mud pump cavitation quotation

Pump cavitation is the formation of bubbles or cavities in the liquid, developed in areas of relatively low pressure around the eye of an impeller. As the bubbles/cavities travel to the discharge side of the pump, moving to a high pressure area, the cavities implode. The imploding or collapsing of these bubbles triggers intense shockwaves inside the pump, causing damage to the impeller, vibration, and excess noise.

So what’s the cause of cavitation? What causes those cavities to develop? Generally, it’s the lack of NPSHA (Net Positive Suction Head Available). Essentially, the pump is being starved of fluid. (Read more about how this can happen: 9 System Changes That Screw With NPSH)

In a perfect world, your pump should be sized properly for the application and your piping design should match. However, that"s not always possible. If you’re dealing with pump cavitation, be aware that there is technology and techniques available to detect, monitor, and prevent pump cavitation. Here"s a list of options available.

With cavitation comes vibration. One of the newest technologies on the market is able to detect higher than normal vibration levels and alert operators to the pump"s upset conditions. The i-ALERT®2 is very cost-effective and can be installed on any pump (or rotating equipment).

One of the best things about this technology is that it has datalogging capability. So if your pump is cavitating or experiencing upset conditions when no one is around, or on another shift, you"ll know about it.

There are a number of things you can try to prevent cavitation. You can re-evaluate the piping design leading to the pump, ensure the pump is running at its best efficiency point, among others. But there is also technology available for cavitation prevention.

ITT"s PumpSmart automatically right-sizes your pump to your system. The video below illustrates how cavitation is prevented when tank level is low, and NPSHA is too low.

Installing gauges within your pump system is always a good monitoring option. You can reference the gauges to understand where the pump is on the curve. If it is starting to fall off track, your pump might be having a cavitation problem that requires investigation.

Clogged filters are a common cause of cavitation issues. As debris and particulates gather in the filter, NPSHA is reduced if the pump is not sized correctly.

Automatic self-cleaning filters like Eaton’s DCF and MCFseries are popular choices for processes where production can’t be stopped. Self-cleaning filters are great because, as the name suggests, they clean themselves. Once installed, they automatically filter any solids or debris from whatever liquid you are pumping. They are known to improve pump efficiency while requiring minimal operator intervention. Click to see a video on how self cleaning filters work.

A duplex strainer, also known as a twin-basket strainer, filters and removes large particles of dirt or debris from liquid pumping systems. They never require downtime for cleaning because a valve is placed between the two baskets, changing the flow of liquid to one strainer while the other is being cleaned.

An ounce of prevention is worth a pound of cure, as they say, so it’s best to properly size your pump from the beginning and ensure the piping design fits the application.

But we know that flows and processes change over time, creating an unintentional chain of events that can sometimes cause a cavitation problem where there wasn’t one before. Take advantage of the technology now available to properly manage and monitor the pumps in your facility.

Still not sure how to resolve a potential cavitation problem? Contact us! We’re happy to provide technical assistance to businesses in Wisconsin and Upper Michigan.

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.

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 trigger intense shockwaves inside the pump, causing significant damage to the impeller and/or the pump housing.

When a pump is under low pressure or high vacuum conditions, suction 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 when suction cavitation is present.

When a pump"s discharge pressure is extremely high or runs at less than 10% of its best efficiency point (BEP), discharge cavitation 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.

As with suction cavitation, 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.

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.

For more information about how to detect and prevent pump cavitation, be sure to check out our post: Technologies To Detect and Prevent Pump Cavitation.

Cavitation is a common problem in pumping systems, but with proper pump sizing, pipe design, and care of filters and strainers, damage to pumps and their impellers can be largely avoided.

Cavitation is an undesirable condition that reduces pump efficiency and leads to excessive wear and damage to pump components. Factors that can contribute to cavitation, such as fluid velocity and pressure, can sometimes be attributed to an inadequate mud system design and/or the diminishing performance of the mud pump’s feed system.

Although cavitation is avoidable, without proper inspection of the feed system, it can accelerate the wear of fluid end parts. Over time, cavitation can also lead to expensive maintenance issues and a potentially catastrophic failure.

When a mud pump has entered full cavitation, rig crews and field service technicians will see the equipment shaking and hear the pump “knocking,” which typically sounds like marbles and stones being thrown around inside the equipment. However, the process of cavitation starts long before audible signs reveal themselves – hence the name “the silent killer.”

Mild cavitation begins to occur when the mud pump is starved for fluid. While the pump itself may not be making noise, damage is still being done to the internal components of the fluid end. In the early stages, cavitation can damage a pump’s module, piston and valve assembly.

The imperceptible but intense shock waves generated by cavitation travel directly from the fluid end to the pump’s power end, causing premature vibrational damage to the crosshead slides. The vibrations are then passed onto the shaft, bull gear and into the main bearings.

If not corrected, the vibrations caused by cavitation will work their way directly to critical power end components, which will result in the premature failure of the mud pump. A busted mud pump means expensive downtime and repair costs.

As illustrated in Figures 1 and 2, cavitation causes numerous pits to form on the module’s internal surface. Typically, cavitation pits create a stress concentration, which can reduce the module’s fatigue life.

To stop cavitation before it starts, install and tune high-speed pressure sensors on the mud suction line set to sound an alarm if the pressure falls below 30 psi.

Accelerometers can also be used to detect slight changes in module performance and can be an effective early warning system for cavitation prevention.

Although the pump may not be knocking loudly when cavitation first presents, regular inspections by a properly trained field technician may be able to detect moderate vibrations and slight knocking sounds.

Gardner Denver offers Pump University, a mobile classroom that travels to facilities and/or drilling rigs and trains rig crews on best practices for pumping equipment maintenance.

Severe cavitation will drastically decrease module life and will eventually lead to catastrophic pump failure. Along with downtime and repair costs, the failure of the drilling pump can also cause damage to the suction and discharge piping.

When a mud pump has entered full cavitation, rig crews and field service technicians will see the equipment shaking and hear the pump ‘knocking’… However, the process of cavitation starts long before audible signs reveal themselves – hence the name ‘the silent killer.’In 2017, a leading North American drilling contractor was encountering chronic mud system issues on multiple rigs. The contractor engaged in more than 25 premature module washes in one year and suffered a major power-end failure.

Gardner Denver’s engineering team spent time on the contractor’s rigs, observing the pumps during operation and surveying the mud system’s design and configuration.

The engineering team discovered that the suction systems were undersized, feed lines were too small and there was no dampening on the suction side of the pump.

Following the implementation of these recommendations, the contractor saw significant performance improvements from the drilling pumps. Consumables life was extended significantly, and module washes were reduced by nearly 85%.

Although pump age does not affect its susceptibility to cavitation, the age of the rig can. An older rig’s mud systems may not be equipped for the way pumps are run today – at maximum horsepower.

Water hammer is a surge of pressure that can arise in pumping systems. The pressure is created when the pumping system undergoes an abrupt change in flow. The main causes of water hammering include opening and closing of valves, pump starts and stops, and separation and closure of the water columns. Due to these factors, the water column undergoes a change in momentum and this abrupt change can produce shock waves that travel back and forth within the system. Depending on the magnitude of the shock wave, physical damage in the system can be severe.

The phenomenon can be understood by an example in which water is pumped in a pipe that has valves on its both ends. The inlet valve is opened and the water column starts traveling towards the discharge valve. At this point, the discharge valve is closed instantly and the leading edge of the water column strikes the closed valve and begins to compress. A pressure wave (shock wave) begins to travel along the backstream (towards the inlet valve). The shock wave travels back and forth between the two valves until it finally diminishes due to friction losses. This water hammer shock wave is so fast that it can make a round trip between the two valves in less than half a second in the case of a 1000 feet pipe. The pressure created by this shock wave depends on the wave velocity (a), the velocity of water in the pipe (V), and the universal gravitational constant (g). Mathematically,

Cavitation affects the performance of water-jet pumps. Cavitation erosion will appear on the surface of the blade under long-duration cavitation conditions. The cavitation evolution under specific working conditions was simulated and analyzed. The erosive power method based on the theory of macroscopic cavitation was used to predict cavitation erosion. The result shows that the head of the water-jet pump calculated using the DCM-SST turbulence model is 12.48 m. The simulation error of the rated head is 3.8%. The cavitation structure of tip leakage vortex was better captured. With the decrease of the net positive suction head, the position where the severe cavitation appears in the impeller domain gradually moves from the tip to the root. The erosion region obtained by the cavitation simulation based on the erosive power method is similar to the practical erosion profile in engineering. As the net positive suction head decreases, the erodible area becomes larger, and the erosion intensity increases.

Compared with traditional propellers, water-jet pumps have the advantages of high propulsion efficiency, good manoeuvrability, and low vibration. It is widely used in the propulsion of high-speed ships. But the performance of the water-jet pump will be affected by the cavitation problem [1–5]. The cavitation plays an important role in the design and operation of hydraulic machinery, and it causes degradation, noise, vibration, and erosion [6, 7]. Cavitation erosion will appear on the surface of the blades when the pump is operated under cavitation conditions for a long time. It will not only affect the reliability of the overall system but also cause high maintenance costs [8–11].

Many scholars have studied the cavitation structure in water-jet pumps and try to explain the effect of cavitation on the performance of water-jet pumps through experiments and simulations. Park et al. [1] conducted experimental research based on PIV technology. The results show that the flow separation phenomenon is easy to occur at the lips of the flow channel when the inflow velocity decreases. Tan et al. [12] observed the formation of perpendicular cavitation vortex (PCV) in the impeller of the water-jet pump by experimental method. The shedding of PCV can cause the head to drop sharply. Motley et al. [13] used high-speed photography to observe the evolution of cavitation on the impeller of a water-jet pump. Cavitation first appeared in the tip clearance of the impeller. Long et al. [14, 15] captured the cavitation structure on the impeller of the water-jet pump at inception cavitation. Wu et al. [16] used the particle image velocity measurement method to study the turbulence structure of the tip leakage vortex of an axial water-jet pump.

Lindau et al. [17] simulated cavitation in the water-jet pump and found that cavitation would cause a sudden drop in thrust and torque. Katz’s research shows that the axial shear vortex structure has an impact on the development of cavitation in the separation zone [18]. The numerical result of Guo et al. [19] showed that the pressure pulsation amplitude of the monitors near the tip increases with the extent of cavitation. Huang et al. [20–23] analyzed the cavitation and vortex structures in water-jet pumps. The results show that the evolution of cavitation has aggravated the generation of vortex and flow instability in pumps. When cavitation occurs, the vortex expansion and baroclinic torque appear as violent fluctuation. Xu et al. [24] found that the viscous dissipation term has a larger magnitude at the tip clearance of water-jet pumps.

Cavitation erosion is a hot research currently [25]. Traditionally, cavitation erosion risk is assessed by experimental methods [26–30]. High-speed videos are used to assess the visual collapsing cavities. And it is complemented by paint test or erosive material test. But these methods are expensive. With the development of computational fluid dynamics (CFD), numerical methods become an attractive alternative. Ochiai et al. [31] used a Lagrangian method to assess the risk of cavitation erosion based on acoustic pressure emitted from bubbles. Peter et al. [32, 33] proposed a new method considering the microjet mechanism and applied the method to predict cavitation erosion around a hydrofoil. Pereira et al. [25, 34–36] used a macroscopic cavitation theory to assess the cavitation erosion risk in experiment and simulation. Usta et al. [37] compared the applicability of intensity function method, Gary level method, and erosive power method and predicted the erosible areas on a ship propeller.

The cavitation mechanism in the water-jet pumps was analyzed based on the RANS method and the DCM-SST turbulence model, and the cavitation erosion was predicted based on the macroscopic cavitation theory in this paper.

The cavitation in the water-jet propulsion pump will destroy the energy exchange between the impeller and the liquid, which results in the decline of the external characteristics. The experiment in this paper was completed on the closed test bench of the water-jet pump of the 708th Research Institute of China State Shipbuilding Corporation by Long et al. [38]. NPSHa is gradually reduced until the pump head drops by 3% through reducing the pressure at the pump’s inlet. High-speed photography technology was used to observe the cavitation structure during cavitation evolution through the plexiglass window in the impeller shell.

To capture the cavitation flow of the pump, the shooting frequency is determined as follows. If the rotating speed of the pump is and the impeller is required to acquire an image once it rotates a certain degree , the shooting frequency is

The ZGB model [41, 42] is a cavitation model based on the mass transport equation, which describes the cavitation phase change process mainly by establishing the transport relationship between the vapor and liquid phases. Its evaporation rate and condensation rate are defined as follows:

This simulation is based on the SSTturbulence model [50]. To more accurately simulate the development of cavitation in the centrifugal pump, the compressibility of the mixing of the vapor and liquid phases is considered, and the mixing density is corrected using the DCM method. Turbulent viscosity is defined as follows:

After completing the 3D modeling, the inlet pipe and the outlet pipe were divided into structural grids by software ICEM. Impeller and diffuser grids were structured using software TurboGrid. The leading and trailing edges of the blades are elliptical. The spline curve is used to fit the contour of each layer of the grids. The tip clearance of the grid was set to 0.3 mm. The grid of tip clearance region was divided into 10 layers to obtain a better flow field. Regarding the grid-independent inspection, the water-jet propulsion pump studied in this paper is similar to the calculation model of Huang et al. [20–23] in terms of geometrical dimensions and operating conditions. In Huang’s research, the mesh-independence check was made for the impeller, and it was pointed out that the mesh of the impeller is larger than 1.06 million to ensure the calculation accuracy. Considering the requirements of the turbulence model for the boundary layer, the of near-wall surface shown in Figure 3 was adjusted below 40. The number of grids used in this paper is shown in Table 3, and the details of the grids are shown in Figure 4.

The net positive suction head and corresponding head under the design flow rate are obtained. The cavitation characteristic curve was obtained by reducing the inlet pressure gradually. The experimental and simulation results are shown in Figure 5. This water-jet propulsion pump’s rated head is 13 m, and the simulated result is 12.5 m. The relative error is 3.8%, and this error can prove that the simulation is reliable. When the simulated head drops by 3%, NPSHa is 7.15 m, while NPSHa is 7.58 m in the experiment for the same situation. This difference may be because noncondensable gases, thermodynamic effects, and other factors were ignored. The existence of this difference does not affect capturing the flow characteristics in the pump by simulation. The cavitation image captured in the experiment is shown in Figure 6. Figures 5 and 6 are compared and analyzed. As can be seen in Figure 6(a), in the initial stage of cavitation, the head of point A basically does not change. It is generally believed that there is little cavitation in the blade channel at this operating condition, but the cavitation in the tip clearance can be seen from the image. As the inlet pressure decreases, the head curve begins to change. During the evolution of cavitation, point B on the cavitation performance curve was selected, as the initial cavitation. As the net positive suction head decreases, the tip leakage vortex (TLV) area becomes larger, and at the same time, the cavity grows on the suction surface, as shown in Figure 6(b). When the head drops by 3%, the point C on the cavitation performance curve is called the critical cavitation point. a~c on the simulated cavitation performance curve were selected to mark the cavitation situations corresponding to A~C on the experimental cavitation curve. The corresponding NPSHa of point A is 9.90 m, point b is 8.16 m, and point c is 7.15 m.

Figure 7 shows the cavitation structures under different net positive suction heads captured by the simulation, which were represented by gray isosurface with vapor volume fraction of 10%. Observed from the side view, the simulated cavity structure is basically consistent with the experimental results. When NPSHa is 9.90 m, tiny cavitation appears near the tip on the suction surface of the blades, as shown in region 1 of the figure. At the same time, the cavitation in the tip clearance was also captured, as shown in region 2 of the figure. At this situation, the appearance of cavitation has no effect on performance. As the net positive suction head drops to 8.16 m, the cavity develops from the tip to the root of the blades. In the flow direction, it covers most of the blade surface. It can be observed that there is “gap” in the cavitation isosurface near the leading edge of blades. As the net positive suction head continues to drop, it can be observed that the “gap” is more obvious, and the cavitation appears to be stratified. Compared with Figure 6, it can be considered that this delamination may be caused by the tip leakage vortex (TLV) (this will be analyzed in Figures 8–10). The existence of leakage flow causes tip leakage vortex cavitation (TLVC). The local pressure near the blades is lower than the saturated vapor pressure, and cavitation appears on the suction surface. The different cavitation structures gathered together, resulting in a decrease in pump performance. Figure 11 shows the change of the cavitation area at different circumferential positions in the blade channel under design conditions. It can be seen from the figure that as increases, the cavitation area first increases and then decreases. When , the maximum cavitation area of 0.186 cm2 appears at ; when , the maximum value of 8.20 cm2 appears at ; when , the maximum cavitation area of 22.8 cm2 appears at . This phenomenon verifies that the cavitation in the water-jet propulsion pump mainly occurs in the area near the tip of the blades. The cavitation coverage area expands from the tip to the root as the net positive suction head drops. The maximum cavitation area in the impeller domain moves from the tip to the root of the blades.

The evolution of cavitation is closely related to the flow field structure. There are many ways to identify the vortex structure [45]. To better analyze the structure of the flow field, the -criterion is introduced to describe the vortex:

Figure 8 shows the cavity distribution on different spanwise sections. When , the cavity in the impeller is divided into two parts: the attached cavity on the suction surface of the blades and the elongated vortex cavitation appearing at the leading edge of blades. When , only attached cavity exists on the blades. When is 80%, the cavity length is greater than that when is 99.5%. Figure 9 shows the distribution of in the spanwise section at different locations. In the far-field region, the values are negative. The results show that the trend of axial deformation dominates the distribution of the value, due to the rotation of the impeller. Also, it can be seen from the figure that the method can accurately identify the core of the tip leakage vortex (TLV) with a higher positive value on the blades’ suction side (SS). The TLV region gradually becomes longer with the decreases of NPSHa. The value near the initial position of the TLV is the highest. As the TLV moves away from the leading edge, the value gradually decreases. But its value is still positive, and the flow is still dominated by the rotational effect. There is an area with extremely low value of around the TLV structure, which reflects the restriction of the rotation effect and the deformation effect. At the position corresponding to the TLV structure in Figure 8(a), there is a slender cavitation on the SS of the blades, and the shapes are basically similar. This cavitation is caused by tip leakage. As shown in Figure 10, the TLV structure seems a “boundary” that defines the cavitation on the leading edge of the blades. The cavitation only develops in this boundary. This is consistent with the results observed through experimental results in Figure 6. Three planes were set to analyze the relationship between tip leakage vortex and cavity structure as shown in Figure 12. The area of tip leakage vortex is small on plane A. The cavitation structure is located at the vortex core. And the value of beyond the phase boundary is also high. The development of tip leakage vortex will be affected by the rotation of the impeller. From plane A to plane C, the TLV region gradually became bigger and the max value of reduced. The position of the phase boundary moves away from the vortex core area.

Cavitation will cause damage to the pump blades, and the erosion profile in practical engineering is shown in Figure 13. Cavitation erosion is a microscopic and transient process, but it is also affected by macroscopic flow conditions. From the energy point of view, the collapse of the cavitation bubble will produce a pressure wave. This pressure wave is one of the factors that cause cavitation damage. Based on the hypothesis of Pereira et al. [25, 34], the potential energy of the cavity structure is defined as

The cavitation volume is reduced when cavity collapses, and the pressure wave is released. The erosive power function can be defined as [25, 27–30, 33, 35, 36, 40, 46]

There are 5 key parameters in equation (8), which are , , , and . In order to better analyze the distribution of related parameters and their influence on cavitation erosion prediction, the distribution information of related parameters at different spans was extracted and drawn in Figures 14–16. Figure 14 shows the cavitation volume fraction distribution under rated flow rate. When , there is a tiny cavity near the tip at and 99.5%. At the position of , the cavity coverage length is longer than that at . But near the root at , 40%, and 60%, no cavity exists. When the net positive suction head decreases to 8.16 m, the cavity volume at and 99.5% near the tip increased rapidly. In the flow direction, the length of the cavity coverage becomes longer, but the length of the cavity at the position of is still greater than that at . Cavitation also exists at , 40%, and 60%. With a change of from tip to the root, both the vapor content and the coverage length decreased. When , the coverage length of cavity at different positions becomes longer. The position with the longest cavity is at . The maximum vapor content at the positions of , 40%, and 60% all becomes larger. Comparing the vapor volume fraction distribution under different NPSHa, it can also be observed that the leading edge of the blade at has lower vapor content. The cavitation volumes at the leading edge (LE) of the blade at position rise rapidly along the flow direction. The cause of this change may be due to the existence of the tip leakage vortex.

can be used as an indicator for cavitation prediction [47]. Figure 16 shows the distribution of under the rated flow rate. When , the variation of mainly appears at the leading edge and trailing edge of the blades. The maximum value of appearing on the leading edge is greater than that appearing on the tailing edge. As the net positive suction head decreases to 8.16 m, the maximum value of still appears at the leading edge of the blade. But the fluctuation area of near the trailing edge of the blade begins to become larger. When NPSHa decreases to 7.15 m, the area with large fluctuations of on the suction surface of the blade expanded, and the maximum value of on the blade also increased from to . This phenomenon shows that as the net positive suction head drops, the pressure fluctuations gradually become larger.

The unsteady simulation captured the transient contours of vapor volume fraction, , , and . The erosion caused by cavitation is the result of a long-duration accumulation. To better predict the erodible region, the Matlab code was used to process the images into time-averaged results. For ease of description, the abbreviated form is expressed as follows: TAV (time-averaged vapor volume fraction), TAA (time-averaged ), TAP (time-averaged ), and TAPP (time-averaged ).

Figure 18 shows the TAA distribution after one rotation at rated flow rate. Based on the hypothesis of macroscopic cavitation, pressure waves will be generated when the vapor content decreases. The threshold of the parameter was set to . When NPSHa is 9.90 m, the large value of appears near the leading edge of the blade near the root. Compared with the other two NPSHa, the maximum value of is smaller and the distribution area is not obvious. As the net positive suction head decreases, the TAA distribution area gradually becomes larger, expanding from the leading edge to the tailing edge along the flow direction and from the tip to the root along the span direction. The maximum value of on the suction surface of the blade gradually becomes larger. Compared to Figure 17, it can be seen that the distribution of TAA matches with the shape of the cavity coverage line.

A pressure pulse with positive amplitude is an important indicator of cavitation erosion [47]. The threshold of the parameter was set to . Figure 19 shows the TAP distribution after one rotation at rated flow rate. It can be seen from the figure that there is a higher value on the leading edge of the blade after the time-averaged treatment. The occurrence of this area is related to the impact of the incoming flow on the blades. At the rear part of the blade, the distribution of TAP is related to the distribution of cavities. The high TAP in the midstream without cavitation is concentrated and is also close to the root of the blade, as shown in Figure 19(c). As the net positive suction head decreases, the maximum value of TAP gradually increases, and it gradually moves to the trailing edge.

Based on the distribution of TAA and TAP, the time-averaged distribution of erosion power after one rotation is obtained based on equation (8), as shown in Figure 20. The shape of the erosion region in the figure matches with the cavitation coverage area in Figure 21. This is consistent with the conclusions obtained by other scholars [49]. The result of cavitation prediction is also consistent with the phenomenon in Figure 13. The light-colored areas on the blades represent cavitation erosion in the figure. Cavitation erosion first appeared near the tip of the blade and developed to the trailing edge of the blade along the flow direction. There is more cavitation erosion near the tip of the blades. And cavitation erosion basically does not exist on the blade roots. The simulation results showed the same trend. When NPSHa is 9.90 m, the region of cavitation erosion appears at the leading edge near the tip with a near-triangular shape. The erosion area expanded when NPSHa is 8.16 m and mainly distributed near the tip. The high value distribution of TAPP is basically located near the cavity closure. As the net positive suction head continues to decrease, the erosion area expands toward the trailing edge of the blade along the flow direction and expands toward the root of the blade along the span direction. It can be observed from Figure 20 that as NPSHa decreases, the maximum value of TAPP gradually increases, and the risk of erosion becomes more serious.

Simulated and experimental methods were used to study the unsteady flow mechanism and cavitation erosion in a water-jet pump, and the following conclusions are obtained:

(1)The head of the water-jet pump calculated by using the DCM-SST turbulence model is 12.48 m. The calculation error at the rated head is 3.8%(2)With the decrease of the net positive suction head, the position where the severe cavitation appears in the impeller domain gradually moves from the tip to the root(3)The erosion region obtained by the erosion power method is similar to the cavity profile. As the net positive suction head decreases, the erodible area and the erosion intensity become larger

Cavitation is potentially the most destructive force at play within a pumping system, however it is the least understood issue by many pump operators. To quote Sun Tsu from the Art of War, “know thy enemy”. In this blog, we will discuss our enemy cavitation and how it relates to pump cavitation.

I have often heard people refer to pump cavitation as the “sucking of air”.  Although, similar in effect, the two problems are completely different and unrelated.

Cavitation is actually the formation and collapseof vapour bubbles (cavities full of vapour) within a liquid – i.e. small liquid-free zones. Although the liquid free zones resemble air bubbles they do not contain air which is a very important distinction that we will elaborate on in future blogs.

The vapour bubbles from cavitation are created by the movement of a mechanical device in a liquid (or the movement of a liquid over a mechanical device) that results in a pressure drop within that liquid to below that liquids vapour pressure. These vapour pockets then collapse as they transition into a higher pressure zone.

The detrimental portion of cavitation is not the creation of the vapour pocket. It is the actual collapse of the vapour pocket that causes destruction. The energy released as the bubble, or pocket, collapses forms a “micro jet” which can be devastating to any surface in contact with the imploding bubble.

Cavitation micro jets have been recorded at pressures as high as 145,000,000 psi. This exceeds the elastic limit for any exotic alloy, thus proving that even the most exotic alloys cannot prevent cavitation damage.

As mentioned earlier in the blog, the vapour bubbles from cavitation are created by the movement of a mechanical device in a liquid (or the movement of a liquid over a mechanical device).  As such, cavitation can take many forms but the most common form in a pump by far is caused by suction.

As the impeller vane rotates, liquid struggles to flow in behind the moving vane lowering the pressure in that area. If the NPSHA at the suction inlet is below the pumps NPSHRvapour bubbles start forming.  As the bubbles progress down the vane the increasing pressure collapses the bubbles… Cavitation has started!

Assuming operational conditions are stable, and cavitation is minor, the only tell tale sign of cavitation may be the odd popping sound close to the suction inlet. Often confused with the sound of a rock entering a pump, this minor cavitation is often overlooked or ignored. Not good, but not disastrous either.

If conditions are such that there is a significant issue with maintaining a solid stream of liquid into the rotating impeller, major cavitation occurs. In this case the NPSHA is well short of the pumps NPSHR and the impeller is effectively alternating between pumping liquid and trying to pump water vapour.  With water vapour having almost no mass, the impeller is constantly loading and unloading as alternating pockets of liquid and vapour pass through.

There are several areas within a pump that cavitation can occur. Each has its own label. In this blog we have singled out suction cavitation and used it to illustrate the concept. I trust this short brief has provided you a basic understanding of the subject of cavitation. In the next blog we will discuss a similar subject: discharge recirculation, sometimes referred to as discharge cavitation!

With the NPSHr rating supplied with the pump, engineers only need to calculate the NPSHa on their own. Anyone can do this with the formula shown below:

HA is the atmospheric pressure affecting the surface of the liquid while it’s in the supply tank. Unless the system involves a closed tank, this is likely the local absolute pressure based on altitude. HZ measures the amount of vertical distance the slurry travels between the supply tank and the pump’s center line. Work from the lowest point the liquid can reach in the tank since draining the volume changes the NPSHa. HF accounts for friction caused by the piping between the tank and pump. Friction coefficients are recorded for most standard piping materials. HV reflects the velocity of the head found at the suction port. Many engineers leave this measurement out since it’s often very small. Finally, insert the HVP into the formula by measuring the vapor pressure of the liquid, which is based on its pumping temperature. Temperature fluctuates in many slurry pumping operations, so use the highest temperature since that will reflect the highest vapor pressure as well.

By using a simple formula to properly size slurry pumps with NPSH, users can keep them running for years with minimal maintenance and repairs. Preventing cavitation may take a little extra work in the beginning, but it will pay off for the entire lifespan of the slurry pump.

Through different hydrodynamic principles, EDDY Pump technology overcomes obstacles including cavitation/NPSH loss, seal failure and clogging. Cavitation, which affects a pump’s ability to deliver high percent solids while maintaining high production rates, is a constant problem in mining and other slurry pumping applications. Through different hydrodynamic principles, EDDY Pump technology can overcome cavitation, so the pump does not suffer from loss of suction or performance.

This phenomenon is accomplished through the synchronized eddy effect generated by the geometrically shaped rotor acting in sync with the hydrodynamic pattern of the volute. Tests show that there is no evidence of cavitation at speeds up to 2,000-rpm. The cumulative effect of this energy gives this pump a greater head than many pumps and the ability to pump more concentrated material over longer distances.

Instead of operating with an impeller, the EDDY Pump uses a patented rotor design, which can avoid wear and tear much longer than many traditional impellers commonly found in centrifugal and other pumps. Due to the shape of the rotor and larger tolerance between the volute, the pump ensures less abusive contact with the pumping material. Wearing plates and wear rings are also not needed to regulate efficiency, which eliminates the problem of wear rings coming into contact. When wear rings contact, it generates a high amount of friction, which produces heat that causes the rings to gall (friction weld). When galling occurs, the pump can seize.

You have just installed a pump and when you turn it on you immediately hear a crackling sound; the pump is cavitating. Technicians know it well, if you keep running with cavitation in pumps it will cost you money. Fortunately, cavitation in pumps can be easily avoided. Just a few small adjustments to the pump installation can make a big difference.

Cavitation in pumps occurs when bubbles form in a liquid as a result of rapid changes in pressure and then implode when the pressure increases further. We can use the example of a centrifugal pump to explain just what causes cavitation in pumps. Due to the high liquid velocity in the suction line, the pressure can fall below the vapour pressure, causing vapour bubbles to form. As soon as the liquid flow reaches the pump impeller, negative pressure changes into positive pressure, causing the vapour bubbles to implode.

Cavitation in pumps is therefore caused by the implosion of vapour bubbles. This is what causes the crackling sound and if left unchecked will also lead to significant damage to your pump.

It all starts by making the right pump choice. If you still hear a crackling sound in an existing or temporary pump installation, then try the following:

There are a number of additional measures that can also be taken to prevent cavitation in pumps, but these often lead to undesirable changes in the pumping process:

Do you have any questions about the above mentioned adjustments to prevent cavitation in pumps? Would you like to learn why these adjustments can make a big difference? If so, then contact us.

* Obstacles can include: unnecessary pump fittings, kinks in the suction hose, incorrectly selected suction strainer, contamination. Anything that can disrupt the flow of liquid on the suction side.

Cavitation arises because the suction pressure is so low that vapour bubbles form in the liquid. You can avoid this by creating a certain absolute pre-pressure on the suction side of the pump. This minimum required pressure to ensure proper functioning and to avoid cavitation in pumps is called the Net Positive Suction Head or NPSHr, with the r standing for “required”. The NPSHr differs for each pump – even each duty point – and is shown in the manufacturers performance curves diagram of the particular pump.

If the centrifugal pump is improperly operated during the start-up process and during the working process or if the liquid is vaporized in the low pressure zone, air binding and cavitation may occur.

Cavitation and air-binding will cause serious damage to the centrifugal pump. It’s important to learn the causes of the two phenomena and the preventive measures, so as to avoid cavitation and air-binding, and ensure the centrifugal pump in a stable condition.

Centrifugal pump is not the filling of liquid to be delivered before starting, or is in the process of operation within the pump into the air, because the density of gas less than the density of liquid, the centrifugal force is small, can"t take air out, the fluid inside the pump casing with motor for centrifugal movement is to produce negative pressure to suction fluid inside the pump casing, so it can not self-priming to transport the liquid, which is called air binding of a centrifugal pump.

The pump can"t transfer liquid, the mechanism produces severe vibration, and it is accompanied by strong and harsh noise. The motor is idling and it is easy to burn the motor. Affect the efficiency of the transport liquid and the normal operation of the centrifugal pump.

Before starting, the pump should be filled and the pump casing filled with the liquid to be delivered, and the outlet valve is closed at the start. In order to prevent the liquid poured into the pump casing from flowing into the lower tank due to gravity, a check valve (bottom valve) is installed at the inlet of the pump suction pipe. If the pump position is lower than the liquid level in the tank, no water must be filled with when start the pump. Sealing is important for the pump. The valve that is filled with water should not leak water and the sealing performance should be good.

When the liquid sucked in the pump casing is justified at the suction port of the pump due to the pressure reduction, it brings a huge hydraulic impact to the inner wall of the pump casing, so that the shell wall is corroded by "gas". This is called cavitation. phenomenon.

The bubble-containing liquid swells or ruptures after being squeezed into the high pressure zone. Due to the disappearance of the bubble, a partial vacuum is generated, and the surrounding liquid flows to the center of the bubble at a very high speed, which instantaneously generates a high-speed impact force of up to several tens of thousands of KPA, causing impact on the impeller and the pump casing, causing the material to be eroded and damage.

From the point of view of the cause of cavitation and air binding: the air binding is the air in the pump body, which usually occurs when the pump is started, mainly because the air in the pump body is not discharged; and the cavitation is due to the liquid reaching at a certain temperature. It has reached its vaporization pressure.

Foliar cavitation is cavitation that occurs on the surface of the blade, mainly because the pump is installed too high, or the flow is deviated from the cavitation when the design flow is too large. Its cavitation formation and collapse occur mostly at the front and back of the blade or at the inner surface of the front wheel and at the root of the blade.

When the water in the gap cavitation pump passes through the suddenly narrowing gap, the speed increases, the partial pressure drops, and cavitation also occurs. For example, in the gap between the outer edge of the axial pump blade and the pump casing, the gap between the seal ring of the centrifugal pump and the outer edge of the impeller is caused by the large pressure of the inlet side and the water outlet side of the impeller, resulting in high-speed reflow and partial pressure drop causing gap cavitation.

Vortex cavitation due to the sump, poor design of the inlet runner or the pump operating under non-design conditions, it is also possible to produce a top-down band vortex (referred to as vortex band) below the impeller, when the vortex center pressure is lower than When the pressure is vaporized, the vortex band becomes a cavitation zone.

Rough cavitation is when the water flows through the uneven inner wall surface and the over-current component in the pump, and the local negative pressure is easily generated downstream of the protrusion to cause cavitation, which is called rough cavitation.

(1) Deterioration of the performance of the pump, a large amount of cavitation will occur when cavitation occurs, and when the water contains a large number of cavitation, the normal law of the water flow is destroyed, the effective flow area of ​​the groove is reduced, and the flow direction changes accordingly. The loss increases, causing a rapid drop in pump flow, head and efficiency, and even a break when cavitation is severe.

(2) Damage to the over-current components, the wall surface of the pump under the repeated action of high-intensity impact force, local deformation and hardening and brittleness of the metal surface, resulting in metal fatigue phenomenon, causing metal cracking and peeling. In addition to the mechanical action, it is also mixed with the chemical corrosion of the metal by the deep and active gas (such as oxygen) escaping from the water body and the electrochemical corrosion of the metal by the water body. Under the combined effect, the wall surface of the pump initially appears as a pitting, and then becomes a honeycomb shape. In severe cases, the wall surface will be hollowed out in a short period of time.

(3) Vibration and noise are generated. When the bubble collapses, the liquid particles collide with each other and also hit the metal surface to generate noise of various frequencies. In severe cases, the violent explosion sound in the pump can be heard and the unit vibration is caused. Under the repeated action of the huge impact of the impeller, there are spots and cracks on the surface, and even the sponge gradually falls off, which reduces the service life of the pump.

Choosing the right material can improve the resistance to cavitation. Metal materials with high strength and toughness generally have good cavitation resistance, and improving the corrosion resistance of the material will also reduce cavitation damage.

The pressure at the inlet of the centrifugal pump should not be too low, but there should be a minimum allowable value. The corresponding NPSH is called the necessary NPSH, which is generally determined by the pump manufacturer through cavitation test and as the performance of the centrifugal pump. Listed in the pump product sample. When the pump is in normal operation, the actual NPSH must be greater than the necessary cavitation margin, which should be greater than 0.5m in China"s standards.

In addition, for the pump manufacturer, it is necessary to improve the anti-cavitation ability of the centrifugal pump itself, such as improving the structural design of the suction port to the vicinity of the impeller; using the front induction wheel to increase the flow pressure; increasing the inlet angle of the blade, minus Bending at the inlet of the small blade to increase the inlet area.

The air binding and cavitation of the centrifugal pump is very unfavorable for the centrifugal pump. Therefore, before using the centrifugal pump daily, it must be carried out according to the operating procedures to avoid the occurrence of air binding. At the same time, it is necessary to regularly inspect and maintain the inlet and outlet pipes and blades of the centrifugal pump to prevent cavitation.

Cavitation is one of the major disadvantages in pumping system, which enhance to form bubbles in the pipeline and it reduces the efficiency of the pump. So it should be identified and take the preventive measure. Machine Learning is a fast and computational method which can easily detect any faults in the pumping system. Still now lots of work has been done on a detection of fault in the pumping system, but mainly those work has done based on vibration details and variation of speed. The paper…Expand

Across any pumping system there is a complex pressure profile. This arises from many properties of the system: the throughput rate, head pressure, friction losses both inside the pump and across the system as a whole. In a centrifugal pump, for example, there is a large drop in pressure at the impeller’s eye and an increase within its vanes (see Figure 1). In a positive displacement pump, the fluid’s pressure drops when it is drawn, essentially from rest, into the pumping chamber. The fluid’s pressure increases again when it is expelled.

If the pressure of the fluid at any point in a pump is lower than its vapour pressure, it will literally boil, forming vapour bubbles within the pump. The formation of bubbles leads to a loss in throughput and increased vibration and noise. However, when the bubbles pass on into a section of the pump at higher pressure, the vapour condenses and the bubbles implode, releasing, locally, damaging amounts of energy. This can cause severe erosion of pump components.

To avoid cavitation, it is important to match your pump to the fluid, system and application. This is a complex area and you are advised to discuss your application with the pump supplier.

The obvious symptoms of cavitation are noise and vibration. When bubbles of vapour implode they can make a series of bubbling, crackling, sounds as if gravel is rattling around the pump housing or pipework. In addition to the noise, there may be unusual vibrations not normally experienced when operating the pump and its associated equipment.

With centrifugal pumps, the discharge pressure will be reduced from that normally observed or predicted by the pump manufacturer. In positive displacement pumps, cavitation causes a reduction in flow rather than head or pressure because vapour bubbles displace fluid from the pumping chamber reducing its capacity.

Power consumption may also be affected under the erratic conditions associated with cavitation. It may fluctuate and will be higher to achieve the same throughput. Also, in extreme cases, when cavitation is damaging pump components, you may observe debris in the discharged liquid from pump components including seals and bearings.

Under the conditions favouring cavitation, vapour bubbles are seeded by surface defects on metal components within the pump: for example, the impeller of a centrifugal pump or the piston or gear of a positive displacement pump. When the bubbles are subjected to higher pressures at discharge they implode energetically, directing intense and highly focussed shockwaves, as high as 10,000MPa, at the metal surface on which the bubbles had nucleated. Since the bubbles preferentially form on tiny imperfections, more erosion occurs at these points.

When a pump is new, it is more resistant to cavitation because the metal components have few surface imperfections to seed bubble formation. There may be a period of operation before any damage occurs but, eventually, as surface defects accumulate, cavitation damage will become increasingly apparent.

Classic (or classical) cavitation occurs when a pump is essentially starved of fluid (it is also called vaporization cavitation and inadequate NPSH-A cavitation). This can occur because of clogged filters, narrow upstream pipework or restricting (perhaps partially closed) valves. If the pump is fed from a tank, the level of liquid (or pressure above it) may have fallen below a critical level.

In a centrifugal pump, ‘classic’ cavitation occurs at the eye of the impeller as it imparts velocity on the liquid (see Figure 1). In a positive displacement pump, it can happen in an expanding piston, plunger or suction-side chamber in a gear pump. Reciprocating pumps, for example, should not be used in self-priming applications without careful evaluation of the operating conditions. During the suction phase, the pump chamber could fill completely with vapour, which then condenses in a shockwave during the compression phase.

Vane Passing Syndrome, also known as vane syndrome, is a type of cavitation that occurs when the spacing between the vanes of a centrifugal pump’s impeller and its housing is too small, leading to turbulent and restricted flow and frictional heating. The pumped liquid expands as it passes beyond the constriction and cavitation occurs.

Suction recirculation (also called internal recirculation) is a potential problem observed with centrifugal pumps when operated at reduced flow rate. This might occur, for example, when a discharge valve has been left partially closed or when the pump is being operated at a flow below the minimum recommended by the pump manufacturer. Under these conditions, liquid may be ejected from the vanes back towards the suction pipe rather than up the discharge port. This causes turbulence and pressure pulses throughout the pump which may lead to intense cavitation.

Air can be sucked into a pumping system through leaking valves or other fittings and carried along, dissolved in the liquid. Air bubbles may form within the pump on the suction side, collapsing again with the higher pressure on the discharge side. This can create shock waves through the pump.

To avoid cavitation, the pressure of the fluid must be maintained above its vapour pressure at all points as it passes through the pump. Manufacturers of centrifugal pumps specify a property referred to as the Net Positive Suction Head Required or NPSH-R – this is the minimum recommended fluid inlet pressure. The documentation supplied with your pump may contain charts showing how NPSH-R varies with flow.

In fact, NPSH-R is defined as the suction-side pressure at which cavitation reduces the discharge pressure by 3%: a pump is already experiencing cavitation at this pressure. Consequently, it is important to build in a safety margin (about 0.5 to 1m) to take account of this and other factors such as:

Positive displacement pumps require an inlet pressure to be a certain differential greater than the vapour pressure of the fluid to avoid cavitation during the suction phase. This is discussed in terms of Net Positive Inlet Pressure (NPIP) in a similar manner to NPSH for centrifugal pumps. NPSH is measured in feet or meters and NPIP is measured in pressure such as psi or bar. When converted to the same units, NPSH and NPIP are the same. Manufacturers may quote NPIP-R as the recommended inlet pressure and provide charts showing how it varies with pump speed. The available or actual inlet pressure on an operating system is termed NPIP-A.

Cavitation is a potentially damaging effect that occurs when the pressure of a liquid drops below its saturated vapour pressure. Under these conditions it forms bubbles of vapour within the fluid. If the pressure is increased again, the bubbles implode, releasing damaging shockwaves. This can cause severe erosion of components. A common example of cavitation is when a centrifugal pump is starved of feed: vapour bubbles form in the eye of the impeller as it imparts velocity on the liquid and collapse again on the discharge side of the vanes as the fluid pressure increases. This can lead to damage to an impeller’s vanes, seal or bearing. Cavitation can also occur in positive displacement pumps such as gear pumps and plunger pumps.

With a centrifugal pump, ensure that NPSH-A is at least 0.5m greater than NPSH-R during operation. For example, if the pump is fed from a tank, ensure that the level of liquid in the tank (or pressure above it) is sufficient. For a positive displacement pump, make sure that the inlet pressure complies with the manufacturer’s NPIP requirements.

Pumps fail for dozens of reasons. No single change can eliminate all pump failures, but one small change can help to make your pumps more reliable, efficient, and safe. The change can take place at the time of repair and does not require any other modifications to your installation. That change is to upgrade the stationary wear components (wear rings, shaft bushings, throttle bushings) to a composite material such as DuPont™ Vespel® CR-6100 and reduce the internal running clearances within the pump. The rotating parts remain whatever metal you are currently using with the same surface finish you currently use. Long-term studies of this upgrade have shown that this small modification can double the life of “bad actor” pumps. Once the upgrade is complete, the metal-to-metal interfaces within the pump are eliminated and replaced by composite materials. The metal-to-composite interfaces do not seize, minimizing the risks associated with metal-to-metal contact in the pump. Because seizure risk is minimized, clearance at the wear interfaces can be significantly reduced, improving pump reliability and efficiency. Here are the top ten cntrifugal pump problems this upgrade will help you solve:

In a perfect world, pumps would not run dry. The reality of many applications, however, often creates a dry-running situation. Tanks need to be emptied, light products vaporize, and sometimes debris (or a closed valve) blocks the pump suction. In these situations, metal wear components can seize and cause severe damage. Conversely, Vespel CR-6100 wear rings can survive running dry for significant periods of time, minimizing damage to pump internals. In some cases, the pump can continue running without repair.

Pumps running dry is only one cause of pump seizure. A wide range of off-design conditions can also lead to high-energy pump seizures. Foreign objects can enter the pump, low flow operation can cause excessive shaft deflection, or fatigue stresses can cause a shaft to break. Again, a pump with metal wear components runs the risk of seizure and excessive damage. Replacing the wear components with Boulden’s B-Series,  Metcar®, or Vespel CR-6100 mitigates the damage under such scenarios. For example, in the photos a piece of metal is lodged in the impeller of the pump, causing extreme vibration. Operators immediately shut down the pump and switched to the installed spare. Fortunately, the pump had been rebuilt with composite case rings, which did not seize. The rotor spun down, the seals did not leak, and the plant continued to operate at full capacity. Even after being exposed to such extreme loads, the composite rings were intact with minimal wear.

In classic pump cavitation, the net positive suction head available (NPSHA) is less than the net positive suction head required (NPSHR). Sometimes, the problem can be easily solved by raising the level in the suction tank or making other small modifications to the suction system. Unfortunately, the easy fixes are rarely available. More commonly, the choices are a complete redesign of the suction system, a hydraulic rerate of the pump, or a complete replacement of the pump. In this evaluation, the wear rings are often overlooked, but they shouldn’t be. The NPSHR for a pump is directly related to the wear ring clearance. Reduce the wear ring clearance and the NPSHR is reduced. Increase the clearance and the NPSHR increases. Worn-out pumps that have been in service for many years begin to cavitate because their wear rings are worn out. By upgrading the wear rings, you can dramatically reduce the wear ring clearance to 50 percent of the API610 minimum values and reduce the NPSHR of the pump. The magnitude of the change is a function of the pump-specific speed and the percentage by which the clearance can be reduced. In many situations, the extra margin from tighter wear ring clearance is all that is needed to avoid pump cavitation.

Although less dangerous than a full pump seizure, pumps that gall during alignment can be a major annoyance. The most common type of pump to experience this problem is a horizontal multi-stage pump. These pumps rely upon the wear rings to create hydraulic forces that “lift” the rotor once the pump is running at full speed. While the rotor is being turned during alignment, the rotor experiences shaft sag, which often causes the center-stage bushing and middle-