FUNDAMENTALS OF CORIOLIS METERS (FOR GAS MEASUREMENT) · 2015-03-02 · 10% of the worldwide market...

2001 PROCEEDINGS PAGE 37AMERICAN SCHOOL OF GAS MEASUREMENT TECHNOLOGY

gas composition must be accurately measured orcontrolled, the devices regularly maintained (orifice plateschecked, turbine bearings rebuilt), and adequate flowconditioning provided for profile-sensitive technologies.Since Coriolis measures the flowing mass of the gas,and accuracy is independent of composition and flowprofile/swirl, the meter is more accurate under a widerrange of operating conditions and is often lower cost toinstall and maintain.

Coriolis is a smaller line-size technology: the largestoffering from any vendor for gas applications is a 6” meter(handling natural gas flows up to 8-10” lines). While capitalcost, installed cost, and “cost of ownership” of newerdesigns are very low for this technology (e.g. MicroMotion’s R-series™ is under $2800 in 1/4”, 1/2”, and 1” sizesand $3700 for 2”), Coriolis cannot serve all applications.

Micro Motion R-series™ (1/4” to 2”)

Installed R050 (1/2”) meter, preferred installation for gas(tubes “up,” prevents accumulation of condensate)

Coriolis meters are very cost competitive with othermetering technologies on an installed cost basis, whereinstalled cost includes:

• Instrument purchase price• Temperature and pressure compensation

(volumetric technologies)

ABSTRACT

Coriolis meters have gained worldwide acceptance inliquid applications since the early 1980’s with an installedbase or more than 350,000 units. Newer designs haveshown greatly improved low-flow sensitivity, lowerpressure drop, and immunity to noise; factors which nowenable their successful use in gas-phase fluidapplications. With more than 20,000 units on gas aroundthe world, groups including the AGA, API, MeasurementCanada, German PTB, and Dutch NMi are all involved inwriting standards for this “emerging” gas flow technology.

An overview of theory, selection, installation andmaintenance of Coriolis meters will be presented.Application details will be presented to illustrate boththe range of natural gas applications, includingproduction, fuel flow control to gas turbines, mastermetering, and city/industrial gate custody transfer as wellas third-party test data. Laboratories include theColorado Engineering Experiment Station Inc. (CEESI),Southwest Research Institute (SwRI), and Pigsar(Germany).

INTRODUCTION

Coriolis is one of the fastest growing technologies, andgrowth in gas phase applications is approximately fourtimes faster than for liquid applications. Older designswere known to have some fairly well-justified limitationsfor use on gas; in general, a relatively high pressure drop(around 1000” H2O) was required to obtain a highaccuracy flow reading, and large meters (3”-4” meter)did not work well due to sensitivity to noise and effectsof process pressure. Since the market was rapidlygrowing, Coriolis vendors focused mostly on liquidapplications. Note that the extensive range of liquidapplications will not be addressed here. Please contactthe authors for more information, or log ontowww.micromotion.com.

Newer designs and technology developments since theearly 1990’s have changed this, allowing accurate gasflow measurement for even low-pressure gases (50-100psi). Sensitivity has been dramatically improved, andpressure drop lowered (a typical 500 psi distributionapplication can now be sized as low as 90” wc ). All inall, it can be argued that Coriolis technology solves moreproblems and offers even more value for gas than liquidmeasurement. This is because gases are compressible,and with traditional technologies (dP/orifice, turbine,rotary, diaphragm), process pressure, temperature, and

FUNDAMENTALS OF CORIOLIS METERS(FOR GAS MEASUREMENT)

Tom O’Banion, Micro Motion, Inc.Karl Stappert, Daniel Industries, Inc.

2001 PROCEEDINGSAMERICAN SCHOOL OF GAS MEASUREMENT TECHNOLOGY

• Flow conditioning and straight runs (profilesensitive technologies)

• Flow computer (volumetric technologies) formass flow & standard volume calculations

Application “sweet spots” include:

• Line sizes 8-10” and smaller (single 6” meter, orsmaller meters in parallel)

• High turndown requirements (20:1 up to 50:1 iscommon), eliminating parallel metering runs ofother technologies

• Dirty or wet gas where maintenance can be anissue

• No room for adequate straight-runs (re: AGA-3revision)

• Changing gas composition and density• Critical phase fluids such as Ethylene (C2H4) or

Carbon Dioxide (CO2), where volumetric metersare very expensive, especially where custodytransfer or process control is mass based

Currently, as measured by flowmeter units sold, around10% of the worldwide market for Coriolis meters is forgas phase applications. This is in a flow market that isapproximately one-fourth (26%) gas, not including steam(Process gas is thought to be approximately 16% withNatural gas being 10%, and steam being 10%). Coriolisis primarily a single-phase flowmeter, although promisingearly results from wet-gas testing will be mentioned.

Coriolis offers an improved primary element, with familiaroutputs. Much like liquid petroleum applications, usersdesire improved reliability and accuracy, but in traditionalunits such as MMscfd.

CORIOLIS STANDARD OR NORMAL VOLUMEOUTPUT:

Coriolis technology measures the mass of fluid (gas orliquid) flowing through the primary element. For liquidapplications, the on-line density from the Coriolis meter(similar to other vibrating element densitometers) is usedto output actual volume. This is useful for fiscal transfersof liquid petroleum, and if often corrected to baseconditions, such as barrels of oil at 60 deg F.

For gas applications, the meter output can be configuredfor familiar standard or normal volumetric flow units, suchas MMscfd or NM3/hr. The on-line density from the meter

is not used; rather the standard or normal density of thegas is entered into a flow computer from either a sampleor on-line analysis, using a gas chromatograph (GC).Coriolis technology uses the following approach tooutput a highly accurate standard or normal volumetricoutput, in common usage throughout much of the world:

Somewhat like multipath Ultrasonic technology a fewyears ago, Coriolis “caught the eye” of the natural gasindustry. Now, work is actively underway by (amongothers) Task Group #11 of the AGA’s TransmissionMeasurement Committee (TMC). As of the time of thiswriting (June 2001), the “for publication” Tech Note hasbeen submitted to AGA.

Coriolis has been used since the late 1970’s for liquidprocess applications, and has now been used since 1992for process gas with more than 10,000 installed units.Another 10,000 have been used for Compressed NaturalGas (CNG), natural gas at 3000+ psi for vehicle fueling.

This paper will discuss why the technology is now a bonafide option for natural gas applications. Status of majorworldwide standards will be presented, with an emphasison the Americas and Europe, plus a sampling ofapplications from “wellhead to burner tip.”

HOW THE TECHNOLOGY WORKS: THEORY OFOPERATION

A Coriolis meter is comprised of two main components,a sensor (primary element) and a transmitter (secondary).Coriolis meters infer the gas mass flow rate by sensingthe Coriolis force on a vibrating tube or tubes. Theconduit consists of one or more tubes and is forced tovibrate at a resonant frequency. Sensing coils locatedon the inlet and outlet sections of the tube(s) oscillate inproportion to the sinusoidal vibration. During flow, thevibrating tube(s) and gas mass flow couple together dueto the Coriolis force, causing a phase shift between thevibrating sensing coils. The phase shift, which ismeasured by the Coriolis meter transmitter, is directlyproportional to the mass flow rate.

Note that the vibration frequency is proportional to theflowing density of the fluid. For gas applications, theflowing or “live” density is not used (standard or normalvolumetric flow rate is based on gas composition, butnot conditions of P and T). For liquids, the “live” densityis often used.

g g

Precision Flow Market $2.6B

(1.2 million units/yr)

Liquid

Gas (26%)

Steam

Coriolis Industry $300M/yr

(50,000 units/yr)

Gas (10%)

Standard or Normal

volume flow

Gas standard density

MMI

Transmitter

FlowComputer

Mass

QSTP = QMass / standard density

Q (scfm) = Q (lb/min)/ (lb/ft3)

Input by userMeasured

by MMI

Flow Computer


Note the transmitter can either be integral or remote,depending on the design. The transmitter provides bothdrive power to sustain the sensor resonance as well asinterprets and configures the Coriolis signal.

For a more complete discussion of the Coriolis theory ofoperation, please see Micro Motion’s website at:

www.micromotion.com (TUTOR™)

STANDARDS WORK AND APPROVALS

Coriolis meters have long been used for process control,and a number of worldwide approvals or documents existfor fiscal (custody) transfer of liquids. These include:

• USA NIST C.O.C.• USA API• German PTB• Dutch NMi• Numerous other countries, including Canada,

Switzerland, Belgium, Austria, and Russia

Beginning in the mid-1990’s, some of these groups andthe industry also began studying the technology forgaseous applications. The German weights and measuresgroup (PTB) extended custody transfer approval to includeboth gas and liquid phase fluids in 1999. As well, Dutchweights and measures (NMi) has performed testing andpublished a statement that the flow calibration factorestablished on water transfers without field calibrationto gas phase applications, within a certain tolerance.

In spring of 2001, Measurement Canada granted MicroMotion Coriolis meters approval for use in fiscal transferof natural gas.

Copies of these approval documents are available fromthe author/speaker.

Shown below are two recent calibration curves on 3”custody transfer meters (model CMF300). These arebeing used in “Industry Gate” applications in Australia.

Laboratory is Pigsar-Dorsten, with natural gas at 725 psi.Flow rates ranged from 350 to 7300 scfm (0.5 to 10MMscfd). Accuracies were better than ±0.2% over the20:1 test range.

Right

Left

∆ t

Right Pickoff Coiland Magnet

Flow Tubes

Drive Coiland Magnet

Case

RTD

Process ConnectionFlanges

Leftt Pickoff Coiland Magnet

Process ConnectionFlanges

��


Most exciting to the natural gas industry in the“Americas” and world areas influenced by “Americas”expertise, is Task Group #11 of the AGA TMC. At thetime of this writing (June 2001), a Tech Note coveringCoriolis meters for custody transfer of natural gas hasbeen submitted to AGA for publication. It is expectedthat this document will be available before the date ofthe American School (Sep 2001).

As well, testing work toward a formal report is underwayat SouthWest Research Institute in San Antonio, TX. Afterbaseline testing (Coriolis meters with no flowdisturbances and clean, dry gas), the two main focusareas of study are:

1) Flow Profile: Establish if Coriolis is trulyinsensitive to flow profile and swirl. Test matrixincludes “AGA-3 like” testing for single anddouble elbows in and out of plane. Early resultsfrom Micro Motion show there is no effect at leastwithin 5d; one researcher (Dr. Umesh Karnik ofTCPL/NOVA) found some profile dependence,as reported at the 4th International Fluid FlowSymposium (July, 1999 Denver, CO USA). OtherCoriolis vendors have indicated some smallprofile sensitivity, so this testing is critical to theestablishment of a technical note and/orstandard.

Results from a 1995 study (T. Patten; North Sea FlowWorkshop) using hot water. Within a few pipe diametersof the primary element, no effect of flow profile or swirlwas found.

Gas testing at CEESI during product development ofMicro Motion’s R-seriesTM is shown in the prior column.Note the installation details: meters are mounted flange-to-flange and a Micro Motion ELITE™ is used as areference meter.

2) Wet Gas: The industry is very interested toquantify the amount of entrained liquid Coriolistechnology can handle, while maintaining goodaccuracy. Initial results from a consortium studyin Canada indicate good tolerance for entrainedliquids, but additional testing is necessary anddesired. Produced or wellhead gas are the targetapplications.

The AGA work focuses on two “sweet spots” for Coriolis:

1. Medium to high pressure distribution meteringpoints. These city-gates or industry-gates oftenhave high turndown requirements, and newinstallations must meet the recently revised,more stringent AGA-3 straight run requirements,adding additional cost to using traditionalmeasurement technologies. As well, Coriolis’proven stability over time offers reducedmaintenance and field proving/verification.

2. Wet gas measurement. If Coriolis is capable ofmeasuring wellhead or produced gas, prior toprocessing, significant operational savings arepossible.

SIZING AND SELECTION

Selection of a Coriolis meter for gas applications is quitestraightforward but different than traditional technologiesused on natural gas, such as orifice and turbine meters.There are two reasons for this; one being that Coriolis isavailable in discrete sizes (like turbine or rotary); thesecond being that the Coriolis meter can be sized for amuch higher pressure drop than is the industry norm.This can be useful as it increases useable turndown.

Coriolis can be installed upstream of a pressure regulator,resulting in a smaller and less expensive primary (sensor)and increased turndown.

Coriolis flow meters for gas measurement are currentlyavailable in line diameters from 1/4 to 6 inches. There arethree major considerations when sizing a Coriolis meter:

1) Pressure loss,

2) Velocity of the gas inside the sensor, and,

3) Total meter error.

PRESSURE DROP

The sensor geometry, gas density and velocity determinethe permanent pressure loss through the meter.

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

10000 100000 1000000

Reynolds Number

Ma

ss

Flo

w R

ate

Err

or,

%

Baseline

Single El

Double El

Single El

Double El


Pressure loss is also determined by any pipe fittingsrequired for meter installation. Pipe reducers, valves, andadditional straight pipe requirements should beconsidered when calculating the loss in pressure for theselected meter.

VELOCITY

Some Coriolis meters have performance limitations athigher gas velocities due to noise imposed on the metersignal. Such signal noise can affect meter accuracy andrepeatability. The gas velocity at which signal noisebecomes a problem is design (vendor) specific. Seldomis signal noise a concern when the gas velocity in themeter is below about 200 ft/s. A Mach number limit isusually provided by the meter manufacturer to definethe maximum recommended velocity.

If abrasive contaminants are present in the gas flowstream, erosion of the wetted meter components maybe a concern when the meter is exposed to high gasvelocities. This effect is design specific.

METER ERROR VS. FLOW RATE

Meter error versus flow rate is determined from aperformance curve, similar to the one shown below. Theerror versus flow rate curve is based on the results oflaboratory calibrations. Most manufacturers state theprobable meter error as a percentage of rate, plus the zerostability value. Third party meter error data are referencedin Appendix B. The error is typically expressed as:

% Error = ± (0.50% ± (zero stability/flow rate) x 100)

The base error value (0.50%) in equation (9) was chosenfor illustrative purposes. The actual meter error can beestablished from laboratory calibration. This shouldinclude the effects of laboratory uncertainty, linearity,hysteresis, and repeatability.

ZERO STABILITY

The zero stability value defines the limits within whichthe meter zero may drift during operation, and is constantover the operating range (all other affecting parametersfixed). It may be given as a value in flow rate units, or apercentage of a stated nominal mass flow rate. The zerostability value is the limiting factor when establishingmeter turndown ratio. The stated zero stability value isachievable when the Coriolis flow meter is installed, andre-zeroed at operating conditions.

Because process temperature or pressure changes, and/or, environmental temperature changes will affect themeter zero stability, the estimated value of the zerostability is usually limited to meters at thermal andpressure equilibrium. Limits for changes in theseparameters may be given, which when exceeded, willrequire a re-zeroing of the meter to reestablish thespecified zero stability.

TEMPERATURE AND PRESSURE COMPENSATION

Both pressure and temperature affect the meter vibrationcharacteristics, hence the magnitude of the sensedCoriolis force. In comparison to zero stability, these effectsare small, but should be compensated for to achieveoptimum meter performance. Most meter designscompensate for temperature effect automatically, usingthe temperature of the tube(s), and for pressure effectseither continuously using an external pressure transmitter,or by entering a fixed adjustment for the known pressure.

Other meter designs periodically check meter sensitivityby applying a waveform reference force to the tube(s),during field operation, and compare the system responseto that achieved under reference flowing conditions. Thissystem will compensate for both pressure andtemperature effects.

Errors and compensation methods for pressure andtemperature effects should be stated in the meterperformance specifications, and included, if necessary,when establishing meter performance for sizingconsiderations. Errors are estimated from data for eachmeter design and with the use of error propagationmethods.

TURNDOWN RATIO

Flow meter turndown ratio is the ratio of the acceptablemaximum mass flow rate to the acceptable minimummass flow rate. The turndown ratio is application specificand dependent on gas conditions, allowable pressureloss across the meter, and allowable meter error.

The maximum pressure loss (at maximum flow rate)across the meter can be determined once the meterdiameter, piping installation configuration, and maximumallowable gas velocity are specified. Typically, the meterselected is one line diameter smaller than the size of the

Meter Error and Pressure Loss Versus Flow Rate

Flow Error and Pressure Loss vs. Flow Rate, 3" Bending Mode Coriolis Meter

Gas Conditions: Gr = 0.59, Tf = 60 F

-3.00

-2.00

-1.00

0.00

1.00

2.00

3.00

0 200000 400000 600000 800000 1000000

Flow Rate, SCFH

Flo

w R

ate

Err

or,

%

0

100

200

300

400

500

600

Pre

ssure

Lo

ss, in

che

s H

2O

Error limit

Pressure loss-1000 psig

Pressure loss-500 psig


pipe in which the meter is installed. This usually providesmore accurate measurement at lower flow rates.However, the resulting permanent pressure loss for agiven flow rate is higher than if the meter diameter is thesame as the pipe diameter. Because higher pressure gashas a higher mass flow rate for the same velocity, higherpressures will produce higher flow turndowns for thesame meter arrangement.

A family of curves can be generated showing flowturndown for different gas pressures. The figure abovepresents example values of this relationship for apressure loss of 1% of pipe pressure at the meter inlet.

INSTALLATION & MAINTENANCE

Mounting

Proper mounting of the sensor is required. Considerationshould be given to the support of the sensor, thealignment of the inlet and outlet flanges with the sensor.A spool piece should be used in place of the meter toalign pipe-work during the construction phase.

Piping should follow typical industry piping codes. Meterperformance, specifically zero stability, can be affectedby axial, bending, and torsion stresses from pressure,weight and thermal effects. Utilizing properly alignedpipe-work and properly designed supports can minimizethese stresses and associated loads.

Mount the Coriolis transmitter so that it may be easilyaccessed to attach communications equipment, to viewdisplays, and to use keypads. Coriolis meters areconfigured in two basic ways – the transmitter mountedto the sensor or the transmitter mounted remotely.

Orientation

As a general rule, orient the sensor tubes in such a wayas to minimize the possibility of “settling” heaviercomponents, such as condensate, in the vibrating portion

Error at Minimum Flow Versus Flow Turndown Ratio

of the sensor. Solids, sediment, plugging, coatings ortrapped liquids can affect the meter performance,especially when present during zeroing. Allowablesensor orientations will depend on the application andthe geometry of the vibrating tube(s).

Fluid Swirl and Flow Profile Effects

The effect of fluid swirl and non-uniform velocity profilescaused by upstream and downstream pipingconfiguration on meter performance differs from onemeter design to another. Flow conditioning and straightupstream and downstream piping lengths may or maynot be required. Further testing is recommended toevaluate these potential effects.

Effects of Contaminants, i.e. Compressor Oil, Liquidsand Free Mists

Testing has shown that liquid carried in a gas stream maynot have the same adverse affect on performance as gascarried in a liquid stream. However, the meter will measurethe mass flow rate of the total flow stream, including theliquid i.e., condensate, glycol and compressor oil. Theallowable liquid fraction will depend on the applicationand sensor geometry. Care should be taken to removeliquid slugs before measuring the gas flow.

Vibration and Fluid Pulsation

During product development, extensive analysis andtesting have resulted in meter designs that are inherentlystable under a wide range of “normal” plant vibrationconditions.

Operation and Maintenance Considerations

Other than the vibrating sensor element(s), Coriolis metershave no moving parts, requiring minimal maintenance.

There are three common types of field checks whichinclude meter zero, sensor checks, and transmitterchecks.

COMMON FIELD CHECKS

Meter Zero

Should be checked periodically and adjusted if it doesnot meet the manufacturer’s specifications. If the meterzero check shows degradation, then the following checkshould be performed. Physical internal inspection maybe needed.

Sensor Checks

Product buildup, erosion or corrosion will affect the meterperformance.

Product buildup (coating) may bias the meter zero. If thebuildup is causing a zero drift, cleaning and re-zeroing

Error at Minimum Flow vs Turndown

3" Bending Mode Coriolis Meter, Natural Gas, Gr = 0.59, Tf = 60 F

Pressure Loss at Max Flow = 1% of Line Pressure

0

20

40

60

80

100

0.5 1.0 1.5 2.0 2.5 3.0

Error at Minimum Flow, Percent

Flo

w T

urn

dow

n

1480 Psig

1000 Psig

500 Psig

100 Psig


the meter should bring performance within specification.If coating of the sensor continues, the zero will continueto drift.

Although rare, erosion or corrosion will permanently affectmeter calibration and will compromise sensor integrity.When used within the specified fluid and ambientcondition limits, fatigue of the sensing tubes of a Coriolismeter due to vibration during the stated meter lifetime israre and does not need to be considered when inspectinga meter. However, operating the meter in more extremecorrosive or erosive applications will shorten theexpected lifetime.

Secondary Element (Transmitter)

Diagnostic LED(s) and display may be provided toindicate operating status of the primary and secondaryelements. See the manufacturer’s documentation fordetailed description of secondary element diagnostic andtrouble shooting procedures. In addition, refer to APIManual of Petroleum Measurement Standard (MPMS)Chapter 21.1 for security issues.

Density Checks

As of this writing, operating density measured by themeter should not be used to convert mass flow rate tovolume flow rate. However, it is useful as a diagnostictool to monitor changes in meter performance oroperating conditions.

Checking and Adjusting Meter Zero

Improper zeroing will result in measurement error. In orderto adjust the zero of the meter there must be no flowthrough the flow sensor, and the sensor must be filledwith gas at process conditions. The meter zero must beestablished at process conditions of temperature,pressure and density. Even though the stream is notflowing, the flow meter may indicate a small amount offlow, either positive or negative. Causes for the zero errorare usually related to the differences between thecalibration conditions and the actual installation, whichinclude but are not limited to the following:

• Differences between the calibration mediadensity and the gas density

• Differences in temperature• Differing mounting conditions

The meter should read a mass flow rate that is less thanthe manufacturer’s zero stability specification under theno-flow condition.

The zeroing of the meter must be performed at nominaloperating condition with no flow through the meter. Onceit has been confirmed that there is no flow through themeter, the zeroing procedure specified by the metermanufacturer should be followed.

APPLICATION EXAMPLES

Coriolis meters have been used in a wide variety ofapplications, from the “wellhead to the burner tip”.Coriolis meters are primarily a smaller line size meter,ideally suited to these “sweet spots”:

• Line sizes 8-10” and smaller• High turndown requirements• Dirty, wet, or sour gas where maintenance can

be an issue with other technologies• There is no room for long straight-runs• Changing gas composition and density

Coriolis meters can be sized for very low-pressure drop(100” H2O), but can also be installed upstream of thepressure regulator for increased useable turndown. Forinstance, in one application for custody transfer ofnitrogen, a 50-psid drop (2000” H2O) was taken throughthe primary element, and the pressure regulator adjustedaccordingly. This allowed the use of a 1” primary elementinstead of a 3” element, and a 40:1 useable turndown(better than 2% accuracy at minimum flow). The onlyother alternative was two to three parallel orifice runs atmuch greater installed cost ($8k vs. $20k).

Test/Production Separators: The application shownbelow is a “before and after” scenario. Coriolis meterson both the liquid (oil/water) and gas streams streamlinedthe separator design, saving over $100k in design,engineering, and fabrication. As well, numerous parallelorifice runs were eliminated by the superior turndown ofthe Coriolis meter.

Saudi Aramco Separator Gas: Saudi Aramco uses anumber of Coriolis meters on both the liquid and gasside. This application is of particular note becausethe gas stream is wet with entrained condensate.Measurement of this stream is within a few percent overa wide range of conditions, greatly enhancing separatoroperation.

Fuel Control: A major US vendor of gas turbines designsa high-efficiency, low emissions offering. This designutilizes a trio of Coriolis meters to measure the naturalgas burned in each of three combustion zones (fuel


“rails”). The combination of high turndown, high accuracy,immunity to vibration in a very high vibration environment,along with ease of installation due to no straight pipe runrequirement, makes Coriolis technology a perfect fit. Notethat this fuel-consumption application is a non custody-transfer application, allowing the use of Coriolis meters inthe field, without any formal AGA standard or guideline.

Natural Gas Fiscal Transfer ExampleOne specific example of gas measurement capability isat a natural gas utility in Western Australia. Two 3” metersare used in parallel with a third used as a “hot spare”.

The justification for using the Coriolis meters was basedon installed and calibration/maintenance costimprovements over the more traditional turbine meteringsystems. Since Coriolis meters require no straight runsor flow conditioning the installed costs were reduced by

five times, even with the parallel meters required tohandle the highest flows.

Additionally, periodic maintenance costs were muchreduced due to the intrinsic reliability of Coriolis meters(i.e. no moving parts). Similarly, reliability improvementshad a very positive effect on calibration and proving costs.

Internal checks by the customer have shown agreementto better than 0.1% on all gas transfers. The meters havebeen installed and operating for over four years: 48months of proving data is being requested from this utilityas part of the AGA TMC#11 effort.

PROVING

The data shown in Figure 1 was taken on natural gas,but the meter was calibrated (i.e. the meter factor was

Coriolis meters on low NOx gas turbine for pipelinecompressor

Western Australia: Previous installation using turbinemeters for 50:1 turndown

“After” installation since 1996, with two operating andone “hot spare” meter for 80:1 turndown. Custody

transfer between utility and cogeneration project. 0.3 –24 MMSCFD at 500 psia


established) on water at the factory. Based on anextensive database of water vs. gas calibration data,there is no change in calibration between water and gas.In addition, a history of over 250,000 installed meters onliquid and gas indicates no change in meter factor overtime (barring corrosion or erosion issues).

Since proving any gas meter in-situ is difficult, the stabilityof Coriolis meters makes them ideal for use on gas. Byutilizing the transferability of water calibration to gas andthe meter stability over time, an extremely accurate andstable metering system can be established. The followingmethodology was proposed by the Australian utility inthe previous example to establish traceability for high-value gas transfers:

1. Establish the meter factor on water2. Validate the meter factor on gas (i.e. natural gas

at Pigsar)3. Periodically remove the meter from service and

verify the meter factor on water

Although this methodology requires that the meter beremoved from service, it defines very accurately the in-situ performance of the meter. Since steps 1 & 2 establishthe meter traceability between water and gas, verifyingwater performance in step 3 automatically validates themeter in-situ (gas) performance. After some experience,it is likely that the period to repeat step 3 would belengthened from every year to every two or three years.

A variation of this proving methodology is to use a Coriolismeter as a master meter. By establishing the traceabilitybetween water and gas measurement on the mastermeter, it can be used to prove other meters (of any type).Figure 3 shows a 1” Coriolis meter being used as an in-situ master to prove an in-line turbine meter. Since inthis example the master meter is moved from oneinstallation to another validating the measurement onwater periodically can be easily accommodated.

Distribution metering: Of special interest to the AGAworking group are medium to high-pressure distributionapplications as shown below. In this example, naturalgas at 1000 psig is transferred, and requires Modbuscommunications protocol.

Energy Metering: Coriolis meters have the potential tobe excellent energy meters, with or without on-linechromatography. Three basic levels of energy meteringexist, as follows:

1. Small lines, where volumes do not justify on-line GC. Coriolis meter alone can be used tomeasure energy content typically within 1-3%.

2. Medium lines, where volume justifies an on-lineGC. Single or parallel Coriolis meters can be tiedto the GC, per the sketch below:

3. Large lines, with on-line GC and multipathultrasonic meter.

Energy per scf varies tremendously, depending onmolecular weight, with ethane having almost twice theenergy content of methane. If energy is measured per unitmass, it can be seen that energy varies only 4%. For naturalgas energy metering, if composition is relatively constant(especially of inerts such as nitrogen), the Coriolis meter byitself offers a very affordable method of measuring energytransferred. In this application, the meter’s output isconfigured to represent energy per time (e.g. BTU/hr)instead of mass or standard volume (lb/hr or scfh).

FIGURE 3. Master meter proving of a turbine meterusing a 1” Coriolis meter.

Distribution metering: 3.5MMscfd, 1000 psia, ModBus,high turndown

GC

Flow Computer

Energy flow

(BTU/hr, etc)

Mass/time

E/mass

Key advantage is that mass measurement is more accurate

Core Processor

“Puck”

Air required for CombustionHeat of Combustion

BTU / scf BTU / lb scf air / scf fuel lb air / lb fuel

M ethane 911 21600 9.6 17.2

E thane 1630 20500 16.8 16.1

P ropane 2360 20000 24.3 15.7

n-Butane 3110 19700 32.1 15.5

H ydrogen 273 51900 2.4 34.3


Combustion Control to Boilers: In this application, a Pulpmill in Quebec sought a more reliable way to meet EPAemissions requirements. Combustion control was easier,based on the mass (standard volume) ration betweenthe natural gas and combustion air, over wider turndownswith no flow conditioning.

Check Metering: An emerging application is in the useof Coriolis meters to “check” the billing from utilitycompanies. In this application, a major US specialtychemical manufacturer on the West Coast used a largeamount of natural gas for process boilers and furnaces.When plant energy balances did not match utilitycompany billing, check meters were installed. Note thatthis user relied on the NIST-traceable water calibrationfactor transferring to gas applications in their discussionswith their utility supplier.

Ethylene Gas Transfer: Ethylene is commonly viewed as adifficult to measure gas, due to its highly non-ideal nature.In this application, Coriolis meters are used for intra-planttransfers, helping to meet both unit mass-balance goals,

as well as reactor feed rate requirements. Ethylene is fedcontinuously to a polymerization reactor, where variousgrades of polyethylene (LDPE, etc) are made.

SUMMARY

Although relatively new technology for natural gasapplications outside of compressed natural gas (CNG),Coriolis have gained worldwide acceptance for otherfluids and other industries. With a worldwide installedbase of around 300,000 units, Coriolis technology isseeing expanded use for both liquid petroleum andnatural gas. A number of countries and groups haveeither drafted standards or are in the process of studyingthe technology. Most notably the AGA Task Group #11has published its Tech Note covering Coriolis for CustodyTransfer of Natural Gas.

Technology limitations of earlier designs have beenlargely overcome, with high accuracy measurement nowpossible at low-pressure drop, typically 90” wc. Coriolis“sweet spots” are mainly in lines of 8-10” and smaller,where high turndown is needed, flow conditioning tomeet new AGA-3 requirements is costly, and/or the gasis of dirty, sour, or of changing composition. Also, goodpotential exists for “simple” energy metering, using theCoriolis meter output directly, scaled for energy units.

Coriolis is an ideal complement to multi-path ultrasonicmeters. These two technologies overlap 4” to 8” line sizerange.

Third-party data from CEESI, Pigsar, SwRI, and othersshow little if any effect of flow profile, and little if anyshift in meter factor from factory calibration to naturalgas application. As the AGA Task Group progressestoward a formal Report, testing to quantify these effectswill be done. Testing is also planned to quantify theeffects on accuracy for wet and dirty gas.

Common Coriolis gas applications range from wellheadand separator gas to turbine and boiler combustioncontrol to medium/high pressure distribution metering.

Several Coriolis vendors are currently active in supportingthe AGA’s work to develop both a Technical Note as wellas a formal Report. Strong continuing support fromindustry, vendors, AGA and API will help assure theformal Report is published in calendar 2002. Until then,Coriolis technology merits serious consideration as abona fide contender to complement Ultrasonic for naturalgas applications.

FUNDAMENTALS OF CORIOLIS METERS (FOR GAS MEASUREMENT) · 2015-03-02 · 10% of the worldwide market...

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Transcript of FUNDAMENTALS OF CORIOLIS METERS (FOR GAS MEASUREMENT) · 2015-03-02 · 10% of the worldwide market...