The effects of vapor pressure on pump performance

When cavitation occurs in a pump, its efficiency is reduced. It can also cause sudden surges in flow and pressure at the discharge nozzle. The calculation of the NPSHr (the pump's minimum required energy) and the NPSHa (the system's available energy), is based on an understanding of the liquid's absolute vapor pressure.

The effects of cavitation are noise and vibration. If the pump operates under cavitating conditions for enough time, the following can occur:

■ Pitting marks on the impeller blades and on the internal volute casing wall of the pump.

■ Premature bearing failure.

■ Shaft breakage and other fatigue failures in the pump.

■ Premature mechanical seal failure. These problems can be caused by:

■ A reduction of pressure at the suction nozzle.

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■ An increase of the temperature of the pumped liquid.

■ An increase in the velocity or flow of the fluid.

■ Separation and reduction of the flow due to a change in the viscosity of the liquid.

■ Undesirable flow conditions caused by obstructions or sharp elbows in the suction piping.

■ The pump is inadequate for the system.

The focus should be on resolving cavitation problems by increasing the external pressure on the fluid or decreasing its vapor pressure. The external pressure could be increased by:

■ Increasing the pressure at the pump suction.

■ Reducing the energy losses (friction) at the entrance to the pump.

The vapor pressure of the fluid is decreased by:

■ Lowering the temperature of the fluid.

■ Changing to a fluid with a lower vapor pressure.

At times, simply removing aspirated air venting the pump will have the same effect.

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