Cavitation

To boil any liquid, there are always two choices; increasing liquid temperature or decreasing its pressure. As the liquid temperature increases, its molecules receive more energy and at a certain temperature (boiling temperature or saturation temperature) the molecules obtain very high energy that allows them to get free from the liquid (evaporate). On the other hand, decreasing the liquid pressure allows the molecules to get free although still having the same energy (at the same temperature). This means we can boil water at the room temperature by only decreasing its pressure. The pressure at which the liquid evaporates at the working temperature is called the “vapor pressure”. The vapor pressure of water at 20o C is 0.023 bar (always absolute value). The flow area at the eye of the pump impeller is usually smaller than either the flow area of the pump suction piping or the flow area through the impeller vanes. When the liquid being pumped enters the eye of a centrifugal pump, the decrease in flow area results in an increase in flow velocity accompanied by a decrease in pressure. The greater the pump flow rate, the greater the pressure drop between the pump suction and the eye of the impeller. If the pressure drop is large enough, or if the temperature is high enough, the pressure drop may be sufficient to cause the liquid to flash to vapor when the local pressure falls below the saturation pressure for the fluid being pumped. Any vapor bubbles formed by the pressure drop at the eye of the impeller are swept along the impeller vanes by the flow of the fluid. When the bubbles enter a region where local pressure is greater than saturation pressure farther out the impeller vane, the vapor bubbles abruptly collapse. This process of the formation and subsequent collapse of vapor bubbles in a pump is called cavitation. Cavitation in a centrifugal pump has a significant effect on pump performance. Cavitation degrades the performance of a pump, resulting in a fluctuating flow rate and discharge pressure. Cavitation can also be destructive to pumps internal components. When a pump cavitates, vapor bubbles form in the low pressure region directly behind the rotating impeller vanes. These vapor bubbles then move toward the oncoming impeller vane, where they collapse and cause a physical shock to the leading edge of the impeller vane. This physical shock creates small pits on the leading edge of the impeller vane. Each individual pit is microscopic in size, but the cumulative effect of millions of these pits formed over a period of hours or days can literally destroy a pump impeller. A cavitating pump can sound like a can of marbles being shaken. Other indications that can be observed from a remote operating station are fluctuating discharge pressure, flow rate, and pump motor current.

Preventing Cavitation

Raising the suction tank level (hss): From a rough estimation for water flow (hatm – hvap = 10 m, Vs = 2 m/s and NPSH = 4 m), we find that the suction level should not drop more than 4 m below the pump level (hss > – 4 m). Some liquids such as gasoline are volatile (hatm – hvap = 4 m). For these liquids the suction level should be much higher than the pump level, so a hole is dug in the ground to place the pump in it.

Decreasing the losses (friction and eddy types) in the suction type: That means placing the pump as close as possible to the suction tank, although theoretically its operating point is not affected by its place along the pipeline, to decrease suction pipe length. Also increasing the suction pipe diameter (sometimes it is designed larger than the delivery pipe) to decrease losses and kinetic energy. Finally, one important point is excluding any unnecessary fittings (e.g. elbows) and using suction valves of types that make minimum losses when fully open (e.g. gate valve and ball valve).