adjustable solenoid valves
Angle pattern / globe valves
Rotary Valves, ball & butterfly
Rotary pneumatic actuators
Pneumatic pilot valves
Rotary electric actuators