Waterhammer

Engineers dealing with hydraulics are quite familiar with waterhammer phenomenon. There are different causes for waterhammer but the one that is related to machinery engineers is water hammer in pumps discharge lines because of a power failure.

When there is an unwanted shutdown of the pump, its speed is reduced quickly. This leads to a lower flow passing through the pump and a lower head developed by it. At some point, because of the higher discharge line head, the flow reverses toward the pump. This specifically happens for electric motor driven pumps as electric motors require no flywheel and their energy-to-mass ratio is high. Because of this, the reduction in the pump speed is very quick and so would be the change of flow and head.

If there is a check valve which closes soon after power failure, a shock wave is produced at the valve which then travels through the line, expands it and can damage it. For instantaneous closure of the check valve, pressure head generated at it can be calculated from Joukowski's law;

In which "a" is shock wave velocity, "g" is acceleration due to gravity and "∆V" is change in fluid velocity. This equation is presented graphically below

Waterhammer determination chart [Source: "A Practical Approach to Water Hammer Prevention" by Parco Engineering corporation, Medfield, Massachusetts]

In case of no check valve or check valve failure, flow goes back to the pump, pump speed becomes zero and then begins to rotate in the reverse direction. Both the head produced at the pump from back flow and possibility of pump over speeding can cause damages.

The most detailed analysis of waterhammer can be found in "Waterhammer Analysis" by John Parmakian. Chapter XI of this book is dedicated to waterhammer caused by pump failure. It states that "In order to determine the transient hydraulic conditions at the pump and discharge line subsequent to a power failure at the pump motor, three effects must be considered; namely, the pump and motor inertia, the pump characteristics and the waterhammer wave phenomena in the discharge line"; Then discusses each of these effects in details.

There is also a "Waterhammer" section in Karassik's pump handbook which is also written by John Parmakian. Discussions of this section seem less mathematical and theoretical and more operational and practical. It discusses some factors affecting waterhammer such as high and low-head pumping systems, discharge line profile, pipeline size, number of pumps, flywheel effect, specific speed of pumps, etc. Some of the important points of these discussions are:

• If there is a malfunction at one of the pumps or check valves, a multiple pump installation on each discharge line would be preferable to a single pump installation because the flow changes in the discharge line due to such a malfunction would be less with multiple pumps;
• When a simultaneous power failure occurs at all of the pump motors, the fewer the number of pumps on a discharge line, the smaller the pressure changes and other hydraulic transients;
• A method for reducing the waterhammer effects in pump discharge line is to provide additional flywheel effect (WR²) in the rotating element of the motor;
• The radial-flow (low specific speed) pump will produce the highest head rise upon a power failure if the reverse flow is permitted to pass through the pump;
• During a power failure with no valves, the highest reverse speed is reached by the axial-flow (high specific speed) pump.

There are some solutions for preventing waterhammer damages such as check valves, controlled valves, surge suppressors, etc. Of course the most conventional solution is still surge tanks; although it may not be cost-effective for high-head pumping installations.