What is Water hammer? Water hammer in a steam system occurs when water inside of a pipe or vessel is accelerated and forcibly impacts a piping elbow, transition, or downstream piece of equipment. This results in a knocking sound similar to hitting the pipe or vessel with a hammer, hence the name water hammer. The issue is that these slugs of water can be accelerated to very high speeds and the impacts that cause these hammer sounds can contain a lot of force. This can damage equipment or cause piping to fail, which is dangerous to any operators in that area.