When the pressure of flowing liquids drops to, or below, the liquid’s vapour pressure, the liquid boils and vapour cavities (bubbles) form locally inside the liquid. If the pressure within the flow path subsequently increases above the vapour pressure, the vapour cavities implode, releasing energy. The formation and sudden collapse of these bubbles is called cavitation.
What can be done?
The obvious answer is to ensure proper pump selection at the initial stage. Many users would agree that most pump vendors are sufficiently competent in giving customers what was asked for in the specifications. It is also imperative that the issue of NPSHA vs NPSHR is properly understood and considered by both user and supplier.
The solutions to existing cavitation conditions can be determined by considering both NPSHA (the system) and NPSHR (the pump). On the system side, the NPSHA can be increased by one or more of the following:
- Increase the static liquid level above the pump or reduce the suction lift. This can be done in the case of a flooded suction by raising the liquid level in the suction tank, raising the suction tank to a higher level, or lowering the pump. In the suction lift situation, the liquid level in the sump or suction tank can be raised or the pump can be lowered – for example, mounting the pump off the sump side or building a dry sump beside the existing sump.
- Reduce the friction losses by increasing pipe sizes and reducing the length of pipe runs and the number of fittings, e.g. tees, bends, valves. Selection of fittings with lower friction loss, such as long radius elbows and full flow ball valves, should also be considered. In particular, resist the use of suction strainers that can clog.
- Reduce the vapour pressure by reducing the temperature of the product. This can be done by reducing the operational temperature of the process (if feasible) or cooling the temperature in the suction line – for example, cooling annulus on the suction pipework. It must be noted the reduction of vapour pressure by reducing the temperature is rarely possible.
The remedies detailed below can be applied to the pump (NPSHR):
- Reduce the flow rate by throttling on the pump discharge. This will generally reduce NPSHR (always check the pump curve) and increase NPSHA (due to reduced friction losses). Care must be taken to ensure that the flow rate is not reduced below the minimum flow rate recommended by the manufacturer.
- Reduce the pump speed as this reduces NPSHR. This will require the user to accept reduced pump performance.
- Reduce the pump speed and install a larger diameter impeller. This will have a two-fold effect as lower speed means lower NPSHR and in many cases the larger impeller diameter has lower NPSHR characteristics.
- Install a different pump. This would normally mean installation of a larger pump as they generally have a better NPSHR value for the same flow rate. The selection of a larger pump is sometimes required with speed reduction.
- Change the impeller material to one that is more resistant to erosion – for example, from cast iron to stainless steel. This does not eliminate cavitation but will reduce the impact of cavitation.
With a vast array of pumping designs and solutions available, ensuring you have the right pump for the right application involves a thorough set of considerations. Moreover, proper pump selection at the initial stage is essential in preventing cavitation. Consider consulting your pump supplier to discuss your pumping requirements.
Cavitation damage to an impeller with segments chipped away
This sponsored editorial is brought to you by Kelair. For more information, visit www.kelairpumps.com.au.
