Net positive suction head
In a hydraulic circuit, net positive suction head (NPSH) may refer to one of two quantities in the analysis of cavitation: NPSH is particularly relevant inside centrifugal pumps and turbines, which are parts of a hydraulic system that are most vulnerable to cavitation.
If cavitation occurs, the drag coefficient of the impeller vanes will increase drastically—possibly stopping flow altogether—and prolonged exposure will damage the impeller.
In a pump, cavitation will first occur at the inlet of the impeller.
[1] Denoting the inlet by i, the NPSHA at this point is defined as:
Applying the Bernoulli's equation for the control volume enclosing the suction free surface 0 and the pump inlet i, under the assumption that the kinetic energy at 0 is negligible, that the fluid is inviscid, and that the fluid density is constant:
For simple impeller systems, NPSHR can be derived theoretically,[2] but very often it is determined empirically.
[1] Note NPSHAand NPSHR are in absolute units and usually expressed in "m" or "ft," not "psia".
Experimentally, NPSHR is often defined as the NPSH3, the point at which the head output of the pump decreases by 3 % at a given flow due to reduced hydraulic performance.
On multi-stage pumps this is limited to a 3 % drop in the first stage head.
In a reaction turbine, cavitation will first occur at the outlet of the impeller, at the entrance of the draft tube.
[4] Denoting the entrance of the draft tube by e, the NPSHA is defined in the same way as for pumps:
[1] Applying Bernoulli's principle from the draft tube entrance e to the lower free surface 0, under the assumption that the kinetic energy at 0 is negligible, that the fluid is inviscid, and that the fluid density is constant:
Vapour pressure is strongly dependent on temperature, and thus so will both NPSHR and NPSHA.
Careful design is required to pump high temperature liquids with a centrifugal pump when the liquid is near its boiling point.
The violent collapse of the cavitation bubble creates a shock wave that can carve material from internal pump components (usually the leading edge of the impeller) and creates noise often described as "pumping gravel".
Additionally, the inevitable increase in vibration can cause other mechanical faults in the pump and associated equipment.
The NPSH appears in a number of other cavitation-relevant parameters.
The suction head coefficient is a dimensionless measure of NPSH:
Remember that positive or negative flow duty will change the reading on the pump manufacture NPSHR curve.
Lifting out of a well will also create negative NPSH; however remember that atmospheric pressure at sea level is 10 metres!
This helps us, as it gives us a bonus boost or “push” into the pump intake.
The minimum requirement is 0.6 metres above NPSHR), so the pump should lift from the well.
Using the situation from example 2 above, but pumping 70 degrees Celsius (158F) water from a hot spring, creating negative NPSH, yields the following: Example Number 3: A well or bore running at 70 degrees Celsius (158F) with an operating level of 5 metres below the intake, minus a 2 metre friction loss into pump (pipe loss), minus the NPSHR curve (say 2.4 metres) of the pre-designed pump, minus a temperature loss of 3 metres/10 feet = an NPSHA (available) of (negative) -12.4 metres.
Remembering that the minimum requirement is 600 mm above the NPSHR therefore this pump will not be able to pump the 70 degree Celsius liquid and will cavitate and lose performance and cause damage.
To work efficiently, the pump must be buried in the ground at a depth of 2.4 metres plus the required 600 mm minimum, totalling a total depth of 3 metres into the pit.
A minimum of 600 mm (0.06 bar) and a recommended 1.5 metre (0.15 bar) head pressure “higher” than the NPSHR pressure value required by the manufacturer is required to allow the pump to operate properly.
Serious damage may occur if a large pump has been sited incorrectly with an incorrect NPSHR value and this may result in a very expensive pump or installation repair.
NPSH problems may be able to be solved by changing the NPSHR or by re-siting the pump.
If an NPSHA is say 10 bar then the pump you are using will deliver exactly 10 bar more over the entire operational curve of a pump than its listed operational curve.
Some pumps can have up to 150 stages or more, in order to boost heads up to hundreds of metres.
