Understanding Pump Cavitation
If a pump starts making a noise like gravel or stones passing through it, it may be more than just a mechanical fault.
One possible cause is cavitation.
Cavitation occurs when the pressure of a liquid falls below its vapour pressure, causing vapour bubbles to form. As these bubbles move into areas of higher pressure, they collapse rapidly. In a centrifugal pump, this can happen around the impeller where local pressure can fall significantly as the liquid accelerates through the pump.
The result can be noise, vibration, loss of performance and, over time, damage to the impeller and other internal components.
What causes pump cavitation?
Cavitation is fundamentally a pressure problem. For a pump to operate correctly, there needs to be sufficient pressure at the suction side to prevent the liquid from reaching its vapour pressure as it passes through the pump.
Several factors can cause this pressure to become too low, such as:
- Insufficient pressure at the pump inlet
- Excessive suction pipework losses
- A suction pipe that is too small or restrictive
- Blocked strainers or filters
- Partially closed valves
- Excessive flow rates
- High liquid temperatures
- Excessive suction lift
- Low liquid levels in a supply tank or break tank
An important point is that the pump does not necessarily have to be faulty for cavitation to occur. The conditions around the pump can create the problem.
The process of cavitation
As liquid enters the pump impeller, its pressure can fall locally as the flow accelerates around the impeller blades. If the local absolute pressure falls below the liquid’s vapour pressure, vapour bubbles begin to form.
These bubbles are carried through the pump with the liquid. As they reach areas where the pressure is higher again, they collapse or implode.
When this happens close to the impeller surface, the collapse can produce highly localised pressure effects. Repeated exposure can cause small pits to form on the metal surface, a process known as cavitation erosion. Over time, this can seriously damage the impeller and other wetted components.
Cavitation can also cause the pump to run rough, with increased vibration and noise. As it becomes more severe, the pump may lose head and overall efficiency.
Why liquid temperature matters
The temperature of the pumped liquid can have a significant effect on cavitation risk.
As liquid temperature increases, its vapour pressure increases too. This reduces the margin between the absolute pressure within the pump and the liquid’s vapour pressure, making cavitation more likely if suction conditions are not adequate.
For example, hot water applications require particular attention to suction conditions because the available pressure margin can be reduced as temperature rises.
This is one reason why a pump that operates normally with cold water may require different suction conditions when handling a hotter liquid.
What is NPSH?
NPSH stands for Net Positive Suction Head and is a key factor when assessing cavitation in centrifugal pumps. Two values are particularly important: NPSH available (NPSHa) and NPSH required (NPSHr).
NPSH available (NPSHa) is determined by the system. It is influenced by factors including the pressure at the liquid source, liquid temperature, elevation and losses in the suction pipework.
NPSH required (NPSHr) is a characteristic of the pump and varies with the pump’s operating conditions, particularly flow rate and speed.
For a pump to operate without unacceptable cavitation, the available NPSH needs to be greater than the pump’s required NPSH, with an appropriate margin for the application.
This is why NPSH needs to be checked at the actual operating conditions rather than treated as a fixed value. Pump manufacturers determine NPSHr through testing, and the value is normally shown on the pump’s performance data or NPSH curve.
How does flow rate affect cavitation?
Flow rate is another important factor. As flow through a centrifugal pump increases, the NPSH required by the pump will generally increase. If the system cannot provide adequate NPSH available, operating at a higher flow rate can therefore increase the risk of cavitation.
This is particularly relevant when a pump is operating away from its intended duty point.
Selecting a pump based purely on flow rate and head isn’t enough. The operating point, NPSH requirements and suction conditions all need to be considered together.
Cavitation symptoms
Cavitation can produce a few recognisable symptoms, although none of them should be treated as conclusive on their own.
Common signs include:
- Unusual rattling, crackling or gravel-like sound
- Rougher operation and vibration than normal
- Lower flow or head than expected
- Pitting or erosion on the impeller
These symptoms can also be caused by other issues, so it’s worth stressing that a noisy pump isn’t automatically a cavitating pump.
However, if unusual noise or vibration is accompanied by a reduction in performance, the suction conditions and NPSH should be looked at.
How can pump cavitation be prevented?
Preventing cavitation starts with making sure the pump has suitable suction conditions for the required duty.
Depending on the application, this can involve:
Reduce suction-side losses
Keeping the suction pipework correctly sized and minimising unnecessary bends, valves, strainers and other restrictions can help reduce pressure losses before the pump.
Check the pump duty
The pump should be selected to operate at the required flow and head, while also providing suitable NPSH characteristics for the application.
Maintain adequate inlet pressure
The pressure available at the pump inlet needs to remain adequate under the worst expected operating conditions.
Consider liquid temperature
The vapour pressure of the liquid increases with temperature, so the highest operating temperature needs to be taken into account when assessing cavitation risk.
Maintenance
Blocked strainers and filters, partially closed valves and other restrictions can gradually increase suction losses. Regular inspection and maintenance can help prevent these conditions from developing.
Pump selection starts with the application
Pump cavitation is a good example of why pump selection can’t be considered in isolation.
A pump may have the correct flow and head for an application, but if the system cannot provide sufficient NPSHa at the operating point, cavitation can still happen.
At Trebles, we look at the wider application when selecting and supplying the right pump. Pump duty, operating conditions, pipework and system requirements all need to work together to provide reliable performance.
Need advice on pump selection or system requirements? Get in touch with the Trebles technical sales team: