an effect, also known as water hammer, in which a pressure wave can be generated behind a rapidly closed valve. The pressure wave travels at sonic speed, reflecting off of the pipe end or the bottom of the well and returning to the valve. If the valve is closed before the wave returns, a hydraulic impact is produced on the valve. Extreme cases are seen with slam closures of sub surface safety valves. Much smaller effects may be produced in front of the valve in a few cases.