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Conservation of Flow?

Conservation of Flow?

Conservation of Flow?

(OP)
I have system that consists of the following:
1.  Permanently installed flow meter for whole system (incoming and outgoing)
2.  Manifold where whole system is split into three branches
3.  Orifice devices in two of the three branches to limit flow to approximately 1/2 of the 3rd branch
4.  Temporary flow meters on each branch
5.  Similar manifold where all three branches converge (but with no orifices)

A given set of data looks like this (all in GPM):
Permanent flow meter incoming = 3.1
Permanent flow meter outgoing = 3.1
Temporary flow meter incoming = 3.0 (test for meter calibration)
Temp flow meter exiting branch 1 = .6
Temp flow meter exiting branch 2 = .6
Temp flow meter exiting branch 3 = 1.1

Because these values total 2.3 GPM, and the flow into the manifold = approximately 3 GPM, there is a feeling that the manifold is somehow limiting flow.  I understand that while it is probably limiting flow of the SYSTEM, it cannot cause a localized flow restriction that somehow disappears after reconvergence.  How do I explain to my peers the measurement differences they are seeing?

Thanks,
Jay

RE: Conservation of Flow?

What is supplying the flow and pressure?  How far downstream of the orifices are the flow meters?   

-Mike

RE: Conservation of Flow?

If there are no leaks, I look at this and conclude:

(1) The "total flow" meters are in close agreement and are probably reasonably correct in that they verify each other.
(2) If the flow branches are geometrically similar *and dynamically similar* (refer to fluid mechanics definitions of dynamic similarity) then the two branch meters reading 0.6 appear to verify each other.
(3) I tend to suspect the third branch meter is off calibration; it seems to be the only one in the system without a benchmark to follow.

You could probably rent a temporary Doppler meter for a day to confirm the rates.

Regards,

SNORGY.

RE: Conservation of Flow?

(OP)
Flow and pressure are supplied by a central pump unit located fairly far away.  For this question, I've excluded all other parts of the system.

The third branch SHOULD read differently than the other two, since the orifices in the three branches are different.  Branch 3 has a larger orifice than the other two in order to induce more flow to that branch.

Thanks,
Jay

RE: Conservation of Flow?

Is the fluid air? or water?

Water is essentially incompressible, so 3.1 gpm in must result in 3.1 gpm out, assuming no leaks.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: Conservation of Flow?

(OP)
Water.  I agree that flow in = flow out.  The problem is, how do I explain the discrepancies in the flow meters on each branch vs. the total flow meter on the system?

Thanks,
Jay

RE: Conservation of Flow?

An old adage in flow measurement: "Measure it once, you have measurement, measure it twice you have an argument".

On the surface, it would appear to me that one or more of  your "temporary flow meters" would be the culprit since your ingoing and outgoing meters agree and your "calibration test  meter" agrees with your permanent incoming meter within roughly 3% (3.0 vs. 3.1). At 3 GPM, I assume these are very small lines (maybe 1/4" or 1/2" ?), so I assume you left yourself plenty of straight-run before the meter (no disturbances from elbows, the orifice plates, wall taps, etc.). What flow metering technology are you using for the main and branch lines? Whenever you "mix" metering technology, you have the potential for discrepencies.

I don't know your complete piping arrangement. Is there any way to block off #1 and #2 and run 100% flow through Branch 3 (the one with no restrictive o-plates). A 25% discrepancy in flow between your branch lines and the main lines is huge. Isolating the flow through one line should at least indicate whether or not the error is cumaltive.  

Since the units are in GPM, I assume you are running a non-compressible fluid (density compensation not required at measurement points).

I'm sure we can help trouble-shoot your application, but I would ask for the following:

1) Start with Mike's questions above.
2) Identify what fluid you are measuring
3) Identify what type of meters you are using on all lines.
4) Indentify all line sizes and equations you are using to calculate flow (for example, if it is DP, are you taking the square root of the DP to calc flow).
5) Are the flow rates you are measuring within the specs of the meter (turndown of 3.1 GPM down to .6 GPM)
6) Is there adequate straight-run up and downstream of the meters?

There are hundreds of meters claiming to have accuracy of 1% or better. Only a small percentage can produce independent flow lab data to back it up.

Bottom line: If the flow in = flow out, and there are only 3 branch lines in between (all are being metered), then you should not have a 25% error.

I hope we can help.

RE: Conservation of Flow?

I was writing my long post while others were posting. I apologize if anything is redundant. There was only one response when I started. I guess I was a little long-winded.

Jake

RE: Conservation of Flow?

My fluids prof in grad school explained the theory of continuity the best I've ever seen:  If flow in does not equal flow out, what happened to the the excess inflow or where did the excess outflow come from?  If you don't have a place to stack up or leak the missing liquid then it must be leaving down the pipe.  Period.

Since the inflow physically equals the outflow, your difference is measurement.  I would first look at the two meters reading 0.6 gpm.  I've measured the flow on seemingly identical legs hundreds of times (looped flow meters on big gas wells, but it is the same principle) and have never once had accurate, repeatable meters read the same on the two "identical" legs.  It never happens.  20% difference is quite common, 30% is not rare.  

Any chance you can put a 3-ball manifold downstream of your meters to drop in a prover?  One, two, or three of your outlet meters is really wrong.  Your inlet meters may also be wrong, but it is rare for two meters in series to both be wrong the same amount in the same direction.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

"Life is nature's way of preserving meat"  The Master on Dr. Who

RE: Conservation of Flow?

I would suspect the branch flow meters are not reading correctly.  By your stated intended flow distribution, the flowmeters should read about 1.5, .8, and .8 gpm.

Unless there is a leak or storage, what comes out must equal what goes in.

Ted

RE: Conservation of Flow?

without knowing what type of temporary meter you have.  Is it linear or square root?  or something else (a micromotion is a cubic equation)

RE: Conservation of Flow?

dcasto,
"Micromotion is a cubic equation"?????????  Micromotion infers flow from linear displacement of the piping--i.e., a linear equation since the only thing they are measuring is the shift in the piping.

David

RE: Conservation of Flow?

Um, at low flows (below I think 10% of rated max) they go non-linear David.  At least, we always showed them to be like that, but we were also using them at 1% and less of rated flow, and so had to calibrate them for the "off label" prescription.

RE: Conservation of Flow?

Shift is only linear in the make-believe world. smile

**********************
"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: Conservation of Flow?

Jay (OP), we are doing a lot of speculating. Can you tell us what meters you are using and the pipe sizes?? At this point, we really do not know if you are measuring "high" flow rates or "low" flow rates ("measuring in the mud"). For example, water flow of 3.1 GPM in a pipe with a 1/2" ID is moving at a velocity of roughly 5 FPS. At .6 GPM the velocity is slightly less than 1 FPS. Pipe with 1" ID would be 1.27 FPS at max flow (3.1 GPM) and only .25 FPS at the minimum flow rate (.6 GPM). Given that many flow meters are Reynolds # dependent and, with liquid flow, the point at which the flow trips from laminar to turbulent can vary according to velocity, temp., pipe roughness, etc., it is certainly possible that you could be measuring flow in different regimes (laminar, transitional, or turbulent).
 

RE: Conservation of Flow?

By "linear" I meant that flow is inferred from the measurement of a single parameter (displacement).  I would say that a squared-edged orifice is cubic because it infers flow from static pressure, differential pressure, and temperature.  Or that a turbine meter is a random number generator linear because it infers flow from a pulse counter.

I was only referring to the number of variables sampled, not to the shape of a graph of flow.

David

RE: Conservation of Flow?

Ah.  I was thinking of the raw device "response" to varying flow rate.  Orifice output (pressure gage) changes by the square of flow, micromotion (vibrating u-shaped beam amplitude) is linear within its rating, and gets wacky at the low end (but so do all amplified signals, I used to point out to our rocket lab's "flow guru"...he did not follow and did not appreciate the kibitzing from a junior engineer).  Agree that turbines and paddlewheels are random number generators.  Some thermo-based meters are square-rootish, but I've never used those.  A fun read is the NIST flow calibration website.  Their calibrated flow totalizer is a tipping bucket...shades of Chinese water clocks!  For what it's worth, all flow calibrations, at their roots somewhere, are based on a guy with a bucket and a stopwatch.

RE: Conservation of Flow?

More like a guy with a leaking bucket and an hour glass, but I get your point.

David

RE: Conservation of Flow?

Or, in my case, a 5-gallon pail and a wristwatch.

I was once involved with automating a batch cooling process where utility water would cool "stuff" in a batch cooling tank.  The instrument and process engineers were trying to establish the sizing basis for a full-port snap-acting dump valve on the outlet of a 1.5" utility water line.  I asked if the utility water was hazardous or toxic, and they looked at me like I was some kind of idiot and said, no, it's only water.  So I picked up a 5-gallon pail, hung it on the end of the pipe that was being dumped to sewer, opened the ball valve, and filled the bucket in 10 seconds.  I told them to size the valve for about 30 usgpm.  Again, they looked at me like I was some kind of idiot.

They didn't ask me for any more help, so I never did find out what they actually installed.

Regards,

SNORGY.

RE: Conservation of Flow?

Change temp flow meters' position (branch 1 to branch 2 and vice versa) and check whether measurements are still the same

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