Analysis And Prevention Of Vane Pumps Cavitation
Cavitation is a widespread phenomenon in vane pump products. It damages vane pumps very seriously, which seriously affects the service life of water pumps, causing social waste that cannot be underestimated and safety accidents. Therefore, it is necessary to study the cavitation of the vane pump to prevent the occurrence of cavitation, or to minimize the impact caused by cavitation.
Causes of Cavitation
How does cavitation occur?
To put it simply, the pressure in the pump somewhere drops below the saturated steam pressure below this temperature, and air bubbles are generated in the liquid. In addition, the gas dissolved in the water will also precipitate. The separated steam and gas bubbles are sent to the high-pressure area by the flowing liquid, where they burst quickly (the time duration T ≤ 0.001s at this time), in addition to the excessive increase in pressure, which in turn causes this The process has all the characteristics of bursting, and the shock phenomena generated in this way will soon spread in sequence, and then the bubbles close to the eroded surface and the bubbles in the holes (micropores, fractures, cracks, etc.) will burst. In this way, cracks may occur in places with relatively low strength. Instead, the bombardment of the surface caused by the collapse of the bubble causes the material layer close to the cavitation zone to vibrate and be damaged.
Cause of cavitation
There are several possible reasons for cavitation:
1) The highest point of inhalation is too high or the highest point of perfusion is very low.
2) Exceeding the rated flow, so the water flow in the pump rises and the pressure drops.
3) From the perspective of kinematics, the direction of the flow line is never correct, the flow direction changes rapidly and the liquid flow at the inlet of the impeller is never uniform.
The formation of cavitation depends on the operation of the suction area of the pump. Changes in suction conditions will directly affect the occurrence of cavitation and continue to occur.
2. Cavitation damage location
Both the impeller blade and the wall of the over-flow part of the pump may be damaged due to corrosion, erosion and cavitation. Based on the observations of various vane pumps for many years, the location of the damage and the position of the flow channel in the pump, It is concluded that the position damaged by cavitation is the place where the bubble collapse position moves a little further in the flow direction.
In the initial stage of cavitation, the bursting of air bubbles is within the range of the impeller. In the expansion stage of cavitation, most of the bubbles have been destroyed within the range of the impeller, and the rest will continue to the guide vane, the volute and the lower stage of the multi-stage pump. The bubbles burst there and cause steam at the place. Erosion occurs.
1) The damage caused by cavitation of the centrifugal pump (see Figure 1), the damage point is not limited to the blade, so at a above the cover of the impeller, the lowest pressure area is located on the front of the impeller blade close to the inlet b, Sudden pressure rises there, which can cause cavitation. When cavitation is severe, cavitation damage will also occur at the blade outlet, the tongue of the pump, and on the induction wheel that pre-spins the liquid.
2) The oblique flow pump and axial flow pump are at the damage point of the open impeller cavitation (see Figure 2): close to the impeller inlet c.
The impeller blade tail and the pump body corresponding to the end are at d.
Suction above the horn tube, just close to the impeller inlet e.
At the inlet f of the guide vane.
3. Signs of cavitation
If the pump's head and efficiency are significantly reduced, noise and vibration can be generated, and cavitation can basically be determined. Once the pump flow exceeds the rated value, the probability of cavitation will increase.
4. Prevention of cavitation
Measures to prevent cavitation and reduce cavitation damage can be divided into three categories:
(1) Structural precautions
1) In the case of the same flow rate, use a double suction pump instead of a single-stage pump. This will reduce the flow through the impeller and the required cavitation allowance.
2) Pre-spinning is used. If the pre-spinning along the impeller rotation direction is not very strong, cavitation can be reduced and the suction performance of the pump can be improved. In addition, the efficiency of the pump can be improved.
3) The impeller with large exit width can achieve good results.
4) Avoid sharp turns behind the impeller.
5) When the number of blades is even, the inlet of the second blade is shortened each time.
6) Adopt the front cover with gentle bending.
7) Increase the number of blades with high specific speed.
8) Thinn the inlet end of the blade.
9) Use a small blade inlet angle.
10) A spiral induction wheel is used behind the centrifugal pump impeller inlet.
11) An ejector is installed behind the impeller inlet, so that the pressure at the inlet of the impeller can be increased, and the suction height of the pump can be increased (the required filling height is reduced).
(2) Preventive measures for installation and operation technology
1) The highest suction point of the pump is as small as possible.
2) The temperature of the conveyed medium is reduced as much as possible.
3) Sufficient pouring height should be guaranteed when transporting hot water.
4) The pump should be operated at the point of highest efficiency.
(3) Precautions on materials Although there is no material that is completely resistant to cavitation, the cavitation resistance of various materials is still very different.
1) For metals subject to cavitation, the forging metal is much better than the casting metal.
2) A smooth surface is better than a rough surface.
3) High surface hardness is better than low surface hardness.
4) The surface is sprayed (plastic sprayed, silicon carbide sprayed, etc.).