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Preface: Cavitation occurs frequently in many different pumping applications; it is a phenomenon that can cause severe damage to centrifugal pumps, and it is a direct result of improper operating conditions. The generally accepted view at present is that vaporization cavitation is caused by two changes resulting from pressure variations as the liquid passes through the impeller: 1) The pressure drops slightly as the liquid enters the pump through the inlet pipeline. The amount of decrease depends on the geometry and size of that part of the specific pump, and varies from pump to pump. Then, the liquid enters the inlet of the rotating impeller, where a significant pressure drop occurs. If the pressure at the inlet of the impeller is lower than the vaporization pressure of the liquid being pumped, the first phase of vapor cavitation occurs, which is the formation of bubbles. 2) When the bubbles are carried to the impeller blades by the centrifugal force of the impeller, they are immediately re-pressurized and collapse in a series of implosions, resulting in the second phase of this process, namely the collapse of the bubbles. At the moment when the bubble condenses, shrinks, and bursts, the liquid surrounding the bubble rapidly fills the cavity formed by its condensation and rupture, generating a strong shock wave. Although a single such shock wave is insignificant, their increasing frequency and severity can generate energy levels far exceeding the yield strength of most impeller materials, causing damage to them. Drawing on various engineering application experiences, this article starts with the typical symptoms of vaporization cavitation, differentiates it from backflow cavitation and air entrainment cavitation, and lists various monitoring methods to determine whether vaporization cavitation has begun, is occurring, or is about to occur. The ultimate goal is to provide owners with sufficient time windows for inspection and maintenance, thereby shifting from reactive maintenance to predictive maintenance. Typical symptoms of vaporization cavitation: For many users of centrifugal pumps as well as those who work with such pumps, there is generally a basic understanding of the symptoms associated with vaporization cavitation, which are as follows: 1) Pressure fluctuations. A necessary condition for the occurrence of vaporization cavitation is that the inlet pressure of the pump (impeller) be close to or no greater than the vaporization pressure of the fluid being pumped; in other words, cavitation arises due to a decrease (fluctuation) in the inlet pressure ； 2) Noise occurs. Cavitation noise usually sounds like a high-pitched crackling sound; as cavitation becomes more severe, it turns into a loud grinding noise, yet the sound remains relatively steady and regular ； Along with 3) decreased performance. The pump's head and flow rate will decrease ； And 4) increased vibration occurs. Cavitation usually comes with a significant increase in vibration ； And it causes 5) cavitation damage. Due to a low or insufficient NPSH margin, vaporization cavitation can cause corrosion of the flow-through components – such as the formation of small pits on the low-pressure side surface of the impeller inlet blades. Symptoms that can lead to confusion: Cavitation remains a problem that is difficult to diagnose accurately, as the typical symptoms of vaporization cavitation are the same as those of the other three conditions. This means that when we encounter unique noise and high vibration levels, they may also be caused by backflow or entrained air at the inlet or outlet, independent of the inlet pressure. 2.1 Inlet recirculation This condition is caused by various types of instability, such as the combined effect of turbulence, recirculation, and vortices that may occur in the impeller when the pump operates at a low flow rate. Sometimes referred to as \"decoupled\" or \"hydrodynamic\" cavitation, these flow patterns intensify at low flow rates. The flow rate at which this occurs varies depending on the impeller. 2.2 Outlet recirculation: Outlet recirculation is a similar situation that can cause pitting damage at the tips of the blades; it can also lead to pitting damage at the pump volute in (volute-type pumps). It could also be caused by the pump operating at a low flow rate. 2.3 Entrained air Entrained air encompasses various situations, in which steam bubbles are already present in the liquid before reaching the pump. When they reach the inlet of the impeller, exactly the same thing happens, as if they were created at that point. In other words, once the steam bubbles reach the blade, they begin to experience increasing pressure and collapse as a result, causing damage at the same location as that caused by cavitation. Since entrained air causes the same pitting damage to the impeller at exactly the same location as vapor cavitation, it can lead to confusion in fault diagnosis, especially when both phenomena may occur simultaneously under the same operating conditions. However, a quick comparison of NPSHA and NPSHR, combined with a visual inspection of the pipeline conditions, usually helps to determine the root cause of the so-called \"cavitation\" and resolve the air entrainment problem. 2.4 Similarities and differences: Conventional vapor cavitation, air entrainment, and backflow cavitation all cause pitting damage on the impeller, which is resulting from the formation of bubbles and their subsequent collapse. The difference between them lies in the method of bubble formation and the location of impeller damage resulting therefrom. As the severity of all these conditions increases, noise, vibration, and impeller damage also increase. Under severe conditions, pitting damage can spread throughout the impeller and may also extend to the pump casing. All these conditions have some similar symptoms. Therefore, they may be misdiagnosed. However, they are caused by three different situations; by focusing on these root causes, diagnosis can be simplified and made more accurate. It must be recognized that damage to the impeller is merely a consequence of the three situations mentioned above. Determining cavitation: Generally speaking, the biggest issue is figuring out which of the three aforementioned hydraulic conditions is at play when encountering common symptoms of noise and vibration. The pump flow rate can be reduced by adjusting the opening degree of the pump outlet valve, and then the effects on noise and vibration can be assessed based on these changes in flow rate. Adjusting the outlet valve of the pump to reduce the flow rate through it can result in three possible outcomes: 1) The noise and vibration will decrease significantly, or may even disappear entirely. 2) Noise and vibration will become more severe. 3) Noise and vibration changes are minimal or absent. In the first case, the pump operates more quietly and smoothly as the flow rate decreases, indicating that vapor cavitation is being eliminated. In the second case, noise and vibration become more severe, indicating backflow in the pump at low flow rates. In the third case, when noise and vibration changes are minimal or absent, it indicates a problem with trapped air, and it will not be affected immediately by changes in flow rate. Furthermore, some experience shows that damage caused by normal vaporization cavitation occurs on the low-pressure side (the non-working surface) of the impeller inlet blades, whereas damage caused by backflow cavitation occurs on the pressure-bearing side (the working surface) of the impeller blades. At the same time, the noise generated by backflow cavitation is an irregular popping sound. Cavitation monitoring: The most common forms of monitoring for centrifugal pumps are temperature monitoring (such as bearing temperature) and vibration monitoring. Due to investment cost considerations, the vast majority of centrifugal pumps are not equipped with specialized cavitation monitoring devices. Cavitation monitoring can be divided into two main categories: 1) Passive monitoring. Cavitation has occurred and is severe, accompanied by very obvious external symptoms such as a significant increase in noise and vibration, as well as a decline in performance. 2) Active monitoring. Cavitation may occur in the near future, or it may have already started but is still in its early stages, with no very obvious symptoms of cavitation (such as a small number of bubbles appearing, with no distinct external signs of cavitation). 4.1 Pressure monitoring: If the inlet pressure of the pump decreases significantly or drops suddenly, and falls below a certain value, cavitation may occur. 1) By subtracting the vaporization pressure (converted to head) from the pressure value measured by a pressure measurement instrument at the pump inlet (such as a pressure gauge or pressure transmitter), it is possible to determine whether NPSHA is greater than or equal to the NPSHR required on the manufacturer’s characteristic curve for the pump. This is the simplest and most commonly used passive cavitation monitoring method. 2) The pressure transmitter installed at the inlet of the pump (as close as possible to the impeller’s suction inlet) can also be used to monitor pressure fluctuations; if these fluctuations exceed a certain range or value, the pump may be at risk of cavitation. Through continuous online monitoring via pressure transmitters, it is possible to obtain trends in pressure changes, thereby enabling predictions of potential cavitation. For this reason, passive cavitation monitoring has been turned into active monitoring. 4.2 Noise monitoring When steam bubbles reach high-pressure areas, they undergo implosion, collapse, condense into liquid, and generate shock waves. A rapid implosion produces a characteristic popping sound associated with cavitation, which sounds like stones passing through the pump body. Cavitation noise is part of the total noise generated by pumps and systems. By using appropriate sound measurement equipment for continuous online monitoring, it is possible to analyze the trends in this noise and predict when cavitation is likely to occur, its intensity, as well as the potential for corrosion. This is active cavitation monitoring. The frequency of cavitation noise is generally between 10 kHz and 100 kHz, whereas the frequency of cavitation noise caused by backflow is around a few hundred Hz. 4.3 Performance Monitoring When cavitation occurs, in addition to generating significant noise, it is also accompanied by a decrease in the pump’s flow rate and head. Flow monitoring devices (such as venturi tubes, flow nozzles, etc.) and head monitoring devices (such as differential pressure transmitters) can be used to monitor the trends in flow rate and head over time. 4.4 Vibration monitoring: When cavitation occurs, in addition to generating significant noise, it is also accompanied by an increase in pump vibration. Vibration sensors are used for vibration monitoring. Two types of vibration sensors are commonly used on centrifugal pumps: 1) Proximal vibration sensors, which are used to measure the relative vibration of the pump shaft. Shaft vibration measurement should be peak-to-peak displacement, in μm. 2) Acceleration vibration sensor, used to measure the absolute vibration of the pump bearing housing. Vibration measurement of the bearing housing should use root mean square (RMS) velocity, with the unit being mm/s. For pumps equipped with hydrodynamic bearings, approach-type (eddy current) vibration sensors are commonly used ； For pumps equipped with rolling bearings, acceleration vibration sensors are typically used ； If the pump’s radial bearings are hydrodynamic sliding bearings, and the thrust bearings are 7300 series angular contact ball bearings mounted back to back, then the optimal configuration is to use proximity-type vibration sensors for the radial bearings, while using contact-type acceleration vibration sensors for the thrust rolling bearings. However, this configuration is rarely encountered in engineering practice. The aforementioned vibration monitoring method is typically used together with noise to indirectly determine whether cavitation has occurred in the pump. Cavitation can also be directly determined by measuring the vibration frequency of the pump (the main frequency of vibrations caused by cavitation is generally around 1 kHz). The vibration frequencies generated by different hydraulic causes (vibration excitation sources) are shown in Table 1. Table 1: Vibration frequencies generated by different excitation sources. Vibration frequency, Excitation source: 0.1 times the operating speed – vanes/diffuser flow separation (internal recirculation); 0.8 times the operating speed – impeller flow separation (internal recirculation); Number of blades × operating speed; Blade/volute clearance and cavitation. In current engineering applications, vibration monitoring is essentially a form of passive cavitation monitoring. By continuously monitoring the vibration frequency of the pump online, an early warning of cavitation can be issued as soon as those vibration frequency components are detected (indicating that cavitation has begun to occur, but in its early stages). The user can then take appropriate action or pay close attention depending on the actual situation. In this way, passive cavitation monitoring becomes active monitoring. 4.5 Bubble monitoring Bubble monitoring can only be carried out on transparent pump housings, using a high-speed camera to observe the formation and development of bubbles at the impeller inlet. For pumps used in high-value, critical applications, such as the main feed water pumps in the conventional island of nuclear power plants with million-watt units, in order to ensure that cavitation does not occur under any specified operating conditions and to guarantee their long-term safe and reliable operation, KSB uses the moment when bubbles first appear (i.e., the onset of cavitation) as a criterion for determining cavitation, and relies on this to select the head required of the booster pump. However, for various reasons, bubble monitoring has not yet been directly applied in engineering practice. Internationally, only a very few multinational pump companies (such as Germany’s KSB) conduct bubble tests on pumps used in critical applications. 4.6 Motor torque monitoring For pump sets equipped with variable-frequency drives, the functionality of intelligent variable-speed drives (VSDs) – also known as variable-frequency drives (VFDs) – can be used for monitoring. In addition to the benefits of energy efficiency and various useful functions (such as cleaning and removing blockages in pumps), some VSDs now also incorporate cavitation prevention software. This makes it possible to prevent cavitation without incurring any additional costs or increasing system complexity. The anti-cavitation software built into the intelligent VSD uses algorithms to measure the pump’s torque and speed in order to detect any specific patterns that indicate the occurrence of cavitation. Since measurements are taken directly from the pump shaft, there is no delay in the detection process, so the response is almost immediate. When cavitation is detected, the VSD can automatically adjust the pump speed in response to pressure changes. Once the pump stops cavitation, it will return to normal operation. Figure 1 shows the working principle behind the cavitation prevention algorithm. Figure 1: The cavitation prevention algorithm monitors how the pump motor torque changes over time. The VSD algorithm detects the onset of cavitation by comparing the torque measured on the shaft with the nominal torque. If cavitation has begun, the torque curve shown in red starts to rise. The software responds by adjusting the speed reference displayed in blue to a lower level. This will result in a decrease in the actual motor speed, as shown in green. The algorithm can be adjusted to adapt to different operating conditions. In many cases, if the cavitation duration after the motor speed decreases exceeds a set time, the VSD will be set to stop the motor from running. In other applications, it may be necessary to stop the motor from operating immediately upon detecting cavitation. Now, the new generation of intelligent VSDs makes it possible to resolve pump cavitation issues in real time within the drive. The anti-cavitation software built into the drive means that no additional components such as sensors or programmable logic controllers (PLCs) are required. The only additional work required is to set the operating parameters. As a result, potential flow problems caused by cavitation can be detected immediately by monitoring the torque changes on the pump motor shaft. This provides an accurate and immediate response to eliminate cavitation. This is a new type of active cavitation monitoring. Summary 1) The necessary condition for vapor cavitation to occur is that the inlet pressure of the pump (impeller) is close to or no greater than the vaporization pressure of the fluid being pumped. 2) Cavitation remains a problem that is difficult to assess accurately; both vaporization cavitation and cavitation caused by inlet recirculation, outlet recirculation, and entrained air can lead to pitting damage on the impeller, as well as a significant increase in noise and vibration. The difference between them lies only in the method of forming bubbles and the location of pump damage resulting therefrom. 3) The pump’s flow rate can be reduced by adjusting the opening degree of the outlet valve, and cavitation can then be identified based on the effects of these flow rate changes on noise and vibration. 4) Cavitation monitoring can be divided into passive monitoring and active monitoring. Active monitoring can issue early warnings before cavitation causes damage, thereby providing enough time for repairs. 5) For pump sets equipped with variable-frequency drives, the new generation of intelligent VSDs makes it possible to resolve pump cavitation issues in real time within the drive.