Cavitation is one of the most common phenomena that shorten the life of centrifugal pumps - both in home installations and in large industrial and agricultural systems. In our earlier article What is cavitation and how to prevent it we explained the basics of this phenomenon. In this text, we go a step further - as a manufacturer of brand water pumps IBO i IPRO we show how to recognize cavitation in practice at the operational stage, how to calculate the NPSH reserve when designing a suction installation, and what accessories - controllers, inverters, valves, filters and pressure gauges - actually help reduce the risk of its occurrence. The text is addressed primarily to installers and service technicians who select and run centrifugal pumps on a daily basis.
The phenomenon of cavitation in centrifugal pumps - what happens inside the impeller
The formation of cavitation in a centrifugal pump has a very specific physical cause. When the liquid enters the impeller inlet, its flow rate increases rapidly, and according to Bernoulli's principle the liquid pressure drops at this point. If the fluid pressure drops below the saturated vapor pressure of a given liquid at a given temperature, local evaporation occurs - tiny vapor bubbles appear in the liquid. It is this moment - the liquid pressure below the vapor pressure - that is the starting point of the entire cavitation phenomenon.
Vapor bubbles cause a local disturbance in the continuity of the liquid stream, but the real problem appears a moment later. The pump impeller moves the pumped liquid from the low pressure area (suction side) to the higher pressure area (discharge side). When steam bubbles hit an area of higher pressure, the steam bubbles implode violently - collapsing at a speed comparable to microscopic water hammers. The energy released during this process is so high that, when repeated thousands of times per minute, it leads to erosive damage to the cavitation material on the surface of the impeller and pump body.
It is worth remembering the relationship: the lower the saturation pressure of a given liquid under the operating conditions of the installation, the smaller the safety margin of the pump. Therefore, the temperature of the pumped liquid has a direct impact on how easily the evaporation limit is exceeded.
Gas cavitation and classic steam cavitation – two different mechanisms
In installation practice, it is worth distinguishing between two phenomena that are often confused. Classic steam cavitation occurs when the drop in steam pressure is so large that the liquid evaporates locally inside the pump - this is the mechanism described above. Gas cavitation has a different origin: it is the release of gases dissolved in a liquid (usually air) as a result of a pressure drop, without the need to reach saturated vapor pressure. Both phenomena lead to similar effects - pressure fluctuations, loss of efficiency and noise - but their root causes are different, and therefore, different methods of counteracting them.
Gas cavitation most often occurs when there are leaks in the suction pipeline, where air enters the installation, and when the flow rate is incorrectly selected in relation to the diameter of the pipeline. Classic steam cavitation, in turn, is closely related to the operating conditions of the pump's suction side - suction height, medium temperature and flow resistance.
What increases the risk of cavitation in the installation - the suction side of the pump under a magnifying glass
The vast majority of cavitation cases in installation practice originate on the suction side of the pump, not on the discharge side. The most common causes that increase the risk of cavitation include:
- Pump suction height too high – the deeper the pump has to suck the liquid, the lower the static pressure on the suction side, and therefore the smaller the reserve for evaporation pressure.
- Suction pipe too long or too narrow – causes pressure losses proportional to the square of the flow speed, which additionally reduces the pressure at the impeller inlet.
- Increased liquid temperature – as the temperature increases, the saturated vapor pressure increases, which means that the safety margin decreases even with the same installation geometry.
- Leaks sucking in air – typical of suction installations with threaded connections or worn seals.
- Flow rate too high in relation to pump parameters – operation outside the characteristic curve, with efficiency significantly exceeding the optimal point.
In practice, this means that a suction centrifugal pump operating on a long suction pipeline, at elevated medium temperature and high suction height is much more exposed to cavitation areas than the same pump powered from a tank located close to the installation level. By selecting Submersible pumps instead of pumps suctioning from the surface, in many cases the problem of the suction side can be completely eliminated - the impeller then works flooded with liquid, which significantly reduces the risk of the steam pressure falling below that. Our overview IBO submersible pumps shows solutions used in such, more hydraulically difficult, applications.
NPSH, NPSHav and NPSHr – how to design cavitation-resistant pumps
The most important parameter that allows you to assess the risk of cavitation at the installation design stage is NPSH (Net Positive Suction Head), i.e. the excess of suction pressure over the evaporation pressure of the liquid. In the technical documentation of centrifugal pumps, we find two values that need to be compared:
- NPSHav (available) – available excess pressure, resulting from the specific installation: suction height, length of the suction pipeline, liquid temperature and atmospheric pressure at the installation site.
- NPSHr (required) – required excess pressure, given by the pump manufacturer for a given operating point on the characteristic curve. This value results from the design of the impeller and the centrifugal pump system used in a given model.
The basic rule that every installer should know: pumps should be designed so that NPSHav is always clearly higher than NPSHr - in practice, a safety margin of 0.5-1 meter of liquid column is recommended. If this condition is not met, pump operation almost inevitably leads to cavitation, regardless of how good the quality of the centrifugal pump itself is. Therefore, when selecting surface pumps or submersible pumps for a specific installation, calculating the real NPSHav for given operating conditions should precede the selection of a specific model, and not the other way around.
Symptoms of cavitation during pump operation
Cavitation can be recognized even before permanent damage to the pump components occurs. Common symptoms include:
- a characteristic noise resembling the operation of a pump pumping gravel or small pebbles - this is the sound of implosion of steam bubbles,
- increased vibration of the pump body and pipeline, beyond normal operating levels,
- uneven, pulsating operation of the pump and pressure fluctuations visible on the pressure gauge on the discharge side,
- noticeable drop in pump efficiency with unchanged installation parameters,
- faster than usual increase in the temperature of the bearings and mechanical seal.
Installing a pressure gauge on the suction and discharge sides of the pump is one of the simplest diagnostic tools - a sudden, repeated drop in pressure on the suction side often precedes the appearance of audible symptoms of cavitation by several hours or days of installation operation.
Effects of cavitation - damage to pump components
Long-term operation in cavitation conditions leads to permanent, expensive damage to the pump. The most characteristic are the so-called cavitation pits - irregular, spongy losses of material on the rotor surface, in places where the implosion of steam bubbles occurs most intensively. In advanced cases, cavitation damage to the material also affects the pump body in the area of the flow channels.
Cavitation rarely works in isolation - it is usually accompanied by additional effects:
- accelerated wear and leakage of the mechanical shaft seal resulting from vibrations and local pressure changes,
- damage to engine bearings due to vibrations transmitted to the shaft,
- reduced pump efficiency and increase in electricity consumption in relation to the effect achieved,
- in extreme cases - complete loss of impeller patency and the need to replace it.
Importantly, damage to pump components resulting from cavitation is usually not covered by the manufacturer's warranty because it results from incorrect selection or operation of the installation, and not from a manufacturing defect of the device. It is all the more profitable to prevent this phenomenon already at the stage of designing and commissioning the system.
How to prevent cavitation in practice – equipment and installation selection
Effectively reducing the risk of cavitation rarely comes down to one action - usually it is a combination of the proper selection of the pump and the appropriate installation equipment. In our offer, in addition to circulation pumps, circulation pumps, submersible pumps, submersible pumps and surface pumps of the IBO brands and IPRO, there are elements that directly support stable, cavitation-free operation of the system:
- Inverters (frequency converters) – by regulating the pump motor speed to the current water demand, they limit the pump's operation at points in the characteristics where the risk of cavitation is greatest. We write more about the principle of their operation in the article Submersible pump – types, installation and advantages of the inverter, and specific models and accessories can be found in the section inverters and accessories.
- Pump controllers – monitor pressure and operating parameters in real time, turning off the pump before long-term operation in cavitation or dry running conditions occurs. The phenomenon of dry running, closely related to the topic of cavitation, is described in more detail in the text What is pump dry running and how to protect yourself against it?.
- Check and shut-off valves – properly selected and installed in the suction pipeline, they reduce the risk of air jams and sudden pressure drops when starting the pump.
- Filters – installed on the suction side, they prevent solid contaminants from entering the impeller, which indirectly protects the working surfaces, already weakened by cavitation, against additional mechanical erosion.
- Pressure gauges – enable ongoing pressure control on the suction and discharge sides, allowing you to detect the first symptoms of cavitation long before audible and visible damage occurs.
When choosing a specific model, it is also worth using our general guide Dambat water pumps – guide to types and applications (IBO / IPRO), which helps match the type of pump to the nature of the installation before calculating the NPSH parameters.
Selecting a pump for a specific application – what to pay attention to
When designing or modernizing the installation, it is worth remembering that the selection of the brand or engine power alone will not be enough if the hydraulic balance of the suction side is ignored. The recommended sequence of actions when selecting a cavitation-resistant pump is as follows:
- Determining the actual suction height of the pump and the length and diameter of the suction pipeline in a specific installation.
- Calculation of pressure losses resulting from flow resistance, elbows, valves and the length of the suction pipe.
- Determining the temperature of the pumped liquid and reading the corresponding saturated vapor pressure from the tables.
- Determining the available NPSHav for a given installation and comparing it with the NPSHr value provided by the manufacturer for the planned operating point.
- If the margin is too small, consider changing the location of the pump (e.g. submersible installation instead of surface installation), shortening the suction pipeline or using a model with a lower NPSHr.
The offer includes both models intended for home use, as well as solutions for agricultural and industrial installations - incl IBO surface pumps for shallow shots and submersible pumps dedicated to deep wells, where the correct calculation of the suction height is particularly important. If in doubt, our technical department will be happy to help you choose a model for specific installation parameters.
Frequently asked questions (FAQ)
How does cavitation differ from normal pump noise?
The operating noise of the pump is usually constant and monotonous, while the sound accompanying cavitation resembles the pressing of gravel or small stones - it is irregular and appears suddenly, most often when the pump operating point changes, for example when water consumption is higher.
Can cavitation occur in a submersible pump?
The risk is much lower than in surface suction pumps because the impeller works flooded with liquid, which eliminates the problem of suction height. However, cavitation in submersible pumps may occur when the pump is operated close to the water table or at a flow rate that is too high compared to the capacity of the source.
How quickly does cavitation destroy the pump?
It depends on the intensity of the phenomenon. If the NPSHr is slightly, periodically exceeded, the first traces of erosion on the rotor may appear after a few months of operation. In case of strong, continuous cavitation, serious damage to the pump components may occur after just a dozen or so hours of operation.
Does the inverter completely eliminate the risk of cavitation?
The inverter limits the pump operation at unfavorable points in the characteristic curve and helps maintain stable pressure, but does not replace proper design of the suction side. If the NPSHav of the installation is significantly below the NPSHr of the pump, simply adjusting the engine speed will not eliminate the problem.
How important is water temperature for the risk of cavitation?
As the liquid temperature increases, the saturated vapor pressure increases, reducing the available NPSHav margin. Therefore, the same installation that works safely with cold water may already generate areas of cavitation at elevated medium temperatures.
Is a pressure gauge enough to diagnose cavitation?
A pressure gauge on the suction and discharge sides is a very helpful diagnostic tool, showing pressure fluctuations typical of cavitation, but a full diagnosis should be supplemented with the observation of noise, vibration and analysis of installation parameters (suction height, pipeline length, liquid temperature).
