For a given pump, at a given speed and flow must have the necessary to have the necessary cavitation margin, commonly known as NPSHr. Also known as the pump cavitation margin, is required to achieve the performance of the pump cavitation performance parameters. The internal flow of NPSHr and pump, is composed of a pump head itself, its physical meaning is expressed in part of the pump inlet pressure drops of liquid level, which is in order to guarantee the pump cavitation does not occur, with surplus energy exceeds the vaporization pressure head at the inlet of the pump unit weight of liquid. It is necessary that the cavitation margin has nothing to do with the device parameters, only with the pump inlet part of the motion parameters (VO, wo, wk, etc.), these motion parameters in a certain speed and flow rate is determined by the geometric parameters. This means that the NPSHr is determined by the pump itself (the geometric parameters of the suction chamber and the inlet section of the impeller). For the fixed pump, regardless of the type of medium (viscous medium due to the impact of velocity distribution in a certain speed, except) import and traffic flowed through the pump, because the speed of the same size with the same pressure drop, which is similar to NPSHr. So NPSHr has nothing to do with the nature of the liquid (but not the thermodynamic factor). NPSHr smaller, indicating that the pressure drop is small, the requirements of the device must be provided by the NPSHa small, so the pump anti cavitation performance is better.
Effective cavitation margin refers to the pump installation conditions determined by the cavitation margin, commonly used NPSHa. Also known as the device cavitation allowance, is provided by the suction device in the pump inlet unit of the weight of the liquid has more than the vaporization pressure head of the surplus energy. The bigger the NPSHa, the more difficult the pump will occur. The size of effective cavitation margin is related to the device parameters and liquid properties (P, PV, etc.). Because the hydraulic loss of the suction device is proportional to the square of the flow rate, the NPSHa decreases with the increase of the flow rate.
