What is cavitation in a pump?
Cavitation is the formation of vapor bubbles in a liquid when its vapor pressure equals the pressure acting on it. Boiling water is the most familiar example. Inside a pump this phase change is harmful: it can cause loss of flow and physical damage to the impeller and casing, and it gets worse the longer the pump runs.
What does a cavitating pump sound like?
A cavitating pump makes a loud rumbling noise, often described as sounding like gravel is going through it. The noise comes from vapor bubbles collapsing back to liquid as they move into higher pressure areas of the impeller. If you suspect cavitation, do not run the pump for any length of time; find and correct the cause.
What is vapor pressure and why does it matter?
Vapor pressure is the absolute pressure a liquid exerts on its surroundings. It depends on the liquid and its temperature, and it rises as temperature rises. When the pressure on a liquid drops to its vapor pressure, the liquid boils. That can happen from heating, or from lowering the pressure, which is what happens at a pump suction.
| Water temperature | Vapor pressure (absolute) |
|---|---|
| 20 °C (68 °F) | 0.023 bar (about 0.34 psia) |
| 100 °C (212 °F) | 1.013 bar (14.7 psia) |
At 100 °C, water boils because its vapor pressure equals atmospheric pressure. If the pressure on the liquid is increased, boiling stops, and the temperature can rise until vapor pressure again equals the pressure on the liquid. The reverse is also true: boiling can occur at much lower temperatures when pressure falls below atmospheric.
Where does cavitation start inside a centrifugal pump?
Cavitation starts in the impeller eye. A centrifugal pump relies on pressure at the surface of the suction supply to push liquid into the eye. The pump creates a low pressure zone there, below the free surface pressure, which draws liquid in. That same low pressure zone is where the liquid is most likely to boil.
The sequence goes like this:
- If the liquid's vapor pressure is higher than the pressure in the eye, the liquid vaporizes. At 20 °C, water expands about 43,000 times in volume when it turns to vapor.
- Past the vane inlets, the fluid reaches higher pressure and the vapor condenses back to liquid.
- Each collapsing bubble releases a large amount of energy. Over time this breaks small pieces from the impeller and leaves a pitted surface.
- Pump head drops rapidly as cavitation progresses.
How do you prevent pump cavitation?
Keep the minimum absolute pressure anywhere on the suction side above the vapor pressure of the liquid being pumped. In practice, that means making sure net positive suction head available exceeds the pump's net positive suction head required, with margin, at every operating condition the system will see.
Pmin > Pvp
Ways to add suction pressure or reduce the pressure the pump needs:
- Raise the liquid level in the supply tank or lower the pump.
- Shorten the suction line, enlarge its diameter and remove unneeded fittings to cut friction losses.
- Lower the liquid temperature, which lowers vapor pressure.
- Avoid running the pump far out on its curve, where NPSH required rises.
- Select a pump with a lower NPSH required at the operating point.
See how to calculate NPSH available for the full method. If a pump is already cavitating, PCT can review the system and recommend a fix or a better suited pump. Request a quote.