When a valve is suddenly closed, shock waves are generated and cause damage to valves due to the high pressure caused by the mass of flowing water, which is so-called positive water hammer. On the contrary, when a closed valve is suddenly opened, it will also produce water hammer, called negative water hammer that is also destructive but is not as much as the former.
Although water hammer makes a lot of noise, the real damage is caused by mechanical failure. Because the kinetic energy is converted rapidly into static pressure in pipeline, water hammer can cause pipe to break or damage pipe supports and pipe joints. For valves, water hammer can produce vibration through valve core, which may lead to failure of valve core, gasket or packing.
For valves, the way to prevent water hammer is to prevent any sudden pressure change in the system, including slowing down the closing speed of the valve itself or providing a greater degree of tension and rigidity when the closure element approaches the valve seat. To prevent pressure fluctuation, the closing speed of the valve should change evenly.
In some cases, when the quick open characteristic is used, it may be required to change the former to an equal percentage characteristic. Because control valves must be used for throttling flow when approaching the seat, actuators with enough thrust output should be used (e.g. pneumatic piston actuators or hydraulic actuators), which may prevent water hammer.
Water hammer can also be reduced with a pressure relief valve or a buffer bucket. In addition, gas can be injected into the system, which can reduce the density of the fluid.