Water hammer is the audible noise resulting from the hydraulic shock that occurs when a valve shuts off the flow of water or any other liquid too quickly. The high-pressure spike or pressure transient produced in this situation can cause permanent damage to pipes and other equipment. A rather straightforward equation makes calculating the water hammer pressure quite easy.
The equation to calculate the pressure increase due to water hammer is:
PWH = PI +(0.07V*L)/t
Where:
PWH = water hammer pressure (in psi)
(0.07) = conversion constant
V = change in velocity of the liquid in the pipe (in ft/sec)
L = upstream pipe length (in feet)
t = valve closing time (in seconds)
PI = inlet pressure of the fluid (before water hammer) (in psi)
With an initial inlet pressure of 80 psi, closing a valve in 4 seconds with water flowing at 10 ft/sec in a 500-foot length of pipe results in a water hammer pressure increase of 87.5 psi. Added to the initial pressure, the total pressure is 167.5 psi. With the same parameters, except reducing the closing time to 0.5 seconds, the added pressure is 700 psi for a total pressure of 780 psi. Fast acting valves found in appliances such as washing machines or dishwashers can be a possible cause of this reduced closing time.
At these and higher pressures, the water hammer effect can easily damage tanks, valves, and appliances. Travelling at the speed of sound (over 4,800 ft/sec in 70°F water), the shock wave moves backwards until it hits a pump, a check valve, a filter tank, or some other solid object, and then forward and back until the energy dissipates.
The water hammer effect can also occur in wells at pump start up. In this case, an improperly installed or leaky non-return valve can create a partial vacuum in the downpipe. When the pump starts, water fills the vacuum at a high velocity and presses against the closed non-return valve and the stationary water column in the pipe causing the water hammer.
In situations where a water hammer suddenly occurs without changes in the plumbing, the water system’s air chambers could be blocked. Blockage can occur due to residue buildup from minerals found in water. When this occurs, the blockage will stop the air chamber from absorbing the pressure in the system and, as a result, the water hammer is heard.
In any case, the very high hydraulic pressure or vacuum development caused by the momentum of the moving fluid produces a very definitive hammering sound. With the audible signature created by water hammer, a microphone could sense its presence and provide an indication of its severity — if a human was not present to hear it. More directly, a pressure sensor in line with logging recorder could also provide the value of the water hammer while normally monitoring the line pressure for other abnormalities such as unusually high operating pressures or reduced pressure from leaks. This approach could be specifically used in a commercial or industrial environment where the water hammer could develop, and human observers do not immediately report its occurrence.
No matter how the water hammer is detected, by human observers or sensors, the next step is the most important — correcting the situation in a timely manner to prevent plumbing failures.
References
Hydrodynamic Design, Part 11: The Water Hammer – WCP Online
What does water hammering mean, and what causes it? | Grundfos
Featured image source: The Most Common Causes of Water Hammer (And How to Stop It) | Anchor Pumps
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