High Viscosity

[metalonis]

Is there a viscosity value (or range) that differentiates low viscosity fluids from high viscosity fluids? Or is it all more complicated than that?

 

[BigInch]

Probably less complicated. I think "high viscosity" could be entirely relative to what you're going to do with whatever fluid you've got. Squirting mustard on a hotdog is a little different than pumping it around a food processing building, but anyway I suppose you could say the viscosity of water is a good reference point, which at normal ambient temperature just happens to be around that most convenient number of 1 (centiPoise) and water is the accepted reference fluid for pump curves as well.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Latexman]

It is all relative. One engineer's low viscosity is another engineer's high viscosity. It is a product of one's experience.

Good luck,
Latexman

 

[25362]

Fluid mechanics considers a viscous fluid as that whose viscosity is sufficiently large to make the viscous forces a significant part of the total force field in the fluid.

 

[BigInch]

Usually if its not enough to affect a pump curve, its not significant for me... but as Latexman says, "all is relative". I could see where sometimes 1 or 2 cP might be just enough to keep me from clearing a mountaintop, in which case THAT would cause me to change my definition of "sufficiently". So, I'll have to stick to my original statement, "its relative to what you have to accomplish with the fluid and how [ν] affects that goal", not to any actual value itself or to some percentage of field force, as that may be highly significant for cool fluid near a pipe wall, but have little effect on a hotter faster stream flowing in the middle of the pipe, but 1 cP is still a convenient number... especially for pump curves, but not for compressors.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[JLSeagull]

I calculated control valves for a mountainous crude oil pipeline over a decade ago. The crude was considered waxy. I think that the viscosity was around 250 cP. Playing with the calculations I found that viscosity had little impact on the control valve coefficient until the viscosity exceeded 1000 cP. Consider an exercise such as this to form your judgment.

 

[Scipio]

A good example of 'sufficiently', the 250 cP JLSeagull states had little impact on his control valves would dramatically impact the performance of a centrifugal pump. Generally (at least in terms of pumping) anything more than 5 cP I check for viscosity impact on performance (though at that viscosity, it's frequently not much).

 

[BigInch]

and Venezuelan Heavy Crude, SG 0.96, cP=1300 had pleanty of influence on the pipeline.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[25362]

BigInch and Latexman (vide supra) rightly underscored the relativity of viscosity values. Even water, with 0.89 cP at 25 Celsius, is considered a high-viscosity fluid when referred to its molecular weight. Compare with pentane having 0.23 cP at the same temperature. The comparative anomalous high cohesivity of water is attributed to hydrogen bonding.

 

[BigInch]

stranger than strange
See,
Sixty-three Anomalies of Water

http://www.lsbu.ac.uk/water/anmlies.html

Actually surpirzing we can drink the stuff.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[DesignerMike]

As everyone else has mentioned it really depends on the application.
I currently have a problem with a customer that told me their product was about 8000 CPs, and I sized the pump and suction line accordingly. The engineer that I worked with has moved on, and the new engineer was researching his flow meter selection. I talked him into letting me test the product, and....it's actually closer to 30,000 Cps! It's a short discharge line so I'm very close to acceptable on that end, but the suction conditions and inlet side of the pump are never going to work. The good thing is that nothing is installed yet so they can take the one black eye and modify it before they installed everything and have it not function correctly.... then they would really take a beating.

Bottom line is ALWAYS pay attention to the viscosity. I hate surprises...and they can be VERY expensive.

 

[Latexman]

DesignerMike,

I hope the shear rate was varied, and then the shear rate profile reversed, during the test to determine if the product is Newtonian, non-Newtonian, and/or time dependant.

Good luck,
Latexman

 

[ThePumpGuru]

My opinion on this (and the basis for my opinion are as follows:

Low Viscosity = 0 - 500 SSU (~100 cps)
Most Centrifugal pumps start to have serious degredation of Flow, Head, and efficiency over 500 SSU. The rule of thumb is over 500 SSU = don't use a centrifugal pump. Now before I get people started, I know that you can use a centrifugal pup at higher viscosities it is just not typically practical.

Medium Viscosity = 500 - 20000 SSU (~100 - 4300 cps)
This viscosity range is outside of typical centrifugal applications but can be moved by most all positive displacement pumps. ie. Gear, Piston, Diaphragm, Screw, etc.

High Viscosity = >20,000 SSU (>4300 cps)
At these thicknesses if you have a glass full of the liquid and you turn the glass sideways you have to wait for the liquid to start pouring out.
Here special consideration must be taken with the pump. Make sure the pump is large enough and running slow enough that it takes a bite of the fluid. For example, a gear pump may appear appropriate but when running the gear teeth just push the fluid back instead of taking a bite out of the liquid. Valves in piston & diaphragm pumps need additional time to seat before pump reverses it's stroke. Pumps will need to be oversized, slowed down and/or specialty pumps such as Progressing cavity or peristaltic pumps may be required.

2,000,000 cps is the highest pumpable viscosity that I have ever seen a pump rated for. That's about the viscosity of chunky peanut butter. Anything thicker than that.... get a shovel.

Hope this helps.
ThePumpGuru

 

[DesignerMike]

I would say that's a pretty decent analysis.

I did an application about 3 years ago pumping some recycled gum...customer claimed it was 4,000,000 Cps....and it was the most nasty thick stuff I have worked with. The pump was turning about 8 rpm, and taking .75 gallon per rev.

 

[BigInch]

So, somewhere between 1 and 500

BigInch

http://virtualpipeline.spaces.msn.com