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Accuracy of fluid resistance calcs
3

Accuracy of fluid resistance calcs

Accuracy of fluid resistance calcs

(OP)
Hi all,
In your vast collective experiences, what accuracy can one expect between theoretical piping dynamic head loss and actual values.  What is a reasonable margin of error for these sort of things.  I am designing a small hydronic system and have painstakingly calculated the pressure drop at the desired flow rate around the system.  Now I am selecting a pump and am trying to get as close to peak efficiency as possible.  Is this a fools errand.  should I select the pump with a reasonable middle sized impeller and fine tune with trimming or replacement afterwards?
cheers

RE: Accuracy of fluid resistance calcs

The equations are pretty good as long as you have good input values for all the parameters.  I've got a pipeline model with several hundred wells and many hundreds of miles of pipe that I can calibrate to within +/- 5 psia at every well (i.e., input each well's flow rate and each compressor station's inlet pressure then adjust pipe roughness until the model can generate wellhead pressures to the desired sensitivity) and then take another snapshot and it will still be within +/- 10 psia at every well.

For smaller systems I've gotten +/- 1 psia in the calibration run and +/- 3 psia in the subsequent runs.  

The equations do a very good job within the constraint of the quality of the input data.

On the other hand, your goal of reaching a peak effeciency in the pump is just silly.  Conditions change continiously.  A section of pipe will accumulate some scale, another section will partially plug off, another section will have a bad weld create a von Karmen Vortex Street that screws up pressure drop.  The best I've ever done is to design a system with the design conditions in about the middle of the pump operating range and add enough hp to operate a fair bit on either side of the design point.

David
 

RE: Accuracy of fluid resistance calcs

He should be able to run BEP/OFF for a small hydronic system.

**********************
"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/

RE: Accuracy of fluid resistance calcs

For which 10 psi may be his whole system loss, so let's say normally 5% of maximum value ...or so.

**********************
"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/

RE: Accuracy of fluid resistance calcs

(OP)
Hey biginch,
what is BEP/OFF? I have calculated the system loss at 83kpa or 12psi.  My flow is .6 l/s.  Any ideas for pump selections.  I want to go for the grundfos magna series, they have a power regulation unit attached, it matches power input to system resistance.  Any other ideas?
cheers

RE: Accuracy of fluid resistance calcs

zdas was suggesting that its difficult to run a pump at best efficiency point, BEP, because many system variables are changing all the time.  With a very large system, that would tend to be slightly true, but IMO most of those variations would just be an indication that system maintenance is needed, so I feel it is possible to run at BEP, or within a very acceptable margin thereto.  Besides, if you don't spec and buy your pump using BEP characteristic values, what can you use (no question mark).

BEP/off means simply that you're simply either running the pump at its Best Efficiency Point flowrate, or its turned off.  You won't try to control the flow to adjust to flowrates other than that BEP rate, so no control valves would be needed, just on/off types.

There are thousands of pumps that fit this application and maybe you can find one at your local Home Center.  I'd look for a small centrifugal pump (plastic components will be fine) with as close as possible to a B. E. Point of 36 l/minute (10 gpm) at a head of 8.5 to 10 meters (25 to 33 feet) and at least an operating pressure of maybe around 20-25 psig, which shouldn't be a problem.  The farther away from the 10 gpm & 30 ft head B.E.P., the more likely you are to need some valve for adjustments to flow and a little bit more money for electricity costs.




 

**********************
"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/

RE: Accuracy of fluid resistance calcs

Of course you look at your design point and try to get a pump to match it.  My point is that any pumped system is dynamic and ever changing so getting a pump that can handle 36 L/min +/-0.005 L/min is a bad idea.  Getting a pump that is at a good place on the pump curve at 36 L/min and can handle 30-45 L/min makes more sense.  

When I hear "trying to get as close to peak effeciency as possible" alarm bells go off.  I've seen too many "effecient" systems that couln't handle the change in load caused by a low pressure weather cell.

David  

RE: Accuracy of fluid resistance calcs

Pumps and hydraulic efficiency are like rust on steel...it gets worse with age.  The best efficiency your likely to get is when you run it's all new...downhill from there because of exactly what zdas04 said.  Just for the cliche festival...you're tripping over dollars to pick up nickels.

RE: Accuracy of fluid resistance calcs

What?  Come on.  Guys, change your medication.
What pump does 36 +/- 0.005 L (no question mark).

BEP is a target. If you can't operate at 78% efficiency, OK try to hit 77% or 76 or 75.   You can operate at increasingly worse efficiencies away from BEP until you run out of dollars to pay for the increase in fuel or electricity price.  If pipe roughness gets too much, then you spend some money on maintenance, including looping or replacing the pipe, if that's the economical thing to do. If the impeller gets worn too much, replace it.  If its still pumping at reasonable efficiency, let it go.

Simple engineering economics can tell you when its worth the cost to replace, or maintain and how far away from BEP you can afford to operate.

**********************
"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/

RE: Accuracy of fluid resistance calcs

2
Lets get real with what the OP is asking  - we are talking about a pump that at best will be something like 40-50% requiring 0.1Kw to handle the hydraulis requirement, this certainly isn't rocket science. Find a pump that will get somewhere near to the duty and put it to work, does it really matter if the power consumption is 0.1 or 0.105 Kw.

RE: Accuracy of fluid resistance calcs

I have seen more than a few 10 hp applications get a 10 hp motor.  When everything is new and perfect, the pump would do just its job.  A bit of wear, a bit of scale and the application was out of hp and flow was under the amount required.  In every case, the engineer said "the system load calculated out to around 10 hp, why shouldn't I install a 10 hp motor?"  In one case the system load calculated out to 10.5 hp with some very optimistic pipe roughness numbers.  The 10 hpo motor and pump never moved design volume.

For a 10.5 hp application I'll generally install a 15 hp motor.  "Effecient" applications would install a 10.5 hp motor.  "Peak Effecient" applications would install a 10 hp motor and claim it was the pump's fault that it can't meet design specs.

David

RE: Accuracy of fluid resistance calcs

For small pump motor combinations it is usual to size the motor for end of curve run-out plus motors normally come in standard frame / power ratings which also limits or dictates the motor size selection.  

RE: Accuracy of fluid resistance calcs

No great harm in installing a 15 HP, it will still only use 10.5 HP at BEP.

But ratio your same factor up for 5000 HP required at BEP and I bet you won't get your wish list filled.

**********************
"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/

RE: Accuracy of fluid resistance calcs

For motor sizes upto maybe 250 - 300 HP you don't have much choice as they are standard size - so you install what is availble however, for 5000 HP you would expect a motor designed for the application plus a fair amount of engineering to establish all transients for the system, careful safeguards and monitoring for power input with undercurrent, overcurrent, phase monitoring etc etc.   

RE: Accuracy of fluid resistance calcs

Artisi,
In other words, the more economic risk, the more Engineering is justified.  I sure agree with that.  I see a lot of 5-25 hp applications that are not Engineered at all.  Above 250 hp the price tag goes up and the interest of Engineering management goes up with it.

David

RE: Accuracy of fluid resistance calcs

For one brief moment I had to check to see if I was still in the Piping & Fluid Mechanics Forum.  

RE: Accuracy of fluid resistance calcs

JohnGP- yes still here - the subject has drifted which seems to be fairly normal and we sure have come along way from a 0.1Kw happy-weed irrigation system.

RE: Accuracy of fluid resistance calcs

Well, it's not unrelated, and it was the OP that headed the discussion that way, so I'm certainly not calling foul.

RE: Accuracy of fluid resistance calcs

OK
"see a lot of 5-25 hp applications that are not Engineered at all."  Most likely why you've seen a lot of them fail to meet expectations.

But, if you don't start with a BEP target, where do you start?   The pumps at Home Center I've seen have motors sized for the full curve, which is why I didn't mention HP required.

**********************
"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/

RE: Accuracy of fluid resistance calcs

Agreed, after Q and H is established next step is selecting a pump or pumps based on best efficiency available from pump/s ranges available - followed by NSPHr/a, a review of max and min flows and impact of Eff.and NSPHr/a and motor selection etc. Pump style, materials of construction etc are a seperate issue - which in most cases is the first step dictated by the application.

 

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