When it comes to pumps and their derivatives, some frequently encountered terms in search engines include: suction mode, NPSH, NPSH pump, NPSH calculation, NPSH value, and NPSH curve. Here is a brief summary of NPSH.
What is NPSH?
NPSH stands for Net Positive Suction Head. It refers to the inlet pressure that allows a pump to perform transfer according to its curve. Also called the NPSH curve, it represents the minimum inlet pressure required to prevent cavitation by stopping the liquid from vaporizing inside the pump. The NPSH curve is expressed in meters ("m"). For a specific flow rate, the available NPSH at the pump suction should ideally be at least 0.5 m higher than the required NPSH.
What is Cavitation?
Cavitation occurs when bubbles form in the liquid inside the pump. It is one of the most common problems in pumps. Cavitation is more of an indicator than a problem itself. It can happen for two reasons: the nature of the pumped liquid or air being sucked into the pump during suction. The NPSH value and curve play a critical role in preventing cavitation. In short, NPSH represents the minimum pressure required at the pump inlet for centrifugal pumps to operate without cavitation. It is the pressure necessary to keep the fluid in a liquid state and prevent vaporization.
NPSH Calculation
NPSH indicates the minimum required pressure at the pump suction side. Centrifugal pumps cannot create suction by themselves, so a minimum inlet pressure is required for proper operation. This value varies from pump to pump. If the NPSH value is too low, cavitation increases, which can damage the pump, reduce efficiency, and eventually render it nearly unusable.
The NPSH formula is:
NPSH = Ha + Hs – Hvpa – Hf
Where:
- NPSH = Net Positive Suction Head; the minimum pressure required at the pump suction.
- Ha = Absolute Pressure of the liquid (atmospheric pressure); the pressure applied externally on the liquid surface.
- Hs = Static Suction Pressure (or static suction head); the height above or below the pump suction inlet.
- Hvpa = Vapor Pressure of the liquid; varies with temperature and type of liquid, increases with higher temperatures.
- Hf = Friction losses in the suction line; depends on flow rate and should be minimized to ensure proper NPSH.