Have a pump that makes popping sounds, or sounds like it’s pumping marbles? If so, you may have a cavitation problem.
Pump cavitation can cause a number of issues for your pumping system, including excess noise and energy usage, not to mention serious damage to the pump itself.
Description of the phenomenon
Simply defined, cavitation is the formation of bubbles or cavities in liquid, developed in areas of relatively low pressure around an impeller. The imploding or collapsing of these bubbles in areas where fluid pressure increases, causes intense shockwaves inside the pump, causing significant damage to the impeller and/or the pump housing.
If left untreated, pump cavitation can cause:
- Damage to the pump housing, impeller and/or shaft
- Excessive vibration level – leads to premature sealing loss and bearing failure
- Higher than necessary power consumption
- Decreased flow and/or pressure
There are also two phenomena that exhibit similar behavior to cavitation and is necessary to be reported: re-circulation and aeriation or air entrapment.
CAVITATION
When a pump is in low pressure or high vacuum conditions, cavitation occurs. If the pump is “starved” or is not receiving enough flow, bubbles or cavities will form at the eye of the impeller. As the bubbles carry over to the discharge side of the pump, the fluid conditions change, compressing the bubble into liquid and causing it to implode against the face of the impeller.
An impeller that has fallen victim to suction cavitation will have large chunks or very small bits of material missing, causing it to look like a sponge. Damage to the impeller appears around the eye of the impeller.
Possible causes:
- Clogging of the suction area
- Pump is running too far right on the pump curve
- Poor piping design
- Poor suction conditions (NPSH requirements)
RE-CIRCULATION
When a pump’s discharge pressure is extremely high or runs at less than 10% of its best efficiency point (BEP), re-circulation of fluid occurs. The high discharge pressure makes it difficult for the fluid to flow out of the pump, so it circulates inside the pump. Liquid flows between the impeller and the housing at very high velocity, causing a vacuum at the housing wall and the formation of bubbles.
The implosion of those bubbles triggers intense shockwaves, causing premature wear of the impeller tips and pump housing. In extreme cases, discharge cavitation can cause the impeller shaft to break.
Possible causes:
- Blockage in the pipe on discharge side
- The pump operates too far left on the pump curve
- Poor piping design
AERIATION(OR AIR ENTRAPMENT)
Air entrapment happens when bubbles are in the liquid before reaching the impeller. This can happen when:
- The liquid is aerated ( for whatever reason) near the pump inlet.
- Liquid is near its boiling point, such as in a condensate pump.
While this problem is not always as damaging (or as loud) as the previous ones, it can certainly damage the impeller if left unchecked.
Preventive treatment actions
- Check filters and the check valves – clogs on the suction, or discharge side can cause an imbalance of pressure inside the pump.
- Reference the pump’s curve – Use a pressure gauge and/or a flowmeter to understand where your pump is operating on the curve. Make sure it is running at its best efficiency point. Running the pump off its best efficiency point not only causes excess recirculation, expect excessive heat, radial loads, vibration, high seal temperatures, and lowered efficiency.
- Re-evaluate pipe design – Ensure the path the liquid takes to get to and from your pump is ideal for the pump’s operating conditions. Designs with inverted “U”s on the suction side can trap air, while designs with a 90° immediately before the pump can cause turbulence inside the pump. Both result in suction problems and pump cavitation.
Cavitation is a common problem in pumping systems, but with proper pump sizing, pipe design, and care of the whole system, damage to pumps can be largely avoided.
A satisfactory method to determine which of these problems are taking place using ultrasound or vibration analysis is to slow throttle the discharge valve closed.
As the discharge valve is being throttled closed:
- If noise and vibration get better – expect cavitation.
- If noise and vibration get worse – expect re-circulation.
- If noise and vibration stay the same – expect aeriation (or air entrapment).