What happened to thread on Centrifuagal Pumps and head squared

[amptramp]

Where did the thread go on the discussion of centrifugal pump head versus the rotational speed squared. I started the thread and hate to lose it because there was good info contained within.

 

[BigInch]

"Much ado about nothing." I believe is how the saying goes.


**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[SNORGY]

I looked for it this morning as well.

It was becoming a somewhat passionate discussion. It might (quite justifiably) not have withstood "moderator scrutiny". In any event, I think the thread had run its course.

Regards,

SNORGY.

 

[BigInch]

If you believe, you can, was the take away lesson.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[amptramp]

Oh well. Thanks to all who participated in a somewhat "passionate" and "imaginative" but very informative discussion. It would seem I should have received notification my thread had been pulled and by whom, but I did not receive any such notification. Thanks again!

amptramp

 

[Artisi]

Yep, gone to the happy hunting ground where it can be discussed over and over without any hard feelings, whether it is ever resolved is another discussion.

 

[electricpete]

I apologize if anyone didn't like my tone. The regulars on this forum all know a lot more about pumps then I ever will.

A small attempt to summarize the conclusions:

For a centrifugal pump attached to a fixed fluid system, the "operating point characteristics" (*) will follow the complete pattern Q~N, DP~N^2, FHP~N^3 if and only if the fluid system satisfies DP~Q^2

I think most agree with the above. Some comments/objections were raised:
1 – It is not likely that many fluid systems are fixed – often there is some control valve.
2 – Perhaps there are not a lot of systems that obey DP~Q^2 to begin with. Very turbulent systems do on a first view, although Snorty had some more comments on that. In any case more in depth analysis is usually in order than trying to give a simple generalization as this.
3 – Who cares
* "operating point characteristics" represents change in operating point of the fixed system as a functio of speed. It is not to be confused with pump laws. Pump laws depend on the pump only, nothing to do with the system.

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[electricpete]

I am very susceptible to typo's as you guys may have noticed. The g key is next to the t. Sorry about that Snorgy.

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[electricpete]

I certainly subscribe to one of Snorgy's last comments - no harm no foul. Today is a new day. I have no emotional investment in this topic anymore.

If anyone wants to correct, revise, or addend my summary, please do.

Thanks.

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[hydtools]

My take away and search results are that the pump characteristic curves follow the affinity laws. The operating point is dependant on the system curve and where it intersects the pump curve.
The system curve does not follow the affinity laws.
Predicting the operating point does not necessarily follow the affinity laws since it is dependant on both the pump and the system characteristics. It is necessary to know both the system curve and the pump curve(s) to determine the new operating point for the new pump curve.

Thanks to the participants for making me review what I used to know are research to understand the rest.

Ted

 

[BigInch]

It wasn't me that flagged it.

Likewise I don't mean anything personal in anything I say, nor take it personally in what I see. Just calling it like I see it. Hope no harm was done.

If by chance anyone wants to discuss this, or anything else for that matter, in private, my contact info is in the link.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[SNORGY]

No worries, Electricpete.

Sincerely,

Snorty.



It's actually pretty funny...I mean...how much worse than "Snorgy" could it get, anyway?



Regards,

SNORGY.

 

[Artisi]

Electripete
- I would suggest that many many thousands probably millions of installations are fixed, within very small variations - fixed inlet conditions and disharge conditions, no change to SG, Vis. Temp. etc - the only significant change woud be increasing head overtime due to pipeline fouling.


Hydtools
- The friction component to a system curve does follow the Q^2 law all things being equal with no change in conditions other than flow

Given a flow, the total head and knowing the static head component of a system curve you can calculate a head new for an increase in flow using the following parabolic equation:

H = Hs + KQ^2

H = total head
Hs = static head
K = constant
Q = Capacity / flow

 

[BigInch]

If its 1 phase flow, only 1 fluid in the pipe, newtonian ...

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

Agreed, but I can only think water - so no problem to me as I like to keep everything simple.

 

[25362]

Allow me to be the devil's advocate when referring to some special cases.

Pressure drop is admitedly proportional to the friction factor [f] multiplied by velocity squared [V²], and since [f] is sometimes proportional to Re^-0.2, as for water flowing through tube banks, the resulting pressure drop is proportional to V^1.8.

Any comments ?

 

[electricpete]

There is only one system characteristic that allows us to predict change in operating point with change in speed WITHOUT consulting the pump curve: and that system is:
DP~Q^2 (same as DP~V^2)

For all others we would need to utilize the pump curve to make any prediction about changes to the operating point with speed.

The best you can do for DP~Q^1.8 is say it is CLOSE to the result predicted above.... OR ELSE dig out the pump curve, adjust it for speed and compare to system curve.

The fact that DP~Q^2 is unlikely does not contradict my summary statement which has DP~Q^2 in the IF part of IF/THEN logic, but perhaps makes it of less practical value to some.

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[electricpete]

The best you can do for DP~Q^1.8 is say it is CLOSE to the result predicted above

For what little it's worth, you could also predict the direction of the deviations as well.... for example when doubling speed, the DP~Q^1.8 system will have a new DP somewhere less than factor of 4 above original operating point and new Q somewhere less than factor of 2 above the original operating point.

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[electricpete]

CORRECTION IN BOLD:
"For what little it's worth, you could also predict the direction of the deviations as well.... for example when doubling speed, the DP~Q^1.8 system will have a new DP somewhere less than factor of 4 above original operating point and new Q somewhere MORE than factor of 2 above the original operating point"

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[electricpete]

To support my previous statement (in case it's not already obvious), attached is a picture.

Assumed a form of system characteristic: DP~Q^m

Plotted three different systems m=1.8, m=2.0, m=2.2... all connected to identical pumps and all initially at the same operating point (V=4.698, DP=44.14).

Then double the speed of the pump. We know that where the m=2 operating point ends up: twice the flow rate and 4 times the DP as initial operating point. The new m=2.2 operating point is above and to-the-left of the new m=2 operating point. The new m=1.8 operaring point is below and to-the-right of the new m=2 operating point.

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