The reason for the cavitation phenomenon of the self priming trash pump is that the local pressure in the pump is lower than the saturated steam pressure of the water, or that vaporization is a prerequisite for the formation of cavitation. To understand the causes of cavitation caused by self-priming pumps, first of all, we must know what cavitation is.
Cavitation makes the pump sound as if pumping gravel. Cavitation damages the pump faster than any other accident. The liquid entering the pump must flow through the impeller, where the liquid suddenly turns at an angle and then enters the rotating blade space. If the pressure in the inlet pipe is not too high, bubbles will form, but it is not air. These bubbles reach the high-pressure area along the impeller, and they are easily compressed in this short distance. The high-speed water flow breaks the bubbles, and the water flows into the space originally occupied by the bubbles, crackling, as if a stone block hit the pump casing. If many air bubbles burst at the same time, the noise is like the stirring of gravel in the pump.
Blasting bubbles hit the metal surface to produce a very high pressure shock wave, and the grain structure of the metal on the inner wall of the pump casing will appear loose and flake off. The appearance of coarse-grained surfaces on the inner surface of the pump casing and the impeller blades is damaged by cavitation. The smooth wear surface is mostly caused by erosion. Cavitation and erosion together, the pump will be severely worn in a very short time. The coarse-grained surface formed by the cavitation effect on the inner wall of the pump casing is then eroded by the solid matter in the mud and smoothed. It should be noted that the only similarity between cavitation and air lock is that both have air bubbles at the impeller. But they are caused by different reasons and have different causes.
The air lock phenomenon causes air bubbles to be often brought into the pump by the mud and accumulates at the inlet of the impeller. When the large bubbles gradually expand and expand between the blades of the impeller, the head will be reduced. (Because the impeller is not filled with mud). Air bubbles also prevent mud from entering the pump. But such bubbles do not make noise.
Cavitation damage occurs when the concentrated mass does not flow along the metal surface. It may appear at the inner wall of the pump casing near the impeller inlet, at the end of the impeller blade, or at a valve that is not fully open. If cavitation continues to occur, these parts will be destroyed quickly. Since the pump can still continue to run in the event of severe cavitation, its head or flow rate will not be significantly reduced, so the cavitation phenomenon may still not be noticed.
The cause of cavitation in the self priming trash pump
1. The installation height is too high. As the installation height of the self-priming pump increases, the vacuum at the pump inlet increases, causing the pressure in the pump to drop too low.
2. The actual operating conditions deviate far from the design operating conditions. When the pump is running under non-design conditions, it may also generate a band-shaped vortex from below to below the impeller. When the center pressure of the vortex belt drops to the saturated steam pressure, the vortex belt becomes a cavitation belt. When this vortex belt extends into the pump, it can not only promote and aggravate the cavitation of some parts of the pump, but also cause strong vibration of the unit and make a roar.
3. Poor water conditions. For pumps with elbow-shaped inlet flow channels, the uneven distribution of the flow velocity and pressure in the bend causes uneven distribution of the inlet flow velocity of the impeller of the pump. The vortex in the inlet pool will cause the water entering the pump to pre-rotate or bring air into the pump. The flow velocity and pressure at the inlet are unevenly distributed, which leads to the occurrence of cavitation and cavitation. According to foreign cavitation test data, the greater the gas content in the water, the earlier the cavitation starts.
4. The altitude of the installation site is higher, the temperature of the water delivered by the self-priming pump is higher. At high altitudes, the lower atmospheric pressure will cause the pump suction pressure to be correspondingly lower; and the higher the water temperature, the higher the saturated steam pressure, and the easier the water to vaporize.
How to solve the cavitation phenomenon of self priming trash pump:
Measures to improve anti-cavitation performance of centrifugal pump
1. Improve the structural design of the suction port of the single-stage centrifugal pump to the vicinity of the impeller. Increase the flow area; increase the radius of curvature of the inlet section of the impeller cover, reduce the rapid acceleration and pressure reduction 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. Reduce the acceleration and pressure reduction of the blades around the flow; improve the surface finish of the impeller and the blade inlet part to reduce the resistance loss; extend the blade inlet edge to the impeller inlet, so that the liquid flow receives work in advance and increase the pressure.
2. The front induction wheel is adopted to make the liquid flow work in advance in the front induction wheel to increase the pressure of the liquid flow.
3. 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.
4. 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.
5. 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.
Measures to increase the effective cavitation margin of the liquid inlet device
1) Increase the pressure of the liquid level in the storage tank before the centrifugal pump to increase the effective cavitation allowance.
2) Reduce the installation height of the suction device pump.
3) Change the suction device to the backfill device.
4) Reduce the flow loss on the pipeline in front of the centrifugal 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.