Water hammer is a water pressure surge that occurs inside water pipes.
Water hammer occurs in water supply piping when a valve or faucet shuts quickly. The flowing water suddenly hits the closed valve or faucet and a wave bounces back. This bounce back continues until the water hits a point of impact and the energy from the wave of water is distributed more evenly in the piping system. It is important to note that many valves, fittings, and plumbing fixtures are designed to operate and to resist damage up to 150 pounds per square inch. This gives a margin of safety against unexpected pressure increases since 60 pounds per square inch is the average pressure found in a city water system.