Net Positive Suction Head or NPSH for pumps can be defined as the difference between liquid pressure at pump suction and liquid vapor pressure, expressed in terms of height of liquid column. Suction head is the term used to describe liquid pressure at pump suction in terms of height of liquid column. When vapor pressure is also expressed in terms of equivalent height of liquid column, and subtracted from the suction head, the difference is npsh available at the pump suction.
where, hL is the head loss between 0 and 1,
p0 is the pressure at the water surface,
pV is the vapour pressure (saturation pressure) for the fluid at the temperature T1 at 1,
Δz is the difference in height z1 − z0 (shown as H on the diagram) from the water surface to the location 1,
and ρ is the fluid density, assumed constant, and g is gravitational acceleration.
Importance of pump NPSH
If the NPSH available at pump suction falls below the required head (NPSHr), there is risk of formation of vapor bubbles at the pump suction. Presence of vapor in the pump suction can lead to several problems such as - pump cavitation, damage to impellers, dry running of pump etc.
However, these problems can be prevented by ensuring that the NPSHa remains sufficiently about NPSHr specified by the pump manufacturer. This can be done when designing the pumping system. Factors such as elevation of the pump, suction piping, inlet tank etc. are important determinants for the available net positive suction head.
Next, let's look at the NPSH calculations and how they will influence the design of a pumping system.
Net Positive Suction Head Calculation
Normally the NPSH required by the pump at its suction (NPSHr) is specified by the pump manufacturer. Hence when you design a pumping system, you would most likely need to calculate the NPSH available (NPSHa).
You can easily calculate the available NPSHa using the equations from above.
Here are some more tutorials, references and calculators to help with those calculations.
- Net positive suction head - definition
- Difference between NPSHr (required) and NPSHa (available)
- Here is an NPSH calculation tutorial that makes extensive use of the EnggCyclopedia calculators for quick calculations.
- Tutorial for using the pump sizing calculator
- Pipe pressure drop calculation tutorial