The cavitation or cavitation process of a multistage centrifugal pump is a process in which a flowing liquid generates bubbles and then ruptures. When the absolute velocity of the fluid increases, due to the decrease of the static pressure of the fluid, for some specific particles of the fluid at a certain temperature, although no heat is input from the outside, they have reached the vaporization pressure, causing the particles to vaporize and generate bubbles. . Along the flow path, if the static pressure of the fluid rises again, which is greater than the vaporization pressure, the bubble bursts rapidly, producing a large condensation shock that is an inward explosion property. If the bubble burst does not occur at the flowing liquid, but occurs at the wall of the flow guiding assembly, the cavitation can cause the wall material to be etched.
When the multistage centrifugal pump is operated under cavitation conditions, even if the erosion of the wall material does not occur, it is found that the noise of the centrifugal pump increases, the vibration is intensified, the efficiency is lowered, and the lift is lowered.
Device NPSH: Also known as effective NPSH. The NPSH of the device is provided by the inhalation device, and the unit weight liquid at the inlet of the centrifugal pump has excess energy exceeding the vaporization pressure and the head. It is said to be an effective net positive suction head, that is, the liquid at the pump inlet (zero position water head) has a full head minus the vaporization pressure and the net head value, called NPSHa. Its size and device parameters are related to the nature of the liquid. Since the hydraulic loss of the inhalation device is proportional to the square of the flow rate, the NPSHa decreases as the flow rate increases. NPSHa-Q is a decreasing curve.
The NPSH of the multistage centrifugal pump is independent of the device parameters and is only related to the motion parameters of the pump inlet. The motion parameters are determined by geometric parameters at a certain speed and flow rate. This means that the NPSHr is determined by the pump itself (the geometry of the suction chamber and the inlet portion of the impeller). For a given pump, no matter what kind of liquid (except for the viscosity, which affects the speed distribution), it flows through the pump inlet at a certain speed and flow rate. Because of the same speed, it has the same pressure drop, and the NPSHr is the same. Therefore, NPSHr is independent of the nature of the liquid (regardless of thermodynamic factors). The smaller the NPSHr, the smaller the pressure, the smaller the NPSHa that the device must provide, and the better the cavitation resistance of the pump.
The NPSH of the centrifugal pump is related to the flow in the pump. It is the pressure drop of the inlet part of the balance pump determined by the centrifugal pump itself. In order to ensure that the pump does not cavitation, the unit weight liquid at the pump inlet is required. Excess energy exceeding the vaporization pressure head. Foreign countries call this the necessary net positive tip. The physical meaning of the pump NPSH indicates the extent to which the pressure drops in the pump inlet section. The so-called net positive suction head means that the inhalation device must provide such a large net positive suction head to compensate for the pressure drop and ensure that the pump does not cavitation.
