meterrundesign_bestpractices

8/7/2019 MeterRunDesign_BestPractices

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International SE Asia Hydrocarbon Flow Measurement Workshop9 - 11 March 2005

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METHODS FOR REDUCTION OF TRANSFER UNCERTAINTY IN FLOWMEASUREMENT

Mr. Jim Hill, Kodai Flow ResearchMr. Andreas Weber, RMG Messtechnik

Mr. Tri Budi Pramana, Pratiwi Putri Sulung

1 INTRODUCTION

The goal in designing any metering facility is to make a measurement that, when incorporatedwith other facilities through-out the measurement system, provides an accurate accounting ofthe distribution of product. To this end, the design of the metering facility has the requirementsto minimize the uncertainty of the final product flow measurement and to quantify the exactconfidence interval around that measurement. Only with this confidence interval establishedcan system managers determine what levels of unaccountables require action.This paper proposes some standard practices for design of the meter run section of the skid andcommunicating system requirements to the meter manufacturer in a form that can be comparedto experimental performance data.

2 UNCERTAINTY PROPAGATION

The expansion of uncertainty from primary standards is a dry subject that is commonly skippedover. However, the value of data from a metering station is totally dependent on its ability to betraced to a common point of reference with the other data in the system. Because of the criticalnature, a detailed development of the expanded uncertainty of the measurement is an importantfirst step.

2.1 Propagation of Uncertainty through the Standards Trace

Uncertainty, like entropy, can only increase, never decrease. Fig. 1 shows the expansion ofuncertainty of a normalized volumetric flow as it is traced back through the multiple levels ofderivation from primary standards.

The primary standards of length are used to derive a standard volume which, in turn, iscombined with a time standard to create a volumetric rate transfer standard. This transferstandard, usually a small precision turbine meter, outputs actual (not standard or normalized)volumetric flow rate. Banks of these transfer standards in parallel are used by facilities tocalibrate field meters, again on an actual volumetric basis. While an actual volumetric ratecalibration is adjusted by measurements of pressure and temperature to compensate fordifferences in fluid density between the reference location and the test location.

The uncertainty of the field measurement is further increased as the meter is moved into

position in the measurement skid with new installation conditions. Finally, measurement of thegas composition is combined with measurements of pressure and temperature, each with their

Primary Standards Standard VolumeVolumetric Rate

Transfer Standard

Calibration inActual

Volumetric FlowRate Units

InstallationEffects

Composition andSuper -

CompressibilityCalculation

Normalization

Fig. 1 Propagation of Uncertainty of a Measurement of Normalized Volumetric Flow


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own uncertainty, to calculate the final output in normalized or standardized volumetric flow rate.Rules for calculating the uncertainty and confidence levels are set by governing authorities

1,2.

2.2 Effect of Harmonized High Pressure Natural Gas Standards

Every commercial calibration facility operates under a regulating authority which specifies theprotocol for creating derived standards from primary standards. While all these authorities useidentical primary standards, differences in methodology and unavoidable experimental errorlead to small differences in the volumetric transfer standards used by the calibration facilities.These differences are usually within the documented uncertainty of the calibration facilities butlead to conflicts in a system where meters have been calibrated at multiple facilities.

To prevent these types of conflicts, several authorities, led by PTB and NMI, established aprogram of inter-comparisons of high-pressure, high-flow transfer standards. This resulted in amatching transfer standard, which is commonly referred to as a Standard Cubic Meter of HighPressure Natural Gas. Care should be taken when reading the documented expandeduncertainty as it is traced to this harmonized volumetric rate standard and not all the way backto primary standards. This practice is correct if the accounting for the distribution network is

being done strictly on volumetric rate basis. If the data from the metering station is used tocalculate mass totals or energy content, the total uncertainty in reaching the harmonizedstandard must be included. This would occur if a gas product is distributed in liquefied form insome segment of the network.

3 A TYPICAL ULTRASONIC METER SYSTEM

With all other sources of uncertainty accounted for, we must focus on the effects of transferringthe meter from the calibration facility to the point of use. To estimate the potential shiftintroduced, we must first estimate the deviation of the flow conditions between the calibrationfacility and the metering skid. For an example, a simplified redundant ultrasonic meter skid withpiping for cross-comparison was analyzed.

3.1 Redundant Ultrasonic Meter Skid with Cross-Comparison

Fig. 2 is a schematic of a simplified ultrasonic metering skid. This design is typical of stations inwestern Europe. The skid is designed so that normally the station flow is diverted through the

bypass so that the flow runs through both meters (Fig. 3). In this mode, the two meters form aredundant system cross checking the output. When maintenance on the meter station isrequired, either the left or right branch of the skid can be isolated (Fig. 4, Fig. 5) withoutinterrupting flow or losing measurements. If drift due to fouling or corrosion is a concern of theoperator, they may chose to select either the left or right branch as the primary route. Onlyduring operational checks would the secondary meter be brought into service to verify that theprimary has not drifted.

Fig. 2 Simplified Redundant Ultrasonic Meter Skid with Serial Bypass


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The use of multiple configurations adds a new complication to the estimation of the installationeffects on the measurement uncertainty. Not only will the flow fields entering the individual runsbe different than the fields at the calibration facility, but they will change with the variousoperating conditions.

3.2 Individual Meter Run Sub-Components

Designing and constructing a metering skid is a complex project that requires coordination besystem engineers, an array of vendors, and specific system rules. Fig. 6 illustrates a typical

meter run where a make-up spool is used to span the difference between the length of thecalibrated USM assembly and the skid opening. The specific combination of flow conditioner,

Make-Up PieceInlet Pipe w/ Captive

ConditionerUSM Outlet pipe

Meter Run Length (Specified by Skid Designer)

Calibrated Prefabricated USM Assembly

Fig. 6 Prefabricated USM Assembly with Make-Up Piece to Fit in Custom Meter Skid

Fig. 3 Bypass Engaged to Route the Flow through Both Meter Runs Serially

Fig. 5 Skid Configured to Divert Flow through Left Meter Run

Fig. 4 Skid Configured to Divert Flow through Right Meter Run


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inlet pipe, USM, and outlet should be proven as an assembly by the meter vendor. Sinceproving the insensitivity of a specific combination to various upstream conditions is expensiveand time-consuming, vendors prefer to use a few standard combinations. Also, the assemblywith the flow conditioner captivated in the inlet pipe is preferably shipped from the vendor to thecalibration facility (and on to the skid assembly location) in a single piece. This restricts the sizeof the assembly to practical limits.

3.3 CFD Modeling for Multiple Valve Configuration

At this point a system engineer faces a standard paradox. To finish the skid layout, they needto know what combination of inlet, conditioner, and outlet should be used, but to write the bidspecification to get this information, they must provide layout information to the meter vendors.One way to get started is to assume the free run between the major components is 30D. Thisis the worst case with 20D upstream and 7D downstream. The total layout now can be dividedinto sub-sections, depending on the flow routing, and modeled.

Fig. 7 below shows the results of a CFD model of the left branch of the skid at 5 m/s. Thesystem was analyzed using Fluent 6.0software package. The mean node spacing along the

pipe wall is 15mm (the model is based on 200mm pipe). The total mesh size is 180,000tetrahedral volume cells. A standard K-epsilon turbulence model was selected for thesesimulations. A blank end was used to simulate the dead legs created by closing the isolationvalves. It is important to note that these CFD only provide mean flow field information andtesting for standing wave resonance must be done separately by analytical methods.

A similar model was created for both the right branch and the left branch when fed by the serialbypass. The bypass poses a problem since it is assumed that a flow conditioning device will beused in the right branch of the skid, but the exact type and location is unknown. For the initialanalysis, we assumed that the flow conditioner is effective and that the flow entering the bypassis fully developed and irrotational. This may not be a good assumption, but the input flow fieldcan be revised once the flow conditioner is selected. Fig. 8, 9, and 10 display the velocity inaxial and transverse directions for the right branch, left branch, and left branch in bypass,

respectively.

Fig. 7 Simplified CFD Model of the Left Branch of Meter Skid


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The contour and vector formats are good for visually displaying information; however, it is not aformat that can easily be interpreted by a meter vendor to make an installation recommendation.For that, it is more convenient to select a model to and fit coefficients to the data. One way todo this is to select several key chords in the data plane and plot the velocity vectors for thiscord. Fig. 11 shows the transverse velocity vectors along chords bisecting the pipe in thevertical and horizontal axis at the measurement plane of the left branch of the skid. The data iseasily described by a solid body rotation model, which can be expressed by a swirl and cross-flow angles. This process was repeated for the other branch conditions. The model coefficientsfor the three flow fields are summarized in Table 1 in section 4.2

Fig. 8 - Axial and Transverse Velocity Distribution at the Entrance of the Right Branch of Meter Skid

Fig. 10 - Velocity Distribution at the Entrance of the Left Branch of Meter Skid (Bypass Engaged)

Fig. 9 - Axial and Transverse Velocity Distribution at the Entrance of the Left Branch of Meter Skid


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4 COMPARISION TO PERFORMANCE DATA

For some vendors, it will be possible to recommend installation specifics based on the

description of the inlet flow conditions. Others will only have data on the level of shifts in flowreading caused by installations behind a group of standard disturbances. If this is the case, thedesign engineer will have to determine which disturbance best resembles the conditions in themeter skid.

4.1 Typical Inlet Disturbance Tests

One of the most common tests for installation effects is the combination of piping elementsdescribed in OIML R 32

3. While this is a test standard for turbine meters, these disturbances

were adopted for acceptance testing by PTB4. Fig. 12 depicts four of the disturbances, three

mild and one severe. The final element of all four disturbance is an expansion one standard sizeup to the meter diameter.

a) Single Elbow

b) DoubleElbow In-Plane

c) Double ElbowOut-of-Plane

Expansion (1 size)

d) Double Elbow Out-of-Plane with HalfMoon Plate(Severe Disturbance)

Fig. 12 Four standard flow disturbances specified by OIML R 32

Fig 11 Plots of the Orthogonal Velocity Components Along the Centerline Axis (Right Branch)

VZ (y) VY (z)


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4.2 Cross-Comparison of Flow Fields

The level of flow fields created by the four OIML disturbances was measured using a 6-pathultrasonic meter

5. Table 1 is the comparison between the experimental results and the CFD

calculations for the three valve configurations. While the three flow configurations generate

significantly lower conditions than the severe disturbance, both the left and right branch exceedthe levels of a mild disturbance. However, the left branch in by-pass exhibits levels that can beclassified as mild. In this case, it would be prudent to select an inlet/conditioner/metercombination that has been demonstrated to have a shift in observed flow between a mild andsevere disturbance of less than required by the applicable standard. Under AGA-9 forexample

6, the combination should have documentation showing a shift of less than 0.2%

between these two conditions.

Inlet Condition SwirlAngle(Degree)

CrossflowAngle(Degree)

Asymmetry(Percent)

OIML Single Elbow 4.6 0.3 2.2%

OIML Double Elbow In-Plane 5.7 0.2 3.0%

OIML Double Elbow Out-Plane 11.4 2.4 5.0%

OIML Double Elbow Out-Plane with Half-Moon 31.7 16.5 9.0%

CFD Left Branch 16.0 1.1 8.0%

CFD Right Branch 22.3 0.6 2.0%

CFD Left Branch with Series Bypass 4.3 2.3 6.4%

5 CONCLUSION

The use of CFD to estimate the general level of disturbed flow is a convenient procedure. Careshould be taken when using the data though, as exact results are subject to subtle influenceswhich can effect the orientation of the results. A major shortcoming of this analysis is a solidunderstanding of how typical flow conditioners affect the flow fields in a pipe. The standard K-epsilon turbulence model is acceptable for this application due to the high Reynolds numbersand smooth joints. It is unclear how a perforated-plate flow-conditioner effects turbulenceintensity decay.

Because of this, it would be prudent to use experimental data to confirm at least several pointsin the model. Since it is extremely expensive to conduct flow experiments in natural gas, datamay have to be acquired in water and air flows and related back to natural gas through modelsimilitude.

6 ACKNOWLEDGMENT

The authors would like to express their appreciation to RMG Messtechnik for the use of itslaboratory facilities. We also thank the Department of Mechanical Engineering at the Universityof Massachusetts Lowell, specifically Dr. J.W. McKelliget and Mr. J.P. McInerney, for theirexpertise and assistance with the computational fluid dynamics.

7 REFERENCES

[1] B. TAYLOR and C. KUYATT. Guidelines for Evaluating and Expressing the Uncertaintyof NIST Measurement Results, Technical Note 1297, NIST, 1994

Table 1 Comparison of Flow Fields Generated by OIML Tests and Example Meter Station


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[2] Guide to the Expression of Uncertainty in Measurement, International Organization forStandardization, 1995

[3] Rotary Piston Gas Meters and Turbine gas Meters, International Recommendation R32, International Organization of Legal Metrology, 1988

[4] Messgerte fur Gas, Technische Richtlinien G13, Physikalish TechnischeBundesanstalt, 1994

[5] J. HILL, A. WEBER, AND J. WEBER. Qualification of Ultrasonic Flow Meters forCustody Transfer of Natural Gas Using Atmospheric Air Calibration Facilities, North SeaFlow Measurement Workshop, 2002

[6] Measurement of Gas by Multipath Ultrasonic Meters, A.G.A. TransmissionMeasurement Committee Report No. 9, American Gas Association, Arlington, Virginia,June 1998.

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