How to solve the cavitation phenomenon in sewage pump

Cavitation is an important factor affecting the performance of the pump system. Its triggering conditions mainly include excessive liquid temperature, insufficient inlet pressure and excessive flow rate. When the pump inlet pressure is lower than the saturated vapor pressure of the liquid, the gas dissolved in the liquid will precipitate to form bubbles. As the liquid flows into the high-pressure area, these bubbles will burst and generate instantaneous high-pressure shock waves. Studies have shown that the energy released by a single bubble when it bursts can reach 10^5 Pa. This high-frequency impact can cause honeycomb corrosion on the impeller surface. In severe cases, the metal surface will show sponge-like shedding.

The harm of cavitation to sewage pumps is mainly reflected in three aspects: first, the performance of the pump will decline significantly, which is manifested as a decrease in flow, head and efficiency; second, the structure will be damaged, and the service life of the impeller may be shortened to less than one-third of the normal value; finally, the operating risk increases, and severe vibration may cause equipment shutdown and even cause pipeline rupture. According to statistics from a municipal pump station, the cost of impeller replacement caused by cavitation accounts for 40% of the annual maintenance cost, and the economic loss caused by shutdown can be as high as 5,000 yuan per hour.

In order to effectively deal with cavitation, it is necessary to solve it from multiple technical paths.
Pump body structure optimization
Optimizing the pump body structure is the key to improving anti-cavitation performance. By improving the impeller design, the use of a double-suction impeller can significantly increase the inlet cross-section and reduce the inlet flow velocity, thereby reducing the formation of local low-pressure areas. In a certain engineering case, the double-suction impeller increased the cavitation margin by 1.2 meters and extended the operating life to 8,000 hours. In addition, extending the blade inlet edge to the impeller inlet allows the liquid flow to receive work in advance, thereby increasing the inlet pressure by 0.5 to 1.0 bar.
The application of the front inducer technology can increase the pressure of the liquid flow by 15% to 20% before entering the main impeller by adding a pre-pressure device. After adopting this technology, the effective cavitation margin (NPSHa) of an industrial sewage pump increased from 2.5 meters to 3.8 meters, and the cavitation risk was completely eliminated. At the same time, optimizing the curvature radius of the impeller inlet section can reduce the degree of rapid acceleration and pressure reduction of the liquid flow, thereby reducing the flow velocity gradient and the probability of bubble generation.
Operation parameter regulation
Regulating the operation parameters of the pump is an effective means to increase NPSHa. Lowering the installation height of the pump can directly increase NPSHa. For every 1 meter reduction in installation height, NPSHa can increase by 0.1 bar. After a pump station reduced the installation height from 5 meters to 3 meters, the cavitation phenomenon completely disappeared. In addition, reducing pipeline resistance is also the key. Suction loss can be effectively reduced by shortening the pipeline length, reducing the number of elbows and increasing the pipe diameter. Experiments show that for every 90-degree elbow reduction, NPSHa can be increased by 0.05 bar.
Controlling liquid temperature is also an important measure to prevent cavitation. When the temperature of the conveying medium exceeds 40°C, the saturated vapor pressure increases significantly. A sewage treatment plant installed a cooling device to reduce the medium temperature from 45°C to 35°C, which reduced NPSHr by 0.8 meters. In addition, avoiding long-term operation at high flow can also effectively reduce flow losses, thereby reducing the risk of cavitation.
Material and process upgrade
Choosing anti-cavitation materials is an effective way to increase the life of the impeller. The hardness of high chromium alloy (Cr26) can reach HRC60 or above, and its cavitation resistance is three times higher than that of ordinary cast iron. After a pump station replaced its impeller with a high chromium alloy one, the number of annual replacements dropped from 6 to 1. In addition, through surface coating technology, spraying tungsten carbide coating on the impeller surface can form a 0.2 mm hard protective layer, which significantly enhances the resistance to bubble impact.