Cavitation in Centrifugal Pumps: Causes and Prevention Strategies
Cavitation in centrifugal pumps is a big problem for many industries. It happens when bubbles form and pop inside the pump. This can damage the pump and make it work poorly. Cavitation occurs when the pressure in parts of the pump drops below the liquid’s vapor pressure.
Pump makers and users need to know about cavitation. It can cut pump life and raise costs. Learning how cavitation works helps stop it from happening. This keeps pumps running well for a long time.
Stopping cavitation means looking at the whole pump system. This includes the pump itself, the pipes, and how it’s set up. Cavitation is one of several performance topics covered across our guides to centrifugal pumps. By watching for signs of cavitation, people can fix issues fast. This helps pumps work better and last longer.
Key Takeaways
- Cavitation in centrifugal pumps can cause damage and reduced performance
- Understanding cavitation helps prevent it and extend pump life
- Proper system design and monitoring are crucial for avoiding cavitation issues
What Causes Cavitation
Cavitation is a suction-side problem. It begins whenever the local pressure at the impeller eye falls to the vapor pressure of the liquid being pumped, which makes the liquid flash to vapor inside the pump. The comparison that governs this is between net positive suction head available (NPSHa), a property of the system, and net positive suction head required (NPSHr), a property of the pump at a given flow.
NPSHa is calculated from the absolute pressure on the liquid surface, minus the vapor pressure of the liquid, plus or minus the static suction lift or head, minus friction losses in the suction piping:
Cavitation appears when NPSHa falls to or below NPSHr. The Hydraulic Institute recommends keeping a margin above NPSHr rather than simply matching it, with larger margins for higher-energy pumps and continuous-duty service.
Common Field Causes
- Excessive suction lift: Placing the pump too far above the liquid level reduces NPSHa directly.
- Restricted or undersized suction piping: Long runs, undersized pipe, and multiple elbows add friction loss. A partly closed or throttled suction valve does the same thing.
- Clogged strainers or intake screens: Debris buildup raises suction losses over time, so a pump that started out with adequate margin can begin cavitating months later.
- High liquid temperature: Vapor pressure rises sharply with temperature, so hot water or heated process liquid cuts NPSHa even when nothing else changes.
- Operating far right of the best efficiency point: NPSHr increases with flow, so running a pump well beyond its design point can push it into cavitation.
- Vortexing and air entrainment: Insufficient submergence at the intake draws air into the suction, which is why centrifugal pumps wet well design sets minimum submergence depths. The symptoms resemble cavitation, but the cause and cure are different.
Types of Cavitation
Not all cavitation is the classic NPSH shortfall. Suction recirculation occurs at low flow, when part of the liquid reverses direction at the impeller eye; the damage typically shows on the leading edge of the vanes. Discharge recirculation, also a low-flow phenomenon, damages the vane trailing edges and the volute. Vane passing cavitation occurs where the impeller runs too close to the cutwater. Distinguishing between these patterns matters, because low-flow recirculation is cured by raising flow, while NPSH cavitation is cured by improving the suction.
Detecting and Diagnosing Cavitation
The gravel-in-the-casing sound is the best-known symptom, but it is not the only diagnostic tool available. A practical sequence is to confirm the hydraulic conditions before opening the pump.
- Record suction and discharge pressure with gauges close to the pump flanges, then convert to head and compare the duty point against the pump curve.
- Calculate NPSHa from actual conditions, including the current liquid temperature and the measured pressure drop across the strainer, and compare it against the manufacturer’s NPSHr at the operating flow.
- Check where the pump is running relative to its best efficiency point. Cavitation-like noise at low flow usually indicates recirculation rather than an NPSH shortfall.
- Take vibration readings. Cavitation typically produces broadband high-frequency energy rather than the discrete peaks associated with imbalance or bearing defects.
- Inspect the impeller at the next opportunity. Damage location distinguishes classic cavitation, which pits the vane inlet and the low-pressure face, from recirculation damage.
Prevention Strategies
Increase the Suction Head Available
The most durable fixes raise NPSHa. Options include raising the liquid level in the supply vessel, lowering the pump relative to that level, shortening or oversizing the suction pipe, removing unnecessary fittings, keeping the suction valve fully open, and cleaning strainers on a schedule rather than after symptoms appear. Where liquid temperature is the driver, cooling the feed or accepting a lower flow may be the only practical answers.
Reduce the Suction Head Required
The pump side can also be changed. Reducing speed with a variable frequency drive lowers NPSHr substantially, because required suction head falls roughly with the square of speed. Other options include selecting a pump with a larger impeller eye or a lower specific speed, fitting an inducer, splitting the duty between two pumps, or choosing a double suction design so each side of the impeller sees half the flow.
Operate Closer to the Design Point
Running near the best efficiency point protects against both classic cavitation and recirculation. Control strategies that help include variable speed control in place of discharge throttling, staging multiple smaller pumps instead of running one oversized unit at part load, and trimming or replacing an impeller that was oversized for the actual system curve.
Material and Design Choices
Where some cavitation cannot be avoided, harder and tougher materials extend the interval between repairs. Stainless steels, duplex alloys, and specialized coatings resist cavitation erosion better than cast iron or bronze. These measures buy time; they do not remove the underlying hydraulic cause, and the noise and efficiency penalties remain.
Effects of Cavitation
Cavitation in centrifugal pumps can lead to serious problems. It causes damage, reduces performance, and creates noise and vibration. These issues can shorten pump life and increase operating costs.
Damage to Pump Components
Cavitation can severely damage pump parts. It erodes impeller blades, causing pitting and material loss. The collapse of vapor bubbles creates shock waves that impact metal surfaces. This repeated impact weakens the impeller over time.
Pump casings and volutes also suffer damage. Cavitation can create holes in these components. Seals and bearings may fail due to increased vibration and misalignment.
Cavitation damage often starts small but grows quickly. Regular inspections can catch early signs of wear. Addressing cavitation promptly helps prevent catastrophic pump failure.
Impact on Pump Performance
Cavitation significantly reduces pump efficiency. It disrupts fluid flow through the impeller, lowering the pump’s ability to move liquid. This leads to decreased flow rates and reduced head pressure.
Pumps experiencing cavitation often fail to meet their design specifications. They may not deliver the required flow or pressure. This can cause problems in the wider system the pump serves.
Energy consumption typically increases as the pump works harder to compensate. Operating costs rise due to higher power usage and more frequent maintenance needs. Because cavitation shifts the pump away from its design point, it shows up directly in the efficiency of centrifugal pump calculations used to track plant energy use, and repeated impeller repairs feed into the centrifugal pumps lifecycle cost comparison between capital and operating spend.
Noise and Vibration Issues
Cavitation creates distinct noise and vibration. The sound is often described as gravel or marbles in the pump. This noise can be loud enough to require hearing protection for nearby workers.
Vibration from cavitation can loosen bolts and fittings. It may cause misalignment of pump shafts. Excessive vibration can damage seals, bearings, and coupling components.
These issues make the work environment less safe and comfortable. They can also interfere with other nearby equipment or processes. Addressing cavitation helps create a quieter, smoother-running pump system.