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collapses the vapor bubbles. This imploding action attacks the pump components removing chunks of material from their surfaces and causes premature failure of the pump. In order for cavitation to occur, the local pressure must be at or below the vapor pressure of the liquid. When a fluid flows over a surface having a curvature, there is a tendency for the pressure near the surface to be lowered. There is a separation of fluid flow lines where there are different velocity regions. Between these fluid regions, turbulence can form which may cause bubbles to occur if the pressure is low enough. The collapsing of these bubbles can cause noise and vibrations. Sometimes, these pressure changes can be very dramatic and cause extensive damage to impellers, rotors, casings or shafts. If exposed for a sufficiently long time, pitting or severe erosion can occur. Cavitation generally occurs in the first stage of a multistage centrifugal pump, although second stages have also been found to be effected when the suction head is substantially reduced. With positive displacement pumps like the rotary screw, cavitation can also occur. For these pumps it is important to understand the characteristics of entrained and dissolved air with respect to the vapor pressure of the fluid medium. The rotary screw pump shows a greater tendency for cavitation when the total available pressure at the pump inlet is below atmospheric pressure. With both displacement and centrifugal pumps, cavitation can be identified and easily remedied. Many times the inlet piping arrangement can be modified which will cause flow patterns that alleviate the problem. 10.2.2 Vortexinq Vortexing in centrifugal pumps is caused by insufficient fluid height above the suction line entrance or excess fluid velocity at the suction line entrance causing a noisy pump operation and loss of fluid flow. Vortexing of the fluid in a suction sump or pit sounds a lot like cavitation problems and will cause excessive shaft deflection and damage to mechanical seals, bearings and the pump intake structure and piping. Vortexing problems are intermittent as the vortices form as opposed to cavitation which once started tends to be a constant problem. There are several possible causes of a vortexing problem: The pump running at a faster speed than specified A change in the flow or volume to the pump inlet The inlet line is restricted with contaminant solids Excess air in the liquid 10.2.3 Operating Environment The effect of the ambient temperature and altitude on performance is normally independent of the type of pump. Limits for satisfactory performance are established Pumps 10-10 Revision C

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