Understanding and Controlling Water Hammer in a Pumped Water System
We've all experienced water hammer at home -- the shower valve is closed abruptly, or the sprinkler system valve closes at the end of the cycle, or the clothes washer fill cycle ends -- and bam! All the pipes in the house rattle. Any time you have a high flow velocity, particularly in a long pipe run, and there is a sudden change in velocity, water hammer can occur.
The resulting pressure wave is transmitted throughout the system, subjecting every component in the system to pressures approaching 10 times the normal system pressure. The pressure returns to normal only when it is dissipated by friction loss, pipe expansion, or when something gives -- a valve or section of pipe. Let's take a closer look at the causes of water hammer and consider some remedies.
For water hammer to occur, several conditions must be present -- sufficient flow velocity, a sufficiently long pipe run, an abrupt change in velocity and a rigid piping system. Regarding flow velocity, it generally is agreed that water flow velocities below 5 feet per second (fps) preclude the possibility of water hammer. As to an abrupt change in velocity, it can be either rapid acceleration (pump start/valve opening) or rapid deceleration (pump stop/valve closure).
This first formula is used to calculate the velocity change timeframe below which water hammer occurs, based on the length of the pipe run. The second formula offers a means for calculating the maximum pressure surge resulting from water hammer in a rigid piping system.
The formula used to calculate the critical time factor that is the minimum time duration for a valve to open or close, or for a pump to start or stop, without causing water hammer, is as follows:
Critical time factor in seconds: Tc = 2L/S
Where: L = length of pipe in feet
S = Speed of sound in water (4,860 fps)
For example, if we have a 400-foot pipe run from the well to the house, the critical time factor would be:
Tc = 2 x 400 / 4,860
Tc = 0.16 seconds.
This means as long as the pump takes less than 0.16 seconds to start, or if a valve in the house is closed in less than 0.16 seconds, your pressure tank better be in good working order. If it is water logged, you may have a water hammer every time the pump starts or a valve is closed.
What then would be the worst case peak pressure caused from water hammer? The Instrument Society of America suggests the following formula for a rigid piping system based on the assumption that the shock wave will travel at the speed of sound, which is 4,860 fps for water. Of course, no piping system is absolutely rigid, so the actual peak pressure would be a little less than these numbers.
Ps = Po + (64V x SG)
Where: Ps = Maximum pressure surge in psig
Po = Normal system operating pressure in psig
V = Velocity of fluid in fps
SG = Specific gravity of fluid (1 for water)
Ps = 50 + (64 x 7.5 x 1) = 537.5 psig
Now that we know how to calculate the severity of the water hammer problem, how can we protect our system? Start with a good system design. Size the piping such that the velocity does not exceed 5 fps. In the above example, if 1 1/4-inch pipe had been used instead of 1-inch, the velocity at 20 gpm would have been on the order of 4 fps verses 7.5 fps, and water hammer would not have been an issue. Also, make sure the pressure tank is checked regularly and, at the first sign of air loss, fix the problem or replace the tank. Make sure there is not a check valve between the house and the pressure tank. You want any shock waves generated in the house to expand into the pressure tank, and a check valve would prevent that from happening.
Consider using a flow control valve to reduce the peak flow velocity to below 5 fps. Consider installing a stand pipe or small surge tank near the source of the water hammer. Consider replacing 1?inch turn ball valves with gate valves, which open more slowly, or train the ball valve users to open and close them slowly.
In larger systems, pilot-operated flow and pressure control valves (Figure 1) can control the speed at which the velocity changes, be it on the pump end or in the distribution system. Electronic soft starts and variable speed drives are another good solution to water hammer caused by pumps because they can be programmed to ramp up the pump speed slowly.
Whether it be a small residential water system or large commercial piping system, water hammer can be a major problem. With an understanding of the factors contributing to the problem and access to the necessary tools, it is possible to tame this insidious gremlin and provide your customer with a quiet, long-lasting pumped water system.