Some systems are very prone to this, and the use of a simple swing check valve can give severe water hammer. The closure of non-return valves can also cause water hammer. When it collides with the valve or pump, a severe hammer can occur. This suction can cause the liquid to flow backwards and the cavity collapses as it approaches the closed valve or stopped 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 can happen after a valve or downstream of a pump. 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. The pressure wave can travel through pumps, damaging the impellor and drive. This might cause the pipe to move off its supports or transmit the force to its anchors. 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 causes a shock wave which starts at the valve or pump and passes along the pipe, changing the fluid velocity as it goes. The most common cause is a valve closing too quickly, or a pump tripping or starting up suddenly. Water hammer is a shock wave passing down the pipe as a result of a sudden flowrate change. Its presence can also be revealed sometimes by unexpected opening of relief valves. 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. Slight water hammer can be detected by pipe movements, banging noises and pulsing flows. The risk to safety, assets and environment are obvious. In pipe bridges, collateral damage can occur. However under some circumstances, the pressure created can be many tens of bars, and forces on supports can be many tonnes, exceeding their specifications. Normally this pressure is small and the rate of change is gradual, and water hammer is practically undetectable. Increased pressure occurs every time a fluid is accelerated or retarded by pump condition changes or valve position changes. HOW DOES IT OCCUR, AND WHAT ARE THE CONSEQUENCES? It also outlines some of the ways to resolve the problems. 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 can cause pipes to burst and structures to collapse. It usually occurs in liquids, but it can occur in gases. 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 can cause burst pipes, damaged supports and pipe racks, and leakage at joints. It can cause very high pressures in pipes, very high forces on pipe supports, and even sudden reversals of flow. Water hammer occurs when the flowrate of fluid in the pipe changes rapidly.