1. Cavitation diagnosis method for in-service pumps
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, the ultrasonic method, the external noise method of the pump, and the vibration method can also be used to judge whether cavitation occurs in an in-service pump.
1. Observation method
This method is to observe after the fact and judge according to the surface shape of the damage. Due to cavitation, casting pores, erosion wear, corrosion, etc., the shape of the metal surface will be different from the ideal shape. The metal surface damaged by cavitation usually has a honeycomb shape. It is caused by local high-speed water hitting the metal to cause fatigue damage to 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, and sometimes the porosity and pores inside the metal are mistaken for foot cavitation due to the scouring of the water flow, but when the surface is continued to be removed by mechanical methods, it will be found that there are still pores inside. Erosion wear marks often appear grooves in the same direction as the water flow, but pay attention to whether there are eddies of water flow.
2. Noise method
This method is relatively simple and does not need to be in contact with the pump body. However, since the noise method is greatly affected by the surrounding noise, when its intensity is the highest. Generally, the cavitation of the water pump has reached a very strong stage, and the ear can already judge the cavitation working condition through the strong cavitation cracking sound. Therefore, the pump body noise method is not suitable for monitoring the occurrence of cavitation on site.
3. Vibration method
A method of measuring the vibration frequency of the pump body through the accelerometer probe, the method is simple, but the sensitivity is low. Especially for large pumps, the rigidity of the pump body is large. It has a slow response to the excitation vibration generated by the bubble collapse caused by local cavitation in the pump, and at the same time, there are many vibration sources on the pump. Vibrations due to cavitation are often masked by other vibrations. Therefore, the vibration method is only suitable as an auxiliary means for on-site monitoring of cavitation.
4. Ultrasound
The ultrasonic method for measuring cavitation is simple, easy to debug, and is not disturbed by 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 the cavitation damage of in-service pumps
1. Into the pool
In the field of use, it is very necessary and convenient to check the flow status of the inlet pool for pump cavitation (including other faults). Regarding whether a vortex breaker is required, the flow of the inlet pool should be checked on the spot. If a strong vortex can be seen on the surface of the pool, a vortex breaker should be considered. In addition, the geometric dimensions of the nozzle and the inlet pool should also be noted. Such as: whether the distance between the nozzle and the pool wall is appropriate, and whether there are air bubbles entering the pump suction pipe.
2. Water inlet pipeline
In addition to the setting 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 water inlet pipe should not be higher than the pump inlet to prevent the accumulation of gas in the pipe. It can also be said that the resistance of the suction pipe is reduced, such as increasing the diameter of the pipe, reducing the pipe accessories, bottom valve, elbow, gate valve, etc. This method achieves the purpose of increasing the NPSHa value by improving the inhalation conditions, and is also more convenient and practical.
3. Using the ejector structure
The jet device is equivalent to a liquid jet pump in principle. A high-pressure water is drawn out from the outlet of the pump, led 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 to exchange energy, and the energy of the mixed mixed water increases relative to the water energy of the original suction pipe, so as to meet the necessary cavitation margin for the pump inlet. In addition, the purpose can be achieved by increasing the booster pump, increasing the gas phase pressure in the storage tank, reducing the temperature of the conveying medium, and using a double-suction pump, but the actual operation is relatively difficult and the increase in cost is large.
4. Imported qi
The method of supplementing gas can not prevent the generation of cavitation holes, but proper gas supplementation will reduce the damage to the side wall of the flow channel caused by the rupture of the hole, and the supplemented gas is like a layer of sponge to protect the side wall of the flow channel. This method is commonly used in hydraulic turbines, etc., but it is rarely used to supply air to the pump due to the difficulty in grasping the amount of air supply. Some people have studied the cavitation of the air-supply waterproof pump and achieved certain results, but at the same time pointed out that the air-supply prevention and control of the water pump cavitation is very technical. Effect. Otherwise, the flow, head and efficiency of the pump will drop a lot, causing adverse consequences.
5. Adopt anti-cavitation material
There are obvious differences in the cavitation resistance of different materials. There are many factors that affect the erosion resistance of materials. Generally, materials with high hardness and high elasticity have stronger erosion resistance. Low-carbon chromium-nickel alloy steel is recommended abroad, as a hydraulic mechanical material working in a cavitation state, it has good anti-corrosion performance. It is also effective to use cavitation resistant materials when cavitation cannot be avoided. The impeller chamber replaces the original cast steel with stainless steel, which is more effective than using cast iron and copper. Cavitation erosion will be reduced exponentially. Facts have proved that the pump running in our company's low-temperature heating system has a very prominent cavitation phenomenon. Replacing the cavitation-resistant material overcurrent components is the easiest way to solve or reduce cavitation.
6. Impeller protective layer
The method of impeller coating 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 flow channel 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. Comparing several methods of non-metal and alloy (including stainless steel), non-metal coating method is economical, but the phenomenon of coating peeling often occurs, and experience should be constantly summed up in real construction. The use of alloy surfacing welding is generally carried out by the user. The use of alloy powder spray welding has good effect but high cost, and may not be possible in some places. There are many examples of methods on the protective layer. For example, a pumping station adopts the blades of metal alloy powder spray welding, which has achieved good anti-cavitation effect and can prolong the service life.
7. Trimming the blade head
Trimming the blade head has obvious 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 pumps, and sometimes several methods are required at the same time. To sum up, in the installation process of the centrifugal pump, in order to avoid the occurrence of cavitation as much as possible, several principles should be followed: the installation height of the pump must be lower than the allowable suction height of the pump; the suction pipeline 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 gas accumulation at the variable diameter. In short, during the operation of the centrifugal pump, pay attention to whether there is abnormal noise, observe whether the pressure gauge is normal, and at the same time, regularly check the heating of the bearing and shaft seal, and pay attention to whether there is any abnormality in the lubrication and shaft seal. seepage situation.