Everybody who uses pumps regularly will have heard the term “pump cavitation” commonly identified by a rumbling noise (“sounds like there are stones in the pump”) and excessive vibration.  This affects the performance of the pump and prematurely wears and damages the pump. 

The rumbling sound is caused by the creation and implosion of vapour bubbles. 

There are numerous articles available that explain in great detail the physics behind this but in this article, we will discuss the practical actions that can be taken to eliminate this and the characteristics to consider when designing your pumping solution. 

Simply put, if the pump impellor is imparting more energy to the liquid on the exit of the pump than exists at the entry of the pump, the pressure drop between the inlet exceeds the vapour pressure of the liquid causing bubbles that then implode once they move toward the higher pressure side of the impeller. These imploding bubbles produce the recognisable rumbling sound of cavitation. 

When designing a pumping solution, we must consider the NPSH-r. Net Positive Suction Head Required is quoted by pump manufacturers as a result of extensive testing under controlled conditions. NPSH-r is a minimum suction pressure that must be exceeded for the pump to operate correctly and minimise cavitation. 

For our intentions we will only consider atmospheric pressure and assume the pump is operating in an environment that delivers approximately 1 bar of pressure.  If your application is at altitude, then more careful consideration needs to taken and a reference table is available to help this calculation.  

When sizing pumps we all tend to pay a lot of attention to what is going on at the pump outlet side, we consider pipe sizes, friction losses and head losses and how these will affect the flow rate.  

Often, we do not pay enough attention to what is happening at the suction side, and this will affect the overall performance of the pump. 

The pump impeller design also has an impact on this, the more efficient the impeller design, the greater the scope for creating cavitation if the suction side condition is not correctly controlled. When high flow and pressure pumps are required, high-efficiency closed impellers like those used in Pioneer Pumps are essential, however, high-efficiency impellers can be more prone to cavitation from poorly designed systems, this is why we always recommend to our customers a full on-site survey to ensure the system is correctly designed.  

The two most important aspect to consider on the suction side of your pump solution is the head lift and the friction losses in the suction pipework. 

Suction Head Lift 

When we talk about suction head lift we mean the distance between the top of the liquid the centre of the pump inlet (hs) 

Something that has to be considered is the difference in the head height when pumping out lagoons etc. The suction lift when the lagoon Is at it minimum level is much higher than when at its maximum level. When calculating the suction head the minimum level must always be used for this calculation. 

Friction Losses 

As well as considering the friction loss on the outlet side of the pump, it must also be considered on the inlet side of the pump. The type of material, the internal diameter of the suction pipe and flow rate all need to be considered. Just as in the outlet side the velocity flow through the pipe will influence the friction loss, the higher the flow, the higher the loss. To minimise this a large diameter pipe is always recommended. 

Length of the suction pipe 

In many situations the pump is not located next to the lagoon but can be many meters from the edge. The length of the suction pipe must be considered when calculating the suction friction losses. The bigger the diameter, the lower the friction losses. 

Summary

Cavitation has a negative impact on the performance and long-term health of a pump. Correct specification of the pump and its associated pipework is the simplest way to prevent this phenomenon. This is why it is always advisable to get your application reviewed by one of our experts to ensure the overall application is considered before a pump is selected.

Contact our experts today on +44 (0) 800 707 6011 or e-mail sales@pioneer-hire.co.uk