Differerence in flow 1

[SonnieTP]

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.

 

[Latexman]

Check your P&IDs for additional documented sources. Any undocumented sources? I.e., walk the line.

Good luck,
Latexman

 

[SonnieTP]

I did that already. We verify the 9.6 mile pipeline and checked the P&ID. The pipeline configuration is straight and simple. One long pipe with flow meter on the upstream and another flow meter downstream. Flow meter upstream reads 225 gpm and the flow meter downstream reads 166 gpm. No leaks on pipe line, no other connection. Not sure if pressure drop and elevation comes into play but if it does, it shouldn't be this big difference in flow.

 

[Artisi]

If you have X flow into the pipe at source then you will have X flow at the exit. So, the flow meters are reading incorrectly or you have a leak/s.

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.

 

[FlaCivEng]

If you are confident that the water meters are correct, better check for leaks. How old is the pipe? Does it show signs of deterioration?

 

[SonnieTP]

Thanks gentleman, I received good answers so far. Anyway, the pipe is 9 years old. The meters are calibrated annually so I am confident that they are both correct. The pipe does not show any signs of deterioration.By the way I forgot to mention that it also has a 40 micron filter right after the pump discharge so the water flow from the pump, to the valves, to the filter, then to the flow meter; So the aeration and fluid compression I believe is minimized due to filter before the fluid enters the flow meters.

 

[ash9144]

What kind of meters? How were they calibrated? Did they agree last year at calibration time? When did they start disagreeing?

 

[SonnieTP]

It is both a Rosemount 8800 vortex flowmeter and send off to a third party vendor for calibration using a proofer meter calibration system. It was like this from the beginning and never agree at all.

 

[jmw]

What happen if you swap them round?
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

http://www.ViscoAnalyser.com

 

[SonnieTP]

We have tried swapping them as well over the years and the difference is still the same. I have another response from a different group that I may try. I will check the transmitter card calibration and transmitted signal. I may need to boost or lower the transmitted signal on one or the other. Thanks everyone!

 

[micalbrch]

Assuming that both flow meters were correctly calibrated I think Artisi is on the right way. Something (air?) affects the downstream flow meter reading. That would explain why both flow meters were found to work well when they were calibrated (taken out of the system). Is there any chance for you to measure the outflow from the pipe? For example fill a barrell with a known volume and measure the time it takes?

 

[ash9144]

Is the pipe the same size at both locations? Are they both set up for same size pipe?

 

[SonnieTP]

The pipes are the same 6 inches all the way from upstream to downstream.

@micalbrch- we cannot perform that type of experimentation.

 

[btrueblood]

"We have tried swapping them as well over the years and the difference is still the same"

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.

 

[SonnieTP]

@btrueblodd - It is the opposite that is occurring.

Thank you all for your response.

 

[jmw]

I will check the transmitter card calibration and transmitted signal. I may need to boost or lower the transmitted signal on one or the other.

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

http://www.ViscoAnalyser.com

 

[jmw]

PS
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

http://www.ViscoAnalyser.com

 

[BigInch]

Can't you just use the average.
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?

 

[DLite30]

I suggest the following:

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.

 

[BigInch]

Flowrates must be the same!

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.