Water hammer is defined as “a pressure surge or wave caused when a fluid (usually a liquid but sometimes also a gas) in motion is forced to stop or change direction suddenly (momentum change). A water hammer commonly occurs when a valve closes suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe. It is also called hydraulic shock.”
When improperly drained of condensate in a high pressure steam main fills with condensate and completely surrounds the steam, an implosion takes place causing devastating water hammer.
Draining condensate and keeping it away from the steam by using proper steam trapping equipment will prevent this from happening.
The following video, courtesy of Spirax Sarco USA, dramatically demonstrates the principle behind water hammer and its potentially devastating effects.