Water Hammer: The “Belly Flop” of Piping Systems

Mike Lehner Fire Protection Engineerby Mike Lehner, P.E.

Water is a seemingly harmless thing, but in reality, it is quite a force to be reckoned with. Have you ever done a belly flop in a swimming pool and felt the pain afterwards? While the pain of a belly flop results from the surface tension of the water resisting your impact, water’s impact against other things can cause damage. In water piping systems, this impact is called “water hammer.” In some older residential plumbing systems, you can hear the pipes “clank” when you shut the water off fast. That is water hammer. Water hammer is a common problem in water systems with long runs of piping, including plumbing and fire protection systems. Water hammer can be a significant issue in large systems, and it is a real concern that plumbing and fire protection engineers must address.

Water’s Power

Water weighs 62.4 pounds per cubic foot, or 8.3 pounds per gallon. That can be a lot of weight to move; ever lift a gallon of milk? It’s pretty heavy for its size! Once water is moving, it can take a significant force to slow it down or stop it altogether.

Now imagine you have an eight-inch diameter pipe, with water traveling through it at 10 feet per second. That equates to a volume of about 1,500 gallons of water per minute. Now imagine that this section of pipe is 1,000 feet long. At that length, there is about 2,600 gallons, or about 21,600 pounds of water, in this pipe. That is equivalent to a medium sized truck traveling at about 7 miles per hour.

The Pain in the Piping System

If someone quickly closed a valve, all that water mass would have to stop immediately, because water is non-compressible. When a valve is shut quickly, it creates a shock wave which travels back and forth through the piping system until dissipated. This can exert thousands of pounds of force on the valve and the piping. This shock wave from water hammer can be very destructive. It can result in water pressure spikes above the design limits of the system. If this happens, then unpredictable results occur, including broken piping, fittings, and sprinkler head failures. System downtime caused by broken piping can be very costly as well. Not to mention water damage from flooding and damage from flying parts near the point of failure. What can we do?

Eliminating the “Clank” with Air

There are several ways to alleviate the problem. One of them is with air. Air is compressible and can act as a shock absorber to dampen and eliminate the shock wave that develops when water stops quickly. This can be done with an empty pipe or other device that has air inside it and is connected to the system. This air cushion can absorb and dissipate a shock wave.

Where a standpipe fire suppression system has a long supply main that must be filled with water, this creates conditions where water hammer could occur.  The design may include a water hammer arrestor in the system; this is a bladder with compressed air in it. A dead-end section of piping might be used to allow the water “wave” to slow down prior to hitting the end of the piping system.

On a recent bridge project that I worked on, the standpipe system required air release valves to aid in filling the system with water. A small air release valve was located at the end, with a second larger one placed 400 feet back from the end of the system to allow the remaining length of piping to act as the water hammer arrestor. After the water passed the large air release valve, it began to “slow down” since the air in the system was released more slowly.

Many fire protection systems have air trapped in the system, which is fairly normal, and also can serve to passively mitigate water hammer. In the case of a dry pipe fire suppression system, the piping is filled with compressed air, which dampens water hammer while the system is tripped or filling with water. Deluge fire suppression systems have open sprinkler heads that can relieve pressure surges as well.

Slowing it Down

Water hammer can be reduced by simply slowing down the events that create it in the first place by using slow-acting valves and check valves. This can be simply done mechanically, with slow actuators, or electronically, delaying or slowing valve closure with electronic controls.  Where a fire pump is provided, soft start fire pumps are another option, where the fire pump slowly comes up to speed to fill the pipes with water, instead of instantaneously. Finally, a rupture disc can be provided as additional insurance where there is a pressure surge beyond the design pressure of the piping. With rupture discs, the failure point can be controlled and located where less damage could occur and also be replaced fairly quickly to bring the system back on line.

Save Your Pipes!

If not properly considered during the design process, water hammer can cause considerable damage to a plumbing and fire suppression system. There are several design methods that can mitigate water hammer, and it’s important to have the right solution for the type of system, as well as where and how it is used. The next time you hear pipes rattling in an old house, maybe close the valves a little slower!

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