What is NPSH (Net Positive Suction Head)?
To put it simply, the pump draws in the net positive value after deducting the saturated vapor pressure (vapor pressure) at its operating temperature at its suction flange or the impeller inlet of a vertical axis centrifugal pump. The water head is usually expressed as the height of the liquid column. This is a pipeline system that can be used to measure whether the pressure state is adequate. NPSH is closely related to whether cavitation will occur in the pump. If cavitation erosion (also known as cavitation) generation, will affect the performance of the pump and may even cause damage.
The calculation formula of the NPSH value is as follows:
NPSH = NPSHa - NPSHr
NPSH values are usually expressed in meters (m) or feet (ft).
There are two main parameters that must be understood in the NPSH value:
1. Available net positive suction head NPSHa (Net Positive Suction Head Available): refers to the actual pressure state when the fluid enters the suction end of the pump. NPSHa represents the absolute pressure that can be obtained at the pump inlet, ensuring that the fluid can enter the pump stably. Determined by factors such as the design of the piping system and operating conditions.
NPSHa value calculation formula:
NPSHa = ha + hs - hv - ht
- ha: absolute pressure at the suction end
- hs: hydrostatic head of the suction pipe
- hv: friction head of the suction pipe
- ht: Negative pressure head of the suction pipe
2. Necessary Net Positive Suction Head NPSHr (Net Positive Suction Head Required): refers to the minimum pressure value that the pump needs to ensure normal supply of liquid under specific operating conditions. NPSHr is part of the pump design and is typically provided by the pump manufacturer.
The performance of the pump is related to the relationship between NPSHa and NPSHr.
If NPSHa is greater than NPSHr, the fluid can enter the pump stably and avoid cavitation erosion during pump operation.
If NPSHa is less than NPSHr, the liquid may vaporize or form bubbles, causing cavitation, reducing pump performance or causing damage.
Therefore, it should be noted that when the pump is running, it must NPSHa > NPSHr to ensure the safe operation of the pump.
