Differerence in flow
Differerence in flow
(OP)
What could have cause this to differ in flow?
Our waste water centrifugal pump is discharging water at a normal rate, there is an installed flow meter on the upstream of the waste water disposal approximately 15 feet away from the pump discharge and the flow rate is 225 gpm.
On the downstream of the waste water line that goes into the injection well, 9.6 miles away, we also have another flow meter installed. The reading that I am getting on the downstream flow meter is 166 gpm.
I have both flow meter calibrated but the readings are far apart of each other. What could have cause this uncertainty in meter reading?
The downstream meter location is 9.6 miles away from the pump with an elevation of 1000 ft. The upstream location had a site elevation of 1330 ft. The pipe is a 6 inch carbon steel.
Our waste water centrifugal pump is discharging water at a normal rate, there is an installed flow meter on the upstream of the waste water disposal approximately 15 feet away from the pump discharge and the flow rate is 225 gpm.
On the downstream of the waste water line that goes into the injection well, 9.6 miles away, we also have another flow meter installed. The reading that I am getting on the downstream flow meter is 166 gpm.
I have both flow meter calibrated but the readings are far apart of each other. What could have cause this uncertainty in meter reading?
The downstream meter location is 9.6 miles away from the pump with an elevation of 1000 ft. The upstream location had a site elevation of 1330 ft. The pipe is a 6 inch carbon steel.





RE: Differerence in flow
Good luck,
Latexman
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However, just a way-out thought, seeing as how you pumping at fairly high discharge pressure,(330ft + friction losses) is it possible that the product is slightly aerated which is compressed at the pump discharge but has expanded at the exit (plus the elevation change) which is giving a false reading at the flow meter.
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If this is a meter error then the readings will reverse.
If the meters are right it won't matter which one goes where, you'll get the difference as before.
Once you know if it is the meters or the location then you can look at possible causes.
JMW
www.ViscoAnalyser.com
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@micalbrch- we cannot perform that type of experimentation.
RE: Differerence in flow
Sonnie, not to be pedantic, but to clarify:
You have:
meter A normally upstream, reads 225 gpm
meter B normally downstream, reads 166 gpm
Right off the bat, aeration of the flow would cause a reverse of that reading, i.e. expansion of air across the pipeline pressure drop would result in a higher outlet flow meter reading, not lower. Leaks are still possible.
When reversed, does this happen:
meter A (now installed downstream) reads 166 gpm
meter B (now installed upstream) reads 225 gpm
If so, it indicates that installation location is affecting meter reading. If there really are no leaks or losses along the pipe, the next thing I'd look at is placing a de-swirl device between pump outlet and first meter. I assume the Rosemount meters are the reducing type, i.e. the effective pipe diamter at the meter is quite a bit less than 6", in order to generate measureable velocities above the cutoff point of the meter. Vortex meters can be affected by swirl, and pumps are notorious swirl generators, and swirl is intensified by reducing pipe diameter. Another method would be to simply locate the meter further away from the pump, to allow a fully developed pipe flow (no swirl). Also look into signal degradation over the 9.6 miles, but if it's a 4-20 mA current loop or serial digital signal, those signals are unlikely to degrade...
If the opposite is occurring:
meter A (now installed downstream) (still) reads 225 gpm
meter B (now installed upstream) (still ) reads 166 gpm
Then you have a meter error somewhere, either in internal meter programming or at the signal reading devices.
RE: Differerence in flow
Thank you all for your response.
RE: Differerence in flow
What signal are you talking about here? A 1-5volts signal? 4-20mA? Hart? Modbus?
Are both the meter signals displayed at the upstream location? I assume you are suspecting some form of signal loss from the furthest meter.
But, what about local displays?
Are you also totalising?
Is the integration rate consistent with the flowrate at each meter?
If so I would not suspect the signal from either meter but a local installation problem such as Btrueblood suggests.
As micalbrch suggests, try measuring the outflow independently. That way you will find out which meter meter reading, if either, is consistent with the true flow.
JMW
www.ViscoAnalyser.com
RE: Differerence in flow
If the error is consistent over time, and the flow rate signals are transmitted to a simple 3 1/2 digit rate indicator, check that the analogue output span and bias scaling on both meters and both displays match up.
I need a moment to figure out what happens if you swap meters but not displays.... (late night and lack of coffee).... i.e. normally meter A transmits to Display A but when you swap meters, meter A transmits to display B. You might need to match meter A with display A when you swap meter A from upstream to downstream and visa versa.....
JMW
www.ViscoAnalyser.com
RE: Differerence in flow
Reminds me of an electrical engineer that once tried to find out why my right turn signal wasn't working. After 4 hours, I asked him if he checked the light bulb.
The first thing you do is to make a hydraulic model of the pipeline and calculate the pressure drops you expect to have, then try to explain why they don't match.
You do that for each point where you can get Flow and Pressue data and know the difference in elevations.
Hopefully you will have those at the pipeline inlet and outlet. What are the pressures here?
Inlet Outlet
Flow: 225 gpm Flow 166 gpm
Press: ??? Press ???
Elev: Z = +330 Elev: Z = 0
You calculate your energy gradient at the inlet,
Do it again at your outlet.
Convert the pressures to head.
Draw lines at the corresponding slope of the energy gradient for each one. Look for your leak where they intersect. If you don't find a leak, then check the signal.
Its also possible that the air entering at a higher elevation will be compressed due to the elevation pressure difference to the outlet, which depending on your pressure drop at those flowrates, your pipeline could actually discharge at a higher pressure at the outlet than it has at the inlet. If I knew the pressures at those points, I could estimate the amount of air entering that would cause the lower flowrate reading at the outlet.
AND, BTW, a filter won't stop air.
Now what are the pressures at the inlet and the outlet?
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Double check to make sure both pipe diameters are indeed identical.
The ratio of flow areas between a 5" and 6" line is almost exactly the same as the ratio between your two flow rates.
5" pipe and fittings are pretty uncommon but not extinct. You may actually have a 5" suction line.
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Slightly compressible liquids in steady state flow would make flowrate independent of pipe diameter. Steady state constant flowrate would make the velocity in each diameter change such that the continuity equation would be valid and hold true at any point in the system.
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However I suspect the most likely cause is bends near the meters. According to the Rosemount manual "No K-factor correction is required if 35 straight pipe diameters upstream (35D) and 10 straight pipe diameters downstream (10D) are available." You have stated that the upstream meter is 15 feet from the pump discharge - which I suspect would be much worse that a pipe bend. Have you corrected the K-factors accordingly?
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Differerence in flow
Let's assume these are the meters installed 9 years ago when the pipeline was installed.
Let's assume they are identical and the displays are identical.
Let's also assume that when first installed they agreed within acceptable limits.
So we need to ask why they now disagree.
Have there been any significant operational changes?
Any significant installation changes?
Has anyone been doing anything to the meters at all for any reason?
Is the upstream meter flow rate (225g/min) consistent with past history?
If it is suddenly indicating higher flows than previously why? If not then assume this meter is accurate. 225g/min is being pumped into the pipe (is there perhaps a small increase in flow? e.g. due to the lower pressure drop caused by some of the flow finding an easier path)
Is the outflow flowrate lower than usual?
The tests suggested above can and should all have been done prior to sending the meters away for testing as that takes time and may be unnecessary. At around 60g/min that's about 600,000gallons a week.... that's a lot of water to go missing.
Why have two meters? On long pipelines it makes sense as this is a good way to check for significant leaks.
I can't think there is a bit of Chernobyl syndrome evident.
Sure, you need to be sure the meters are correct but if the upstream meters reads more or less what it always does and no one has messed with the installations, and swapping the meters shows the error staying with the location, then maybe there is a leak?
These are Vortex meters. They have no moving parts and the accuracy should remain stable over time. You would not expect one or other meter to show any significant calibration shift, especially not one compared with the other. Failure is likely to be complete failure due to component malfunction or there would be some erratic behaviour.
JMW
www.ViscoAnalyser.com
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Sit back for awhile until the OP posts the pressures and probs the dirt. If he doesn't have those, nobody's going to figure out what gives here.
It's not always instrument error.
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SonnieTP, I guess this is an operation that you've inherited....?
You have to wonder why the problem was not investigated before.
Given that no one has ever bothered to find the cause of the error, why have the meters been calibrated every year??
Apart from anything else, I'd guess no one has checked to see by how much calibration shifts each year either - probably, even if they were no differences to account for, it would have been found that the calibration shift was pretty small and far less frequent checks would be acceptable (no moving parts sensors with no erosion and no deposition should exhibit pretty stable calibration).
If we accept there are no leaks, because the difference has remained constant (is this the case?), then either one or both installations are bad or there is some simple span and bias scaling issue.
As has been suggested, it could be the flow conditions downstream of the pump. A couple of bends before the meter and that could generate a lot of swirl. Turbulence will dissipate quite quickly but not swirl.
If there is reason to suspect swirl then you need to install flow straighteners upstream of the meter. These needn't be to big of a problem to install as you can have an straightener assembly mounted to a thin plate to locate between flanges. If there is a straight pipe location further downstream you could install the meters, move the upstream meter further away from the pump.
But I'd first be sure that the error is with the location and not the meters and it would be nice to know which meter is wrong. I'd suggest hiring a clamp on ultrasonic meter and measure the flow rates at various points along the pipe.
JMW
www.ViscoAnalyser.com
RE: Differerence in flow
This sort of problem is caused by one of three things (or a combination of the three):
1. One or both meters is reading wrong.
2. There is actually a different mass flow rate at the two points (i.e., a leak or an undocumented source).
3. You are mistaking a variable stream for a steady stream.
The third one is really common--the pump is surging between 150 and 300 gpm and the software is displaying snapshots. You can generally catch this by using daily cumulative volume instead of snapshot readings.
The second one is really unlikely--someone would have noticed 300 million gallons of water in their basement.
Everyone above is focusing on the first possible source. I would too. A vortex meter infers a flow rate from the frequency of vibration of an element in the downstream side of a bluff body in the flow. The vibrations are proportional to mass flow rate. It becomes a volume flow rate by inputting a fluid density into the computer. Errors in this sort of input are really common. I'd print out the meter station parameters and compare them line for line.
David
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Yeah, we need those pressures BigInch asked for. It occurred to me that because the downstream flowmeter is 330 ft below the upstream meter, and all we know is that it "goes into the injection well", the pipe might not be running full at the downstream meter. We've all assumed that because it is an "injection well" that the waste water is being injected by the pipeline pressure. That may not be the case. Perhaps the pipeline discharges to an injection pump reservoir, so that it is vented to atmosphere and thus allows the elevation drop to accelerate the pipeline flow velocity. 225 GPM equates to 2.5 fps. I don't have any rules of thumb or quick calcs handy, but 330 ft head on a 6" pipe will flow a heck of a lot more than 225 GPM.
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It really only makes sense to look at daily (or weekly) cumulative flow.
David
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Ted
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Nobody mentioned if there is a high point somewhere between inlet and outlet. Without knowing the exact elevation profile (not just at the inlet and outlet) and the pressure profile of this pipeline, the outlet meter might even be in slack, or "cascade" flow, (2 phase flow in a downhill segment) and measuring the passage of both wastewater liquid and its vapor phase too. Even more likely if the pipeline outlet is a turndown into the well.
Cascade flow can be steady state over the long term, but transient over the short term, running full, then with vapor slugs. We just don't have any idea what the hell it might really be doing. I'm just saying that if you do the hydraulic model first, you can tell if you should look for a leak somewhere, change the operating pressures or flow, or fix the instruments.
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Interchanging meters produced the same result, downstream reading less than upstream.
Ted
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At any slope or any backpressure, whether provided by any combination of discharge pressure and pressure drop from a restricted outlet, atmospheric pressure included, is less than the exit pressure of the fluid, theoretically it will discharge across the full cross section. Gravity helping on the end of the pipe will only accelerate out so much. If a pump is feeding it at that rate or more, full cross-sectional flow occurs. Think of a garden hose without a nozzle on it. Turn it downwards and turn on the water. I bet its pretty easy to get full x-sect flow, because the x-sect is so small in relation to the pressure available at the last high point. Cut back the flow enough and you'll eventually see water just coming down in a ring from the connection wall and the water making a coaxial flow pattern with air in the center as the air tries to enter and run back into the hose.
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Anyway, seems SonnieTP may have stopped listening.
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Does it really matter? The only real important thing is that WE work it out one way or another, right?
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Adding the valve to restrict flow, as you say, should be an easy add-on.
Ted
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Say that flowrate was slow and that there was an orifice through an end cap that restricted water outflow enough and still allowed air inflow, so that the bottom of the pipe could fill as you suggest. Any air entering would tend to backflow upstream in relation to the slow liquid velocity. If the air reached a high point in the pipeline and could not travel any further upstream (now downhill), even against the minimal flowrate, it would tend to collect there, at first along the top wall of the overbent pipe. Further collection would at first just increase the velocity of fluid below the bubble, perhaps drawing some air downstream with it, but if still not fast enough to do that, the bubble would grow larger. But while restricting liquid flow and increasing the velocity, pressure drop might also be increased to the point where upstream pressure built up and then just blew out the air bubble and all downstream liquid to restart filling with liquid again. Or .. the bubble could continue to grow and fill up the downhill leg until it reached the next uphill segment and start the bubble up-and back filling process there too. It's possible that a number of air-locking and pressure build cycles would occur and initiate a 2-phase flow pattern that would remain for the duration of that particular flowrate. Of course what exactly would happen would be very dependent on the precise elevation of all points of the pipe along its entire length, and if upstream pressure could build, or remained more or less constant. If it remained constant, perhaps a vapor lock situation might eventually develop over multiple overbends where the sum of water column pressures in uphill segments, without the help of a counteracting dense fluid in the downhill segments, finally balanced the pipeline's inlet pressure.
That's why they usually need a lot of air release valves in water pipelines, because water holds a relatively high amount of air that tends to easily come out of solution with reduced pressure and either increase some local velocities and pressure drops, or tend to cause vapor lock ups. Fortunately liquid hydrocarbons do not absorb so much air and generally operate at higher pressure than water pipelines, so that problem can usually be limited, as long as you don't mechanically introduce air into the line by mistake or by not purging after repairs.
RE: Differerence in flow
Ted