I. Cavitation Diagnosis Method for In-Service Pumps Users of pumps generally or cannot use the method used by the manufacturer to determine whether the cavitation of the pump has occurred, that is, the method of determining whether cavitation occurs by a drop in the head when the flow rate is constant. Whether cavitation occurs in the service pump, in addition to the observation after cavitation damage, it can also be judged by ultrasonic method, pump noise method, vibration method and other methods.
1. Observation method This method is observed after the fact and judged according to the shape of the damaged surface. Because of cavitation, casting porosity, erosion wear, corrosion, etc., the shape of the metal surface is different from the ideal shape. The cavitation-damaged metal surface usually appears as a honeycomb. 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 in communication with the outside, and most of the pits are perpendicular to the metal surface. Looseness of casting defects is often hidden inside the metal. Sometimes, due to the erosion of the water flow, the loose inside of the metal and the pores on the surface are mistaken for foot cavitation. However, when the surface is removed by mechanical means, there are still pores inside. Scouring wear marks often appear in the same direction as the water flow, but pay attention to whether there is a vortex of water flow.
2. Noise method This method is relatively simple and can not be in contact with the pump body. However, since the noise method is greatly affected by the surrounding environmental noise, when the intensity ziu is high. In general, the cavitation of the pump has reached a very strong stage. At this time, the in-ear has been able to judge the cavitation condition by a strong cavitation burst. Therefore, the pump noise method is not suitable for on-site monitoring of cavitation.
3. Vibration method A method for measuring the vibration frequency of a pump body by an accelerometer probe. The method is simple, but the sensitivity is low. Especially for large pumps, the pump body is stiff. The excitation response caused by the bubble collapse caused by local cavitation in the pump is slow, and at the same time, there are more vibration sources on the pump. Vibrations due to cavitation are often masked in other vibrations. Therefore, the vibration method is only suitable as an auxiliary means for monitoring cavitation on site.
4. Ultrasonic ultrasonic method is simple to measure cavitation, easy to debug, and free from interference from other environmental noises. It is sensitive to the occurrence and development of cavitation. Therefore, it is an ideal method for on-site monitoring of cavitation at the pumping station.
Second, to reduce the cavitation damage of the in-service pump 1, the inlet pool at the use of the site, the pump cavitation (including other faults) is very necessary to view the flow of the pool is quite convenient and easy. Regarding whether the vortex plate needs to be broken, the flow of the inlet pool should be mainly observed on the spot. If a strong vortex is seen on the surface of the pool, the vortex plate should be considered. In addition, the geometry of the nozzle and the inlet pool should also be noted. For example, if the distance between the nozzle and the wall of the tank is appropriate, whether there is air bubbles entering the pump suction pipe.
2, in addition to the installation of the inlet pipe of the inlet pipe should be as small as possible (such as the use of elbows and unnecessary valves as much as possible), so that the inlet pipe must not have a higher than the pump inlet to prevent gas accumulation in the pipe. It can also be said that the resistance of the suction pipe is reduced, such as increasing the pipe diameter, reducing the pipe fittings, the bottom valve, the elbow, the gate valve, and the like. This method achieves the purpose of increasing the NPSHa value by improving the inhalation conditions, and is also convenient and practical.
3. The ejector structure injection device is equivalent in principle to a liquid jet pump. A high-pressure water is drawn at the outlet of the pump and introduced 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 mixed mixed water energy increases with respect to the water energy of the original suction pipe, thereby satisfying the necessary cavitation allowance for the pump inlet. In addition, the booster pump can be added, the gas pressure of the storage tank can be increased, the temperature of the transport medium can be lowered, and the double suction pump can be used to achieve the purpose, but the actual operation is relatively difficult and the cost is increased.
4. The method of importing qi and qi does not prevent the generation of cavitation cavities, but proper qi will reduce the damage of the rupture of the cavity to the side wall of the runner. The gas that is added is like a sponge that protects the side wall of the runner. This method is commonly used in turbines and the like, but it is difficult to grasp the amount of gas supplied to the pump because the amount of gas is difficult to grasp. Some people have studied the cavitation of the gas-filled waterproof pump and achieved certain results, but at the same time pointed out that: the gas is cavitation prevention and control, and the technicality is very strong. Only the air supply flow, the gas supply position and the gas supply mode are appropriate, so that good is achieved. effect. Otherwise, the flow, head and efficiency of the pump will be greatly reduced, causing adverse consequences.
5, the use of different materials resistant to cavitation materials has a very obvious difference in cavitation resistance. There are many factors affecting the material's ability to resist erosion. Generally, materials with high hardness and high elasticity have strong resistance to erosion. Low-carbon chrome-nickel alloy steel is recommended abroad as a hydraulic mechanical material working under cavitation and has good corrosion resistance. It is also effective to use cavitation resistant materials when cavitation cannot be avoided. The impeller chamber changed the original cast steel to stainless steel, which is more effective than the comparison between cast iron and copper. Cavitation erosion will be reduced by a factor of two. It turns out that the cavitation phenomenon of the pump running in our company's low-temperature heating system is very prominent. It is a convenient way to solve or reduce cavitation by replacing the cavitation-resistant material overcurrent parts.
6. The impeller protective layer is commonly used for the coating of the impeller. The non-metallic coating is coated with epoxy resin, nylon powder and polyurethane. In addition, the method of surfacing alloy or spraying alloy on the surface of the runner has achieved certain effects on cavitation damage, such as stainless steel electrode surfacing, stainless steel plate welding repair, and alloy powder spray welding. Compared with several methods of non-metal and alloy (including stainless steel), the non-metal coating method is economical, but the phenomenon of coating peeling often occurs, and the experience should be continuously summarized in the actual construction. Alloy surfacing is generally carried out by the user. The alloy powder spray welding effect is good but the cost is high, and some places may not be possible. There are many examples of methods for protecting layers. For example, a pump station adopts metal alloy powder spray-welded blades, which has achieved good cavitation resistance and prolonged service life.
7. Trimming the blade head to trim 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 velocity of the liquid flow at the blade inlet. The method is to thin the back of the blade head and repair it more near the front of the impeller.
Third, the conclusions have been applied to reduce the cavitation damage of the pump, and sometimes several methods are needed at the same time. In summary, in the installation process of the centrifugal pump, in order to avoid the occurrence of cavitation, several principles should be followed: the installation height of the pump must be lower than the allowable suction height of the pump; the suction line should be short and straight, and the pipe fittings should be as far as possible. Less, the diameter of the suction pipe should not be smaller than the diameter of the suction port; there should be no gas volume at the variable diameter. In short, during the operation of the centrifugal pump, it is necessary to pay attention to the abnormal noise, observe whether the pressure gauge is normal, and also regularly check the bearing and shaft seal heating, and pay attention to whether the lubrication and shaft seal are abnormal. Seepage situation.
1. Observation method This method is observed after the fact and judged according to the shape of the damaged surface. Because of cavitation, casting porosity, erosion wear, corrosion, etc., the shape of the metal surface is different from the ideal shape. The cavitation-damaged metal surface usually appears as a honeycomb. 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 in communication with the outside, and most of the pits are perpendicular to the metal surface. Looseness of casting defects is often hidden inside the metal. Sometimes, due to the erosion of the water flow, the loose inside of the metal and the pores on the surface are mistaken for foot cavitation. However, when the surface is removed by mechanical means, there are still pores inside. Scouring wear marks often appear in the same direction as the water flow, but pay attention to whether there is a vortex of water flow.
2. Noise method This method is relatively simple and can not be in contact with the pump body. However, since the noise method is greatly affected by the surrounding environmental noise, when the intensity ziu is high. In general, the cavitation of the pump has reached a very strong stage. At this time, the in-ear has been able to judge the cavitation condition by a strong cavitation burst. Therefore, the pump noise method is not suitable for on-site monitoring of cavitation.
3. Vibration method A method for measuring the vibration frequency of a pump body by an accelerometer probe. The method is simple, but the sensitivity is low. Especially for large pumps, the pump body is stiff. The excitation response caused by the bubble collapse caused by local cavitation in the pump is slow, and at the same time, there are more vibration sources on the pump. Vibrations due to cavitation are often masked in other vibrations. Therefore, the vibration method is only suitable as an auxiliary means for monitoring cavitation on site.
4. Ultrasonic ultrasonic method is simple to measure cavitation, easy to debug, and free from interference from other environmental noises. It is sensitive to the occurrence and development of cavitation. Therefore, it is an ideal method for on-site monitoring of cavitation at the pumping station.
Second, to reduce the cavitation damage of the in-service pump 1, the inlet pool at the use of the site, the pump cavitation (including other faults) is very necessary to view the flow of the pool is quite convenient and easy. Regarding whether the vortex plate needs to be broken, the flow of the inlet pool should be mainly observed on the spot. If a strong vortex is seen on the surface of the pool, the vortex plate should be considered. In addition, the geometry of the nozzle and the inlet pool should also be noted. For example, if the distance between the nozzle and the wall of the tank is appropriate, whether there is air bubbles entering the pump suction pipe.
2, in addition to the installation of the inlet pipe of the inlet pipe should be as small as possible (such as the use of elbows and unnecessary valves as much as possible), so that the inlet pipe must not have a higher than the pump inlet to prevent gas accumulation in the pipe. It can also be said that the resistance of the suction pipe is reduced, such as increasing the pipe diameter, reducing the pipe fittings, the bottom valve, the elbow, the gate valve, and the like. This method achieves the purpose of increasing the NPSHa value by improving the inhalation conditions, and is also convenient and practical.
3. The ejector structure injection device is equivalent in principle to a liquid jet pump. A high-pressure water is drawn at the outlet of the pump and introduced 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 mixed mixed water energy increases with respect to the water energy of the original suction pipe, thereby satisfying the necessary cavitation allowance for the pump inlet. In addition, the booster pump can be added, the gas pressure of the storage tank can be increased, the temperature of the transport medium can be lowered, and the double suction pump can be used to achieve the purpose, but the actual operation is relatively difficult and the cost is increased.
4. The method of importing qi and qi does not prevent the generation of cavitation cavities, but proper qi will reduce the damage of the rupture of the cavity to the side wall of the runner. The gas that is added is like a sponge that protects the side wall of the runner. This method is commonly used in turbines and the like, but it is difficult to grasp the amount of gas supplied to the pump because the amount of gas is difficult to grasp. Some people have studied the cavitation of the gas-filled waterproof pump and achieved certain results, but at the same time pointed out that: the gas is cavitation prevention and control, and the technicality is very strong. Only the air supply flow, the gas supply position and the gas supply mode are appropriate, so that good is achieved. effect. Otherwise, the flow, head and efficiency of the pump will be greatly reduced, causing adverse consequences.
5, the use of different materials resistant to cavitation materials has a very obvious difference in cavitation resistance. There are many factors affecting the material's ability to resist erosion. Generally, materials with high hardness and high elasticity have strong resistance to erosion. Low-carbon chrome-nickel alloy steel is recommended abroad as a hydraulic mechanical material working under cavitation and has good corrosion resistance. It is also effective to use cavitation resistant materials when cavitation cannot be avoided. The impeller chamber changed the original cast steel to stainless steel, which is more effective than the comparison between cast iron and copper. Cavitation erosion will be reduced by a factor of two. It turns out that the cavitation phenomenon of the pump running in our company's low-temperature heating system is very prominent. It is a convenient way to solve or reduce cavitation by replacing the cavitation-resistant material overcurrent parts.
6. The impeller protective layer is commonly used for the coating of the impeller. The non-metallic coating is coated with epoxy resin, nylon powder and polyurethane. In addition, the method of surfacing alloy or spraying alloy on the surface of the runner has achieved certain effects on cavitation damage, such as stainless steel electrode surfacing, stainless steel plate welding repair, and alloy powder spray welding. Compared with several methods of non-metal and alloy (including stainless steel), the non-metal coating method is economical, but the phenomenon of coating peeling often occurs, and the experience should be continuously summarized in the actual construction. Alloy surfacing is generally carried out by the user. The alloy powder spray welding effect is good but the cost is high, and some places may not be possible. There are many examples of methods for protecting layers. For example, a pump station adopts metal alloy powder spray-welded blades, which has achieved good cavitation resistance and prolonged service life.
7. Trimming the blade head to trim 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 velocity of the liquid flow at the blade inlet. The method is to thin the back of the blade head and repair it more near the front of the impeller.
Third, the conclusions have been applied to reduce the cavitation damage of the pump, and sometimes several methods are needed at the same time. In summary, in the installation process of the centrifugal pump, in order to avoid the occurrence of cavitation, several principles should be followed: the installation height of the pump must be lower than the allowable suction height of the pump; the suction line should be short and straight, and the pipe fittings should be as far as possible. Less, the diameter of the suction pipe should not be smaller than the diameter of the suction port; there should be no gas volume at the variable diameter. In short, during the operation of the centrifugal pump, it is necessary to pay attention to the abnormal noise, observe whether the pressure gauge is normal, and also regularly check the bearing and shaft seal heating, and pay attention to whether the lubrication and shaft seal are abnormal. Seepage situation.
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