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This article will mainly introduce ways to reduce cavitation damage in service pumps. These methods have achieved significant results in practical applications.
First, the causes of cavitation Cavitation is caused by liquid vaporization, liquid molecules escape the liquid surface, a process of gas molecules, known as "vaporization."
The degree of vaporization of liquid pressure and the size of the high and low temperature. Dissolved in the liquid gas, pressure and temperature changes will be released, the formation of cavitation. When the internal pressure of the liquid drops below the saturated vapor pressure of the liquid at the temperature, bubbles or cavitation are formed in the local area. When the pressure rises, the bubble is suddenly crushed by the pressure around it, Extruded to the center of the bubble at an extremely high rate, causing a hydraulic shock to the equipment. The entire process of microbubble generation, rupture and physical and chemical effects on the overcurrent surface is called cavitation. If the liquid does not contain any impurities, cavitation does not occur even under low pressure. Foreign cavitation researchers believe that the tensile strength of ultra-pure water (ie, the limit for generating holes) far exceeds the tensile strength of conventional metal materials. However, the usual liquid always contains gas or solid, these impurities become corrosive nuclei, under certain conditions to induce the occurrence of holes. Due to the different proportion of water and sand, the movement path of sand and streamline may speed up the occurrence of cavitation. The author describes in detail the paper "Cavitation and Anti-cavitation Materials for Mud Pumps" (Lubrication and Sealing 1993).
Second, in-service pump cavitation diagnostic methods Pump users are usually unable to use the manufacturer's flow rate drop of a certain time to determine whether the cavitation occurs.
In-service pump cavitation occurs, except in the cavitation damage observed after the law can be used (1) ultrasonic method; (2) pump noise method; (3) vibration method to determine. Observation method: destruction of surface observation method is to observe the method afterwards, according to the destruction of the surface shape to judge. Due to cavitation, casting porosity, erosion, erosion, etc. will cause the metal surface shape and the ideal shape of the difference. Cavitation erosion of the metal surface is usually honeycomb, which is caused by the local high-speed water hit the metal fatigue damage the metal surface, so the honeycomb holes are generally connected to the outside, most of the pit and the metal surface perpendicular to. Loose casting defects are often hidden in the metal inside, and sometimes because of the erosion of water will be loose metal inside, stomatal surface was mistaken for cavitation, but when we continue to work on the surface of the mechanical method will find that there are still internal Stoma. Scour wear traces often appear with the same direction of the water groove, but pay attention to the flow sometimes vortex. Noise method: pump noise method is relatively simple, you can not contact with the pump. However, as the noise method is greatly affected by ambient noise, it shows the highest intensity. The general pump cavitation has reached a very strong stage, when the human ear has been through a strong cavitation crack sound discrimination cavitation conditions. Therefore, the pump body noise method is not suitable for on-site monitoring of cavitation. Vibration method: This method is through the accelerometer probe pump vibration frequency of a method, the method is simple, but the sensitivity is low. Especially for large pumps, pump body stiffness. The partial cavitation caused by the pump bubble rupture caused by the vibration response slow, at the same time, pumps on the vibration source more. Vibrations due to cavitation are often masked by other vibrations. Therefore, the vibration method is only suitable as an aid to on-site monitoring of cavitation. Ultrasonic method: Ultrasonic method of measuring cavitation is simple, convenient debugging, and free from the interference of other environmental noises, and is very sensitive to the occurrence and development of cavitation. Therefore, as a pumping station to monitor cavitation is a more ideal way. In addition, Fushun Petroleum Institute of Chemical Machinery Zhao Huijun introduced the use of electricity to predict cavitation centrifugal pump performance ("Journal of Petrochemical Industry" 1997).
Third, to reduce the pump in service cavitation damage methods:
1, into the pool at the site of use, the occurrence of cavitation (including other failures) of the pump to view the flow of water into the pool is very necessary and convenient. If the pool surface can see a strong vortex, should consider adding broken vortex. In addition, the nozzle and the sink geometry should also be noted. Such as: nozzle from the pool wall is appropriate, whether there is a bubble into the pump suction tube. Note that the pool water level is necessary to raise the water level into the pool can reduce or completely prevent cavitation. Jietai pumping station on the water inlet transformation, erosion reduction of about 45%. Qilu Petrochemical Liu Kewang, Sun Jinghe and others in the import of circulating pump inlet vortex after the pump no longer occurs and the flow from the unstable 8500, 8000m3 / h, respectively, increased to a stable 9700,8200m3 / h.
2, Inlet pipe inlet pipe set In addition to the pipeline should try to make small losses (such as the minimum number of elbows and unnecessary valves), let the inlet pipe may not have higher than the pump inlet parts to prevent the tube of gas. Chishui Natural Gas Fertilizer Plant in 1995 circulating pump imported rubber expansion to 1000mm, while the pipe is 800mm. To this end increased the exhaust pipe, to solve the pressure drop caused by the resistance to solve the cavitation problem.
3, adjust the pump flow Wang Weidong, Xinjiang Electric Power Design Institute suggested that the original resistance calculation formula used in our country from the former Soviet Union, the calculated resistance value is larger than the actual, resistance calculation should adopt the new formula. Otherwise, the pump is prone to cavitation at high flow rates (Northwest Power Technology 2000). In the pump design selection and the actual deviation, the pump cavitation and economic operation can be solved by cutting the impeller, in order to achieve the purpose of eliminating cavitation and operating economy, this practice has proved to be effective.
4, the use of injection structure injection device in principle equivalent to the liquid - liquid jet pump. A high-pressure water is drawn at the pump outlet into the high-pressure water chamber as shown, and 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 energy of the mixed water after mixing is increased relative to the water energy of the original suction pipe so as to meet the required NPSH margin of the pump inlet. Wu Yu, Zhejiang University, who conducted this
Introduction (Zhejiang University Master's thesis 2003), the paper describes the lead injection device back flow should be controlled between 2% -5%. Mr. Zhang Dehuang used a similar method in Changsha to test the multi-stage pump with 100 caliber and achieved remarkable results. The method is to use 1/2 tube to guide the water after the balance plate to the pump inlet, and the pump system's equipment NPSH is reduced by 0.5 ~ 0.8m. Guo Dilong, Wuhan University, who introduced a jet - centrifugal pump device. This device is equivalent to the jet pump and centrifugal pump in series work on the suction pump is very effective, but generally not suitable for the pump in service transformation.
5, imported qi qi gas method does not prevent the cavitation cavitation, but the qi will be appropriate to reduce the bursting holes on the runner wall damage, fill the gas as a protective runner wall sponge. This method is commonly used in water turbines and the like, but very little use is made of gas supplementing the pump due to the difficulty in grasping the gas supply. Wuhan University of Water Conservancy and Electric Power Sun Shou, Yan Jinwen on the water vapor pump waterproofing research and achieved some results, but also pointed out: gas pump cavitation prevention and control, highly technical, only the gas flow, qi location and qi The right way to get good results. Otherwise, the flow, lift and efficiency of the pump will drop a lot, causing adverse consequences.
6, the use of anti-cavitation materials Different materials have a very significant difference in cavitation resistance. There are many factors that affect the anti-erosion ability of materials. Generally, materials with high hardness and high elasticity are more resistant to erosion. Foreign recommended low-carbon chromium-nickel alloy steel, such as 13Cr4N as cavitation in the work of hydraulic machinery, has good resistance to erosion. Zhengzhou Machinery Research Institute of Chen Yan conducted a comparison of the anti-cavitation properties of different materials ("Hot Processing Technology" 2000), the results are as follows Material HT200 QT400-15 QT600-3 ZG230-450 ZG310-570 Weight loss mg 961.4 737.1 481.1 241 155 Test Time h 15 Material ZG40Cr ZG20SiMn ZG1Cr18Ni9Ti ZG0Cr13Ni4Mo ZG0Cr13Ni6Mo Weight loss mg 139.9 122.2 71.5 40.2 25.2 Test time h 15 30 Information provided by Zhu Xu-ren from Changsha Water Pump Factory, whose materials and cavitation loss are shown in the following table. Materials HT200 QT600-3 ZG310-570 18-8 Weight Loss 148.3 76.5 33 16.3 It is effective to use cavitation-resistant materials when cavitation can not be avoided. Such as: Lingcheng pumping station on the 7 # and 10 # units, the impeller room will be replaced by the original cast steel stainless steel, running a few years did not actually find the damage marks. Wuding gate station will be cast iron QT42-10, increased service life of more than 2 times. Jietai pumping station with cast iron and copper contrast, copper impeller anti-cavitation effect is obvious.
7, the impeller protective layer on the impeller coating method is more common, non-metallic coating epoxy resin, nylon powder, polyurethane and so on. The method of surfacing the alloy or sprayed alloy on the surface of the flow channel has also obtained certain effect on the cavitation destruction, such as stainless steel electrode surfacing method, stainless steel inlaid repairing method and alloy powder spray welding. Compare several methods for non-metals and alloys, including stainless steel. Non-metallic coating economy, but should be strictly controlled the operation of its process to prevent the coating off the phenomenon. The use of alloy powder spray good but costly, and some places may not be able to proceed. Such as: a pump station using metal alloy powder spray welding blade, and achieved good anti-cavitation effect, the service life can be extended. China Great Wall Aluminum Company Hao Baishun Hl pump impeller cavitation erosion coating applied research. Dazhuang Zhuang Duanzhuang pumping station pumping station serious cavitation, the use of soft ceramic composite repair 4 years 4000h water inspection found no cavitation damage. Wang Songchen, Songliao Committee of the Ministry of Water Resources, Metallurgical Reservoir Administration, introduced the application of TS216 wear-resisting agent in the anti-cavitation of turbine runner. Lianyungang Water Supply Company MA Yu-East laser cladding method of cast iron and cast steel, the anti-cavitation performance were spray process 1.3 and 1.5 times.
8, trim blade head trim impeller head to reduce the flow rate of leaf imports, reduce impeller import crowding, improve pump anti-cavitation capacity is effective. Practice has proved that despite the thinning of imported leaves, impeller life is often longer in the cavitation environment. General trim the impeller head is thinning the back of the blade head, near the impeller repair some more front cover. 2001 Changsha Pump Plant on the flow rate of 5040m3 / h, lift 17.5m pump dressing (while laryngeal dressing), the test found that the NPS decreased by about 0.5.
Methods to reduce the cavitation damage in service pumps
Cavitation is one of the common faults in hydraulic machinery. Due to the unreasonable design of inlet pool or pipeline and the incomplete consideration of atmospheric pressure, temperature, and the change of gasification pressure of the medium, the pump often fails due to cavitation. Already installed service pumps have little way to completely overcome the cavitation damage caused by poor pump cavities (Pump Booklet, Volume 1).