Axial Thrust and Viscosity (Pumps)

[Vic123]

In centrifugal and axial flow pumps, if we are pumping viscous liquids, does the axial thrust (determined for water) stay the same or does it reduce. If yes, how can we determine the value?

 

[BigInch]

If I'm reading you as I think,

"If the motor slows enough to actually matter to a process it's in trouble." I'd say, it depends on the process, as not all processes run continuously at "1750 rpm".

Pipelines can operate at a wide range of flows, so much so that slow motors are sometimes even required. In those cases, VSDs are added to pump driving motors when the flow variability is sufficient to warrent running them slowly for extended periods of time while keeping the motors from burning up in the process.

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[itsmoked]

Yeah that works!

If you have say, a 1725 RPM motor that means 1725 RPM fully loaded. Unloaded it may spin at 1750 RPM. That's where it spends all its(happy) days, between those two speeds. So yes, if the motor is loaded to 70% of its rated load it might spin at 1735 RPM, viscosity goes higher, the motor's load increases just as you've described, until it hits full load at 1725 RPM. Further loading and the motor starts to dip below 1725 at 120% load it may drop down to 1720 (not much) => borrowed time.

Of course a VFD changes all this but the "stiffness" continues.

VFDs are cool devices. Small ones are dirt cheap.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[BigInch]

I'm lovin' it.

Right. 1725 to 1750 It a small but VERY significant difference.

Much thanks to all that contributed.

Now can we get back with how to wire the 2-switch hallway light? When I turn one switch off, it comes on....

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[25362]

Coming back to the pump subject here is a quote from Sulzer's Centrifugal Pump Handbook:

The economic duty limit for centrifugal pumps is about 150 cSt. Use of a centrifugal pump is possible up to about 500 cSt, but a higher NPSH must be made available for this higher viscosity. Channel impeller and torque flow pumps allow higher values (800 to 1000 cSt) depending on size and type, although an efficiency loss must be taken into account compared with positive displacement pumps with the same service data.
Reciprocating or rotary positive displacement pumps are employed for still higher viscosities.

BTW, 1 cSt = 10^-6 m²/s

I've seen both cS and cSt as abbreviations for the unit of kinematic viscosity.

 

[BigInch]

Let's start a new thread after this,

The way I always see it is
cSt(oke) and
cP(oise)

I have used rotary screw pumps on a hot heavy crude and naptha mixture pipeline with viscosities from 80 to 1300 cSt.

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[electricpete]

I don't ever remember seeing cS, always cSt

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[BigInch]

I've never seen it. Gotta' be a typo.

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[Artisi]

and conversely, when you switch the other one on - it goes off-

Naresuan University
Phitsanulok
Thailand

 

[25362]

FWIW, the article in Wikipedia: Viscosity, confirms my observation on the units cS and cSt.

 

[BigInch]

Have you read Wikipedia's disclaimer?

http://en.wikipedia.org/wiki/Wikipedia:General_disclaimer

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[25362]

BigInch, I followed your advice, read it, and came to the conclusion that Wikipedia disclaimers aren't much different from those of other famous sources which are indeed "peer reviewed" with disclaimers transcribed by Wikipedia.

That is to say the information given by Wikipedia isn't necessarily unreliable and inaccurate.

My observations referred to "reliable" books. Among them the McGraw-Hill Dictionary of Scientific and Technical Terms, which goes even farther and abbreviates centistoke as cs, and centipoise as cp, both in lower case.

In Engineering Tribology by Stachowiak and Bachelor, from the Elsevier Tribology Series, 24, one finds cP, and cS. QED.

 

[electricpete]

That is surprising. I looked on page 13 of Engineering Tribology and indeed you are correct 25362. He uses S for stokes and cS for centistokes. Then there is the Siemens (invere ohm) from electrical engineering which is also S. Apparently there is more than one convention.



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[25362]

I know we're digressing, and the subject pertains to the Engineering Language Forum, but I must tell that I was bewildered to see that S was not only the common symbol for seconds, entropy, stokes and siemens, but also sabins (sound absorption), savarts (sound pitch interval),
scruples (apothecaries' mass), seidels (Austrian volumes),
sieverts (radiation dosis), and svedbergs (ultracentrifuge technology sedimentation interval).

If the context isn't clear, to avoid confusion the key would be to write the whole name of the unit. Agree ?

 

[electricpete]

Yes, that makes sense.

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[BigInch]

I can accept the other references.

Seems like only the "S", unit is being confused.

For centiStokes I'm going with the flow ~~~~~~~ "cSt".

TG I don't use Stokes.

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