Shock arrestors are used to prevent water hammer. Water hammer is caused by shock waves running through the water system which create noise, or in severe cases - pipe movement within the system. Reverberations are a series of shock waves in quick succession. The pressure wave in such circumstances can be up to three times greater than the standing pressure. Water hammer occurs when the flow rate is suddenly changed e.g. when a valve is quickly closed; quarter turn lever operated taps or solenoid valves are two of the main culprits. Reverberations occur when system components have moving parts, which respond to an initial shock wave by trying to open or close. The RWC shock arrestor is a mini expansion vessel, which works by absorbing the shock wave from the fitting so that it does not travel through the system and cause noise. The closer the shock arrestor is installed to the fitting the better.