Centrifugal pump cavitation diagnosis and mitigation method
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1. Observation method This method is observed afterwards and judged based on the shape of the broken surface. As cavitation, casting porosity, erosion wear, corrosion, etc. will cause the metal surface shape and the ideal shape is different. The cavitation-damaged metal surface usually appears as a honeycomb, which is caused by the local high-speed water striking the metal and causing the metal surface to fatigue. Therefore, the honeycomb holes are generally communicated with the outside, and most of the pits are perpendicular to the metal surface. The loosening of casting defects is often hidden deep inside the metal. Sometimes, because the scouring of the water stream will cause loosening and pores on the surface of the metal, it is mistaken for foot cavitation. However, when the surface is removed by mechanical means, there are still pores in the interior. Scour and wear marks tend to appear in the same direction as the water flow, but be careful about whether there is a vortex of water flow.
2, noise method This method is relatively simple, you can not contact with the pump body. However, since the noise method is greatly affected by environmental noise of the surrounding area, when the intensity of the display is the highest. The cavitation of a general pump has reached a very strong stage. At this time, the ear has been able to distinguish the cavitation condition by the strong cavitation burst sound. Therefore, the pump body noise method is not suitable for on-site monitoring of cavitation.
3. Vibration method A method of measuring the vibration frequency of a pump body through an accelerometer probe has a simple method but a low sensitivity. Especially for large pumps, the pump body is stiff. Exciting vibration caused by bubble collapse caused by local cavitation in the pump is dull. At the same time, there are many vibration sources on the pump. The vibration caused by cavitation is often masked by other vibrations. Therefore, the vibration method is only suitable as an auxiliary means for on-site monitoring of cavitation.
4. Ultrasonic method ultrasonic measurement of cavitation method is simple, easy to debug, and is not interfered by other environmental noise, and has a strong sensitivity to the occurrence and development of cavitation. Therefore, it is an ideal method to monitor the cavitation at the pump station site.
Second, to reduce the cavitation damage of the pump method in service 1, into the use of the site, the pump cavitation (including other failures) to see the flow of the pool is very necessary and very convenient. Regarding whether or not the vortex plate needs to be broken, the flow of the water into the pool should be primarily checked at the site. If the pool surface can see a strong vortex, the broken vortex plate should be considered. In addition, the geometry of the nozzle and inlet tank should also pay attention. Such as: whether the nozzle from the pool wall is appropriate, whether there is a bubble into the pump suction tube.
2. In addition to setting the inlet pipe of the water inlet pipe as far as possible to minimize the loss of piping (for example, to use the elbow and unnecessary valves as much as possible), the inlet pipe must not have a part higher than the inlet of the pump to prevent the pipe from being trapped in air. It can also be said that reducing the resistance of the suction pipe, such as increasing the pipe diameter, reducing the pipe accessories, bottom valve, elbow, gate valve and so on. This method is to improve the inhalation conditions to achieve the purpose of improving the NPSHa value, but also more convenient and practical.
3. The ejector injection device is in principle equivalent to a liquid jet pump. A high-pressure water is led out at the outlet of the pump and introduced into the high-pressure water chamber. The high-pressure water enters the suction pipe of the pump through the annular nozzle. The high-pressure water is mixed with the water in the suction pipe to exchange energy, and the mixed water energy after mixing is increased with respect to the water energy of the original suction pipe so as to satisfy the cavitation residual amount necessary for the pump inlet. In addition, it is also possible to achieve the purpose by increasing the booster pump, increasing the gas phase pressure of the storage tank, lowering the temperature of the conveying medium, and using double-suction pumps. However, the actual operation is relatively difficult and the increase in cost is large.
4. The method of replenishing qi without supplementing the air does not prevent the generation of cavitation holes, but proper replenishing will reduce the damage of the cavitation to the side walls of the flow path. The added gas is like a layer of sponge that protects the side walls of the flow path. This method is commonly used in hydraulic turbines and the like, but it is very difficult to control the amount of air used to supplement the air in the pump. Some people have studied the cavitation erosion of air-supply water-repellent pumps and achieved certain results, but at the same time pointed out that air-supplementing pumps prevent cavitation of pumps and are very technical. Only the flow of supplemented air, the position of replenishing gas, and the mode of replenishing qi are appropriate to achieve good results. effect. Otherwise, the flow, head, and efficiency of the pump will be greatly reduced, causing adverse consequences.
5, the use of anti-cavitation materials, different materials, anti-cavitation ability has a very obvious area. There are many factors that affect the anti-erosion ability of materials, and materials with high hardness and high elasticity usually have strong anti-erosion ability. Low-carbon chromium-nickel alloy steel is recommended abroad as a hydraulic mechanical material working under cavitation conditions, and 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 conspicuous than cast iron and copper. Cavitation erosion will decrease exponentially. Facts have proved that the cavitation phenomenon of the pumps running in the low-temperature heating system of our company is very prominent, and replacing the cavitation resistant material flow-through parts is the easiest way to solve or reduce cavitation.
6. The method of coating the impeller with impeller protection layer is more commonly used. Non-metal coating is coated with epoxy resin, nylon powder, polyurethane and so on. In addition, the method of depositing an alloy or a sprayed alloy on the surface of the flow channel has also achieved certain effects on the cavitation damage, such as a stainless steel electrode welding method, a stainless steel plate welding repair method, and an alloy powder spray welding. Compared with several methods of non-metals and alloys (including stainless steel), non-metallic coating methods are economical, but the phenomenon of coating shedding often occurs, and experience should be continuously summed up during practical construction. The use of alloy surfacing is generally performed by the user himself, and the use of the alloy powder has a good spray-welding effect but a high cost, and may not be performed in some places. There are many examples of methods for the protective layer. Such as: a pumping station using metal alloy powder spray welding blade, has achieved a good anti-cavitation effect, the service life can be extended.
7. Trimming the blade head to trim the blade head has obvious effect on reducing cavitation damage. The principle is to reduce the displacement of the blade inlet and reduce the velocity of the flow at the blade inlet. The method is to thin the back of the blade head and make more repairs near the impeller front cover.
CONCLUSIONS <br> <br> to reduce pump cavitation damage has application examples it is sometimes necessary while using several methods. In summary, during the installation of the centrifugal pump, in order to minimize the occurrence of cavitation, several principles should be followed: the 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 should be as far as possible. Less, the diameter of the suction tube should not be less than the diameter of the suction inlet; there can be no gas accumulation at the reduced diameter. In short, during the operation of the centrifugal pump, we must pay attention to whether there is abnormal noise, observe the pressure gauge, etc. is normal, but also should regularly check the bearing, shaft seal heat, and pay attention to lubrication and shaft seal is not normal Exudate condition.