When a valve is closed quickly in a line containing flowing liquid, the inertia of the flowing liquid increases the pressure at the valve. This effect is called surge, and the increase in pressure is called surge pressure. Surge pressure can cause such extremely rapid changes in pressure that it results in the metallic percussions commonly called water hammer. The surge pressure wave then propagates back up the line and can cause mechanical damage. High fluid velocities and long runs of pipe increase the possibility of water hammer.
To minimize surge pressure peaks, specify devices to control the closing rate of the valve and test them during the FAT. Normally, for quarter-turn valves, the speed is controlled by an adjustable needle valve.