Cavitation phenomenon of centrifugal pump and its preventive measures

Centrifugal pump is widely used in petrochemical, metallurgy, water conservancy, electric power, nuclear power and other industrial fields. It carries out power transmission of liquid medium in various production devices. Its performance reliability plays a very important role in the normal operation of the device. Cavitation is an important phenomenon in the operation of centrifugal pump. It is the most common problem affecting the operation reliability and service life of centrifugal pump. At the same time, it is also a huge obstacle to its development in the direction of large flow and high speed. Therefore, cavitation has become an important topic in pump research.

Principle of cavitation

Cavitation is a hydrodynamic phenomenon. The fundamental reason is that the local pressure drop occurs in the flow process of liquid, forming a low-pressure area. According to the knowledge of physics, for a liquid medium, there is a certain saturated vapor pressure PV at a certain temperature, and vaporization will occur when the pressure of the medium is less than PV. When the centrifugal pump is running, after the medium enters the suction port of the pump, the pressure decreases gradually before the impeller does work on the medium. When the pressure decreases to the saturated steam pressure at the corresponding temperature, the medium will boil and vaporize, resulting in a large number of bubbles in the original flowing medium. The bubbles contain the steam conveying the medium and the air originally dissolved in the medium. When the bubble flows from the low-pressure area to the high-pressure area along with the liquid flow, the medium pressure rises rapidly due to the work of the rotating impeller on the medium. When the pressure is greater than the saturated steam pressure PV at the corresponding temperature, the bubble will re condense into the liquid phase, forming a large number of holes instantly, and the surrounding liquid phase media rush to the holes at high speed and collide with each other, The local pressure at the cavity increases sharply. This kind of liquid hammer is a kind of impact with high strength and high frequency. Its pressure can reach more than hundreds of atmospheres, and the water hammer frequency can reach 25000 times/s. The fatigue failure gradually occurs on the material wall due to the repeated load of such high frequency and high pressure. Under some working conditions, active gas (such as oxygen, etc.) may be dissolved in the pumped medium. When the medium condenses from gas phase to liquid phase, it will release a lot of heat, produce electrochemical corrosion to the metal, accelerate the speed of corrosion damage, and cause pitting, perforation and even fracture on the metal surface. This phenomenon of vaporization, condensation and impact of liquid medium in the pump, resulting in corrosion and damage of metal materials is generally called cavitation of centrifugal pump.

Hazards of cavitation

Cavitation will affect the normal operation of centrifugal pump and cause many serious consequences.

Cavitation will degrade the performance of centrifugal pump

Centrifugal pump transfers the energy to the medium through the rotation of the impeller and converts it into the pressure energy of the medium, but cavitation will seriously interfere with the energy transfer between the impeller and the liquid. When cavitation occurs, a large number of bubbles will be generated in the medium, blocking the impeller channel, and generating vortices locally, increasing the flow loss, reducing the flow, head and efficiency of the pump. In serious cases, it will also lead to cut-off and make the centrifugal pump unable to work normally. From the performance curve of centrifugal pump when cavitation is serious in Figure 1, when cavitation is serious, all performance indexes drop sharply.

Fig.1 cavitation performance curve of pump

Cavitation will damage overcurrent components

In the flow passage parts of centrifugal pump, the impeller is the part most affected by cavitation. When cavitation occurs, many small pits will be gradually generated on the surface of metal materials, and then the pits will continue to develop and expand in honeycomb and groove shape. In serious cases, perforation will be formed, and even the impeller will be broken, which will seriously affect the service life of the pump.

Noise and vibration of pump caused by cavitation

When cavitation occurs, high-frequency liquids collide with each other, which will produce various noises. In serious cases, the pump will produce crackling explosion sound, and induce the vibration of the pump unit, which will accelerate the generation and rupture of bubbles. When the frequency of liquid hammer is the same as the natural frequency of the pump unit, strong cavitation resonance will occur and the amplitude will increase rapidly. At this time, if you want to protect the centrifugal pump from greater damage, you must stop immediately for inspection.

Cavitation restricts the development of centrifugal pump

With the continuous progress of science and technology, modern industry requires the centrifugal pump to develop to large flow and high head, which requires to improve the flow rate of the medium. According to hydrodynamics, the higher the liquid flow rate, the greater the inlet pressure loss, and more prone to cavitation. Therefore, improving the anti cavitation performance of pump and studying the cavitation mechanism are important research topics in the development of centrifugal pump.

Identification of cavitation of centrifugal pump

Cavitation is one of the main reasons for the decline of performance and efficiency of centrifugal pump. The occurrence of cavitation can be identified in time to take corresponding preventive measures. In actual production, the following methods can be used to judge whether cavitation has occurred.

Identification according to lift

This is a simple and widely used method in the industry. It can be seen from Figure 1 that when cavitation occurs, the head of the centrifugal pump will drop sharply. In API610 standard, the head of centrifugal pump (the first stage head for multi-stage pump) is reduced by 3% as the sign of performance fracture, and the value of necessary NPSHr of centrifugal pump is determined according to this. Generally, when the head on the characteristic curve of centrifugal pump drops by 3%, we think this point is the critical point of cavitation. However, in the initial stage of cavitation, the change of centrifugal pump head is not very obvious. When the change of head is obvious, cavitation has developed to a certain extent. Therefore, judging the cavitation of centrifugal pump by head has a certain lag.

Identification according to noise

When cavitation occurs, various noises will be generated due to liquid impact, and when cavitation is serious, the crackling sound similar to firecrackers can be heard in the pump. We can use this as the judgment of cavitation.

Vibration identification

The cavitation of the centrifugal pump is accompanied by the vibration of the pump body, so a vibration sensor can be added to the pump body. When the pump is running, it is found that the vibration is different from the normal, whether cavitation has occurred should be considered first. In actual production, we can feel the difference of pump body vibration according to experience, so as to preliminarily determine whether cavitation has occurred.

Preventive measures for pump cavitation

According to the conditions of cavitation, to avoid cavitation of centrifugal pump, npsha ＞ NPSHr shall be ensured, and a certain margin shall be reserved. Therefore, cavitation can be avoided by increasing npsha or reducing NPSHr in the design, manufacture and use of centrifugal pump.

Improve the structural design of the pump

To improve the NPSH performance of the pump, we can start with reducing the necessary NPSH of the pump according to the formula of the necessary NPSH of the centrifugal pump:

In the formula:

• V0 – average flow velocity at impeller inlet, usually refers to absolute liquid velocity at impeller throat, M/S;
• ω 0 – relative velocity of liquid at impeller inlet, M/S;
• λ 1 – correction coefficient of energy loss caused by the increase of liquid speed and change of flow direction from pump inlet to impeller inlet;
• λ 2 – pressure drop coefficient of fluid bypassing blade head, which is related to angle of attack, number of blades, shape of blade head, etc;
• G — gravitational acceleration, M/S2.

It can be seen from formula (1) that NPSHr is only related to the structure of the pump itself and has nothing to do with the properties of the medium. Therefore, the structure of the pump can be improved from the following aspects to reduce NPSHr:

• (1) Increasing the impeller inlet diameter d0 can reduce the impeller inlet velocity V0; Alternatively, the relative velocity of the liquid at the inlet of the impeller can be increased by increasing the width B1 of the inlet edge of the impeller blade ω 0 decreases. However, it should be noted that d0 and B1 are not the bigger the better, but have the best design range, otherwise the efficiency of the pump will decrease.
• (2) Properly increase the curvature radius of the inlet section of the impeller cover plate; Extend the blade properly to the inlet edge of the impeller and make the blade at the inlet as thin as possible; Improve the surface finish of impeller and blade inlet; Increase blade inlet angle and adopt positive attack angle; These measures can reduce the flow loss and make the medium flow more stable, so as to reduce the NPSHr of the pump.
• (3) The double suction impeller is selected, and the medium flows in from both sides of the impeller, which is equivalent to increasing the inlet area of the impeller and reducing the flow passing through each side of the impeller, so as to reduce the V0 ω 0 and λ 2. The anti cavitation ability of the pump is improved.
• (4) The installation of inducer for centrifugal pump can pre pressurize the medium, increase the medium head at the inlet of impeller, and significantly reduce NPSHr. However, the axial installation size of the inducer will be increased, and the head of the centrifugal pump equipped with the inducer will be reduced during small flow operation, showing a “Hump” on the curve. Therefore, it is not recommended to add the inducer to the centrifugal pump in API610 standard.

Increase the effective NPSH of the unit

During the design of the device, optimize the design as much as possible to improve the effective npsha at the pump suction:

• (1) Appropriately increase the diameter of the pump suction pipe, adopt the shortest suction pipe length as possible, reduce the roughness of the inner surface of the pipe, and reduce unnecessary elbows and valves, so as to reduce the pipe loss of the pump inlet pipe section, so as to improve npsha.
• (2) Increase the medium pressure of the pump suction tank to improve npsha.
• (3) When the npsha provided by the device can not meet the pump requirements, a suitable pump type, such as barrel bag pump, can be selected to reduce the installation height of the pump and increase the pressure at the suction inlet of the pump.

Use anti cavitation materials or conduct coating treatment on overcurrent parts

When the centrifugal pump is limited by working conditions and other factors and can not completely avoid cavitation, materials with good cavitation resistance can be used to manufacture the impeller to prolong the service life of the impeller. Practice has proved that the higher the strength and hardness of the material, the better the toughness, the more stable the chemical properties, and the better the cavitation resistance of the material. Commonly used materials such as stainless steel containing nickel chromium, aluminum bronze, high nickel chromium alloy, etc. In addition, it is more economical to use anti cavitation and wear-resistant materials based on epoxy resin to coat the surface of flow passage parts of centrifugal pump than expensive alloy steel.

Strengthen the operation management of the pump

During the operation of centrifugal pump, pay attention to the correct operation of the pump. Improper operation will artificially induce cavitation of centrifugal pump.

• (1) Ensure that the centrifugal pump works in the allowable working area.
• (2) Avoid using inlet throttling to regulate the flow of the pump.
• (3) The time for closing the valve and starting the pump shall not be too long.
• (4) For pumps with variable speed regulation, avoid excessive pump speed.

Conclusion

Cavitation is an important factor affecting the normal operation and service life of centrifugal pump. Understanding its generation principle and adopting appropriate measures to avoid cavitation can reduce or avoid the harm caused by cavitation. This paper introduces the common measures to avoid cavitation. According to the specific process requirements and operating environment, appropriate measures should be taken to improve the anti cavitation performance of the pump.
Authors: Xu Jinpeng, Qu Peng, Li Yang

Source: China Pipe Fitting Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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