Cavitation is not a new phenomenon that impacts a pump system, but it is an issue that occurs far too often. If you’ve passed by a pump and heard what sounds like rocks or marbles going through it, this noise is internal cavitation. Pump cavitation occurs as a result of the inlet pressure not being high enough to maintain a static pressure higher than the vapor pressure of the liquid. Cavitation can cause loss of flow, higher energy usage, seal leakage, and internal metal damage.
What is Pump Cavitation?
Cavitation is the rapid formation and collapse of vapor bubbles within a liquid. As a liquid passes through a static pressure—which is lower than the liquid-vapor pressure—bubbles are formed. These bubbles can be up to 50,000 times larger than the original liquid size. As the bubbles move back to a static pressure higher than their liquid’s vapor pressure, they rapidly collapse or implode back to a liquid state.
Pump cavitation is most commonly found with centrifugal pumps. As the liquid enters the pump’s suction port, the impeller’s rotation creates a change in speed. Bernoulli’s principle states that as a liquid’s velocity increases, the static pressure decreases. In the case of cavitation, the liquid changes to bubbles. Eventually, the pump’s the static pressure will go back up causing the cavitation: the implosion that happens from the bubble collapsing back to liquid.
Identifying Pump Cavitation
Pump cavitation can be heard and seen. It will sound like rocks or marbles going through the pump. You can also see cavitation damage to your pump’s internal surfaces. The continual metal loss cavitation causes can form pitting in the metal.
Why is Pump Cavitation a Problem?
The collapse of the vapor bubbles erodes the impeller surface and pump casing. These issues decrease pump efficiency—resulting in higher energy use, additional repair costs, and reduced pump lifespan. If strong cavitation occurs at the pump inlet, pump performance decreases, which can lead to premature pumping failure.
4 Types of Pump Cavitation
- Air Aspiration Cavitation
Air is unpredictable and can sometimes be sucked into a pump through failing valves or other weak components. The air will eventually start to form bubbles that then gets popped under pressure by the pump impeller. Some tips to prevent this type of cavitation include:
- Check all O-Rings and mechanical seals
- Ensure all piping is crack-free
- Make sure joint rings have not perished on any suction piping
- Internal Re-Circulation
This type of cavitation prevents the pump from discharging at the desired rate, meaning that the liquid will now re-circulate around the impeller. The liquid travels through low- and high-pressure zones resulting in heat and high velocity. This, in turn, creates vaporized bubbles. This can be causes by operating too close to shut-off head, a closed discharge valve or even an over-pressurized header.
Some tips to prevent this type of cavitation include:
- Opening a restricted discharge valve on the pump
- Checking for clogs in the downstream filter
- Checking pressure at the discharge line
If the system has been designed with parts that are inadequate for the amount of liquid being pumped, it will in turn create vortexes. These vortexes will become turbulent and experience major pressure differences throughout the system. This all leads to erosion of solid materials over time. Some tips to eliminate or avoid turbulence in your system include:
- Design pump suction piping and routing to avoid excess turbulence
- Ensure the system sufficient Net Positive Suction Head (NPSHa)
- Work within maximum allowable flow limits
- Increase pump suction line size as necessary
- Vane Syndrome
This cavitation occurs if the pumps impeller uses too large of a diameter or the housing coat is too thick. Both problems here create less space throughout the pump housing. The pump will then have an increased velocity in the liquid from the small amount of free space available, which in turn leads to lower overall pressure. Cavitation bubbles will occur because the lower pressure is now heating the liquid. Some tips to prevent this cavitation include:
- Ensure there is ample free space between the impeller blade tips
- Ensure the sufficient free space between your impeller and its housing. It is recommended this be at least 4% of the impeller’s diameter.
What to do if Pump Cavitation is Suspected
If cavitation is suspected, it is best to have this condition reviewed by a qualified liquid handling specialist. The specialist will investigate whether the pump’s inlet pressure is higher than the inlet pressure required by the pump manufacturer. To do so, they will look calculate the pump’s inlet pressure and compare this to a pressure reading at the pump’s discharge port. This information, coupled with manufacturer’s pump curve to identify the GPM , will allow them to make recommend any operational improvements or repairs.