Suction Capacity
A common misconception is that pumps “suck” water.
In reality, a self priming pump reduces the pressure above the water column by creating a vacuum, that in turns allows atmospheric pressure to push the water up into the pump.
The theoretical maximum suction occurs when the pump creates a perfect vacuum above the water column, corresponding to a water height of 10.3 meters, which equals the static pressure of 1 atmosphere.
In practical applications, we start with this 10.3 meters, that is the absolute maximum, then calculate the various pressure losses, as show here under, to come to the residual pressure available in the system that feeds the pump, known as NPSH available (NPSHa).
NPSH and Pump Operation
For optimal operation, the NPSH provided by the pump feed system — the NPSH available or NPSHa — must always be greater than the NPSH required by the pump — NPSHr — to ensure that the pump operates without cavitation, thereby maintaining efficiency and preventing damage.
To calculate NPSH available, follow these steps:
Calculating NPSH Available
Start with 10.3 meters
As explained above, this is the theoretical maximum suction height at 1 atmosphere, from which we will take out the various causes for pressure drop.
At first, good practice tells to reduce the available NPSH by 1 metre of security (e), as you do not want to take the risk of having your pump operating too much on the limit of the pump NPSH.
Subtract Geometric Lift and Head Losses
Deduct the height corresponding to the vertical lift plus any head losses in the suction pipe.
At this point, you also have to take into account the pressure losses causes by the piping and the various pipe curves.
Remember that the pressure losses in the piping system feeding the pump varies with the viscosity of the fluid.
Take out the effect of Altitude
As altitude increases, atmospheric pressure decreases, reducing NPSH available.
As an example, at an altitude of 1400 meters, atmospheric pressure is lower, and this must be subtracted from the NPSH available.
The chart provided here tells you that a pump installed at 1400 m of altitude looses 1.6 m in suction.
Temperature and Vapour Pressure
The temperature of the fluid affects its vapor pressure. When the fluid in the pump reaches its vapour pressure, cavitation occurs, and the pump cannot draw the fluid effectively.
This creates can damages and can potentially destroy the pump.
As an example, for water at 60°C, reduce the NPSH available due to the vapour pressure of water at this temperature by 2 metres as shown on the chart.
Density
The density of the fluid determines the possible suction height. Higher density fluids will not rise as high for a given suction vacuum.
As an example, a liquid with a density of 1.4 will reduce the NPSH available by 2.8 meters as shown on the chart.
Conversely, if the fluid’s density is inferior to 1, the available NPSH will be higher than 10.3 m that is available for water.
NPSH Formulas
By understanding these factors and how to calculate the available NPSH, you can ensure that your pump operates efficiently and avoid problems such as cavitation, which can damage the pump and reduce its life.
As a result, the available NPSH of the system feeding the pump with the fluid as calculated here under must be higher that the NPSH required by the pump. Please refer to the pump technical data.
NPSHa = 10.3 – (a) – (b) – (c) – (d) – (e)
NPSHa > NPSHr
SPS Pumps is There to Help
There are many application for self priming and non self priming pumps — for water, loaded water, chemicals…
Some require to be abrasive resistant, some must be made in thermo plastic to offer corrosion resistance to certain chemicals.
This where the Expertise of the SPS Pumps teams can help.
Feel free to call us at +32 2 657 23 53 or via email info@sps-pumps.com
The content of this webpage is inspired by and sourced from the Grundfos Pump Handbook, including the accompanying charts.
