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How to prevent cavitation in stainless steel corrosion-resistant centrifugal pump?

18 Feb 2021Email

   Stainless steel corrosion-resistant centrifugal pumps are widely used in petrochemical, metallurgical, water conservancy, electric power and nuclear power industries. The power transmission of liquid media in various production devices, and its performance reliability plays a very important role in the normal operation of the device. Cavitation is an important phenomenon in the operation of stainless steel corrosion-resistant centrifugal pumps. It is the most common problem affecting the reliability and service life of corrosion-resistant stainless steel centrifugal pumps. 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 the current pump research.

 

  1. The principle of cavitation in stainless steel corrosion-resistant centrifugal pumps

   Cavitation is a kind of fluid dynamic phenomenon, the root cause is that the liquid has a partial pressure drop during the flow process, forming a low pressure zone. According to the knowledge of physics, for a certain liquid medium, it corresponds to a certain saturated vapor pressure Pv at a certain temperature, and vaporization occurs when the pressure of the medium is less than Pv. When the centrifugal pump is running, after the medium enters the pump suction port, the pressure is gradually reduced before the impeller does not work on the medium. When the pressure drops to the saturated vapor pressure at the corresponding temperature, the medium will boil and vaporize, making the original flow A large number of bubbles appear in the medium, and the bubbles contain the vapor of the conveying medium and the air that was originally dissolved in the medium and escaped. When the bubbles flow from the low-pressure zone to the high-pressure zone with the liquid flow, the pressure of the medium rises rapidly due to the work of the rotating impeller on the medium. When the pressure is greater than the saturated vapor pressure Pv at the corresponding temperature, the bubbles will condense into a liquid phase again. A large number of cavities are formed instantly, and the surrounding liquid medium rushes to the cavities at high speed and collides with each other, causing the local pressure at the cavities to increase sharply. This kind of liquid hammer is a kind of high-strength, high-frequency impact. Its pressure can reach hundreds of atmospheres and the frequency of water hammer is as high as 25,000 times per second. The material wall is subjected to such high-frequency and high-pressure repeated loads. Fatigue failure occurs gradually. Under certain working conditions, active gas (such as oxygen, etc.) may be dissolved in the pumped medium. When the medium is condensed from the gas phase to the liquid phase, a large amount of heat will be released, which will cause electrochemical corrosion of the metal and accelerate the rate of corrosion damage. , Resulting in pitting, perforation and even fracture on the metal surface. This phenomenon of vaporization, condensation, and impact of the liquid phase medium in the pump, resulting in the corrosion and damage of metal materials, is collectively called the cavitation of the centrifugal pump.

 

   2. The harm of cavitation of stainless steel corrosion-resistant centrifugal pump


  2.1 Cavitation will reduce the performance of the centrifugal pump

  The centrifugal pump transfers energy to the medium through the rotation of the impeller, and converts it into pressure energy of the medium, but cavitation will cause serious interference to the energy transfer between the impeller and the liquid. When cavitation occurs, a large number of bubbles will be generated in the medium, which will block the flow path of the impeller, and generate vortices locally, which will increase the flow loss, which will reduce the flow rate, lift and efficiency of the pump. Flow, making the centrifugal pump unable to work normally. Judging from the performance curve of the centrifugal pump when the cavitation is severe in Figure 2, when the cavitation is severe, the performance indicators drop sharply.

 

  2.2 Cavitation will damage the overcurrent parts

Among the flow parts of the centrifugal pump, the impeller is the most affected part by cavitation. When cavitation occurs, the surface of the metal material will gradually produce many small pits, and then the pits will continue to develop and expand into a honeycomb or groove shape. At times, perforations will be formed, and even the impeller will be broken, which will seriously affect the service life of the pump.

  2.3 Cavitation causes the pump to produce noise and vibration

When cavitation occurs, high-frequency liquids collide with each other to produce various noises. In severe cases, crackling explosions will be emitted in the pump, and the vibration of the pump unit will be induced at the same time, and the vibration of the pump unit will accelerate the generation of bubbles and rupture. When the frequency of the liquid hammer is the same as the natural frequency of the pump unit, a strong cavitation resonance will occur, causing the amplitude to increase rapidly. At this time, if the centrifugal pump is to be protected from greater damage, it must be shut down immediately for inspection.

  2.4 Cavitation restricts the development of centrifugal pumps

   With the continuous advancement of science and technology, modern industries require centrifugal pumps to develop with large flow and high head. This requires increasing the flow rate of the medium. According to fluid mechanics, the higher the liquid flow rate, the greater the inlet pressure loss, which is more likely to produce cavitation. Therefore, improving pump anti-cavitation performance and studying cavitation mechanism are important research topics in the development of centrifugal pumps.

 

   3. Stainless steel corrosion-resistant centrifugal pump centrifugal pump cavitation identification

   Cavitation is one of the main reasons that cause the performance and efficiency of centrifugal pumps to decline. The occurrence of cavitation can be identified in time to facilitate the adoption of corresponding preventive measures. In actual production, the following methods can be used to determine whether cavitation has occurred.

  3.1 Identify according to head

   This is a simple and easy method that is widely used in the industry. It can be seen from Figure 1 that when cavitation occurs, the head of the centrifugal pump will drop sharply. In the API610 standard, the centrifugal pump head (first stage head for multi-stage pumps) is reduced by 3% as a sign of performance failure, and the required NPSHr value of the centrifugal pump is determined accordingly. Usually when the head of the centrifugal pump characteristic curve drops by 3%, we think that this point is the critical point where cavitation occurs. However, in the initial stage of the pump cavitation, the change of the head of the centrifugal pump is not very obvious, and when the head changes When it is obvious, the cavitation has developed to a certain degree, so the cavitation of the centrifugal pump has a certain hysteresis when judged by the head.

   3.2 Identify based on noise

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

  3.3 Identification based on vibration

   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 vibration is found to be different from normal when the pump is running, it should first consider whether cavitation has occurred. In actual production, we can feel the difference in pump body vibration based on experience, so as to preliminarily determine whether cavitation has occurred.

 

   4. Preventive measures against cavitation in stainless steel corrosion-resistant centrifugal pumps

   According to the conditions of cavitation, if you want to avoid cavitation in the centrifugal pump, you should ensure that NPSHa>NPSHr, and there should be a certain margin. Accordingly, cavitation can be avoided by increasing NPSHa or reducing NPSHr during the design, manufacture and use of centrifugal pumps.

  4.1 Improve the structural design of the pump

  Improving the cavitation performance of the pump can start from reducing the necessary cavitation margin of the pump. According to the formula of the necessary cavitation margin of the centrifugal pump:

The necessary cavitation allowance formula for stainless steel corrosion-resistant centrifugal pumps

   Where: v0——the average flow velocity at the impeller inlet, usually refers to the absolute velocity of the liquid at the impeller throat, m/s;

  Ω0——The relative velocity of the liquid at the inlet of the impeller, m/s;

  Λ1-the correction factor for energy loss caused by the increase in speed and flow direction of the liquid from the pump inlet to the impeller inlet section;

  Λ2——The pressure drop coefficient of the fluid bypassing the blade head, which is related to the angle of attack, the number of blades, and the shape of the blade head;

  G——acceleration of gravity, m/s2.

   It can be seen from the 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 flow velocity v0; or increasing the impeller blade inlet edge width b1 can reduce the relative velocity ω0 of the liquid at the impeller inlet. But it should be noted that D0 and b1 are not the bigger the better, but the best design range, otherwise the efficiency of the pump will decrease.

(2) Appropriately increase the radius of curvature of the inlet section of the impeller cover; extend the blades to the inlet of the impeller appropriately, and try to make the blades at the inlet as thin as possible; improve the surface finish of the impeller and the inlet part of the blade; increase the blade inlet angle and use Positive angle of attack; these measures can reduce the flow loss and make the medium flow more stable, thereby reducing the NPSHr of the pump.

Stainless steel corrosion-resistant centrifugal pump impeller structure diagram

(3) 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, reducing the flow through each side of the impeller, thereby reducing the v0, ω0 and λ2 of the impeller, and improving the pump performance. Anti-cavitation ability.

  (4) is a centrifugal pump with an inducer, which can pre-pressurize the medium, increase the medium pressure head at the entrance of the impeller, and significantly reduce NPSHr. But the inducer will increase the axial installation size, and the centrifugal pump installed with the inducer will reduce the head when running at a small flow rate. From the curve, there is a "hump". Therefore, the centrifugal pump is not recommended in the API610 standard. Plus inducer.

  4.2 Improve the effective cavitation margin of the device

   When designing the device, optimize the design as much as possible to increase the effective NPSH NPSHa of the suction port of the pump:

(1) Appropriately increase the diameter of the suction pipe of the pump, use the shortest possible length of the suction pipe, reduce the roughness of the inner surface of the pipe, reduce unnecessary elbows, valves, etc., to reduce the pipeline loss of the pump inlet pipe section , Thereby improving NPSHa.

  (2) Increase the medium pressure of the pump suction tank to increase NPSHa.

  (3) When the NPSHa provided by the device cannot meet the pump requirements, a suitable pump type, such as a barrel pump, can be selected to reduce the installation height of the pump and increase the pressure at the suction port of the pump.

  4.3 Use anti-cavitation materials or coat the wetted parts

   When the centrifugal pump is restricted by working conditions and other factors, the occurrence of cavitation cannot be completely avoided, the impeller can be made of materials with good cavitation resistance to extend 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 and chromium, aluminum bronze, and high nickel-chromium alloys. In addition, it is more economical to use epoxy-based anti-cavitation and wear-resistant materials to coat the surface of centrifugal pump flow parts than to use expensive alloy steel.

  4.4 Strengthen pump operation management

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

  (1) Ensure that the centrifugal pump works in the allowable working area.

  (2) Avoid using the inlet throttling method to adjust the flow rate of the pump.

  (3) The time for the pump to close the valve and start cannot be too long.

  (4) For pumps with variable speed adjustment, avoid excessively high pump speed.

 

   Cavitation corrosion is an important factor affecting the normal operation and service life of stainless steel corrosion-resistant centrifugal pumps. Understanding its principles and adopting appropriate measures to avoid cavitation can reduce or avoid the harm caused by cavitation. This article introduces the common measures to avoid cavitation. According to the specific process requirements and operating environment, appropriate measures should be adopted to improve the anti-cavitation performance of the pump.

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