What is “Water Hammer”?

Water Hammer Effects for Plastic Valves and Strainers

A significant, nearly instantaneous pressure shockwave may be generated when a valve opens or closes too quickly, or when a pump starts with an empty discharge line or suddenly shuts down. This phenomenon is the result of the sudden change in velocity of the fluid flow in combination with the characteristics of the piping.

This shockwave is manifested by a series of hammer-blow-like sounds, often referred to as water hammer, which may have sufficient magnitude to cause catastrophic failure within the piping system. To avoid water hammer conditions, consider the following:

• Fluid velocities in excess of five feet per second in plastic piping systems increase the hydraulic shock effect resulting from the starting and stopping of pumps and rapid opening and closing of valves. Fluid velocity not exceeding 5–8 feet per second is considered safe and will minimize the effects of water hammer.
• Slow-closing actuated valves should be installed to control the speed at which valves open and close. They can be controlled electrically or pneumatically, eliminating the chances of human error.
• Install pressure relief valves to dampen the effects of water hammer and relieve excess pressure and flow.

The pressure rise created by water hammer is added to the nominal actual working pressure of the system. Please refer to Hayward’s online water hammer calculator to help ensure all your system components are rated for the maximum pressures that might occur.