1. Cavitation diagnosis method of in-service pump

The user of the pump generally or cannot use the method adopted by the manufacturer to determine whether the cavitation of the pump occurs, that is, the method of determining whether the cavitation occurs by the drop of the head when the flow rate is constant. In addition to the observation method after cavitation damage, ultrasonic method, external noise method, vibration method and other methods can also be used to judge whether the pump in service has cavitation.

1. Observation method

This method is to observe afterwards and make a judgment based on the shape of the damaged surface. Cavitation, casting porosity, erosion, wear, corrosion, etc. will cause the metal surface shape to be different from the ideal shape. The metal surface damaged by cavitation usually shows a honeycomb shape, which is caused by local high-speed water hitting the metal to fatigue the metal surface. Therefore, the honeycomb holes are generally connected to the outside, and most of the pits are perpendicular to the metal surface. The porosity of casting defects is often hidden deep in the metal. Sometimes the porosity and pores in the metal appear on the surface due to the erosion of water, which is mistaken for foot cavitation. However, when the surface is removed by mechanical methods, there are still pores inside. The scouring and abrasion marks often appear as grooves in the same direction as the water flow, but pay attention to whether there are vortices of the water flow. 2. Noise method

This method is relatively simple and does not need to be in contact with the pump body. However, because the noise method is greatly affected by Zhou Wei's environmental noise, when the intensity is the highest. Generally, the cavitation of water pumps has reached a very intense stage. At this time, the ear can judge the cavitation working condition by the strong cavitation cracking sound. Therefore, the pump noise method is not suitable for on-site monitoring of the occurrence of cavitation.

3. Vibration method

A method of measuring the vibration frequency of the pump body through the accelerometer probe is simple, but the sensitivity is low. Especially for large pumps, the rigidity of the pump body is large. The pump has a slow response to the vibration caused by bubble collapse caused by local cavitation in the pump. At the same time, there are many vibration sources on the pump. The vibration caused by cavitation is often buried in other vibrations. Therefore, the vibration method is only suitable as an auxiliary method for on-site monitoring of cavitation.

4. Ultrasound

The ultrasonic method of measuring cavitation is simple, easy to debug, and not subject to interference from other environmental noises, and is highly sensitive to the occurrence and development of cavitation. Therefore, it is an ideal method for on-site monitoring of cavitation in pumping stations.

2. Methods to reduce cavitation damage of in-service pumps

1. Into the pool

At the use site, cavitation of the pump (including other faults) is necessary and convenient to check the flow status of the inlet tank. Regarding whether a vortex breaker is needed, the flow into the pool should be checked on site. If strong vortices can be seen on the surface of the pool, a vortex breaker should be considered. In addition, attention should be paid to the geometric dimensions of the nozzle and the inlet tank. For example, whether the distance between the nozzle and the pool wall is appropriate, and whether there are bubbles entering the suction pipe of the pump.

2. Water inlet pipeline

In addition to the installation of the inlet pipeline, the loss of the pipeline should be as small as possible (such as the use of elbows and unnecessary valves as little as possible), and the inlet pipe must not be higher than the pump inlet to prevent air accumulation in the pipe. It can also be said to reduce the resistance of the suction pipe, such as increasing the pipe diameter, reducing pipe accessories, bottom valves, elbows, gate valves, etc. This method achieves the purpose of increasing the NPSHa value by improving the inhalation conditions, which is also more convenient and practical.

3. Use ejection structure

The injection device is equivalent to a liquid injection pump in principle. At the outlet of the pump, a high-pressure water is drawn into the high-pressure water chamber, and the high-pressure water enters the suction pipe of the pump through the annular nozzle. The high-pressure water mixes with the water in the suction pipe and exchanges energy. The energy of the mixed mixed water after mixing is increased relative to the water energy of the original suction pipe, so as to meet the cavitation allowance required by the pump inlet. In addition, the purpose can be achieved by increasing the booster pump, increasing the gas phase pressure of the storage tank, reducing the temperature of the conveying medium, and using double suction pumps, but the actual operation is relatively difficult and the increase in cost is relatively large.

4. Imported Qi

The gas supplement method cannot prevent the generation of cavitation holes, but proper gas supplement will reduce the damage to the side wall of the flow channel caused by the rupture of the cavity. The added gas is like a layer of sponge that protects the side wall of the flow channel. This method is commonly used in water turbines, etc., but it is rarely used because it is difficult to control the amount of air supply to the pump. Someone has conducted research on the cavitation of air-supplementing waterproof pumps and achieved certain results, but at the same time pointed out that: air-supplementation is very technical to prevent water pump cavitation. Only when the air-supply flow rate, air-supplement position and air-supplement method are appropriate can we achieve good effect. Otherwise, the flow, head and efficiency of the pump will drop a lot, causing undesirable consequences.

5. Use anti-cavitation materials

There are obvious differences in the cavitation resistance of different materials. There are many factors that affect the material's anti-erosion ability. Generally, materials with high hardness and high elasticity have strong anti-erosion ability. It is recommended abroad that low-carbon chromium-nickel alloy steel is used as a hydraulic mechanical material that works under cavitation, and has good anti-exfoliation performance. It is also effective to use cavitation-resistant materials when cavitation cannot be avoided. In the impeller chamber, the original cast steel is replaced by stainless steel, which is more effective than using cast iron and copper. Cavitation erosion will be reduced exponentially. Facts have proved that the pumps running in our company's low-temperature heating system have very prominent cavitation. Replacement of cavitation-resistant material flow-through parts is the easiest way to solve or reduce cavitation.

6. Impeller protection layer

The impeller coating method is more commonly used, and non-metallic coatings are coated with epoxy resin, nylon powder, polyurethane, etc. In addition, the method of surfacing alloy or spraying alloy on the surface of the runner has also achieved certain effects on cavitation damage, such as stainless steel electrode surfacing method, stainless steel plate inlay welding repair method, and alloy powder spray welding. Compared with several methods of non-metallic and alloy (including stainless steel), non-metallic coating method is economical, but the phenomenon of coating peeling off often occurs. We should continue to sum up experience in the actual construction. The use of alloy surfacing is generally implemented by the user. The spray welding of alloy powder has a good effect but the cost is high, and it may not be possible in some places. There are many examples of methods for protecting layers. For example, a certain pumping station adopts metal alloy powder spray-welded blades, which have achieved better anti-cavitation effects and prolonged service life.

7. Trim the blade head

Trimming the blade head has a significant effect on reducing cavitation damage. The principle is to reduce the displacement of the blade inlet and reduce the speed of the liquid flow at the blade inlet. The method is to thin the back of the blade head and repair more of the front cover near the impeller.

3. Conclusion

There are application examples for reducing the cavitation damage of the pump, and sometimes several methods are required to be adopted at the same time. In summary, in order to avoid cavitation as much as possible during the installation of the centrifugal pump, several principles should be followed: the installation height of the pump must be lower than the allowable suction height of the pump; the suction pipe should be short and straight, and the pipe fittings should be as short as possible. The diameter of the suction pipe should not be smaller than the diameter of the suction port; there should be no air volume at the reduced diameter. In short, during the operation of the centrifugal pump, pay attention to whether there is abnormal noise, observe whether the pressure gauge, etc. are normal, and check the heating of the bearing and shaft seal regularly, and pay attention to whether there is abnormal lubrication and shaft seal. Exudation situation.