5 replies to this topic

[slashragnarok]

Can centrifugal pumps be used to pump gases? If not why?

Let me explain my dilemma. My understanding is that centrifugal pumps are constant head machines. Meaning that as long as impeller speed and diameter are constant, it will produce the same head. Now I also recognize that head is related to discharge pressure as

Head*Sp gravity = Disch Pressure

So the same pump powered by the same motor will generate a greater discharge pressure for a denser fluid, which is why a centrifugal pump doesn't work if not primed. Also the greater discharge pressure for a denser fluid comes at an expense of power of the motor.

So my question is should a pump full of some gas, say Nitrogen, be able to pump it just like it would pump any other fluid like water, albeit generating a lower discharge pressure.

[ankur2061]

Sure a centrifugal pump can pump a gas. If we consider the example of pumping air using a centrifugal pump the head it will generate cannot exceed the static pressure of the atmospheric air at the location of the pump.

Regards,
Ankur.



Regards,
Ankur.

[slashragnarok]

Thank you very much for your reply.

[kkala]

Let us see an example. Assume a centrifugal pump, with suction pressure close to 1 Atma, transfers water of density ρ=1000 kg/m3 and develops a head of 50 m of liquid. Discharge pressure will be P=50*1000 kgf/m2 g = 5 kgf/cm2 g.
If the pump transfers air of ρ=1.2 kg/m3, assuming same head of 50 m would result in a discharge pressure of 50*1.2 kgf/m2 g = 60 kgf/m2 g = 0.006 kgf g, which is practically not noticeable. Besides this is only in theory, actually a pump running dry (that is without being filled with liquid) will be out of order in a short time, see http://www.cheresour...h__1#entry42357. Exceptions (if any) must be few concerning pumps of special design.

[slashragnarok]

Hi,

Thanks for the example. It really made things clear and the link to "dry run" of centrifugal pumps was great. But I saw lubrication and cooling of pump internals being mentioned in that thread. I don't see how dry run is related to lubrication because as far as I know, lubrication is provided by Lube Oil system. And I really didn't see how exactly dry run leads to damage of pump internals.

[kkala]

I have witnessed centrifugal pumps running dry and then going to maintenance shop every so often (mentioned in the link). Explanation on pump overheating can be that friction increases temperature on the moving parts, while there is no liquid to take heat out. Besides, lubrication is realized not only by the lubricant, but also by the pumped liquid. The latter is not present, when the pump runs dry.