Water hammer occurs when the flowrate of fluid in the pipe changes rapidly. It is also known as “surge flow”. It can cause very high pressures in pipes, very high forces on pipe supports, and even sudden reversals of flow. It can cause burst pipes, damaged supports and pipe racks, and leakage at joints. Water hammer can occur for any fluid, in any pipe, but the severity varies depending on the detailed conditions of the fluid and the pipe. It usually occurs in liquids, but it can occur in gases. It can cause pipes to burst and structures to collapse. This article will describe the conditions most likely to lead to water hammer problems and the issues that pump and pipe designers and operators can face. It also outlines some of the ways to resolve the problems.

Increased pressure occurs every time a fluid is accelerated or retarded by pump condition changes or valve position changes. Normally this pressure is small and the rate of change is gradual, and water hammer is practically undetectable. However under some circumstances, the pressure created can be many tens of bars, and forces on supports can be many tonnes, exceeding their specifications. In pipe bridges, collateral damage can occur. The risk to safety, assets and environment are obvious. Slight water hammer can be detected by pipe movements, banging noises and pulsing flows. Serious water hammer gives the same effects but these might be large enough to cause serious damage, and might only occur once! Pipe systems that show the characteristics that can lead to serious hammer should be analysed by computer software, especially if hazardous chemicals are being carried in them. Its presence can also be revealed sometimes by unexpected opening of relief valves.

Water hammer is a shock wave passing down the pipe as a result of a sudden flowrate change. The most common cause is a valve closing too quickly, or a pump tripping or starting up suddenly. This causes a shock wave which starts at the valve or pump and passes along the pipe, changing the fluid velocity as it goes. This is the cause of high pressure. If the wave is sharp and it passes through pipe bends, the pressure step change can cause out-of-balance forces which move the pipe. This might cause the pipe to move off its supports or transmit the force to its anchors. The pressure wave can travel through pumps, damaging the impellor and drive. Water hammer can also be caused by cavitation due to the pressure dropping below the vapour pressure, and then the bubbles collapsing as the pressure swings back up. This can happen after a valve or downstream of a pump. As the valve closes or the pump trips, the pressure downstream can fall to a level that the fluid boils, creating a vapour cavity. This suction can cause the liquid to flow backwards and the cavity collapses as it approaches the closed valve or stopped pump. When it collides with the valve or pump, a severe hammer can occur.

The closure of non-return valves can also cause water hammer. Some systems are very prone to this, and the use of a simple swing check valve can give severe water hammer. Some companies manufacture non-return valves which minimise water hammer caused by their operation. The formation of cavities in the high points of pipes due to exceeding the barometric height of vertical legs can also cause water hammer as flow is restarted. It’s not possible to give simple, infallible rules for spotting water hammer potential. Computer programs exist that allow pipe systems to be modelled and any potential for water hammer problems to be revealed. In experienced hands they can also be used to find the best solution to any such problems. Simple hecks can be done by hand calculation, and some vendors have nomographs to help predict hammer and design suitable alleviators. However most systems need good computer software to do this accurately.