Causes of a Pulsating Water Hammer


When you shut off a faucet, you should never hear any clanging or banging. If you do, water hammer is present. A water hammer is very detrimental to pipes and fixtures. Only one cause exists for a water hammer: a sudden change in water pressure. When the pressure changes, pulsating or pressure waves are induced in the plumbing system. These waves go back and forth, causing damage in the process. Understanding the phenomenon of water hammer is the first step in eliminating it.

Induced Pressure Waves

  • When you open a faucet, the water pressure drops. When you close the faucet, the water is still trying to run. The water gets "crammed together" at the faucet. This induces a pressure wave that reflects back and forth between the faucet and the water supply valve. According to Dr. Joseph Evans of "Pumps and Systems Magazine," the speed of the traveling pressure wave approaches the speed of light.

Contributing Air Factor

  • Water in a plumbing system is seldom pure water. Rather, it has dissolved air in it. Dr. Evans explains that the dissolved air is a contributing factor. Water is incompressible, but a gas is. When the pressure wave hits the dissolved air, the air becomes compressed. When the wave passes, the air springs back, and the stored kinetic energy is released.

Contributing Machinery Factor

  • Machinery contributes to the formation of water hammer as well. Faucets can be opened and closed slowly to prevent water hammer, but a dishwasher or washing machine cannot. The water control valves open and close very quickly. When a valve opens, the pressure drops suddenly. When the valve closes quickly, an intense pressure surge is developed. The surge contributes to water hammer formation.

Effects of Pressure Waves

  • The pressure wave itself may last for only a split second. This is similar to a gun firing or a hammer striking a nail once. A pressure wave raises the pressure of the system to beyond its design limits. The Plast-o-Matic Valve Corporation points out that the wave can raise the pressure up to five times the designed pressure of the plumbing system. Dr. Evans notes that if the shock is strong enough, damage is done. You hear the aftershocks of the wave by the clanging noise in the pipe.

Curing Water Hammer

  • The cure for a water hammer problem is easy: You install devices called water hammer arresters, which act like shock absorbers in a car. They are small--about the same size and shape of a hot dog. Usually a plumbing professional determines the exact location of a water hammer arrester. The Jay R. Smith Manufacturing company recommends installing one on both the hot and cold water lines, especially near washing machines.


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