Pump Flow VS Pump Motor Amp
Pump Flow VS Pump Motor Amp
(OP)
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!!
Thanks!!





RE: Pump Flow VS Pump Motor Amp
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.
RE: Pump Flow VS Pump Motor Amp
What kind of pump are you talking about? Piston? Centrifugal? Gear?
RE: Pump Flow VS Pump Motor Amp
Johnny Pellin
RE: Pump Flow VS Pump Motor Amp
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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RE: Pump Flow VS Pump Motor Amp
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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RE: Pump Flow VS Pump Motor Amp
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
RE: Pump Flow VS Pump Motor Amp
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
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"Why not the best?"
Jimmy Carter
RE: Pump Flow VS Pump Motor Amp
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
RE: Pump Flow VS Pump Motor Amp
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.
RE: Pump Flow VS Pump Motor Amp
"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
RE: Pump Flow VS Pump Motor Amp
"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
RE: Pump Flow VS Pump Motor Amp
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.
RE: Pump Flow VS Pump Motor Amp
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.
RE: Pump Flow VS Pump Motor Amp
RE: Pump Flow VS Pump Motor Amp
rmw
RE: Pump Flow VS Pump Motor Amp
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
RE: Pump Flow VS Pump Motor Amp
Anything else becomes a dream and wishful thinking.
RE: Pump Flow VS Pump Motor Amp
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.
RE: Pump Flow VS Pump Motor Amp
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
RE: Pump Flow VS Pump Motor Amp
"If everything seems under control, you're just not moving fast enough."
- Mario Andretti- When asked about transient hydraulics
http://virtualpipeline.spaces.msn.com
RE: Pump Flow VS Pump Motor Amp
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??
RE: Pump Flow VS Pump Motor Amp
1 - First check out Artisi's post 20 Jul 08 1:12
2 - Then check out the original post:
3 - Then check out the definition of estimate
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.
RE: Pump Flow VS Pump Motor Amp
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.
RE: Pump Flow VS Pump Motor Amp
I apologize if my tone offended you.
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Pump Flow VS Pump Motor Amp
You may not have intended it to sound like criticism, but that's what it comes accross as.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Pump Flow VS Pump Motor Amp
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.
RE: Pump Flow VS Pump Motor Amp
The original poster asked
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
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"Why not the best?"
Jimmy Carter
RE: Pump Flow VS Pump Motor Amp
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
RE: Pump Flow VS Pump Motor Amp
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.