Which viscosity (kinematic or absolute) for piston and diaphragm pump ?

[stevetremblay]

Hello pumps specialists

Which viscosity should I look at it (kinematic or absolute) if I use a piston or diaphram pump to displace same volume of diffent fluids in a same piping ? The reason why the piston or diaphram pump were chosen, because the fluid could be newtonian or non newtonian.

Thanks

 

[electricpete]

My two cents fwiw:

If non-newtonian, then the viscosity is not constant.

You don't say exactly what you're doing, but I assume you're working with pump curve that has viscosity correction.
Look at the units for a clue. Centipoise or Centistokes should answer your question.

I would think absolute viscosity (Centipoise) has more fundamental relevance to fluid friction and pump performance.... kinematic viscosity (Centistokes) only has significance in the measurement method involving gravity.

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[TD2K]

I don't see it makes a difference. Absolute viscosity in cP = Kinematic viscosity in cSt * SG.

If the fluid is non-Newtonian, both viscosities are going to vary are they not?

 

[stevetremblay]

It's for a small food precessing plant. The pumping system will be use to pump different type of fluids into a big mixer and heater.

I know the relationship between absolute viscosity and kinemattic viscosity. What bother me is the absolute viscosity depends also SG. This means even though the kinematic viscosity is very hight, the absolute viscosity could be very low if SG is very low as well. Or the absolute Viscosity could be very hight even though the kinematic viscosity is very low because of hight number of SG.

 

[electricpete]

My viewpoint is that absolute (dynamic) viscosity has the more fundamental importance to the problem. kinematic viscosity is only of interest if we know s.g. and use it to convert it to absolute. I tend to think kinematic viscosity is used in many different contexts where absolute viscosity is of more interest, only as a result of historical reasons stemming from the fact that kinematic viscosity is more convenient based on measurement method using gravity.

Here’s my logic (I’m coming from wandering through a few first principles, not from detailed knowledge of pumps.)

Assume the important viscous effects are related to flow of high viscosity fluids in small clearances.

Such flow would be laminar.

Therefore friction factor is:
f = 64 / Re = 64 * mu / [rho * V * d] where mu is absolute viscosity

Compare to f = 2*d*DP / [rho * V^2*L]

Set the expressions for f equal to each other:
2*d*DP / [rho * V^2*L] = 64 * mu / [rho * V * d]

Cancel out like terms
2*d*DP / [V*L] = 64 * mu / [ d]
[notice density is gone.... absolute viscosity is still here]

Solve for V
V = DP * 2*d^2 / [L * 64 * mu]

This is velocity that would flow through a tight clearance at a given DP under laminar assumption.

Alternatively we may say that viscosity effects dominate over inertial effects (that is already known the moment we assume laminar flow).

As long as we are viewing pump parameters in terms of velocity, volume flow rate and DP (not head and mass flow rate), then density and kinematic seem like superflous parameters. Kinematic viscosity (mu) should be the one that is of interest for predicting pump performance parameters of DP and volume flow rate without considering density.

If chart gives correction to positive displacement pump flow rate in terms of viscosity, then I’d think it should either be absolute viscosity, or else the assumed density should be identified along with the kinematic viscosity.

At least that’s my thinking, not knowing a whole lot about pd pumps – open to comments or corrections.


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[electricpete]

correction:

As long as we are viewing pump parameters in terms of velocity, volume flow rate and DP (not head and mass flow rate), then density and kinematic seem like superflous parameters. Kinematic viscosity (mu) should be the one that is of interest for predicting pump performance parameters of DP and volume flow rate without considering density

should be:

As long as we are viewing pump parameters in terms of velocity, volume flow rate and DP (not head and mass flow rate), then density and kinematic seem like superflous parameters. Absolute viscosity (mu) should be the one that is of interest for predicting pump performance parameters of DP and volume flow rate without considering density

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(2B)+(2B)' ?

 

[EnergyMix]

Nice way of correcting it Electricpete. Makes it very clear

 

[electricpete]

Thanks. I have a lot of practice correcting my posts ;-)

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[electricpete]

I'm not saying my logic is 100% correct (is it really completely laminar... not sure... open to comment). However the following link supports the conclusion. Specifically it provides detailed discussion for pd pump selection for varying viscosity and the process (while more complicated than I ever imagined) is discussed entirely in terms of absolute viscsosity (centipoise), volume flow rate, and differential pressure. There is no mention of kinematic viscosity (centistokes), density, or mass flow rate.

This was confirmed by word search
Irrelevant parameters for this analysis: kinematic viscosity, head, mass flow rate, density
stoke - 0 hits
density - 0 hits
mass - 0 hits
lb - several hits, but none used as unit of mass (all as unit of force)
head - 9 hits

Rrelevant parameters for this analysis: absolute viscosity, differential pressure, volume flow rate
poise - 3 hits
gpm - 14 hits
differential pressure 4 hits

http://www.fristam.com/DesktopModul...&Command=Core_Download&EntryId=498&PortalId=0

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[stevetremblay]

Thanks electricpete for the explaination. So I guess the kinematic viscosity is only useful for liquid "free fall", Am I right ?

The height between the top of the mixer and the pump is only about three meters. I'm going to check the SG of these fluid and multiplying all them by 2 in order have enough HP for an acceptable volume flow rate.

Thanks again electricpete for helping me and for refreshing my memory. I'm very mad about myself for already forget what I learned in University about 15 years ago. The quote saying that, If you don't use it, you're going to loose it, is so true.

 

[electricpete]

I wouldn't say exactly that.

My main point: a.v. is the one that plays a direct role in viscous friction. k.v. is the one that was more easily measured historically using principles relying on gravity.

To elaborate, a common early measurement technique for viscosity involved using gravity to drain the liquid (oil for example) through a capillary or orifice and the time for a fixed volume to drain was correlated to kinematic viscosity
For example the U-tube viscometer (not you-tube viscometer) shown here:

http://en.wikipedia.org/wiki/Viscometer"

Knowing the geometry and the time (and not the density), we can determine kinematic viscosity uniquely (sometimes by empirical correlations) Two factors play a role in the results of the experiment: 1 - the viscious friction, 2 - the gravity force. We don't need to separate those two effects (or know the density) in order to determine the kinematic viscosity. As stated in the link "The time taken for the level of the liquid to pass between these marks is proportional to the kinematic viscosity

In fact from results of this experiment without knowing density, we cannot separate those two effects (1 - the viscious friction, 2 - the gravity force). And likewise we cannot determine absolute viscosity from the experiment without knowing density.

If we have use the u-tube viscometer to analyse two fluids with same a.v. (absolute viscosity), but different densities, then the fluid with the higher density will flow through the apparatus faster and "appear" to have a lower viscosity... we say it's kinematic viscosity is lower.

Using the relationship
a.v. = k.v. * rho
we can see how the higher rho and lower k.v. multiply to give the same a.v.

So, applying our knowledge of density to results of the viscometer experiment, we separate the two effects of viscous friction and gravity and determine the a.v. which is dependent only upon the viscous friction characteristics of the fluid, not on the gravity force acting on the fulid during the experiment.

We also know absolute viscosity is the one used in the viscous friction equation relating fluid shear stress and shear rate;
Tau = mu * dV/dx where tau is shear stress, mu is absolute viscosity, V is velocity
kinematic viscosity nowhere to be seen in this equation.

That's my simplistic view. There are of course viscometers that measure absolute viscosity directly. And calibration of viscometers like the u-tube type may still require consideration of density for ultra-fine calibration... but the basic idea of kinematic viscosity is combining two separate fluid characteristics (density and absolute) to describe a new parameter kinemantic viscosity which has most direct relevance only in situations where gravity forces flow through a viscous restriction, like the u-tube viscometer.

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[stevetremblay]

Wow you explain better than my fluid mechanics teatcher. It is very clear for me now. But I think we still need to determine the fluid density if the pump has to pump fluids vertically. Because pressure depends also fluid density (P = rho*g*h). However, in my case, the height is only 3 meters. So I don't think this is really a big deal.

Thanks again for your help!

 

[25362]

From the net, on diaphragm pumps and viscous fluids:

Quote:

"If it pours, you can pump it.

A. Use large suction lines, up to three times the size of the pump ports.
B. Position the pump as close to (or below) the level of the fluid as possible.
C. Start the fluid slowly using an air line valve. Set the air pressure and crack the valve open slowly."