What is NPSH, and why is it important for boiler feed pump applications?
Net Positive Suction Head (NPSH) is a critical design factor in any system utilizing pumps, especially when pumping hot liquids. System NPSH is to be evaluated by comparing the “available” head to the “required” head. The experts at BFS Industries are here to discuss these two components below.
NPSH Required (NPSHR)
The NPSH required (NPSHR) is a function of the pump and is flow dependent. The NPSHR of the pump (normally expressed in feet for US applications) is the minimum suction head required to keep the pumped liquid in the liquid phase as it enters the pump, and prevent it from turning to a vapor.
The NPSHR represents the head loss at the pump inlet as the liquid enters the pump suction connection and is directed into the eye of the impeller. There are pressure drops during this change in flow path that need to be compensated for in the system design to prevent cavitation and potential pump failure.
For a specific pump model/size, the NPSHR increases as the pump flow increases. For example, a pump may have a flow rate range of 30 gpm to 60 gpm. The NPSHR of this pump will be lower at a 30 gpm flow rate and will increase as the flow approaches the 60 gpm flow rate.
NPSH Available (NPSHA)
The NPSH available (NPSHA) value is a function of the installation configuration and environmental factors not related to the pump itself. Some of these include:
- The static head of liquid measured from the pump inlet to the water line in the tank
- Friction losses in the suction piping
- Temperature of the liquid
- Vapor pressure of the liquid
- Atmospheric pressure at the installation location
- Pressure in the supply tank (if not atmospheric)
In all design cases, it is important that the NPSHA is greater than the NPSHR. Stay tuned for a future post where we look at NPSH calculations and the effect that the temperature of the liquid has on the NPSHA.
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