Pump cavitation has been a menace to pumps for many years now. It can produce excess noise and energy use that in turn leads to serious pump damage which is not something any business wants. However, it’s easy enough to prevent by doing the right planning or troubleshooting before you get into trouble with pump cavitations!
The engineer’s worst nightmare is a pump that starts to cavitate. Cavitation can severely damage an impeller, housing or other related parts if left unchecked. You’ll know it’s happening when you notice bubbles floating up in the water and it starts to foam at the top of your tank.
The Causes of Pump Cavitation
There are multiple factors that contribute to the formation of cavitation.
Quick drop in pressure at the suction nozzle
The fluid pressure at the suction pump must be higher than the liquid vapour pressure. However, when the vapour pressure becomes very low at the nozzle, vapourisation takes place. This leads to the unlimited formation of gas bubbles, causing cavitation.
Unideal flow conditions
Obstructions in the valves and at sharp elbows can be present. When this happens, especially in the suction piping and other fittings, frictional pressure decreases. This leads to the potential of pressure loss at the suction pump, which eventually leads to pump impeller cavitation.
Unstable pumped liquid temperature
When the temperature of the liquid rises unevenly, the liquid vapour pressure increases. This leads to the operating pressure falling below the vapour pressure limit, causing the formation of bubbles, which leads to pump impeller cavitation.
High fluid velocity at the suction pump
When the liquid flow rates rise higher at the pump suction than at the designed case, frictional pressure drop level rises with the increasing flow rate. This reduces pressure and increases the risk of cavitation at the suction pump.
Types of Cavitation
Also known as inadequate NPSHa cavitation or ‘classic cavitation’, vapourisatin is the most common form of pump cavitation. It can be found when a centrifugal pump issues velocity on a liquid as it passes through the eye of the impeller. If the impeller isn’t working properly, some of the liquid may be boiled quickly (vapourised), creating tiny shock waves.
Vortexes in the pumped liquid are caused when parts of the system – pipes, valves, filters, elbows – are not adequate for the amount or type of liquid you are pumping. This causes the liquid to become turbulent and experience pressure differences throughout, eroding solid materials over time, much as a river erodes the ground.
This type of cavitation, also known as ‘vane passing syndrome’, is found when either the impeller has too large a diameter, or the housing has too thick a coating, both of which creates less space within the housing itself. This reduced amount of free space increases the liquid’s velocity, which in turn leads to lower pressure. This lower pressure heats up the liquid, creating cavitation bubbles.
If internal recirculation is present – such as when a discharge valve has been closed while the pump is running – the pump cannot discharge at the proper rate. The liquid travels through low and high pressure zones, increasing the heat and velocity of the liquid.
Air Aspiration Cavitation
Another common form of cavitation, air aspiration takes place when air is sucked into a pump. This can be due to failing valves or at other weak points such as joint rings. Once inside, the air cannot escape so is processed through the system with the liquid, resulting in bubbles which then get popped under pressure by the impeller.
Effects of Cavitation
Cavitation causes one or a combination of the following:
- Pump vibrations
- Performance deterioration of the pump
- Decreased or lowered flow or pressure
- Impeller erosion
- Seal and bearing failure
- Unpredictable energy use
- High noise levels
- Mechanical damage
How to Prevent Pump Cavitation
- Slow down motor speed (RPMs).
- Install an impeller inducer
- Include a booster pump into your pump system.
- If possible, lower the temperature of your pump, liquid, and/or other components.
- Raise the liquid level around the suction area.
- If possible, try to increase the diameter of the eye at the centre of your impeller.
- Analyse all the components of the pump system to ensure that they can take the strain of the flow rate, volume, and properties of your liquid, and replace components as required.
- Stay within your pump’s manufacturer performance guidelines.
- Increase pump suction line size to reduce turbulence
Vane Syndrome Cavitation
Preventing vane passing or vane syndrome cavitation can be easy if you ensure that the free space between your impeller and its housing is 4% of your impeller’s diameter or more. Any lower than that and cavitation will begin.
Internal Recirculation Cavitation
- Open up your pump’s restricted discharge valve and check the inside for any blockages.
- Unclog the downstream filter of any blockage.
- Ensure that the check valve has been installed correctly (often installed backwards).
- Check the discharge valve and make sure that it is open (not closed).
- Assess pressure at the discharge line – the header may need to be replaced.
Air Aspiration Cavitation
- Regularly check joint rings on any suction piping to ensure they aren’t perished.
- Check whether foaming liquid is creating an accumulation of bubbles. If so, run the system slower, or empty the system of all contents, including air, from time to time.
- Assess all O-rings and any secondary mechanical seals.
- Check that none of the piping has cracks or shows signs of erosion.
- Make sure that all system materials can handle the liquid you intend to transport, as viscous, abrasive, or acidic liquids can eat away at materials, letting air be sucked into the system.
- Regularly inspect valves, joints, and anywhere else that may seem likely to fail at some point, replacing as needed.
Pump Maintenance and Cavitation Prevention is the Key to Prolonged Pump Health
Preventing pump cavitation significantly increases the efficiency and lifespan of your pump. Regular pump maintenance ensures the prevention of breakdowns, costly repairs, downtime, and disruption.
Schedule thorough pump maintenance today and it will save you in the long run.