Methods of Solving Cavitation Problems in Centrifugal Pumps

Category: Water pump knowledge

2024-06-07

When the centrifugal pump is running, due to the pressure change caused by the flow of the liquid, the liquid will appear in the local low pressure area at the inlet of the impeller, causing the gas to precipitate from the surface of the liquid and form the cavitation phenomenon. This phenomenon is called cavitation of centrifugal pump, which will have a serious impact on the performance and life of the pump.

The cavitation margin of the centrifugal pump is generally about 1.5 to 2 meters.

The inlet of the impeller will generate vapor under a certain vacuum pressure. The vaporized bubbles will denudate the metal surfaces such as the impeller under the impact motion of the liquid particle, thus destroying the metal such as the impeller. At this time, the vacuum pressure is called vaporization pressure, and the cavitation margin refers to the excess energy per unit weight of the liquid at the pump suction inlet that exceeds the vaporization pressure.

The smaller the cavitation margin of a centrifugal pump, the better the anti-cavitation performance. For a given pump, the cavitation margin that must be possessed at a given speed and flow rate is called the necessary cavitation margin of the pump, also known as the cavitation margin of the pump. It is related to the internal flow of the centrifugal pump and is determined by the pump itself. Its physical meaning is to indicate the degree of pressure drop of the liquid at the pump inlet, that is, in order to protect the pump from cavitation, it is required that the liquid per unit weight at the pump inlet has surplus energy exceeding the vaporization pressure head. NPSH has nothing to do with the device parameters, only with the pump inlet part of the motion parameters, these motion parameters at a certain speed and flow rate is determined by the geometric parameters.

1. Method for Solving Cavitation Problem of Centrifugal Pump

1. Increase net positive suction height.

2. Increase liquid temperature or reduce liquid volatility. This can reduce the chance of the liquid forming cavitation.

3. Replace the suitable pump model or take improvement measures. Choosing the appropriate pump model or using some new materials to make the pump parts can improve the anti-cavitation ability of the pump.

4. Use a decompression pump. The reduced pressure pump can increase the cavitation margin by reducing the outlet pressure, so as to effectively avoid the occurrence of cavitation.

Concept of 2. NPSH

NPSH is an important indicator of centrifugal pump to prevent cavitation, which refers to the difference between the net positive suction height of the pump and the outlet pressure process. Simply put, the cavitation margin is the maximum value of the net positive suction height allowed by the pump to avoid cavitation.

Calculation Method of 3. NPSH

The calculation method of NPSH is determined according to the performance curve of the pump and the physical properties of the liquid, and is generally calculated according to the following formula:

NPSH = net positive suction height-outlet pressure/liquid density * 9.81

Where, net positive suction height = horizontal distance from the center line of the pump inlet to the water surface-resistance loss of the pump inlet pipeline at the coordinate axis position. The outlet pressure refers to the pressure at the outlet of the centrifugal pump. The liquid density is generally taken as the density of water at 20 ℃ is 1000 kg/m.

4. cavitation phenomenon and its harm

Centrifugal pump in the working process, due to the centrifugal force generated by the rotation of the impeller, the liquid is forced to be sucked in and discharged to the outlet. Therefore, when the pump inlet pressure is lower than a certain limit value, liquid vapor bubbles will be generated, and cavitation will be formed. Cavitation will not only affect the flow and head of the pump, but also may cause mechanical damage to the pump, and even stop the pump in severe cases.

5. the size of the NPSH

In general, the size of the NPSH is related to the working conditions of the pump, the physical properties of the liquid and the rationality of the pump design. According to practical experience, the NPSH of centrifugal pumps is generally about 1.5 to 2 meters. When the cavitation margin is too small, it is easy to cause the cavitation phenomenon to occur, thereby causing damage to the equipment.

The size of the centrifugal pump NPSH directly affects the stability and life of the centrifugal pump. If the cavitation margin is too small, it is easy to cause cavitation phenomenon; if the cavitation margin is too large, it will cause the efficiency of the pump to reduce, thus affecting the work efficiency.

Therefore, in the selection and use of centrifugal pump, must according to the actual needs, determine the required NPSH size, and combined with the pump working state, flow, head and other factors to calculate. Only in this way can the service life and normal operation of the centrifugal pump be ensured.

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