16 November 2010 | DW&BP16 November 2010 | DW&BP| DW&BP

In our line of work, we fi eld questions

from contractors and technicians concerning

repairs, installations, and general backfl ow prevention practices.

We’d like to share some questions that

we receive as well as our answers. Everyone

has different opinions on these subjects and

we would like to hear yours.

Contact us with questions and ideas via email at: imark@

backfl owparts.com or mail us at

American Backfl ow Products Co., PO Box

37025, Tallahassee, Florida 32315.

The Repair Guys

MARK INMAN

QuestionSometimes the relief valve on an RPA will discharge periodically due to water hammer. Can you explain what water hammer is, how to control it, and how it affects backfl ow assemblies?

MARK

Water hammer is common in any piping system that uses valves to control the fl ow of water. Most customers will identify water hammer simply as “banging pipes”. The customer will usually consider the rattling and banging of pipes as annoying but not necessarily dangerous. Water hammer is a destructive high-pressure surge or shock caused by the quick closure of valves or fi xtures within a piping system. This surge in pressure, also known as a pressure spike, can cause damage to system components including backfl ow preventers, water heaters, pressure reducing valves, washing machine hoses, pipe, fi xtures, and fi ttings. So let’s take a closer look at how and why water hammer occurs.

DOUG

We know that water hammer is an abnormal surge or spike in system pressure, but where does it come from and how does it get started? It is important to understand that water is a non-compressible liquid and has kinetic energy, or momentum, as it fl ows. As water fl ows through a piping system it must

dissipate its energy. Normally, the friction of the pipeline or the water discharging will dissipate the energy slowly and no shock is caused to the system. When the fl ow of water is stopped suddenly, the energy has no time to dissipate. When this occurs, a pressure wave will travel back through the system trying to fi nd a point of relief.

MARK

This pressure wave is what causes all the problems. If not controlled, the pressure spikes can reach as high as 300-400 psi. Many times this pressure wave can be absorbed back into the system water supply. If the system has a backfl ow preventer installed, it is considered closed and the pressure wave can bounce back and forth in the piping like a pinball. Every time you hear the pipes bang-ing, that is the pressure wave hitting a barrier in the line. The damaging pressure spikes affect not only the valve that is closed, but the entire system. The best way to protect the pip-ing system is to eliminate or fi x the problem that is causing the hammer. If it is not possible to eliminate the problem, then we must fi nd a way to control or absorb the pressure wave.

DOUG TAYLOR

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November 2010 | DW&BP 17

The Repair Guys

The Repair Guys

DOUG

Water hammer can normally be controlled by an “air chamber” installed within the piping system. In the early days an air chamber could be handmade on the job. The air chamber was simply a capped piece of pipe or tubing that was installed on a tee on the supply line. The chamber was approximately 12” long and the same diameter as the supply line. The air chamber would have a pocket of air that would compress and absorb the pressure spikes. The problem with the simple air chamber is that it could lose air charge and become waterlogged very quickly.

Most manufacturers now offer what is called a hammer arrestor. This mechanical device has male threads and can be installed on a tee in the supply line. The hammer arrestor has a cushion of air or gas that is sealed to keep the water and gas separated. A sealed piston is used to make the separation and maintain the pressure charge. The piston floats up and down inside the housing to absorb the pressure wave. The hammer arrestor should be installed upstream and as close to the source of shock as possible.

Calculated volume of gas

Piston at rest

Water flow

Shock wavedissapated

Shock wavestarts with quick valve closure

Quick closing valve

Piston absorbsshock wave

Watts Series LF15 Lead Free Water Hammer Arrestor

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