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.
Difference between NPSHa and NPSHr
A pump can only accept fluid at suction which above a certain lower limit. This lower limit is specified by the pump maker and it is referred to as the NPSH required OR NPSHr in short.
On the other hand how much suction pressure would be available at the pump suction completely depends on the suction side liquid, flow rate and suction piping. The actual NPSH at pump suction is then referred to as the NPSH available or NPSHa in short.
As NPSHr depends on the pump make and it is specified by pump manufacturer, when we say 'NPSH calculation' it usually refers to calculation of available NPSH, depending on the suction side flow conditions.
For example, consider the following situation.
Here, NPSHa is calculated using the equation,
Here, 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.
Examples of NPSHa Calculations
- Here is an example of NPSH calculation for the suction side of a pumping system which is designed to pump 200,000 kg/hr of water.
- For more clarity on the method, here is another example where NPSHA is calculated for a pumping system using a different calculators available on EnggCyclopedia.
- If you want to quickly try out how the NPSHa equation works, you can check out EnggCyclopedia's Pump Sizing calculator.