Abstract: When steam or condensable vapor contact with its colder condensate or condense, the volume of condensate or condenses is less than 1/100 to 1/1 000 of the volume of original steam or condensable gas with same mass, which lead to the original gas-phase region changing to the low-pressure region. Due to the pressure difference, the surrounding condensate or condenses descend on to this phase at a very high speed, producing a large instantaneous pressure and spreading out the parts of the pipe with condensate water, causing water hammer. Water hammer seriously affects the industrial production and causes safety accidents, which should be highly emphasized. By combining with several specific examples, this study illustrates the hazards of water hammer accidents, analyzes the causes of the accidents, and puts forward the correction measures. For the system of steam and condensate, the pressure and damage induced by water hammer is a function of steam pressure, condensate subcooling degree, bubble size and quantity of noncondensable gases. In general, the induced pressure and damages increases with the increase of the first three items, and decreases with the increase of the latter item. Steam should not be sent to the pipeline containing the cold condensate. When a water hammer accident happens, the steam feed valve should be closed firstly, and then the condensate discharge valve should be opened. For the system of condensable vapor and condensate, water hammer caused by the rapidly condensation of condensable vapor should be avoided in design or operation. Especially for the tower pressure hot vapor bypass control scheme, the subcooled condensate and the hot bypass vapor should be introduced into the liquid and gas phase of the reflux drum, respectively.