The phenomenon of cavitation is generally not valued and understood by people. The cavitation problem generated in actual production is often the main cause of damage to the sewage pump impeller. Generally understood as impeller decay and aging. But this is how it is caused, and its mechanism is not clear. Therefore, it is necessary to understand how cavitation is caused and how to avoid it. Let's talk about cavitation problems and preventive measures.

1 The generation of cavitation Everyone knows that when water is heated to a certain extent, it will become water vapor, and when the water vapor is cooled to a certain temperature, it will condense into water, and, under the action of one atmosphere, the water is heated to 100 °C. It begins to boil and produces a large amount of bubbles (called vaporization). Temperature and pressure are the conditions for their mutual conversion. When the pressure acting on the water surface is high, the temperature required to vaporize water is high; when the pressure acting on the water surface is low, the temperature required to vaporize water is required. It is low; when the pressure acting on the water surface is low to a certain extent, even at normal temperature, water can vaporize to generate a large amount of bubbles, and the pressure at which the liquid starts to vaporize at a certain temperature is called vaporization pressure. The cavitation of the pump is caused by the vaporization of the liquid.
People have a clear understanding of the principle of water absorption from centrifugal pumps. In order for the pump to suck up the water at the lower part, it is necessary to create a vacuum at the inlet of the pump impeller, which creates conditions for the vaporization of water. At a certain temperature, when the pressure at the impeller inlet is low to a certain extent, the water entering the pump begins to vaporize, generating a large amount of bubbles. These bubbles and a small amount of gas dissolved in the water enter the impeller along with the water. Since the pressure changes when water flows in the pump impeller flow path, when the water flows from the low pressure zone (the inlet of the pump) to the high pressure zone (at the outlet of the pump), the bubble is re-energized under high pressure. Condensed into water and lost. When the bubble disappears, the surrounding water impacts at a very high speed on the bubble burst, as if there are countless small warheads, continuously acting on the metal surface of the impeller. Under the continuous impact of the high-speed water flow on the impeller wall surface, honeycomb-shaped cavities appear over time, and their shapes are irregularly distributed. This phenomenon is called cavitation. And people used to regard this phenomenon as the decay of the impeller. The impeller of a centrifugal pump is very sensitive to cavitation. When the time is long, the impeller wall will be eroded. When the pump cavitation occurs, it will generate a lot of noise and vibration. At this time, the pump lift is significantly reduced, the flow rate is reduced, and the more severe cavitation will cause the pump to stop flowing. Therefore, the pump is not allowed to operate under cavitation.
A chemical centrifugal pump, when its speed and flow rate are constant, the generation of cavitation limits the water absorption height of the pump. Generally, the nameplate of the pump refers to “allowing the suction vacuum height” or “necessary NPSH NPSHr”, which is determined by the manufacturer's test according to the principle that the pump does not generate cavitation under the rated speed and flow rate. The maximum water absorption height limits the pressure at the inlet of the pump impeller to a value not to cause cavitation. That is to say, the vacuum at the inlet of the pump impeller cannot be higher than a certain value. Therefore, it is usually the lowest pressure allowed at the inlet of the pump impeller, also called the "allowing vacuum" of the pump, which is the same as "allowing the vacuum height to be absorbed".

2 Preventive measures of cavitation The actual installation height of the shielded pump must be lower than or equal to the actual water absorption height of the pump. The water inlet pipe of the water pump is shortened as much as possible, and the elbow and the valve are less. Some pumping elbows of the production unit are made at right angles instead of using the shrimp shell to bend, which will increase the resistance of the suction pipe. The capacity is evacuated to generate cavitation. Strictly follow the rated parameters on the pump nameplate. The joints and rubber pads of the inlet pipe must be tight and reliable, and the tightening of the joint flange bolts should be checked frequently. The opening and closing of the suction bottom valve should be flexible (especially for long-time unused pump units). Eliminate continuous pumping during pumping. Don't think that if the suction pipe is deeper, it will increase the water absorption height. In fact, this is counterproductive. Because the excessive submersion depth lengthens the water absorption line, the loss along the path is increased, and as a result, the water absorption height is reduced, and it is easy to suck and generate cavitation. The aquaculture production unit that catches the tide and pays more attention should pay attention to the change of seawater level.

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