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Brian2903 (Civil/Environmental) (OP)
14 Jul 08 12:02
I am wondering if there are any relationships to descript pump motor Amp and actual pump flow. That is by measuring the pump motor Amp to eatimate the actual pump flow.

Thanks!!
JRLAKE (Mechanical)
14 Jul 08 12:55
Yes. Given two points (power with impeller diameter) you can estimate the flow using the pump curve at the corresponding RPM.

Find where BHP curve and impeller curve meet, come straight down and you have your flow. You can double check the estimate using the differential head, if you have a way of measuring it. You also need to consider efficiency of the motor and other drive losses as well when calculating the power.

The accuracy of this approach is dependent on how many variables you include that affect pump curve accuracy, ie pump wear, impeller clearance, etc.

If accuracy is key, consider a flowmeter.

   
TenPenny (Mechanical)
14 Jul 08 13:22
The short answer is, yes, there is.

What kind of pump are you talking about?  Piston?  Centrifugal? Gear?
JJPellin (Mechanical)
14 Jul 08 13:56
The previous replies are absolutely correct. I would offer one word of caution based on an experience I had.  Some big vertical turbine pumps actually have a hump in the Brake Horsepower curve.  At extremely high flow, the amp draw actually drops at higher flows. This leads to a situation where one amp draw could relate to two different flow rates; one on each side of the hump.  But this is very rare.   

Johnny Pellin

electricpete (Electrical)
14 Jul 08 21:03
Pump has a bhp vs flow curve a was mentioned.

In very general terms:
-pure radial flow pumps have monotonically increasing BHP vs flow accross their range
- pure axial flow pumps have decreasing BHP vs flow accross their range
- mixed flow have a hump in the range

You also would want to consider motor current vs bhp. The simple approach accurate near full load is to use linear approximation current vs bhp load.   But actual current is higher than this prediction when operating at low load.  At 0 motor load, there might still be ~ 20% FLA current for 2-pole motor and 30% for 6-pole motor.

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electricpete (Electrical)
14 Jul 08 21:07
I should mention the linear approximation above would be based on nameplate FLA current and load.

If you have an accurate measurement of motor speed, and knowledge of line frequency you can also estimate motor load based on slip.

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BigInch (Petroleum)
15 Jul 08 5:14
Pssst.  Clamp-on ultrasonic flow meter.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

Helpful Member!  waross (Electrical)
15 Jul 08 11:22
Well pump horsepower may not be linear with pump flow.
Motor current is not linear with horsepower.
Changes in pump flow may change the dynamic head that will change the relationship between flow and horsepower.
You have to ignore;
1> The pump curve.
2> Dynamic head.
3> Motor power factor.
3> Motor fixed losses.
4> Motor load dependant losses.
Measuring the motor current will give an indication that the flow may be greater or less than it was, but it will take a lot of eaperience and history with the actual pump to get a reasonable estimate of flow, based on motor current.
Oh, and I forgot to mention fluctuations in supply voltage, and similar pumps supplied by feeders with different impedances may show different currents for similar flows. Admittedly a small difference.
I think that the answer is "Probably not, unless you have enough experience with this pump that you don't have to ask the question."

Bill
--------------------
"Why not the best?"
Jimmy Carter

BigInch (Petroleum)
15 Jul 08 18:06
Putting it in perspective, its like figuring how fast you're going from your gasoline consumption rate.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

JRLAKE (Mechanical)
16 Jul 08 13:26
I disagree with the previous two posts.

I estimate pump flow using amps and the pump curve frequently on centrifugal pumps. My accuracy is usually within 5%. Of course, there are exceptions, such as pumping high solids content, entrained air or non-newtonian fluids. However, for the most part, given the critical information, the relationship is quite predictable.


 
BigInch (Petroleum)
16 Jul 08 13:29
And very steady state.


"If everything seems under control, you're just not moving fast enough."
- Mario Andretti
  - when asked about transient hydraulic analysis

http://virtualpipeline.spaces.msn.com

 

BigInch (Petroleum)
16 Jul 08 13:37
JR What do you mean you disagree with the previous 2 posts?  I didn't say it was impossible.  I know I'm going between 100 and 110 kph when my gasoline consumption is 9.5 L/h. I was just commenting on the fact that its a basackwards kinda' way to do it... IMO.   


"If everything seems under control, you're just not moving fast enough."
- Mario Andretti
  - when asked about transient hydraulic analysis

http://virtualpipeline.spaces.msn.com

 

JRLAKE (Mechanical)
16 Jul 08 13:46
I mean, how do you think the manufacturers came up with the pump curves? Do you think they assigned a PhD M.E. to derive an equation using the geometries and CFD?

No. They hooked the thing up on a test stand, ran it and said, "Ok, someone write this down. At 3500 RPM, this pump is delivering X GPM at Y differential head and is drawing Z brake horsepower."

So, if you have Z HP, it makes perfect sense that you can work backwords, as long as you follow proper procedure working from the water reference point defined by the curve, consider reasons for possible curve inaccuracy (wear, etc.) and double check using a second point (differential head for example).

I swear we have a tendency to over analyze everything. But what fun would an engineering forum be if we didn't. I apologize for being cranky, laying people off is no fun and brings about a negative attitude.
JRLAKE (Mechanical)
16 Jul 08 13:49
"JR What do you mean you disagree with the previous 2 posts?  I didn't say it was impossible.  I know I'm going between 100 and 110 kph when my gasoline consumption is 9.5 L/h. I was just commenting on the fact that its a basackwards kinda' way to do it... IMO."

I misunderstood your point was Big Inch. It is backwards I agree. That's why flowmeters were invented.

I don't think I could estimate my car's MPH by looking at fuel consumption, but at the same time, I ain't that smart either.

 
JRLAKE (Mechanical)
16 Jul 08 13:51
...average fuel consumption I mean...different story most likely with instantaneous fuel consumption...I could do that...I see what you are saying now...which brings me back to a point I made in another thread...you have much cooler toys than I do...
rmw (Mechanical)
17 Jul 08 20:51
I have found that if there are other means of determining pump flow such as flow meters (in who knows what kind of calibration) pressure gages, etc and a good cross check method for verification is needed, the pump amps anticipated by the pump curve at that point can either verify the data or bring it into question.  I often rely on the amp readings although you have to have some reasonably accurate information on the PF of the power to the pump.  As I normally used this technique in power stations, that information was usually at hand.

rmw
waross (Electrical)
19 Jul 08 20:40
I stand by this statement.Measuring the motor current will give an indication that the flow may be greater or less than it was, but it will take a lot of experience and history with the actual pump to get a reasonable estimate of flow, based on motor current.
Oh, and I forgot to mention fluctuations in supply voltage,
No offense taken JRLAKE. I've had bad days myself.
The point is if you are familiar with the pump, and you obviously are, you may get an indication but be wary of readings on strange pumps until you have some second source confirmation of your current based estimates. Actually a wattmeter gives a better indication. Some engineers do a shut-in load test and subtract that reading from the flow reading to factor out pump losses. Depends on the specific application though.

Bill
--------------------
"Why not the best?"
Jimmy Carter

Artisi (Mechanical)
20 Jul 08 1:12
I would say that to get somewhere near to a reliable flow using input current to a motor you would have consider and factor in ALL the variables effecting the motor output power and also establish very accurate differential head, this could only then be applied to an actual test curve for that particular pump, this is also dependent on the pump being in exactly the same configuration and internal condition as when originally tested and with ideal flow into the pump etc etc.
Anything else becomes a dream and wishful thinking.

 
JRLAKE (Mechanical)
21 Jul 08 10:33
"That is by measuring the pump motor Amp to estimate the actual pump flow."

The question is can the flow be estimated, not calculated. I guess we all have different definitions of estimate.

I am also making the assumption, and probably incorrectly, that he is attempting a "one-time" quick estimate. If he is trying to implement a permanent system to continuously estimate flow, amps is definitely not the way to do it. But I assumed if he were trying to do this, he wouldn't even consider the amp approach and would be asking for recommendations of flowmeter vendors.

Since he is apparently looking for a non-intrusive and simple way to estimate flow, I assume he is looking for a quick glance at estimated flow. However, I will acknowledge that I frequently make the mistake of assuming others have taken a logical path to the obstacle that produces the question.
rmw (Mechanical)
21 Jul 08 22:13
This is anecdotal and only on one set of pumps but I had a situation once where I had a significant variance between the pump flow indicated by the pump curve based on the differential gages and the outflow weir readings.  They were significantly different than the outflow weir readings indicating that the pump flow wasn't enough (to maintain the measured condenser vacuum in a power plant turbine condenser.)  The pump differential readings were taken over several operating points (river levels) and corrected to pump centerline.  They were even taken pre and post pump(s) overhaul.  The motor amps always verified the differential pressure readings and the pump curve.

As an aside, the piping was such that there was no location accessible to use a Doppler meter and other flow measuring devices were impractical or impossible (96" piping).  It was known that the condenser outflow piping was separated and it was hypothesized (and strongly suspected) that a portion of the flow from the condenser found its way back to the river via the subterranean drain piping system put in place to drain the original plot when it was filled in from the river.  There was good evidence for that.  The fill dirt on which the plant site was located was obtained by building a drainage system and pumping muddy river water in and letting the water drain out after the sediment settled out.  The plant major pieces of equipment were built on pilings.  A civil engineering buddy of mine showed me photos of the original site with the drain piping in place.  I believe that the flow from the broken main found its way to the original drain piping and by passed the weir.  The motor amps proved to be a good verification of the pump differential and heat transfer calculated flow rates.

And, yes, artisi, I has to take every variable in the motor HP formulae into account and factor them in too.  But in the end, it was 3 against 1 - pump curve, motor amps and heat transfer calculations that all agreed - and the weir lost.

rmw
BigInch (Petroleum)
22 Jul 08 4:37
In a steady state situation, I think you can get very close, given a bit of history.

"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
http://virtualpipeline.spaces.msn.com

 

miningman (Mining)
3 Aug 08 10:31
  I suspect everyone here is missing the most relevant point. Pump curves are obviously developed on the basis of a brand new pump.  Any normal wear within the pump essentially means that you are using a different pump and different curve.  From memory, power required is a function of the flow rate and the square of the head. For a worn pump, the flow rate drops as well as the head that the pump can develop so there is an effective cube relationship between power and flow on different pumps... and a worn pump is a different pump from the same model in new condition.

How would one know whether a given installation is a new unit operating at design or a worn unit operating at only a fraction of its design parameters?.  The cubic relationship between flow and power throws this into disarray does it not??
electricpete (Electrical)
3 Aug 08 14:27

Quote (miningman):

  I suspect everyone here is missing the most relevant point.
miningman - Please read a few things:

1 - First check out Artisi's post 20 Jul 08 1:12

2 - Then check out the original post:

Quote (Brian2903):


That is by measuring the pump motor Amp to estimate the actual pump flow.

3 - Then check out the definition of estimate

Quote:

http://www.thefreedictionary.com/estimate
es·ti·mate
tr.v. es·ti·mat·ed, es·ti·mat·ing, es·ti·mates
1. To calculate approximately (the amount, extent, magnitude, position, or value of something).
Then after reading those, you want to come back and affirm your criticism of everyone else who came before, please do.  Until then, I will assume you simply didn't read the original question.

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Eng-tips forums: The best place on the web for engineering discussions.

miningman (Mining)
3 Aug 08 15:54
I might apologise to any previous poster who reads criticism into my previous post but there was absolutely no intent to imply any such criticism. The original poster Brian identified himself as civil / environmental and I might be forgiven for thinking that neither pumping nor electrical engineering are his prime areas of expertise.

There is a huge difference in the theoretical practice of engineering and the use of the same principles to solve problems in the field.  Personally I have run several pumps to the point where discharge volume was zero, because of internal wear. I also have  first hand knowledge of an individual refusing to reverse the rotation on three phase submersible pumps that were not producing the theoretical flow rate .... the argument being that he had done the calculations and in his opinion, said pump could not function at a specific head....... this despite the fact that I had specced, sourced and used the same pumps only a few weeks earlier at much greater heads.

English is my mother tongue and I need no assistance in the definition of commonly used words, and my background allows me to very easily imagine circumstances when a given unit is pulling only 30-50%  of its rated load and producing anywhere from 0-50% of theoretical flow... ie absolutely no correlation whatsoever in the field.

When I have posted queries in this forum I have recieved suggestions ranging from exactly what I was looking for, to suggestions that were useless or irrelevant, and in the mid point, ideas which of themselves were not relevant, but which generated ideas which later proved to be productive.

I never criticised posters who provided suggestions that were less than usefull to me since they obviously believed it worthwhile to try to help.  If Brian2903, or anyone else wants to consider my ideas, fine.  If he wants to ignore my ideas, I will lose zero sleep over it.

 What I tried to do was provide suggestions based on 30 years experience in the field which might have a slightly differing point of view to anyone whose background is essentially office and /or consultancy. Electric Pete obviously has a low opinion as to the relevancy of my experierence...This will be my last post in this particular forum.
electricpete (Electrical)
3 Aug 08 17:19

Quote:

Electric Pete obviously has a low opinion as to the relevancy of my experierence
I said nothing about the relevancy of your ideas or experience.  I was pointed out that the brand new idea you seemed to think you were bringing had (from my perspective) already been covered ad nauseum within this thread.

I apologize if my tone offended you.

 

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Eng-tips forums: The best place on the web for engineering discussions.

electricpete (Electrical)
3 Aug 08 17:25
Might I also add that you should in general expect some reaction when you come into a thread with 21 replies over 6 weeks and lead off with a statement "I suspect everyone here is missing the most relevant point."

You may not have intended it to sound like criticism, but that's what it comes accross as.

 

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dabluffrat (Mechanical)
19 Aug 08 12:08
Yes, it can be done as it was addressed above before everyone got off topic... but it is difficult. If knowing the exact flow is critical, I would look into the flow meters mentioned above.

Couple of things to consider I didn't see already mentioned:

1. Are you measuring balanced 3 phase amps or a single leg?

2. Synchronous or induction motor? Synchronous motors are rare, but used. As the power factor setting moves away from unity, the amps will increase (the V curve).

3. Pump wear over time will cause a decrease in efficiency and increase in amps. You will lose flow as you increase amps.

4. Using typical motor and pump data vs actual test data for your power and BHP calculations. Name plate efficiency is only for the rated full load HP. It's difficult to find actual (accurate) test data on older pumps.
waross (Electrical)
19 Aug 08 22:54
This is getting a little off topic. Posters are responding to other posters rather than going back and reviewing the original post.
The original poster asked

Quote:

I am wondering if there are any relationships to descript pump motor Amp and actual pump flow. That is by measuring the pump motor Amp to eatimate the actual pump flow.

OK, I measured the pump motor amp. My pump is drawing 40 amps. What is the flow rate??
I think that is the point that we are missing.

Bill
--------------------
"Why not the best?"
Jimmy Carter

MikeCool (Mechanical)
20 Aug 08 0:29
I'm a pump designer and manufacturer (www.aspumps.com).  In testing our pumps to confirm design intent, we measure flow rate, total dynamic head, input voltage and amps, rpm, and torque.  Some of these measurement are redundant, but they serve as comparable data, and some measurements are more accurate than others.

In testing to see that production models are performing to specifications, we measure flow, TDH, volts and amps.

In order to determine the pump flow rate, when all we can measure are the motor amps, we must have certified pump and motor performance curves corrisponding to the particular rpm and voltage.  The amp data is used to determine the input motor horsepower to the pump from the motor performance curve, and then the motor BHP is used to determin the pump flow rate on the pump performance curve.  

This method is as accurate as your measurements and factory performance curves.  As Johnny Pellin stated earlier, some pump can have a hump in their performance curves, which obfiscates the solution.  If this is the case as can be detemined by the pump performance curve, you need additional data: suction and discharge pressure.

Good luck!

Mike Cool
Mechanical Engineer
American Stainless Pumps, Inc.
Los Angeles California USA
www.aspumps.com
 

Artisi (Mechanical)
20 Aug 08 0:49
dabluffrat.
This is off topic but I believe that incorrect information  stated in your last posting should be corrected for the benefit of those who believe that eveything on the net is gospel.

" 3. Pump wear over time will cause a decrease in efficiency and increase in amps. You will lose flow as you increase amps."

A reduction in performance for a fixed speed pump eg, less flow and / or less head, usually a combination of both due to internal wear (decrease in efficiency)results in a lower power input(less amps) not more. However, if the pump unit is variable speed drive and the speed is increased to overcome the lower hydraulic efficiency of the impeller due to wear, then and only then will the power input increase.
 

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