Cavitation is the formation and subsequent collapse of vapour bubbles in a flowing liquid and is often responsible for significant damage flow equipment such as pumps. This article will provide an overview of cavitation and provide insight into the identification and prevention of cavitation conditions through the calculation of Net Positive Suction Head (NPSH).
|:||Pressure head at the liquid surface (m)|
|:||Static suction head (m)|
|:||Suction piping friction head (m)|
|:||Vapour pressure head (m)|
Cavitation is the formation and subsequent collapse of vapour bubbles in the pumped fluid. The formation of vapour bubbles in the pumping fluid will occur when the when the fluid pressure drops below its vapour pressure. It should be noted that the vaporisation itself does not cause the damage it is the subsequent collapse of the vapour bubbles when the fluids pressure is increased.
The collapse of vapour bubbles result in shock waves which if occur near solid surfaces such as a pump impeller or pipe wall can result in hardware damage such as pitting. The common signs of cavitation include vibration, noise and impeller pitting or erosion.
Two main types of cavitation may occur in a pump, suction cavitation and discharge cavitation. Suction cavitation occurs when there is low pressure at the impeller eye providing a region in which the pumped liquid can vaporise. Subsequently the formed vapour bubbles move into regions of higher pressure as they travel towards the pump discharge and collapse.
Discharge cavitation occurs when a pump is operating towards the high head end of its pump curve resulting in the majority of the pumped fluid circulating around the pump. When this occurs the fluid is forced through the clearance between the impeller and the pump housing at high velocity resulting in the formation of a low pressure region in which cavitation can occur.
To avoid cavitation when designing a pump system one should pay particular attention Net Positive Suction Head requirements (NPSHr).
Net Positive Suction Head Required (NPSHr)
NPSHr is the head required at the pump inlet for satisfactory operation of a pump. NPSHr is typically included on manufactures pump curves and is determined from performance testing.
NPSHr is dependent on the flow rate and can therefore be determined for a given flow rate if the NPSHr is known for a single flow rate as follows:
Net Positive Suction Head available (NPSHa)
NPSHa is the actual absolute head at the pump inlet less the vapour pressure head. It may be calculated as follows:
The surface pressure head is simply the pressure in the tank the fluid is being pumped from or atmospheric pressure if the tank is open to atmosphere. The static suction head is the difference in height between the surface of the fluid in the source tank and the pump centre line (this will be negative for suction lift conditions). Suction piping friction head may be determined from the calculation of the friction factor.
If the NPSHa is less than the NPSHr than the pump will cavitate and the pumping system design needs to be re-evaluated.
Cavitation may be avoided by increasing the NPSHa and/or reducing the NPSHr. The NPSHa may be increased through the following methods:
- Reduce the length or increasing the diameter of the pump suction line and reducing the number of fittings in the suction line. This will reduce the suction piping friction head.
- Increase the height of the pumping fluid to increase the static pressure head.
- Reduce the temperature of the pumping fluid at the pump entrance for example moving a pump to the return line of a heating fluid system. This will reduce the vapour pressure head.
- Pressurise the pumping fluid supply tank. This will increase the head at the liquid surface.
- Reduce the flow rate and/or fluid velocity. This can be achieved by reducing the pump speed and will reduce the suction piping friction head.
The NPSHr can be reduced through the following methods:
- Throttle the pump discharge using a throttling valve or a restriction orifice. This will increase the pump head by reducing the flow rate and operate the pump in a lower NPSHr region.
- Use an oversized pump.
- Use an impeller with a larger diameter eye.
Some rules of thumb to consider when designing pumping systems are to never throttle the suction line as this reduces NPSHa and to design suction piping so that fluid velocities do not exceed 3 m/s (9.8 ft/s).