Terrible centrifugal pump cavitation

1. What is cavitation

When the partial pressure of the liquid in the pump drops to the critical pressure, bubbles will be generated in the liquid. Cavitation is the whole process of bubbles gathering, moving, splitting and destroying. The critical pressure is generally close to the vaporization pressure.

2. What are the hazards of cavitation?

A    Overflow parts corrosion
First, due to the high-frequency (600~25000HZ) impact when the bubble bursts, the pressure is as high as 49Mpa, resulting in mechanical erosion of the metal surface;
The second is due to the release of heat during vaporization, and the role of thermoelectric batteries to produce hydrolysis, the generated oxygen oxidizes the metal, and chemical corrosion occurs.

B     Pump performance is reduced
During pump cavitation, the energy exchange in the impeller is disturbed and destroyed. The performance of the external characteristics is the Q-H curve, and the Q-P and Q-η curves decline. In severe cases, the flow of the pump will be interrupted and cannot work.

3. The most prone to cavitation in centrifugal pumps

The front cover plate with the largest curvature of the impeller, close to the low-pressure side of the blade inlet edge;
The low-pressure side of the volute septum and guide vane near the inlet edge in the extrusion chamber;
The sealing gap between the outer circumference of the blade tip of the high specific speed impeller without the front cover and the casing and the low pressure side of the blade tip;
The first-stage impeller in a multi-stage pump.

4. Improve anti-cavitation measures

A Measures to improve the anti-cavitation performance of centrifugal pump
Improve the structural design from the suction port of the pump to the vicinity of the impeller. Increase the flow area; increase the radius of curvature of the inlet section of the impeller cover to reduce the rapid acceleration and decompression of the liquid flow; appropriately reduce the thickness of the blade inlet and round the blade inlet to make it close to the streamline shape, which can also reduce the winding Accelerate and lower the pressure of the flow blade head; improve the surface finish of the impeller and blade inlet to reduce drag loss; extend the blade inlet edge to the impeller inlet to make the fluid flow accept work in advance and increase the pressure.
The front induction wheel is used to make the liquid flow work in advance in the front induction wheel to increase the liquid pressure.
The double suction impeller is used to allow the liquid flow to enter the impeller from both sides of the impeller at the same time, the inlet cross section is doubled, and the inlet flow rate can be doubled
The design working condition adopts a slightly larger positive angle of attack to increase the blade inlet angle, reduce the bending at the blade inlet, reduce the blade blockage, and increase the inlet area; improve the working conditions under large flow to reduce the flow loss. However, the positive angle of attack should not be too large, otherwise it will affect efficiency.
Use anti-cavitation materials. Practice has shown that the higher the strength, hardness and toughness of the material, the better the chemical stability and the stronger the resistance to cavitation.
B Measures to increase the effective cavitation allowance of the liquid inlet device
Increase the pressure of the liquid level in the reservoir tank before the pump to increase the effective cavitation allowance.
Reduce the installation height of the suction device pump.
Change the suction device to the backfill device.
Reduce the flow loss on the pipeline in front of the pump. For example, shorten the pipeline in the required range, reduce the flow velocity in the pipeline, reduce the elbows and valves, and increase the valve opening as much as possible.
Reduce the temperature of the working medium at the inlet of the pump (when the working medium is close to the saturation temperature)
Carry out comprehensive analysis according to the conditions of pump selection, material selection and pump use site, and apply them appropriately

5. What is the difference between required cavitation allowance and effective cavitation allowance

The cavitation allowance is divided into effective cavitation allowance NPSHa and required cavitation allowance NPSHr. The required cavitation surplus of the pump is the characteristics of the pump and is determined by the design. The effective cavitation surplus of the pump is determined by the process piping.
For a given pump, the required cavitation surplus at a given speed and flow rate is called the necessary cavitation surplus, which is often expressed by NPSHr. Also known as the cavitation allowance of the pump, it is a parameter that specifies the cavitation performance to be achieved by the pump.
NPSHr is related to the internal flow of the pump and is determined by the head of the pump itself. Its physical meaning is to indicate the degree of pressure drop of the liquid at the pump inlet, that is, to ensure that the pump does not cause cavitation, the unit weight of the liquid at the pump inlet is required. It has excess energy that exceeds the head of vaporization pressure.
The necessary cavitation allowance has nothing to do with the device parameters, only related to the motion parameters (vo, wo, wk, etc.) of the pump inlet part. These motion parameters are determined by the geometric parameters at a certain speed and flow rate. This means that the NPSHr is determined by the pump itself (geometric parameters of the suction chamber and impeller inlet section).
For a given pump, no matter what kind of medium (except for the viscous medium due to the influence of speed distribution), it flows through the pump inlet at a certain speed and flow rate. Because of the same speed, it has the same pressure drop, that is, the NPSHr is the same. So NPSHr has nothing to do with the nature of the liquid (not considering thermodynamic factors).
The smaller the NPSHr, the smaller the pressure drop, and the smaller the NPSHa must be provided by the device, so the better the anti-cavitation performance of the pump. Therefore: r stands for required and is determined by the pump body, which is related to the speed, impeller form, etc.;
The effective cavitation allowance refers to the cavitation allowance determined by the installation conditions of the pump, which is usually expressed by NPSHa. Also known as the device cavitation surplus, is the excess energy provided by the suction device at the pump inlet per unit weight of liquid that exceeds the vaporization pressure head.
The larger the NPSHa, the less susceptible the pump is to cavitation. The size of the effective cavitation margin is related to device parameters and liquid properties (p, pv, etc.). Because the hydraulic loss of the suction device is proportional to the square of the flow rate, NPSHa decreases as the flow rate increases.
Therefore: A stands for available and available, this is determined by the system and the pipeline, and must be strictly calculated;
To ensure that the pump is not cavitation, NPSHa must be greater than NPSHr. The specific value is large, and various types of pumps have empirical values. Generally, the excess energy head that the pump’s necessary cavitation surplus is increased by 0.5-1m is used as the allowable cavitation surplus.

2020-07-15T16:20:59+08:00July 15th, 2020|

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