Fluid Mechanics Solutions

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Vapor Formation.

 

Aside from damaging equipment attached to piping systems, water hammer may cause the pipe to fail from excessive pressure or to fail by collapse due to pressure less than atmospheric. The phenomenon of column separation may occur within a piping system where boundary conditions are such that the pressure is reduced. A reduction in pressure at the upstream end of a pipe with positive flow causes a negative pulse to be transmitted down the pipe, thereby reducing its velocity; the fluid downstream continues at its steady velocity until the wave arrives. This difference in velocity between portions of the flow tends to put the liquid column into tension, which commercial fluids cannot withstand. When vapor pressure is reached, a vapor cavity forms in the pipe. With a pipeline of varying elevation, column separation usually forms near one of the high points in the profile. This cavity will tend to stay on the downstream side of the high point, with the liquid flowing below the cavity. After the cavity is formed it may continue to grow in volume until the flow velocities of the two columns become equal. Usually, the upstream column will be accelerated and the downstream column decelerated by the boundary conditions, and the upstream column overtakes the downstream column. If the difference in velocity at the instant of collapse of the vapor cavity is DV, a pressure increase of may be expected. This pressure increase may be of sufficient magnitude to rupture the pipe or the pump.