Centrifugal pump cavitation causes and preventive measures

Pump cavitation phenomenon is generally not valued and understood by people. The problem of cavitation generated in actual production is often the main reason for the damage of the centrifugal pump impeller. The general understanding is that the impeller decays and ages. But this is how it is caused, and the mechanism is not clear. Therefore, it is necessary to understand how cavitation is caused and how to avoid it.

　　

1 The production of cavitation

　　

When the water is heated to a certain level, it becomes water vapor, and when the water vapor cools to a certain temperature, it will condense into water. Moreover, under the action of an atmospheric pressure, the water is heated to 100°C and begins to boil, producing a large amount of bubbles ( Called vaporization ) . The temperature and pressure are the conditions for their mutual transformation. When the pressure acting on the water surface is high, the temperature required for water vaporization is high; when the pressure on the water surface is low, the temperature required for water vaporization is required. When the pressure on the surface of the water is low, the water vaporizes at room temperature to produce a large amount of bubbles. The pressure at which the liquid begins to vaporize is called the 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 in centrifugal pumps. In order for the water pump to suck up water at a low level, a vacuum must be formed at the inlet of the pump impeller. This creates conditions for the vaporization of water. At a certain temperature, when the pressure at the inlet of the impeller is low to a certain extent, the water entering the pump starts to vaporize, generating a large amount of bubbles. These bubbles and a small amount of gas dissolved in water enter the impeller along with the water. Because water flows in the runner of the pump impeller, the pressure changes, so when the water brings the bubble from the low pressure area ( the inlet of the water pump ) to the high pressure area ( the outlet of the pump ) , the bubble is renewed under high pressure. Condensed into water and lost, when the bubble disappears, the surrounding water will impact at a high speed to the place where the bubble bursts, like countless small warheads, continuously acting on the metal surface of the impeller. The wall of the impeller is continuously hit by high-speed water flow, and honeycomb-shaped pits will appear for a long time, and its shape is irregularly distributed. This phenomenon is called cavitation. However, people often regard this phenomenon as the decay of the impeller. The impeller of a centrifugal pump is very sensitive to cavitation. A long time will erode the wall of the impeller. When the pump cavitation occurs, it will produce a lot of noise and vibration. At this time, the lift of the pump is significantly reduced and the flow rate is reduced. A more severe cavitation will stop the pump. Therefore, it is required that the water pump is not allowed to operate under cavitation. 　

A chemical centrifugal pump, when its rotational speed and flow rate is certain, the generation of cavitation limits the water absorption height of the pump. The nameplate of a general pump mentions “ Allow suction of vacuum height ” or “ Required NPSHr” , which is based on the principle that the pump does not generate cavitation under the rated rotation speed and flow rate. Inclinometer dosing device air The filter moves the pump to a large suction height determined by the manufacturer's test. To prevent cavitation, the pressure at the inlet of the pump impeller must not fall below a certain value. In other words, the vacuum at the inlet of the pump impeller must not exceed a certain value. Therefore, the low pressure allowed at the inlet of the pump impeller is also often referred to as the pump's " permissible suction vacuum level, " which is the same as the " allowed vacuum suction height " .

2 cavitation prevention measures

The actual mounting height of the canned pump must be lower than or equal to the actual water-absorbing height of the pump. The suction line of the water inlet of the pump should be shortened as much as possible, and the elbow and the valve should be less. Some of the aquaculture production units' water pump elbows are made at right angles instead of using the shrimp shell bend method. This will increase the resistance of the suction pipe. The capacity is evacuated and cavitation occurs. Strictly adhere to the rated parameters on the pump nameplate. The joints and pads of the water inlet pipe must be tight and reliable, and the fastening of the joint flange bolts should always be checked. The opening and closing of the suction bottom valve should be flexible ( especially for long-discharged pump units ) . Stop pumping during the pumping process. Do not think that a deeper suction pipe will increase the water absorption height. In fact, doing so is counterproductive. Excessively large depth of submergence lengthens the suction line, which increases the loss along the way. As a result, the height of water absorption is reduced, and it is also easy to evaporate and produce cavitation. The catching-water pumping aquaculture production units should pay more attention to changes in seawater level.

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