Centrifugal pump is a kind of fluid machinery that transports liquid media out by the centrifugal force generated by the impeller rotating at a certain speed. Centrifugal pump in the working often produce a special phenomenon, when the liquid flows in the pump, due to the shape of the blade and the flow in it suddenly change direction and other flow characteristics, there are some local low pressure areas near the non-working surface near the blade. If the fluid pressure in the low-pressure area is reduced to the saturated vapor pressure corresponding to the liquid temperature, the liquid will begin to vaporize and produce bubbles; If the pressure continues to decrease, the bubble and its area will gradually increase and expand; At the same time, some of the active gases that may have been dissolved in the liquid (such as oxygen in water, etc.) will also escape due to the pressure reduction and become bubbles again.
When these bubbles enter the high-pressure area of the pump with the liquid flow, they are quickly condensed or even broken and disappeared under pressure. At the moment when the bubble disappears, the liquid around the bubble quickly enters the hole generated by the bubble condensation, accompanied by local high temperature and high pressure water hammer phenomenon. The process of generation, expansion and collapse of bubbles in the fluid is accompanied by complex physical and chemical phenomena, showing noise, vibration, and accompanied by a decrease in flow, head and efficiency, resulting in a decrease in the performance of the pump, and at the same time, the overflow parts will also be damaged and even unable to work. The above phenomena are collectively referred to as cavitation. When there are not too many bubbles and cavitation is not serious, it will not have a significant impact on the operation of the pump. However, when a large number of cavitation is produced and cavitation continues to develop, serious consequences and hazards will occur. It mainly manifests the following aspects:
1、The pump appears vibration and noise. When the bubble collapses, a strong water hammer is generated, and the noise of a wide band will be generated due to the mutual impact between the fluid particles and the strong impact on the wall of the runner, and cause the vibration of the pump, so that the pump cannot work normally.
2、The performance of the pump is reduced. When cavitation occurs in the pump, the fluid medium is continuously damaged, the lift, flow and efficiency of the pump will drop sharply, and the pump cannot work normally continuously when it is serious.
3、Erosion of overcurrent parts. When the pump cavitation, at the same time as the bubbles condense rupture, the liquid particles fill the holes at a very high speed, which produces a very strong water hammer effect at this moment, and hits the metal surface with a high impact frequency, the impact stress can reach hundreds to thousands of atmospheres, and the impact frequency can reach tens of thousands of times per second, it can make the runner parts become honeycomb or sponge, and the wall thickness will be broken down in severe cases.
(1) Appropriately increase the radius of curvature of the inlet section of the impeller cover; Extend the blade appropriately to the inlet edge of the impeller, and try to make the blade at the inlet as thin as possible; Improve the surface finish of the impeller and blade inlet part; Increase the inlet angle of the blade and adopt the positive punch angle; These measures can all reduce flow losses and make the media flow smoother, thereby reducing the NPSHr of the pump.
(2) The double suction impeller is selected, and the medium flows in from both sides of the impeller, which is equivalent to increasing the inlet area of the impeller, reducing the flow through each side of the impeller, thereby reducing the V0, ω0 and λ2 of the impeller, and improving the cavitation resistance of the pump.
(3) The induction wheel installed for the centrifugal pump can pre-pressurize the medium, increase the medium inlet at the impeller inlet, and significantly reduce NPSHr. However, the induction wheel will increase the axial installation size, and the centrifugal pump with the induction wheel installed will reduce the head when running at a small flow, and the "hump" appears from the curve, so it is not recommended to add an induction wheel to the centrifugal pump in the API 610 standard.