Water hammer occurs in a piping system when the flow is suddenly slowed down or stopped. For a water hammer to occur, the closing time of a valve for instance, must be less than the transit time of the resulting pressure wave to travel to the entrance of the pipe and back. The water hammer pressure rise is
in which ρ is the density of the liquid, vs the velocity of a pressure wave (a sound wave) in the pipe and Δv is the extinguished velocity [Moody ( 1990)].
For cold water vs is about 1400 m/s. In a typical pipe, the elasticity of the walls, in effect, reduces the vs by about 10%. A useful rule of thumb for estimating water hammer pressure is, for each m/s of extinguished velocity, the pressure rise is 106 Pa [Avallone (1987)].
If two phases are present, the velocity of sound in the two phase mixture is greatly reduced so that excess pressure due to a water hammer in such a flow is rarely a problem. Water hammer in steam systems is most likely to occur when a vapor bubble is trapped by cold water and condenses very rapidly. Filling or draining a pipe in which cold water replaces steam or steam replaces cold water very often leads to water hammers and damage to piping supports, valves, or the pipes themselves. This occurs because a steam bubble is sometimes trapped in cold water and condenses very rapidly, accelerating and decelerating a column of water very rapidly.
Water hammer is not a problem in properly designed and operated steam piping systems. Valves should be stroked slowly and cold liquid and warm vapor should never be allowed to contact each other in such a way that a steam bubble is trapped. However, during start-up, cold condensate is often found in pipes that are normally full of vapor. When vapor is admitted too rapidly into such a pipe, a violent bubble collapse can occur leading to damage to the piping system. Similarly, vapor can be trapped near a leaky valve releasing steam into cold, high pressure water so that a water hammer occurs when a pump, for instance, is turned on.
The following is a partial listing of the geometries and processes which lead to water hammer in improperly designed or operated steam systems [Chou (1990)].
Water is admitted into an almost horizontal, steam-filled pipe at a low enough velocity so that the pipe does not run full, a bubble is trapped, it condenses rapidly and causes a water hammer. See Figure 1 [Chou (1989)].
A steam-filled, closed-end pipe of any orientation is filled with cold water rapidly enough so that the liquid hits the end with sufficient velocity to cause a water hammer. See Figure 2 [Chou (1989)].
A vertical steam filled pipe recieves water from above which carries down vapor that condenses rapidly and causes a water hammer.
A horizontal pipe is provided with a source to cold water at one end and a source of steam at the other. For a low enough liquid velocity, a long tongue of cold liquid will extend into the steam, experience a transition to slug flow (due to the high, condensation induced relative velocity of the steam and water) which leads to a water hammer [Bjorge (1984)].
Figure 1. Sequence of events leading to a steam bubble collapse induced water hammer when a short, horizontal steam pipe is filled with cold water with Froude Number less than 1. (Froude Number is , where v is the liquid velocity and D the tube diameter).
Figure 2. Sketch of a short, horizontal steam pipe being filled with the Froude number greater than one. The sudden deceleration of the flow when the surge hits the end of the pipe causes the water hammer.
To design water hammers out of a system, incline all nominally horizontal pipes at least 2.8° from the horizontal, admit any cold water from the lowest point in the system, and keep all changes in velocity or pressure gradual enough so that vapor is not trapped and condensed rapidly.
Avallone, E. A. and Baumeister, T. III. (1987) Marks Standard Handbook for Mechanical Engineers, 9th edn. 3-71, McGraw-Hill.
Bjorge, R. W. and Griffith, P. (1984) Initiation of water hammer in horizontal and nearly horizontal pipes containing steam and subcooled water, ASME Journal of Heat Transfer, 106, 835-840.
Chou, Yuanching and Griffith, P. (1989) Admitting cold water into steam filled pipes without water hammer due to steam bubble collapse, Pressure Vessels and Piping, 156, ASME.
Moody, F. J. (1990) Introduction to Unsteady Thermofluid Mechanics, Ch. 2, John Wiley and Sons.