What is the difference between the necessary NPSH and the effective NPSH of a water pump?

The cavitation head is divided into the effective cavitation head NPSHa and the necessary cavitation head NPSHr. The necessary NPSH of the pump is 

a characteristic of the pump and is determined by the design. The effective NPSH of the pump is determined by the process pipeline.

For a given pump, the necessary NPSH at a given speed and flow rate is called the necessary NPSH, which is commonly expressed as NPSHr. Also known 

as the cavitation margin of the pump, it is the cavitation performance parameter that the pump must achieve.

NPSHr is related to the internal flow of the pump and is determined by the head of the pump itself. Its physical meaning is to indicate the degree of 

pressure drop of the liquid at the pump inlet. That is, in order to ensure that the pump does not cause cavitation, it is required that the unit weight 

of the liquid at the pump inlet is It has excess energy that exceeds the vaporization pressure head.

The necessary NPSH has nothing to do with the device parameters, but is only related to the motion parameters (vo, wo, wk, etc.) of the pump inlet 

part. These motion parameters are determined by geometric parameters at a certain speed and flow rate. This means that NPSHr is determined by 

the pump itself (geometric parameters of the suction chamber and impeller inlet part).

For a given pump, no matter what kind of medium (except for viscous media that affects the velocity distribution), if it flows through the pump inlet

 at a certain speed and flow rate, it will have the same pressure drop because the speed is the same, that is, the NPSHr is the same. So NPSHr has 

nothing to do with the properties of the liquid (regardless of thermodynamic factors).

The smaller the NPSHr, the smaller the pressure drop, and the smaller the NPSHa that the device must provide, so the better the anti-cavitation 

performance of the pump. Therefore: r represents required, which is determined by the pump body and is specifically related to the rotation 

speed, impeller form, etc.;

The effective NPSH refers to the NPSH determined by the pump installation conditions, and is commonly expressed as NPSHa. Also known as 

device cavitation margin, it is the excess energy provided by the suction device that exceeds the vaporization pressure head per unit weight of

 liquid at the pump inlet.

The larger the NPSHa, the less likely the pump is to suffer from cavitation. The size of the effective NPSH 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, NPSHa decreases as 

the flow rate increases.

Therefore: A represents available, effective, and can be provided. This is determined by the system and pipeline and must be strictly calculated;

To ensure that the pump does not cavitate, NPSHa must be greater than NPSHr. There are empirical values for various types of pumps. Generally, 

the necessary NPSH of the pump is increased by 0.5-1m of excess energy head as the allowable NPSH.