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Two-Phase Flow meter
- Thread starter askme
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Here is an research group that has studied 2 phase flow.
You could measure the mass with a coriolis meter and know exactly how many pounds went by, but then you'd have to have a qualitative anlysis of what the componets were. Look here for information on this system.
You could measure the mass with a coriolis meter and know exactly how many pounds went by, but then you'd have to have a qualitative anlysis of what the componets were. Look here for information on this system.
The Coriolis meter is an interesting gadget. What it actually measures is the elastic response of metal to the momentum of the flow. It then assumes a constant density and infers a flow rate. If the density changes dramatically (like it will as you move through the flow-regime map) the reported volume flow rate gets more and more wrong. The spread from mist flow to slug flow can result in really large errors.
You never said what your goal was. If you are looking for custody-transfer measurement in a mixture of gas and oil, then no single device will meet contract specs. To measure for sales you'll need to separate the gas and oil and measure them independently. If you are trying to get a gross material-balance and can handle some random variations, then either a Coriolis meter or a V-cone will give you numbers that can often average out to +/- 10% on a monthly cumulative basis in widely varying flow.
David
You never said what your goal was. If you are looking for custody-transfer measurement in a mixture of gas and oil, then no single device will meet contract specs. To measure for sales you'll need to separate the gas and oil and measure them independently. If you are trying to get a gross material-balance and can handle some random variations, then either a Coriolis meter or a V-cone will give you numbers that can often average out to +/- 10% on a monthly cumulative basis in widely varying flow.
David
UmeshMathur
Chemical
In my own experience, it is impossible to measure flow with any reliability in a two-phase flow line. The solution is to back up to upstream points until you find a segment where you have a single phase and do the measurement there. For example, in a furnace, measure the flow before the feed enters the furnace or, even better, upstream of any feed preheat exchangers. Once the stream starts flashing, it becomes infeasible to measure flow.
In general, Coriolis meters cannot be relied upon for two-phase flow measurements.
In general, Coriolis meters cannot be relied upon for two-phase flow measurements.
Foxboro claim to have a Coriolis meter which reliably handles mixed phase flow. Anyone had practical experience of it?
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Sometimes I only open my mouth to swap feet...
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Scotty,
As I said above, the Coriolis meter infers a flow rate from the metal's elastic response to momentum. Similarly a square-edged orifice meter infers a flow rate from a static pressure, dP, and temp. Either one will take a list of given conditions as fixed and pretend they know the flow rate from the variable inputs, but one of the given conditions of the orifice meter is that there is no free liquid so the rest of the assumptions are really pretty valid.
The Coriolis meter has specific gravity as a fixed input (which is why it can be touted as a "mass flow meter"). In multi-phase flow, flow regime (and therefore specific gravity of the comibined stream) can vary by a large amount within a few seconds. These swings across the flow-regime map kind of average out much of the time so the cumulative production over a day, week, or month is good enough for most applicaitons. A snapshot will always be worthless.
The best I can tell from Foxboro's web page, they have a straight-line tube which is less sensitive than a bent-tube meter (like Micromotion's), but I don't know that that is a bad thing--bottom line is that you are paying for a very expensive random-number generator that will give you data to 9 decimal places, so how could it be wrong?
It looks like Forboro has "solved" the prolem of multiphase flow in software. I haven't seen their algorythm, but I'm really curious about how they categorize a change in momentum (which they can measure very precisely) as a change in mass-flow rate at constant density or a change in density at a constant mass flow rate.
It is a very tough problem, and "simple" solutions will seldom work.
David
As I said above, the Coriolis meter infers a flow rate from the metal's elastic response to momentum. Similarly a square-edged orifice meter infers a flow rate from a static pressure, dP, and temp. Either one will take a list of given conditions as fixed and pretend they know the flow rate from the variable inputs, but one of the given conditions of the orifice meter is that there is no free liquid so the rest of the assumptions are really pretty valid.
The Coriolis meter has specific gravity as a fixed input (which is why it can be touted as a "mass flow meter"). In multi-phase flow, flow regime (and therefore specific gravity of the comibined stream) can vary by a large amount within a few seconds. These swings across the flow-regime map kind of average out much of the time so the cumulative production over a day, week, or month is good enough for most applicaitons. A snapshot will always be worthless.
The best I can tell from Foxboro's web page, they have a straight-line tube which is less sensitive than a bent-tube meter (like Micromotion's), but I don't know that that is a bad thing--bottom line is that you are paying for a very expensive random-number generator that will give you data to 9 decimal places, so how could it be wrong?
It looks like Forboro has "solved" the prolem of multiphase flow in software. I haven't seen their algorythm, but I'm really curious about how they categorize a change in momentum (which they can measure very precisely) as a change in mass-flow rate at constant density or a change in density at a constant mass flow rate.
It is a very tough problem, and "simple" solutions will seldom work.
David
Thanks for the reply - instrumentation isn't my main discipline but I get dragged into it now and then. I'd spotted the Foxboro instrument in the trade press and it made me curious: it seemed to be doing what I with my limited understanding thought was more-or-less impossible, but thay have a reputation to think of so I guess it must be fairly good to be released onto the market.
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Sometimes I only open my mouth to swap feet...
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Montemayor
Chemical
Scotty:
Thank you for the notice from Foxboro. I share your enthusiasm - but with a tongue in cheek.
David:
I share your analysis. I seriously doubt what is being inferred (not said) in Foxboro's propaganda. I would dearly love to discover that they can, indeed, measure a 2-phase mass flow with the same accuracy that they can measure a 100% pure liquid flow. BUT they haven't said that.
Nor have they said a lot of other, important things in the same story. They haven't said anything about "accuracy" or the limits of measurement with respect to a given accuracy. They haven't even mentioned about their being approximately seven (7) different types of "2-phase" flow - and what about the orientation of those different types of flow? Does it make a difference whether the flow is horizontal or vertical? How about sloped? Of course it makes a difference! It makes a big difference.
But I still respect Foxboro. As David implies, as adult and experienced engineers we should be fully aware that there has to be a trade off waiting behind the curtain. Nothing comes free - certainly not when it comes to such a difficult problem like 2-phase flow. Like anyone else, Foxboro would simply rather not talk about all the possible short comings. And I can't say I blame them. I see a lot of potential problems - like David does - in the accuracy levels. But that subject is not about to discussed by manufacturers - at least not for now.
Until manufacturers are ready to address the subject of accuracy levels and other short comings we engineers have little reason to spend too much time on this subject - unless we are willing to experiment. But as David points out, you can kiss off on Natural gas producers ever participating in an experimental custody transfer measuring device.
Thank you for the notice from Foxboro. I share your enthusiasm - but with a tongue in cheek.
David:
I share your analysis. I seriously doubt what is being inferred (not said) in Foxboro's propaganda. I would dearly love to discover that they can, indeed, measure a 2-phase mass flow with the same accuracy that they can measure a 100% pure liquid flow. BUT they haven't said that.
Nor have they said a lot of other, important things in the same story. They haven't said anything about "accuracy" or the limits of measurement with respect to a given accuracy. They haven't even mentioned about their being approximately seven (7) different types of "2-phase" flow - and what about the orientation of those different types of flow? Does it make a difference whether the flow is horizontal or vertical? How about sloped? Of course it makes a difference! It makes a big difference.
But I still respect Foxboro. As David implies, as adult and experienced engineers we should be fully aware that there has to be a trade off waiting behind the curtain. Nothing comes free - certainly not when it comes to such a difficult problem like 2-phase flow. Like anyone else, Foxboro would simply rather not talk about all the possible short comings. And I can't say I blame them. I see a lot of potential problems - like David does - in the accuracy levels. But that subject is not about to discussed by manufacturers - at least not for now.
Until manufacturers are ready to address the subject of accuracy levels and other short comings we engineers have little reason to spend too much time on this subject - unless we are willing to experiment. But as David points out, you can kiss off on Natural gas producers ever participating in an experimental custody transfer measuring device.
WRONG WRONG ZDAS A coriolis meter meters MASS it does not infer it. The meter measures force caused by the Coriolis effect. There is NO inputs for density, gravity nothing is input. The computer will take the derivative of the forces and calculate a desity.
I've measure lots of products with the micromotion, it is great.
I've measure lots of products with the micromotion, it is great.
dcasto is right. For mass flow measurement, there is no extra input required. Density is also measured by the meter independantly and used to divide the mass flow to produce the volumetric flow which is correct for the mixture. The question of inaccuracy creeps in where zero correction for no flow is made as the tube(s) may contain a nonrepresentative gas/liquid mixture. Also the flowing mixed fluid viscosity is a function of gas/liquid ratio and affects the accuracy, when the ratio is not correctly known.
Best wishes
Best wishes
Wait a minute here. You say "the meter measures force caused by the Coriolis effect". As I recall, the units of force are Newtons or LBF. Doesn't it require some inference to convert LBF to LBM/time? I promise, the Coriolis meter DOES NOT MEASURE MASS DIRECTLY. It measures the metal response to a force applied by the fluid momentum and infers a flow rate from that.
Further, I've never seen a fluid densiometer that was reliable in two-phase flow. People construct density values from chromatograph results and know mole weight of the constituents. You can measure density of liquids with a scale and a known volume, but that is either a time consuming process or very inaccurate. It is even more difficult in two-phase flow.
I haven't installed one in 10 years and you could be right about there not being a density input anymore, there used to be a requirement to input fluid analysis and the software took that as constant and calculated density. Since the translation from force to volume flow is made up of a density term and a velocity term, I have to ask myself how the steel decides what part of a change in force is from the fluid density changing and what part is from the velocity changing. The answer used to be that it calculates a new density from the fixed fluid analysis and the current pressure/temperature.
When I was using a lot of Micromotion devices I got good results in reasonably constant-density service. The results in slugging flows or very gassy liquids were much worse (which is consistent with the articles referenced by Scotty above, see especially Process Instrumentation & Control: "Dirty Little Secret").
David
Further, I've never seen a fluid densiometer that was reliable in two-phase flow. People construct density values from chromatograph results and know mole weight of the constituents. You can measure density of liquids with a scale and a known volume, but that is either a time consuming process or very inaccurate. It is even more difficult in two-phase flow.
I haven't installed one in 10 years and you could be right about there not being a density input anymore, there used to be a requirement to input fluid analysis and the software took that as constant and calculated density. Since the translation from force to volume flow is made up of a density term and a velocity term, I have to ask myself how the steel decides what part of a change in force is from the fluid density changing and what part is from the velocity changing. The answer used to be that it calculates a new density from the fixed fluid analysis and the current pressure/temperature.
When I was using a lot of Micromotion devices I got good results in reasonably constant-density service. The results in slugging flows or very gassy liquids were much worse (which is consistent with the articles referenced by Scotty above, see especially Process Instrumentation & Control: "Dirty Little Secret").
David
Dear zdos04,
Please go through the following links:
for curved tube Coriolis meters
and for straight tube Coriolis meters
and
for density measurement by Coriolis meters.
Basically the phase shift between two frequencies is measured as a time difference and used to calculate the mass flow rate. Density is calculated as a function of measured natural frequency of a tube of constant volume which is independant of mass flow rate.
Best wishes
Please go through the following links:
for curved tube Coriolis meters
and for straight tube Coriolis meters
and
for density measurement by Coriolis meters.
Basically the phase shift between two frequencies is measured as a time difference and used to calculate the mass flow rate. Density is calculated as a function of measured natural frequency of a tube of constant volume which is independant of mass flow rate.
Best wishes
monaco8774
Petroleum
ASKME
I work in offshore oil and gas and we do have installed on some facilities what are clainmed to be multiphase meters.
Two that I know of but have no experience was South scott field (paper written OTC 8549) and Gulfaks (paper written OTC 8563) these both came out ion 1997 so quite old now
There is a multiphase flowmeter in use on Captain BLP, I used to work support for it some years ago. Basically it relied on having the flow in a known regime (annular mist or stratified etc) then they use (from memory) radioactive technicques to determine the C12 content and capacitance techniques to get the water content combined with corriolis for mass flow. there are a number of parameters that need to be tuned to the field and so you basically have to fall back on seperation and individual phase measurements for calibration (turndown was poor) and also from memory we never got any decent accuracy or repeatability out of the thing. However that was some years ago and I believe the technology has progressed quite far since then.
More recently these seem to be becoming very fashionable with three of five new facilities we are building calling for multiphase measurement subsea. Again a quick search shows another paper was delivered OTC 15174 in October 2003 on operational experience multiphase flowmeters on susea manifolds.
Personally I would have a back up plan as I think the problem of measuring multiphase fluids through a wide range of operating conditions is too hard
Good luck
I work in offshore oil and gas and we do have installed on some facilities what are clainmed to be multiphase meters.
Two that I know of but have no experience was South scott field (paper written OTC 8549) and Gulfaks (paper written OTC 8563) these both came out ion 1997 so quite old now
There is a multiphase flowmeter in use on Captain BLP, I used to work support for it some years ago. Basically it relied on having the flow in a known regime (annular mist or stratified etc) then they use (from memory) radioactive technicques to determine the C12 content and capacitance techniques to get the water content combined with corriolis for mass flow. there are a number of parameters that need to be tuned to the field and so you basically have to fall back on seperation and individual phase measurements for calibration (turndown was poor) and also from memory we never got any decent accuracy or repeatability out of the thing. However that was some years ago and I believe the technology has progressed quite far since then.
More recently these seem to be becoming very fashionable with three of five new facilities we are building calling for multiphase measurement subsea. Again a quick search shows another paper was delivered OTC 15174 in October 2003 on operational experience multiphase flowmeters on susea manifolds.
Personally I would have a back up plan as I think the problem of measuring multiphase fluids through a wide range of operating conditions is too hard
Good luck
Reena1957,
It has been 3 decades since taking orders was part of my job description. Today they tend to make me stubborn, but I did read a couple of the CBT modules you pointed me towards.
The ingenuity of Engineers never ceases to amaze me. To think that you can infer a total-stream density from a measured vibration frequency is pretty cool. But they are "measuring" frequency and using some lab data to back into a density. This is just like a square-edged orifice meter using P, T, and dP to pretend they know a volume flow rate. If the units of the measurement are anything but mass/unit-volume then you are not "measuring" density, if the units are anything but mass/unit time then you are not "measuring" mass flow, etc.
There is absolutely nothing wrong with using a surrogate measurement to infer a physical parameter, we do it successfully all the time. The one thing that I ask my clients to realize is that the surrogate may have potential for error that is completely isolated from the parameter we're inferring--for example, if you get scale in a Coriolis tube how will the vibration and/or the phase shift be affected? Is there any way to adjust the calibration for salt, upstream pipe scale migration, etc? I always try to make sure that we thorough understand potential effects of the things that can go wrong prior to installing a new inferential device. Having said that, I've installed many Micromotion devices over the years with good-enough results.
What I often wonder about is the infinite number of discrete combinations of gases and liquids that can result in any particular total-stream density--which one does the program use to characterize a gassy liquid flow?
David
It has been 3 decades since taking orders was part of my job description. Today they tend to make me stubborn, but I did read a couple of the CBT modules you pointed me towards.
The ingenuity of Engineers never ceases to amaze me. To think that you can infer a total-stream density from a measured vibration frequency is pretty cool. But they are "measuring" frequency and using some lab data to back into a density. This is just like a square-edged orifice meter using P, T, and dP to pretend they know a volume flow rate. If the units of the measurement are anything but mass/unit-volume then you are not "measuring" density, if the units are anything but mass/unit time then you are not "measuring" mass flow, etc.
There is absolutely nothing wrong with using a surrogate measurement to infer a physical parameter, we do it successfully all the time. The one thing that I ask my clients to realize is that the surrogate may have potential for error that is completely isolated from the parameter we're inferring--for example, if you get scale in a Coriolis tube how will the vibration and/or the phase shift be affected? Is there any way to adjust the calibration for salt, upstream pipe scale migration, etc? I always try to make sure that we thorough understand potential effects of the things that can go wrong prior to installing a new inferential device. Having said that, I've installed many Micromotion devices over the years with good-enough results.
What I often wonder about is the infinite number of discrete combinations of gases and liquids that can result in any particular total-stream density--which one does the program use to characterize a gassy liquid flow?
David
Dear David,
I appreciate your point of view and even more your open mindedness in going thru those links.
The answer to your last question is that there is no particular density used - it is a range between air and water and the actual density is interpolated in between. Of course it will fail in case the density of the mixture falls outside this range.
Best Wishes
I appreciate your point of view and even more your open mindedness in going thru those links.
The answer to your last question is that there is no particular density used - it is a range between air and water and the actual density is interpolated in between. Of course it will fail in case the density of the mixture falls outside this range.
Best Wishes
A useful reminder, zdas04, not to assume too much and to revert back to first principles occasionally, especially when moving outside the comfort zone.
Yes, often what we actually measure is some other property. It isn't always possible to measure what we want to measure so we measure something that we can measure and we characterise the variation of this property against the property we are interested in.
I know my mobile phone works and most times that is enough and quite how actually works, I have no idea. Wherever I am the system finds me and I get my calls and not someone else's.
Do I need to know any more? No, unless, of course, I move outside the normal parameters and I go to the US.
I assume that because it works in Europe that it will work on the US and that isn't a given. It depends on whether I have quad band or whatever.
The same for instruments. I suspect fewer people today try to understand exactly how coriolis meters or density meters actually work because they no longer have that credibility gap to overcome. They work and all any one seems to worry about are features, the physics is accepted.
So we don't actually measure density, we measure resonant frequency.
That means we have to calibrate the resonant frequency against a lab value.
Some density meters are calibrated against picnometers and some against master meters.
Some receive a single point calibration and a fudged offset. Some receive a two fluid calibration (and one of them is air) others a three point cal (sometimes one is air sometimes a liquid).
What does this matter?
Well, the frequency doesn't vary linearly with the density. In fact, the resonant frequency actually varies with the effective mass of the vibrating system which includes the air on the outside, the effective mass of the sensor and the mass of the fluid it contains.
The sensor mass varies only if it is eroded or coated with something.
The fluid mass changes with the volume of the containment which can vary with temperature and with pressure. High pressure can also change the actual density of the fluid. Some sensors are more sensitive to the fluid viscosity than others and to velocity of sound effects and so on.
Oh yes, it also varies with the fluid density.
A variation is used for gases.
This technique is one of the methods used in aircraft altimeters. The vibrating spool sensor was, at one time, used as a reference for pressure sensor calibration and a popular demo on open days was to use it to measure people's heights. Actually, to infer peoples heights and it had to be frequently recalibrated to compensate for atmospheric pressure changes.
OK, height, pressure, density..... what you measure is the resonant frequency. What you infer depends on what you want to know. So this same sensor is sued as part of an altimeter and to determine gas SG and gas density.
Now, to the point of the original post: some (many) vibrating element sensors for liquids are very vulnerable to entrained air.
Some, but not all.
One tube density meter is available for entrained air applications which means it can handle 100% liquid through to 100% gas (the trade off is accuracy). Some significant advances have been made in measuring gas entrained fluids with coriolis meters but the trade off is accuracy. (BP recently ran some trials with a 12" Coriolis on fuel oil and the accuracy fell off quite a bit from the single phase fluid measurement).
But this is a big advance; in the early days, just a few bubbles and you lost the whole measurement.
So how you approach a multiphase flow situation is problematic. You may take quite a hit on accuracy but does it justify itself on cost?
JMW
JMW,
Not sure what your point is. My point was a comment on dcasto's
Back to the OP, I don't see a path to evolve any of today's instruments into custody-transfer measurement of multi-phase flow. If there is ever a reliable device that provides +/-0.5% accuracy with excellent repeatability, I belive it will come from some technology that either doesn't exist today or is way out of the mainstream today.
I could be wrong, it wouldn't be the first time today, but I don't see any of today's technology being able to give me a high-quality, repeatable charactarization of how many SCF's of gas and lbm of liquid went down the line in a given time period.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
Not sure what your point is. My point was a comment on dcasto's
and I was trying to say that I didn't think so. Virtually every device I work with is infering one property from the instrument's response to another property (e.g., an RTD measures change in resistance and reliably infers a temperature change from the resistance readings). Even an old-fashined Bourdon tube pressure gauge is infering a pressure from the distance a curved tube straightens out. These are all very reliable devices.
Back to the OP, I don't see a path to evolve any of today's instruments into custody-transfer measurement of multi-phase flow. If there is ever a reliable device that provides +/-0.5% accuracy with excellent repeatability, I belive it will come from some technology that either doesn't exist today or is way out of the mainstream today.
I could be wrong, it wouldn't be the first time today, but I don't see any of today's technology being able to give me a high-quality, repeatable charactarization of how many SCF's of gas and lbm of liquid went down the line in a given time period.
David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
Technically nobody measure anything. We calibrate back to The "stick and block" locked up in Boulder Colorado. A scale does not measure mass, it measures FORCE and we divide by A to get mass. So, when I stated that the coreolis meter measures mass, it traces it's output via NIST proceedure directly to that little block in Boulder. It does not depend on any other inference, look up tables, jock factors, or intervention.
Zdas, You are correct about two phase flow, the meter will tell you exactly how much mass went by, but if you want that mass on a per componet basis, then to the lab or on line qualitative equipment. To me, if you measure the mass of a two phase HC going into a line, who cares the ratio (except designers) because its all going to change when it comes out, the same mass, but different ratios of vapor to liquid.
Zdas, You are correct about two phase flow, the meter will tell you exactly how much mass went by, but if you want that mass on a per componet basis, then to the lab or on line qualitative equipment. To me, if you measure the mass of a two phase HC going into a line, who cares the ratio (except designers) because its all going to change when it comes out, the same mass, but different ratios of vapor to liquid.
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