In a centrifugal pump, the flow area at the eye of the pump impeller is usually smaller as compared to the flow area of the pump suction piping. When the pumped liquid enters the eye of the pump, the decrease in flow area results in an increase in flow velocity. Due to this increase in flow velocity, liquid pressure decrease at impeller eye. Greater the flow velocity/ flow rate, greater the pressure drop between the pump suction and eye of the impeller. It means flow velocity is inversely proportional to the flow pressure. If this pressure drop is large or if the temperature of the liquid is high enough, the pressure drop causes the liquid to flash to vapor. This vaporization happened when the local pressure of the liquid falls below the saturation pressure. Any vapor bubbles formed at the impeller eye due to pressure drop swept along the impeller vanes. When these vapor bubbles enter in the area where the local pressure is greater than saturation pressure, vapor bubbles collapse abruptly. This process of the formation and subsequent collapse of vapor bubbles in a pump is called cavitation.
“Formation and subsequent collapse of the vapor bubbles in a pump is called cavitation”.
A small number of centrifugal pumps are designed to operate with cavitation conditions. These pumps have a special design to withstand a small amount of cavitation during operation. Most of the pumps not designed to withstand cavitation. Noise is the indication of pump cavitation. Pump cavitation sound just like marbles shaking. Fluctuation in flow rate, discharge pressure, and pump motor current are also indications of pump cavitation.
Pump Cavitation Effects:
Cavitation in a centrifugal pump has many significant effects on pump performance. Some of the cavitation effects are following.
- Cavitation degrades the performance of a pump, resulting in a fluctuating flow rate and discharge pressure.
- Cavitation also causes excessive pump vibration. This excessive vibration can damage pump bearings, wearing rings, and seals.
- Cavitation can also damaging to internal pump part.
Pump Cavitation Prevention:
If the centrifugal pump is cavitating due to some reasons, many changes in operation or system design are necessary to increase the NPSHA above the NPSHR. By achieving NPSHA above the NPSHR, we can stop the pump cavitation.
Increase the pressure at the suction of the pump
NPSHA can increase by increasing the pressure at the suction of the pump. For example, a pump taking suction from an enclosed tank, then raised the liquid level in the tank. Raising the liquid level in the tank causes an increase in the pressure at the pump suction. If we do not want to increase the liquid level in the tank than increase the pressure in the space above the liquid. If the pump takes suction from the open tank or pool, we can also raise pump suction pressure by increasing the pool level.
Reduce the temperature of the liquid being pumped
NPSHA also increases by decreasing the temperature of the liquid. When we decrease the temperature of the liquid, the saturation pressure of liquid also decreases. This decreasing saturation pressure causes NPSHA to increase. So the decrease in temperature of liquid also prevents the pump from cavitation.
Reduce head losses in the pump suction piping
Different methods are available to reduce head losses in the pump suction piping. Suction piping head losses reduced through increasing the suction pipe diameter. Head losses also reduced by using Minimum/reduce the number of elbows, fittings, and valves in the piping. Decreasing length of pipe also reduced head losses at the suction of the pump.
- Reduce the flow rate through the pump
Pump cavitation also stopped by reducing the NPSHR for the pump. NPSHR is not constant for a given pump under all conditions but depends on certain factors. NPSHR is directly proportional to flow rate. It means with an increase in flow rate, NPSHR also increases and vice versa. We can reduce the flow rate through the pump by a throttling valve at the discharge of the pump.
Reduce the speed of the pump impeller
NPSHR also depend on the speed of the pump. Faster the pump impeller rotates, greater the NPSHR. Therefore, if the speed of a variable speed centrifugal pump reduced, the NPSHR of the pump also reduced. So reducing pump speed also protect the pump from cavitation.
The net positive suction head required to prevent cavitation is determined through testing by the pump manufacturer. It depends upon factors including the type of impeller inlet, impeller design, impeller rotational speed, pump flow rate, and the type of liquid being pumped.