Water Hammer in Piping Systems

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You most commonly experience water hammer at home. You open your faucet wide to fill a pot or bathtub, then shut the faucet valve quickly. You hear a banging in the pipes that echoes through the whole house or apartment.

What happens is that a flow of liquid in your home’s water pipes was suddenly stopped — it “hit a wall” when you closed that valve. The impact of the water against the closed valve was transmitted as shock waves back through the pipes, smacking into the inside of the pipes at each turn, until the shock waves lost their energy and dissipated completely.

In industrial systems, the pipe sizes are larger, the flow rates are higher, and the shock waves created are more powerful, all leading to the possibility of significant damage both to piping and to valuable connected components.

Consequences of water hammer

The consequences of water hammer can range from mild to severe. The sometimes-deafening noise associated with water hammer can be a source of distress and concern, especially if people are working nearby when it occurs.

Repeated occurrences of water hammer aren’t just an annoyance, however. Water hammer can blow out gaskets which can cause leaks, and seriously damage pipelines, expansion joints, piping support systems and all the other components of a system (pumps, flowmeters, pressure sensors etc.). The pressure spikes associated with water hammer can easily exceed 5–10 times the working pressure of the system, placing a great deal of stress on system components. This can lead to immediate or fatigue failures, such as pipe wall cracks and sudden ruptures. Repairing or replacing damaged pipeline components and equipment can involve steep costs. If a spill results in an environmental issue, cleanup costs can add up.

Most significantly, water hammer is considered a safety hazard. The extreme pressure of water hammer can blow out gaskets and cause pipes to rupture suddenly. People in the vicinity of such an event could be seriously injured.

Causes of water hammer

The most common cause of water hammer is either a valve closing too quickly or a pump shutting down suddenly.

Water hammer, also known as hydraulic shock, is, in fact, the momentary rise in fluid pressure in a piping system when the fluid is suddenly stopped. As Sir Isaac Newton observed, an object in motion tends to stay in motion unless acted upon by another force. The momentum of the fluid traveling in its forward direction will work to keep the fluid moving in that direction.

Sudden valve closure is most often associated with solenoid valves and automated quarter-turn types of valves. A simple solution is to avoid solenoid valves and close auto-mated quarter-turn valves more slowly. This works in many cases but not all. For example, emergency shutdown valves need to close quickly, so other solutions may be necessary for these types of applications.

The other most common cause of water hammer is sudden pump shutdown. Multiple pumps feeding into a vertical pipe run or a common header, as in cooling tower applications or mine dewatering, either need to be shut down slowly, or need to have in-line silent check valves installed immediately after each pump. Silent check valves can be extremely effective in reducing and often eliminating water hammer.

Water hammer can also result from improper valve selection, improper valve location, and sometimes poor maintenance practices. Certain valves, such as swing check valves, tilting disc check valves, and double-door check valves can also contribute to water hammer problems. These check valves are prone to slamming because they rely on reversing flow and back pressure to push the disc back onto the seat to close the valve. If the reverse flow is forceful, as

in the case of a vertical line with normal flow upwards, the disc is likely to slam with a great deal of force. The resulting shock can damage the alignment of the disc such that it no longer makes full contact with the seat. This leads to leaks that, in the best case, undermine the efficiency of the system. In the worst case, this could do serious damage to other piping system components.

A spring-assisted, silent check valve, because its spring helps to close the valve, shuts before reverse flow is fully developed, and both limits impact of the closing disc and restricts any downstream shock wave from continuing back upstream of the check valve.

Predicting water hammer pressure spikes

It is possible to estimate the magnitude of water hammer pressure spikes based on detailed knowledge of the piping system and the fluid. The actual force of water hammer depends on the flow rate of the fluid when it is stopped and the length of time over which the flow is stopped. For example, consider 100 gallons per minute of water flowing in a 2-inch pipe at a velocity of 10 feet per second. When the flow is quickly brought to a halt by a fast-closing valve, the effect is hundreds of pounds of force slamming into a barrier (the valve disc). If the flow is stopped in less than a half second (which might be the closing speed of the valve), then a pres-sure spike over 100 psi above the system operating pressure can be generated.

The equation for calculating the potential magnitude of the spike is:

p = (a x ∆V) / (g x 2.31) where,

p is the pressure generated

∆V is the change in fluid flow velocity a = acoustic velocity in the media

g = gravitational constant = 32.2 ft/s2 An example is:

a = 4864 feet per second at ~68°F

∆V = 5 feet per second p would be 327 psi

This value is assuming instantaneous valve closure.

Valve closure time

Ensuring that a valve closes gradually can mitigate the effects of water hammer. The basic formula for an absolute minimum safe valve closure time is related to the length of straight pipe through which a pressure wave can travel:

T = 2L/a where,

T = minimum closure time in seconds

L = length of straight pipe between the closing valve and the next elbow, tee, or other pipeline structure

a = 4864 feet per second at ~68°F

For water and 100 feet of straight pipe to the next fitting, T= 41 milliseconds minimum closure time

Solutions to water hammer

There are many ways to mitigate the effects of water hammer, depending on its cause. One of the simplest methods of minimizing damage from water hammer caused by hydraulic shock is to train and educate operators. Operators who learn the importance of opening and closing manual or actuated valves properly can take precautions to minimize the effects. This is particularly true for quarter-turn valves such as ball valves, butterfly valves, and plug valves.

Normally the pressure wave of water hammer is dampened or dissipated in a very short amount of time, but the pres-sure spikes can do enormous damage during that brief period.

Piping design considerations

Water hammer arrestors provide a point of relief for pressure spikes caused by water hammer. These piping system components reduce the characteristic noise and damaging stress on the pipeline system by acting like a shock absorber, dissipating pressure waves in a small branch connection with an air cushion. When sized and installed properly, water hammer arrestors can be an effective solution to water hammer.

On the other hand, pumps that output into a long run of vertical pipe should be avoided. The vertical leg should either be minimized, or silent check valves installed close to the pump. Hydraulic shock resulting from the sudden closure of swing check, tilting disc, and double-door check valves can be remedied by exchanging these valves with silent or non-slam check valves. Silent check valves close upon the decrease of the differential pressure across the closure member of the valve as flowrate decreases, rather than closing from reverse flow. Thus, they are far less likely to slam shut.

System designers must be familiar with best practices and industry standards for minimizing water hammer, such as using slow-closing valves when appropriate, knowing optimal valve locations within a piping system, and giving special piping design considerations for high-operating pressure systems.

When piping systems are properly engineered, the likelihood of water hammer occurring is greatly reduced or even eliminated. In systems that are already in place, the damaging effects of water hammer can be limited in a number of significant ways, such as installing water hammer arrestors, relocating check valves out of vertical lines, installing silent check valves as a primary line of defense and ensuring operating procedures for quarter-turn valves have a slow closing rate. Note that the closure time in automated systems should be at least 10 times what is calculated in the T=2L/a formula.

Conclusion

Water hammer has been studied for many years. Some of the founding research dates back to the late 19th century, and research continues today. Many major universities in the United States, the U.K., and the Netherlands, as well as well-respected valve companies, have authored articles on the comparison of various styles of check valves and their installed dynamic characteristics.

This article only scratches the surface of the subject of fluid transients by exploring some of the causes and solutions of what we commonly call water hammer. Solutions to deal with water hammer problems can be quite costly, and, as always, an ounce of prevention is worth a pound of cure. Pumps feeding into vertical lines or common headers and rapid valve closures can all be avoided during initial system design. Once piping is in place and plant operation is underway, it is more challenging to find ways to eliminate or mitigate water hammer and its effects.

Most manufacturers of in-line silent check valves understand water hammer very well and have engineers on staff who can help. They can be the best source of knowledge when it comes to finding the right solution.

Next time you fill a pot or bathtub, close the faucet gradually. You’ll have a quieter experience — and may prevent damage to your home’s water pipes.

ABOUT THE AUTHOR

Paul Anderson is Director of Engineering at DFT Valves. Reach him at panderson@ dftvalves.com.

Water Hammer in Piping Systems