Water hammer

This phenomenon commonly occurs when a valve closes suddenly at an end of a pipeline system and a pressure wave propagates in the pipe.

This pressure wave can cause major problems, from noise and vibration to pipe rupture or collapse.

[4] In 1796, French inventor Joseph Michel Montgolfier (1740–1810) built a hydraulic ram for his paper mill in Voiron.

In domestic plumbing this shock wave is experienced as a loud banging resembling a hammering noise.

For example, the water traveling along a tunnel or pipeline to a turbine in a hydroelectric generating station may be slowed suddenly if a valve in the path is closed too quickly.

If there is 14 km (8.7 mi) of tunnel of 7.7 m (25 ft) diameter full of water travelling at 3.75 m/s (8.4 mph),[20] that represents approximately 8,000 megajoules (2,200 kWh) of kinetic energy.

[23] It is possible to theoretically explain the 2 millisecond duration 130 bar overpressure peaks with a special 6 equation multiphase thermohydraulic model,[24] similar to RELAP.

Water hammers have caused accidents and fatalities, but usually damage is limited to breakage of pipes or appendages.

Pipelines transporting hazardous liquids or gases warrant special care in design, construction, and operation.

The pulse comes about due to Newton's laws of motion and the continuity equation applied to the deceleration of a fluid element.

where When the valve is closed slowly compared to the transit time for a pressure wave to travel the length of the pipe, the elasticity can be neglected, and the phenomenon can be described in terms of inertance or rigid column theory: Assuming constant deceleration of the water column (dv/dt = v/t), this gives where: The above formula becomes, for water and with imperial unit, For practical application, a safety factor of about 5 is recommended: where P1 is the inlet pressure in psi, V is the flow velocity in ft/s, t is the valve closing time in seconds, and L is the upstream pipe length in feet.

[28] Hence, we can say that the magnitude of the water hammer largely depends upon the time of closure, elastic components of pipe & fluid properties.

This is most likely to occur at specific locations such as closed ends, high points or knees (changes in pipe slope).

[32] Most water hammer software packages use the method of characteristics[27] to solve the differential equations involved.

This method works well if the wave speed does not vary in time due to either air or gas entrainment in a pipeline.