There are many problems occurred during working of the pump. Cavitation is one of them. Let us briefly see it.
What is cavitation?
When the local 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 bubble gathering, moving, splitting and eliminating. The critical pressure is generally close to the vaporization pressure.
What are the hazards of cavitation?
01 corrosion of overcurrent parts
There are two reasons for corrosion:
he first is the high frequency (600 ~ 25000hz) impact and the pressure up to 49MPa caused the mechanical erosion of the metal surface;
The other is due to the heat released during vaporization and hydrolysis due to the action of temperature difference battery. The oxygen generated will oxidize the metal and cause chemical corrosion.
02 pump performance degradation
When the pump cavitation, the energy exchange in the impeller is disturbed and destroyed. The performance of the external characteristics is the Q-H curve. The Q-P and q-η curves decrease. In serious cases, the liquid flow in the pump will be interrupted and can not work. For low specific speed, due to the narrow and long passage between blades, once cavitation occurs, bubbles fill the whole passage, and the performance curve will drop suddenly. For the medium and high specific speed, the flow path is short and wide, so it needs a transition process from the occurrence and development of bubbles to the full flow path. The corresponding performance curve begins to slow down, and then increases to a certain flow rate before sharp decline.
The most easily cavitating part of centrifugal pump
1. The front cover plate with the largest curvature of the impeller is close to the low pressure side of the blade inlet edge;
2. The low pressure side near the inlet edge of the volute tongue and guide vane in the extrusion chamber;
3. The sealing clearance between the outer circle of the blade tip and the shell of the high specific speed impeller without front cover plate and the low pressure side of the blade tip;
4. First stage impeller in multistage pump.
Improve anti cavitation measures
1. Measures to improve the cavitation resistance of centrifugal pump
1) Improve the structure design of the pump from the suction port to the impeller. Increase the flow area; increase the curvature radius of the inlet section of the impeller cover plate to reduce the sharp acceleration and pressure drop of the liquid flow; properly 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 acceleration and pressure drop of the flow around the blade head; improve the surface finish of the impeller and the blade inlet part to reduce the resistance loss; turn the blade inlet side to the impeller inlet Extend, make the liquid flow accept the work ahead of time, increase the pressure.
2) In order to increase the pressure of the liquid flow, the pre inducer is used to make the liquid flow work in advance.
3) If the double suction impeller is used to let the liquid flow into the impeller from both sides of the impeller at the same time, the inlet cross section will be doubled and the inlet flow rate will be doubled.
4) In order to increase the inlet angle of the blade, reduce the bending at the inlet of the blade, reduce the blockage of the blade, increase the inlet area, and improve the working conditions under large flow to reduce the flow loss. But the positive impact angle should not be too large, otherwise it will affect the efficiency.
5) Use anti cavitation materials. The results show that the higher the strength, hardness and toughness, the better the chemical stability and the stronger the cavitation resistance. Use anti cavitation materials. The results show that the higher the strength, hardness and toughness, the better the chemical stability and the stronger the cavitation resistance.
2. Measures to increase effective cavitation allowance of liquid inlet device
Increase the pressure of the liquid level in the liquid storage tank before the pump to increase the effective cavitation allowance.
1) Reduce the installation height of the upper suction unit pump.
2) The upper suction device is changed into a reverse filling device.
3) Reduce the flow loss in front of the pump. For example, shorten the pipeline as much as possible within the required range,
4) Reduce the flow rate in the pipeline, reduce the bends and valves, and increase the valve opening as much as possible.
5) Reduce the temperature of the working medium at the pump inlet (when the conveying working medium is close to the saturation temperature).
The above measures can be comprehensively analyzed and properly applied according to the pump selection, material selection and pump use site conditions.