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Effectively reduce the pump in the operation of cavitation damage method (2)
Third, reduce cavitation damage in service pumps.
1. Inlet pool optimization
At the site of operation, it's essential to monitor where cavitation or other issues occur in the pump. If a strong vortex is visible on the surface of the water, consider installing a vortex breaker to minimize this effect. Additionally, the design of the nozzle and the overall shape of the inlet should be carefully examined. For example, ensure that the nozzle is properly positioned relative to the pool wall and that no air bubbles are entering the suction pipe. Raising the water level in the pool can also help reduce or even eliminate cavitation by increasing the available net positive suction head (NPSH). At the Jietai Pumping Station, after modifying the water inlet system, erosion was reduced by approximately 45%. Similarly, at Qilu Petrochemical, after addressing the inlet vortex issue, flow rates stabilized from 8,500 and 8,000 m³/h to 9,700 and 8,200 m³/h, respectively.
2. Pipeline optimization
The inlet pipeline should be designed to minimize losses, such as by reducing the number of elbows and valves. Also, ensure that the pipe does not rise above the pump’s inlet to prevent air from being trapped inside. In 1995, the Chishui Natural Gas Fertilizer Plant replaced its 800mm inlet pipe with a 1,000mm rubber expansion joint and added an exhaust line, which helped resolve pressure drop issues and eliminated cavitation.
3. Adjusting pump flow
Wang Weidong from the Xinjiang Electric Power Design Institute suggested that the original Chinese formula for calculating resistance was based on Soviet standards and often overestimates actual resistance. Using a more accurate formula can prevent cavitation at high flow rates. Additionally, cutting the impeller during pump selection or design can reduce cavitation while improving efficiency. This method has been proven effective in practice.
4. Using injection structures
An injection device works similarly to a jet pump. High-pressure water from the pump outlet is directed into a chamber and then through an annular nozzle into the suction pipe. This high-pressure water mixes with the suction flow, increasing energy and helping meet the required NPSH margin. Wu Yu from Zhejiang University recommended controlling the backflow between 2% and 5%. Zhang Dehuang in Changsha used a similar technique by diverting water from the balance plate back to the pump inlet, reducing the system's NPSH by 0.5–0.8 meters. Guo Dilong from Wuhan University introduced a jet-centrifugal pump system, which is highly effective but typically not suitable for retrofitting existing pumps.
5. Air injection method
While air injection doesn't prevent cavitation itself, it can reduce damage caused when vapor bubbles collapse. The injected air acts like a protective layer, cushioning the runner walls. This method is commonly used in turbines but less so in pumps due to challenges in controlling the air supply. Researchers at Wuhan University, such as Sun Shou and Yan Jinwen, have studied gas injection for cavitation control, emphasizing that proper air flow rate, location, and injection method are crucial. Improper implementation can lead to significant drops in flow, head, and efficiency, causing operational problems.