If the centrifugal pump in the work of the noise and vibration, accompanied by reduced flow, lift and efficiency, and sometimes can not even work, when the maintenance of this pump, you can often find the blade inlet side of the front cover and blade inlet edge Near the pitted or honeycomb damage. In actual operation, many centrifugal pumps are damaged by cavitation. Centrifugal pump cavitation or cavitation process is the flow of liquid bubbles and subsequent rupture of the process. As the absolute velocity of the fluid increases, due to the drop in the hydrostatic pressure, for certain particles of the fluid at a given temperature, there is no heat input from the outside but they have reached the vapor pressure, causing the particles to vaporize and generate bubbles . Along the runner, if the fluid's hydrostatic pressure rises again, more than the vapor pressure, the bubble will quickly rupture, resulting in a huge intrusive explosive condensation shock. If the rupture of the bubble does not occur at the time of flowing liquid, but at the wall of the flow guiding component, the cavitation causes the wall material to be eroded. When the centrifugal pump is operated under cavitation conditions, the noise of the centrifugal pump is increased, the vibration is increased, the efficiency is decreased, and the head is lowered, even if erosion of the wall material does not occur. NPSH: Also known as effective NPSH. The device NPSH is provided by the suction device. The unit weight of the liquid at the inlet of the centrifugal pump has surplus energy exceeding the vapor pressure and the water head. Abroad called this net positive suction head, that is, the pump inlet (zero head position) liquid full head minus the vapor pressure and net head left, with NPSHa said. Its size and device parameters are related to the nature of the liquid. Because the suction loss is proportional to the square of the flow rate, NPSHa decreases as the flow rate increases. NPSHa-Q is a decreasing curve. Centrifugal pump NPSHr is related to the flow in the pump. It is the pressure drop of the balance pump's inlet part determined by the centrifugal pump itself, that is, to ensure that the pump does not cavitate. It is required that the liquid per unit weight at the pump inlet has Excessive vapor pressure head excess energy. Abroad called this necessary net positive suction head. The physical meaning of the pump nibbling allowance represents the degree to which the pressure of the liquid drops at the inlet of the pump. The so-called net positive suction head is required to require suction device must provide such a large net positive suction head, in order to compensate for the pressure drop to ensure that the pump does not cavitation. Multistage centrifugal pump NPSH has nothing to do with the device parameters, only the pump inlet part of the movement parameters. Motion parameters at a certain speed and flow rate is determined by the geometric parameters. This means that NPSHr is determined by the pump itself (geometric parameters of suction chamber and impeller inlet section). For a given pump, no matter what kind of liquid (except viscous, influencing speed distribution) flows through the pump inlet at a certain speed and flow rate, the same pressure drop is the same for the same speed and the same NPSHr. So NPSHr has nothing to do with the nature of the liquid (without regard to thermodynamic factors). The smaller the NPSHr, the smaller the pressure is and the smaller the NPSHa must be provided by the plant, so the better the pump's cavitation resistance.
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