Net Positive Suction Head

Net Positive Suction Head, NPSH

Net positive suction head is the head above the vapor pressure of the liquid. From a pressure point of view, it is the amount of absolute pressure to overcome the liquid’s vapor pressure at the pump suction during operation under the operating temperature.
The unit of NPSH is given in meters (m) or feet (ft).
Net positive suction head can be further broken down into Net positive suction head available (NPSHa) and Net positive suction head required (NPSHr) where:
  1. NPSHa is the absolute pressure at the pump suction, expressed in meters water column (blue dashed line in the below diagram), defined by
    2. 其中，
    Suction tank liquid surface pressure
    Liquid density
    Acceleration due to gravity
    Suction tank liquid level minus height of the impeller eye
    Suction piping frictional loss
    Vapor pressure of the liquid at the operating temperature
    The value NPSHa is dependent on the suction piping configuration and liquid characteristics.
  2. NPSHr is the minimum suction head at the pump suction before cavitation occurs (solid red line in the following diagram). This value is usually obtained through actual testing and is dependent on the pump design.
NPSH is used to determine if cavitation will occur during operation. If NPSHa is greater than NPSHr, then cavitation will not happen. If NPSHa is smaller than NPSHr, then cavitation is expected (see diagram). For practical purposes, to prevent cavitation from occuring, we usually introduce a safety margin by making sure at the desired flow rate, the NPSHa>NPSHr+0.5 m.