NPSH Definition and applications

The NPSH pumping is a fluid physics concept that stands for "...".Net positive suction head"which translates as net positive suction lift. There are two NPSH different, the available and the required.

This pump variable is the one that determines the proper functioning of the pump and represents the minimum pressure required at the pump inlet to avoid the pump's pumping phenomenon. cavitation which was discussed in depth in a previous post.

In short, underpressure at the suction side of a pump can cause water to boil at low temperature, turning into steam. When the pressure increases at the impeller outlet, the vapour bubbles implode, releasing high energy which erodes the impellers of the equipment very intensely, rapidly reducing the lifetime of the instruments.

TDR 350 | Moisture Meter

-NPSH available:

The NPSHa "available" is calculated as the pressure right at the pump inlet, it can be measured with a pressure gauge placed on the suction side or with remote measurement systems like Regometter. It depends on different factors such as:

• Head of the sheet of water. measured from the cistern to the centre line of the pump inlet. Hs
• The pressure at the surface of the water, usually at atmospheric pressure, but if the cistern is pressurised it can be increased, although this would have an unbearable energy cost. Ha
• The vapour pressure of the water, this factor depends on the temperature, and to lower it it is necessary to cool the fluid, so it is unfeasible in irrigation. Hvp
• Head losses between the cistern inlet and the pump inlet. Hf

NPSHa=Hs+Ha-Hvp-Hf

If the pumps are not submerged, it is always advisable to work with a safety margin of about one metre of water column.

If the design is adequate, there should be no problems, as the whole system should be designed so that the pump inlet pressure never drops below 1 metre of water column. In irrigation systems with a design problem, it is very difficult to fight against a very low available NPSH, as it would be necessary to carry out a work such as:

• Increasing the height of the cistern, or the height of the sheet of water
• Lower the pumping equipment.
• Reduce head losses by increasing pipe cross-section, reducing pipe length, installing large radius bends or replacing valves with knife gate valves.

NPSH required:

The "required" NPSHr is the minimum pressure that must be present at the pump inlet to protect the pump from cavitation. The way to know the NPSHr of the pump is given by the pump curve. Each manufacturer's model has its own.

Specifically, for the system to work perfectly and to avoid cavitation, the NPSHd must be greater than the NPSHr and if possible, its margin must be one metre.

NPSHd>NPSHr

An incredibly useful method to know the pressure at the pumping inlet, in addition to any other monitorable variable of the system such as energy consumption, flow rates or water quality variables such as electroconductivity, turbidity, ORP or pH is our Regometer monitoring system.

Don't hesitate and find out how to turn your pumping equipment into a smart pumping system.

Intelligent pumping