NPSH and maximum vaccum of the pump

[adrich91]

Good afternoon,

I would like to ask you a question about the NPSHr of a pump and the NPSHd.

Let's imagine a pump that has a NPSHr of 1 m. This means that the difference between the pressure at the inlet of the pump and the vapour tension needs to be at least 1 m so that it does not cavitate. Ideally, it is designed so that NPSHd is greater than NPSHr.

However, if we assume the attached drawing, in which the tank is at atmospheric pressure and the difference in height between the pump axis and the level is 40 cm and assuming that the head loss in the entire suction line is 500 mbar, that the fluid has a density of 1 g/cm3 and a very low vapour pressure at that temperature (let us say 10 mbar) and that the NPSHr of the pump is 2.8 m, the calculation of the NPSHd would give as a result (approximate):

P atm = 1 bar --> 10 m
Pvap = 10 mbar --> 0.1 m
z = 40 cm = 0.4 m
F = 500 mbar --> 5 m

NPSHd = 10 - 0.4 - 5 - 0.1 = 4.5 m

As NPSHd > NPSHr, it will not cavitate, but my question is: Now I have to see what will motivate the fluid to move towards the pump inlet. The pump will create a vacuum that will cause the fluid to move, but.... What if the pump is not able to overcome the head loss despite having a NPSHd > NPSHr? How is the NPSHd and/or NPSHr and the maximum suction capacity of the pump linked?



I would like to analyse this situation and find out what I should look for when buying a pump to know if it will be able to overcome the friction and the height and be able to move the fluid to its suction.

Thank you!

Regards

 

[LittleInch]

Jeez, there are a few things here to unpack.

1) What is NPSHd? Nearly everyone I know uses NPSHA - Net pump suction head AVAILABLE. This is by convention always measured at the centreline of the pump inlet flange. Often the centre of the pump but not always.

2 "This means that the difference between the pressure at the inlet of the pump and the vapour tension needs to be at least 1 m so that it does not cavitate." NO. The NPSH figure / curve you get from a pump vendor is the point at which the differential head drops by 3%. It can be cavitating like mad, but still not lose performance by 3%. I've been there and done that and trust me - you need to add at least 1m to the NPSH curve to prevent cavitation and maybe more than that. The slower the impellor and the closer to BEP the lower this margin needs to be. Why 3% TDH drop to define NPSH? No idea, but that's what the pump codes say and that's what everyone uses.

This graph gives you an idea. The NPSHR 0% line is essentially the cavitation line. The figure and graph you get from a pump vendor is the NPSHR 3% TDH line.

The difference at low flow or high flow can be several metres head.



3) "What if the pump is not able to overcome the head loss despite having a NPSHd > NPSHr?" Then you've calculated it wrong.

4) "How is the NPSHd and/or NPSHr and the maximum suction capacity of the pump linked?" Well they are close to the same thing. Note that the pump is assumed here to be centrifugal and will only lift a fluid if it has a fully flooded suction before it starts or during operation. Most times this doesn't work is when the pump and inlet line are not primed and full of liquid when you start it. Nothing to do with NPSH.

The atmospheric pressure or the pressure above the liquid ( if the tank is pressurised or is below atmospheric pressure) is what gives the liquid the energy to be lifted into the pump inlet.

Does that make sense?