a sharp, sometimes very high force and pressure load that is created when a valve is closed too rapidly in a flowing stream. The major force occurs behind the valve. Most common in a production well when sudden closure (slam closure) of a subsurface safety valve can create a load of over 50,000 lbs force in the tubing tensile/compressive loading. In an injector, the water hammer effects of a rapid shut-in are lower, but some effects may be seen on the formation.