Why do centrifugal pumps cause cavitation?

Centrifugal pump. Causes of cavitation:

When a liquid is at a certain temperature, the pressure is reduced to the vaporization pressure at that temperature, and the liquid bubbles are formed. This phenomenon of creating bubbles is called cavitation. The bubbles produced during cavitation decrease in size and burst when they flow to high pressures.

This phenomenon of bubbles disappearing into the liquid due to rising pressure is called cavitation collapse.

The absolute pressure at which the pump is pumped if for some reason a localized area of its flow section (usually somewhere after the inlet of the impeller blades) is in operation.

When the vaporization pressure of the liquid at the current temperature is reduced, the liquid begins to vaporize there, producing a large amount of vapor and forming bubbles, and when the liquid containing a large number of bubbles passes forward through the high-pressure area in the impeller, the high-pressure liquid around the bubbles causes the bubbles to shrink sharply and even burst. At the same time as the bubble condenses and bursts, the liquid particles fill the cavities at a very high speed, which produces a very strong water hammer at this moment, and hits the metal surface at a very high impact frequency, and the impact stress can reach hundreds to thousands of atmospheres.

The impact frequency can reach tens of thousands of times per second, and in severe cases, the wall thickness will be broken down.

The process of creating bubbles and bursting bubbles in the pump, causing damage to the overflow parts is the cavitation process in the pump. After cavitation, the pump will not only have a destructive effect on the flow parts, but also produce noise and vibration, and lead to the performance of the pump to decline, and in serious cases, the liquid in the pump will be interrupted and cannot work normally.

1. Inlet reflux.

It is usually caused by various instability, and if the pump is operated at a low flow rate, the impeller may experience a combination of turbulence, backflow and vortex, which can quickly intensify at small flow rates.

2. Outlet reflux.

Outlet backflow is a similar case and can cause damage at the tip of the blade, pitting cavitation, and sometimes pitting damage at the tongue of the pump, which can also be caused by the pump operating at a low flow rate.

3. Entrainment of air.

Entrained air encompasses a number of different situations, the vapor bubbles are present in the liquid before they reach the centrifugal pump, and when they reach the inlet of the impeller, exactly the same thing happens, the vapor bubbles begin to experience increasing pressure after reaching the vanes and collapse, causing damage in the same position that communicates with cavitation.

Centrifugal pump cavitation: due to the suction height of the pump is too high or the temperature of the pump inlet medium is too high, so that the pressure in the pump is equal to or lower than the saturated vapor pressure at the temperature of the conveyed liquid, the liquid vaporization, bubble formation, rupture and other processes caused by the erosion phenomenon, called "cavitation" phenomenon, "cavitation" occurs when the liquid due to impact and noise, vibration, so that the flow rate is reduced, and even no liquid is discharged.

The root cause of cavitation in centrifugal pumps is liquid vaporization.

When a liquid is at a certain temperature and the pressure is reduced to the vaporization pressure at that temperature, the liquid produces bubbles. This phenomenon of creating bubbles is called cavitation.

The causes of cavitation in centrifugal pumps are:

The centrifugal pump is installed too much above the suction level.

The atmospheric pressure at the centrifugal pump installation site is too low.

The liquid conveyed by centrifugal pumps is hot.

Structural measures: double suction impeller is used to reduce the flow rate through the impeller, thereby reducing the NPSH of the pump; Install a booster pre-pump in front of a large high-head pump to increase the inlet pressure; When the gas reaches the high-pressure area, the vapor condenses, the bubble bursts, and the disappearance of the bubble leads to the creation of a local vacuum, and the liquid particles quickly rush to the center of the bubble, and the particles collide with each other, resulting in a high local pressure.

Increase the density of liquids.

The higher the density of the liquid, the smaller the suction height of the pump, when the installed pump that conveys the less dense liquid is used to transfer the denser liquid, the pump may produce cavitation, but when the pump that conveys the denser liquid is used to transfer the less dense liquid, the inlet pressure of the pump is higher and cavitation will not occur.

Increase the temperature of the liquid being delivered.

When the inlet pressure of the centrifugal pump is less than the saturated vapor pressure of the liquid at ambient temperature, a large amount of vapor escapes from the liquid and mixes with the gas to form many small bubbles; Cavitation occurs when the temperature of the conveyed liquid rises as the temperature of the conveyed liquid rises, and the pump cavitation occurs.

Factors influencing the occurrence of cavitation

The essential cause of cavitation is that the inlet pressure is less than the saturated vapor pressure at the fluid transport temperature. Cavitation mainly occurs at the outer edge of the impeller blade and cover plate, vortex shell or guide wheel, and does not occur at the blade inlet, for example, when the flow rate is greater than the design flow rate, it occurs at the front of the blade near the front cover plate of the blade inlet.

When the pressure at the inlet of the impeller drops to the saturated vapor pressure of the liquid to be sent at the working temperature, the liquid will be partially vaporized, and the generated bubbles will enter the high-pressure area with the liquid from the low-pressure area, and the bubbles will shrink and condense sharply in the high-pressure area, and the surrounding liquid rushes to the space occupied by the original bubble at a very high speed, generating a high-intensity shock wave that impacts the impeller and the pump casing, and causes noise and vibration.

The causes of cavitation in centrifugal pumps are mainly related to the following aspects:

Inlet pressure is too low: Inlet pipes that are too long or too small in diameter, or there are too many devices such as elbows and valves in the inlet pipeline, can cause the inlet pressure to decrease, which can cause cavitation.

Excessive flow rate: When the flow rate of a centrifugal pump is too large, the flow rate of the liquid at the inlet is accelerated and the pressure decreases, resulting in cavitation.

Gases in liquids: Dissolved gases in liquids can precipitate under centrifugal force, forming bubbles that further cause cavitation.

Liquid temperature is too high: Hot liquids are prone to cavitation because the solubility of gases in hot liquids is reduced and bubbles are easy to precipitate.

In order to reduce the hazards of cavitation in centrifugal pumps, the following measures can be taken:

Optimize the design of inlet pipelines to minimize the length of pipes and the number of resistance devices such as valves and elbows.

Reasonably select the model of centrifugal pump to make it work within the rated flow range and avoid overflow.

Regularly remove gases from the liquid to prevent the formation of bubbles.

Control the temperature of the liquid to avoid overheating.

Hello dear! The main causes of cavitation in centrifugal pumps include the following:1

The inlet pressure of the centrifugal pump is too low. If the inlet pressure of the centrifugal pump is too low, it will cause vacuum bubbles to appear in the pump chamber, which will cause the liquid to boil instantaneously and produce gas, which will reduce the pump efficiency or fail to work normally. 2.

The design of the pump inlet pipeline is not reasonable. Improper pipeline design will lead to unstable liquid flow in the pipeline, which is easy to produce liquid oscillation and aggravate the generation and diffusion of bubbles. 3.

Centrifugal pump impeller or housing is worn or corroded. Abrasion or corrosion of the impeller or casing can lead to an unstable flow of liquid in the pump chamber, creating an area of low pressure that promotes gas formation and diffusion. 4.

The liquid level is too low when the centrifugal pump is running. If the liquid level is too low, the liquid in the pump chamber will be thin, and the gas will easily dissolve into the liquid to form bubbles, which will cause cavitation. 5.

Centrifugal pumps use liquids that contain gases or dissolved gases. The presence of gases or dissolved gases in liquids further promotes gas dissolution and evaporation, which can lead to cavitation.

Centrifugal pump cavitation refers to the cavitation phenomenon at the suction end of the centrifugal pump, which leads to the decline of pump performance, increased noise, burial and even damage of the pump, affecting the normal operation of the equipment. The causes of cavitation of centrifugal pumps mainly include the following aspects:1

Too high suction pressure: If the suction pressure of a centrifugal pump is too high, it will cause the liquid to boil, resulting in cavitation. 2.

Gases in liquids: If there is too much gas in the liquid, cavitation can form at the suction end of the centrifugal pump. 3.

Liquid temperature is too high: If the temperature of the liquid being conveyed is too high, it will cause the liquid to boil and produce cavitation. 4.

Unreasonable design of centrifugal pump: If the design of centrifugal pump is unreasonable, such as too long inlet pipe and too many elbows, it will increase the resistance of the pump and cause cavitation. The above are the main reasons for cavitation of centrifugal pumps, and it is necessary to pay attention to the comprehensive consideration and control of these factors when using centrifugal pumps to avoid cavitation.

In summary, the causes of cavitation in centrifugal pumps include excessive suction pressure, gas in the liquid, excessive liquid temperature and unreasonable centrifugal pump design. The comprehensive consideration and control of these factors can effectively avoid the cavitation phenomenon of centrifugal pumps. I'm glad to answer for you, and I hope I can help you.

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The main cause of cavitation in centrifugal pumps is because the gas or vapor in the fluid reaches a saturated state, and when the pressure drops, the gas or vapor will precipitate out of the liquid and form cavitation. Methods to address cavitation in centrifugal pumps include:

1.Increase the inlet pressure: Increase the inlet pressure by increasing the height of the inlet pipe or installing an inlet vacuum pump to reduce the occurrence of cavitation.

2.Reduce the flow rate of liquids: Reduce the flow rate of liquids by adjusting the diameter of the pipe, increasing the length of the pipe, etc., so as to reduce the occurrence of cavitation.

3.Change the structure of the pump: use a pump with good cavitation performance, such as deep well pump, self-priming pump, submerged pump, etc.

4.Increase the liquid level height: By increasing the liquid level height, the liquid pressure can be increased, which reduces the occurrence of cavitation.

5.Increase the outlet pressure of the pump: By increasing the degree of closure of the outlet valve of the pump, the outlet pressure of the pump can be increased, thereby reducing the occurrence of cavitation.

In short, to prevent cavitation of centrifugal pumps, it is necessary to start from many aspects, including increasing the inlet pressure, reducing the liquid flow rate, changing the structure of the pump, increasing the liquid level height and increasing the outlet pressure of the pump.

A method for determining whether cavitation occurs in a centrifugal pump

1.Noise method

This method is relatively simple and can be done without contact with the pump body. However, due to the fact that the noise method is greatly affected by the ambient noise of the surrounding area, when the intensity is the highest. Generally, the cavitation of the centrifugal pump has reached a very strong stage, and the cavitation condition can be judged by the strong cavitation cracking sound in the ear.

Therefore, the pump body noise method is not suitable for on-site monitoring of cavitation.

2.Ultrasound

The ultrasonic method is simple to measure cavitation, easy to debug, and is not disturbed by other environmental noises, and has strong sensitivity to the occurrence and development of cavitation. Therefore, it is an ideal method for on-site monitoring of cavitation at pumping stations.

3.Observational method

This method is based on the observation after the fact, and the judgment is made based on the surface shape of the damage. Due to cavitation, casting porosity, erosion and wear, corrosion, etc., the shape of the metal surface will be different from the ideal shape. The metal surface damaged by cavitation usually appears honeycomb-shaped, which is caused by the fatigue and destruction of the metal surface caused by the local high-speed water hitting the metal, so the honeycomb hole is generally communicated with the outside, and most of the pits are perpendicular to the metal surface.

The looseness of casting defects is often hidden deep inside the metal, and sometimes the looseness and pores of the metal are mistaken for cavitation due to the erosion of the water flow, but when the surface is removed by mechanical methods, it will be found that there are still pores inside. Traces of erosion wear often appear in grooves in the same direction as the flow of water, but pay attention to whether there is a vortex of water flow.

4.Vibration method

A simple but less sensitive method of measuring the vibration frequency of a centrifugal pump body by means of an accelerometer probe. Especially for large pumps, the pump body stiffness is large. The excitation response caused by the bubble burst caused by local cavitation in the pump is sluggish, and there are many vibration sources on the pump.

Vibrations caused by cavitation are often masked by other vibrations. Therefore, the vibration method is only suitable as an auxiliary means for on-site monitoring of cavitation.

It has the following hazards. The performance of the centrifugal pump decreases. The pump's flow, head, and efficiency are all reduced.

If a large number of bubbles are generated, air binding may occur, forcing the centrifugal pump to stop working. Noise and vibration are generated, which affects the normal working environment of the pump. The materials of the pump casing and impeller were damaged, reducing the life of the pump.

Cavitation occurs because the static pressure near the impeller suction port is lower than a certain value. There are many reasons why the static balance pressure is too low, such as the installation height of the pump exceeds the allowable value, the temperature of the pumped liquid is too high, and the local resistance of the suction pipeline is too large. To avoid cavitation, try to keep the pressure near the impeller inlet lower than the saturated vapor pressure of the liquid at the conveying temperature.

In general, according to the anti-cavitation performance of the pump, it is an effective measure to prevent cavitation by reasonably determining the installation height of the pump.

Cavitation in centrifugal pumps can affect the performance and life of the pump, so measures need to be taken to prevent and prevent cavitation from occurring.

Here are some ways to prevent and prevent cavitation:

Choose an appropriate pump: The appropriate centrifugal pump should be selected according to the specific conditions and requirements of use. For example, choose the right pump model based on flow and head requirements, and avoid pump models that are too small or too large.

Reduced inlet pressure: If there is a high-velocity flow in the inlet pipe or a sudden contraction of the pipeline, the inlet pressure will decrease, which will lead to cavitation. Therefore, it is possible to reduce the inlet pressure and prevent cavitation by installing a pressure reducing valve or installing a diffuser at the inlet.

Installing an air valve or inlet pipe: Installing an air valve or air intake pipe can effectively prevent cavitation. This allows gas to be introduced to fill the air in the duct after the pump is stopped, preventing air and water from mixing and thus preventing cavitation.

Change the installation position of the pump: If the inlet pipe of the pump is too long or too high, it will also cause the inlet pressure to decrease, which can lead to cavitation. The inlet pressure can be increased by changing the installation position of the pump or increasing the diameter of the inlet pipe to prevent cavitation.

Regular maintenance and inspection: Regular maintenance and inspection of centrifugal pumps, including cleaning debris inside the pump and regularly replacing worn parts, can ensure the normal operation of the pump and prevent the occurrence of cavitation. Li Yunqi.