Centrifugal pumps: Viscosity and gas/liquid mixtures

[Vic123]

How does viscosity affects a multistage centrifugal pumps ability to handle air/liquid mixtures. As viscosity increases, can the pump handle more air or less?

 

[Artisi]

An interesting question to which I don't know the answer however, my gut feel is that as viscosity increases the capability to handle fully entrained air could well increase.

This is for air that is fully entrained within the pumped liquor and not free air entering the pump inlet from low suction levels or from other sources ie, slugs of air entering the inlet along with the product.

 

[BigInch]

The handling capacity with entrained gas reduces considerably as efficiency in moving volume is lost to compressing gas, somthing that pumps don't do well. You may notice less of a loss in volumetric efficiency as viscosity increases, but I wouldn't expect it to be much more than barely noticable in an academic environment. Basically the fluid would have to become very stiff in order to not compress any gas held within and allow the pump to maintain efficiency, and if it could do that, sooner or later the fluid and gas must equalize to the same pressure anyway, so any gains (or lessened losses) you might find are likely to be very temporary and only in the pump itself, lost within the pipe quite soon.

 

[Artisi]

I still think this is an interesting question and to help the discussion develop in place on any other meaningful input coming forth I will add the following.

Assuming the viscous liquor contains air within the body of the product (the OP has stipulated air/liquid mixtures,not free air entrained at the inlet) - as it enters the impeller eye it may or may not expand depending on NPSHr/NPSHa, if there is an NPSHa margin over the NPSHr (there must be some difference in pressure at the impeller eye otherwise flow won’t take place) the air will remain within the product and likewise we can assume that the air will remain at approximately the same volume. Once it fully enters the impeller the air will be gradually compressed (pressurised) as it flows thru’ the impeller, this means that the "product" will reduce in apparent volume translating to a reduction of volumetric output performance.

However, if the air remains within the product, the pump will remained primed - which I guess is the main aim anyway and a pump that keeps pumping at whatever the overall hydraulic efficiency is, in my mind 100% efficient compared to a pump that is not pumping.

This may well be a bit of nonsense but it is the middle of the silly session and it might get some interesting comments anyway.

 

[BigInch]

Are you trying to differentiate between a mixture being entrained or in solution. I would have thought a mixture is entrained, otherwise it wouldn't be a mixture, it would be a solution and there would be no reason to have to think in terms of volume vs viscosity.

On another equally as interesting topic, when does wet dirt become mud?

 

[btrueblood]

"On another equally as interesting topic, when does wet dirt become mud?"

When the water comes to the surface, due to shear forces acting on the mixture, thus dropping the coefficient of friction between wheels and/or shoe soles and the mud.

You can quote me on this, I spent a lot of time in mud as a youth, and that experience impelled me to get an engineering degree.

 

[btrueblood]

Oh, and that stuff the petro guys call mud just isn't, until somebody tries to walk or drive thru a pile of it.

 

[BigInch]

I should know better by now than to make up those kind of questions on Eng-Tips. At least you didn't say "When the plasticity index is greater than 8", or something.