Cavitation diagnosis method of split case pump in service
Pump users generally or can not use the method adopted by the manufacturer to determine whether the cavitation of the pump occurs, that is, the method to determine whether the cavitation occurs by the drop of the head when the flow is fixed. In addition to the observation method after cavitation damage, the ultrasonic method, pump external noise method and vibration method can also be used to determine whether the pump in service has cavitation.
1. Observation Method
This method is to observe after the event and judge according to the shape of the damaged surface. Due to cavitation, casting porosity, erosion, corrosion and so on, the shape of metal surface will be different from the ideal shape. The metal surface of cavitation damage usually appears honeycomb, which is fatigue damage caused by local high-speed water hitting the metal surface, so the honeycomb hole is generally connected with the outside, most of the pits are vertical to the metal surface. The porosity of casting defects is often hidden in the metal. Sometimes, the porosity and pores in the metal are presented on the surface due to the erosion of water flow, which is mistaken as sufficient cavitation. However, when the surface is removed by mechanical method, it will be found that there are still pores in the metal. Scour and wear traces often appear grooves with the same direction of water flow, but pay attention to whether there is vortex of water flow.
2. Noise Method
This method is relatively simple and can not contact with the pump body. However, because the noise method is greatly affected by the ambient noise, when it shows the highest intensity. Generally, the cavitation of water pump has reached a very strong stage. At this time, the working condition of cavitation can be distinguished by strong cavitation burst sound. Therefore, the double suction pumpcase noise method is not suitable for monitoring the occurrence of cavitation on site.
3. Vibration Method
It is a simple method to measure the vibration frequency of pump body with accelerometer probe, but its sensitivity is low. Especially for large pumps, the rigidity of the pump body is large. At the same time, there are many vibration sources on the pump. The vibration caused by cavitation is often concealed in other vibrations. Therefore, the vibration method can only be used as an auxiliary means to monitor cavitation on site.
4. Ultrasonic Method
The ultrasonic method is simple, easy to debug, and free from the interference of other environmental noise. It is sensitive to the occurrence and development of cavitation. Therefore, it is an ideal method to monitor the cavitation in the pumping station.
Methods to Reduce Cavitation Damage of Split Case Pumps in Service
1. Water Inlet Tank
At the use site, it is very necessary and convenient for pump cavitation (including other faults) to check the flow condition of the water inlet tank. As for the need of vortex breaking plate, it is mainly necessary to check the flow of the inlet tank on site. If the strong vortex can be seen on the surface of the tank, the vortex breaking plate should be considered. In addition, attention should be paid to the geometric dimensions of the nozzle and the water inlet. For example: whether the distance between the pipe orifice and the pool wall is appropriate, and whether there is bubble entering the pump suction pipe.
2. Water Inlet Pipe
In addition to minimizing the pipeline loss (such as minimizing the use of elbows and unnecessary valves), the inlet pipe shall not be higher than the pump inlet to prevent air accumulation in the pipe. It can also be said that the resistance of the suction pipe can be reduced, such as increasing the pipe diameter, reducing the pipe accessories, bottom valve, elbow, gate valve, etc. This method can improve NPSHa value by improving suction condition, which is also convenient and practical.
3. Using the Ejector Structure
The injection device is equivalent to the liquid injection pump in principle. A high-pressure water is led out at the outlet of the pump to 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 and the water in the suction pipe mix and exchange energy. The mixed water energy increases compared with the water energy of the original suction pipe, so as to meet the necessary cavitation allowance at the pump inlet. In addition, it can be achieved by increasing the booster pump, increasing the gas phase pressure of the storage tank, reducing the temperature of the transmission medium, and adopting the double suction pump, etc., but the actual operation is relatively idle and difficult, and the increased cost is large.