Irene Pool
V.P. Legislative
May 2020

From the 2018 UPC Illustrated Training Manual, Chapter 6, WATER SUPPLY AND DISTRIBUTION: 609.10.1 Mechanical Devices. Where listed mechanical devices are used, the manufacturer’s specifications as to location and method of installation shall be followed.

Although water hammer is a subject usually left up to plumbing engineers, the effects of water hammer must be dealt with every day by plumbing contractors everywhere. Water hammer is easily recognized by the banging or thumping noise that is heard when valves are shut off. Although this is an easy way to recognize the problem, water hammer does not always make these telltale noises. Water hammer occurs when the flow of moving water is suddenly stopped by a closing valve. This sudden stop results in a tremendous spike of pressure behind the valve, which acts like a tiny explosion inside the pipe. This pressure spike reverberates throughout the plumbing system, rattling and shaking pipes, until it is absorbed. Normally, a sufficient pocket of air will absorb such a pressure spike, but if no pocket of air is present, expensive fixtures and appliances within the plumbing system will be damaged as they are left to absorb this pressure spike.

It used to be thought that an air chamber, or capped stand pipe, was an effective solution to controlling water hammer. However, within an air chamber, nothing separates the air from the water. It only takes a few short weeks before the air is absorbed into the water, leaving the air chamber waterlogged and completely ineffective against water hammer. Laboratory tests confirm that the air is depleted by simple air permeation and by interaction between static pressure and flow pressure. In the diagram shown in Figure 609.10.1a, notice the difference in water level between “Static Line Pressure” and “Postcycle Static Level.” One can easily see that this method of protecting against water hammer is ineffective.

The most effective means of controlling water hammer is a measured, compressible cushion of air which is permanently separated from the water system. The arrester model shown in Figure 609.10.1b employs a pressurized cushion of air and a two O-ring piston, which permanently separates this air cushion from the water system. When the valve closes and the water flow is suddenly stopped, the pressure spike pushes the piston up the arrester chamber against the pressurized cushion of air. The air cushion in the arrester reacts instantly, absorbing the pressure spike that causes water hammer.

Many other styles of water hammer arresters are manufactured. There are residential installation models and industrial models that are also represented in Figure 609.10.1b. Always consult the manufacturer’s installation and product manuals for the proper size and installation location for a water hammer arrester.