1、 AGA Report No. 6 Field Proving of Gas Meters Using Transfer Methods Prepared by Transmission Measurement Committee First Edition, March 2013 AGA Report No. 6 Field Proving of Gas Meters Using Transfer Methods Prepared by Transmission Measurement Committee First Edition, March 2013 Copyright 2013 Am
2、erican Gas Association All Rights Reserved Catalog # XQ1302 DISCLAIMER full name of the document; suggested revisions to the text of the document; the rationale for the suggested revisions; and permission to use the suggested revisions in an amended publication of the document. A form to propose cha
3、nges has been added at the end of the document. Copyright 2013, American Gas Association, All Rights Reserved. iii ACKNOWLEDGEMENT This report was developed by a Transmission Measurement Committee (TMC) task group initially under the chairmanship of Walt Seidl formerly with Colorado Engineering Expe
4、riment Station, Inc. (CEESI). Following Walts retirement, the leadership was taken over by Jim Witte, the then representative to TMC from El Paso Corp., to finish the document for balloting. Subsequently, the leadership was provided by John Hand with Spectra Energy, who worked hard in reaching conse
5、nsus for resolution of ballot comments. Terry Grimley with Southwest Research Institute (SWRI) provided substantial help in preparing the document for balloting and writing the final version. AGA acknowledges and sincerely appreciates their hard work and contributions. Members of the task group who
6、devoted an extensive amount of their time and deserve special thanks are Paul LaNasa, CEESI Measurement Solutions Dan Rebman, Universal Ensco Dan Rudroff, Welker Flow Measurement Systems Phil Whittemore, Dresser Meters and Instruments Others who also contributed and deserve thanks are Khalid Al-Fadh
7、l, Saudi Aramco Mike Bermel, Southern California Gas Company Jim Bowen, Sick Oil also referred to as the “pulse factor.” It is associated with the electronic pulse output of the meter to produce a volume for an ancillary device (flow computer). If a meter has more than one electronic pulse output, t
8、hen more than one K-factor may be appropriate. Mach number the ratio of the fluid speed to the speed of sound at a set of specified operating conditions. Mass flow rate the flow rate through a meter stated in units of mass per unit of time. Master meter a meter (transfer standard or reference), mete
9、ring system, or CFVN with known characteristics used to prove another meter (MUT). Measurand The quantity intended to be measured. Meter accuracy Actual VolumeMUTActual VolumeREF100% or MassMUTMassREF100% or Standard VolumeMUTStandard VolumeREF100% Note that “meter accuracy” is a specific numerical
10、quantity defined by the above equations and should not be confused with the qualitative concept of “accuracy” (when the word is used alone) as defined previously. Meter error Actual VolumeMUTActual VolumeREF100% 100% or MassMUTMassREF100% 100% or Standard VolumeMUTStandard VolumeREF100% 100% Meter f
11、actor a number by which the result of a measurement is multiplied to compensate for systematic error. The non-dimensional multiplying value is determined for each flow rate at which the meter is proved. The number is calculated by dividing the value from the 3 master meter by the indicated value fro
12、m the meter under test. Meter Proof Actual VolumeREFActual VolumeMUT100% or MassREFMassMUT100% or Standard VolumeREFStandard VolumeMUT100% MUT Meter Under Test or Metering System Under Test. NIST National Institute of Standards and Technology, United States of America. Primary flow standard a flow s
13、tandard with direct traceability to fundamental units, such as mass, length, temperature and time. Proving the process of determining the relationship between the output (or response) of a MUT to the value produced by a master meter. Rangeability for a flow meter, this is the ratio of the maximum to
14、 the minimum flow rates in the range over which the meter meets specified error limits. Repeatability the closeness of the agreement between the results of successive measurements of the same measurand carried out under the same conditions of measurement. These conditions include the same measuremen
15、t procedure, the same observer, the same measuring instrument, used under the same conditions, the same location and repetition over a short period of time. Reproducibility the closeness of the agreement between the results of measurements of the same measurand carried out under changed conditions o
16、f measurement. The changed conditions may include principle of measurement, method of measurement, observer, measuring instrument, reference standard, location, conditions of use and time. Reynolds number a non-dimensional parameter that compares the inertial with the viscous forces in a flowing flu
17、id. Useful for correlating flow meter performance. Secondary flow standard a flow measuring device with documented traceability and uncertainty that is traceable to a primary flow standard. Sonic relating to the local speed of sound of the gas. 4 Stagnation pressure the pressure a fluid attains when
18、 brought to rest isentropically. Related term: inlet stagnation pressure. Standard volume a specified volume of gas at specified standard conditions of pressure and temperature. This volume of gas has a specific mass, related to the standard pressure, temperature, and gas composition. Contrasting te
19、rm: actual volume. Standard volume flow rate the rate of flow in units of standard volume per unit time. Contrasting term: actual volume flow rate. Static pressure the potential pressure exerted in all directions by a fluid or gas at rest. For a fluid or gas in motion, static pressure is measured in
20、 a direction at right angles to the direction of flow. The pressure measured using a properly manufactured perpendicular wall tap. Related term: inlet static pressure. Throat the narrowest internal cross-section of the CFVN. Traceability “requires the establishment of an unbroken chain of comparison
21、s to documented references each with a documented uncertainty,“ as defined by NIST. Transfer prover a portable, self-contained system used for proving meters in the field consisting of a master meter, instrumentation, and data collection equipment all of which have both their traceability and uncert
22、ainty documented. Uncertainty a parameter, associated with the result of a measurement that characterizes the dispersion of the values that could reasonably be attributed to the measured quantity. The dispersion could include all components of uncertainty, including those arising from systematic eff
23、ect. The parameter is typically expressed as a standard deviation (or a given multiple of it), defining the limits within which the measured value is expected to lie with a stated level of confidence. Verification the process of confirming or substantiating that the output of a device is within the
24、specified requirements. Contrasting term: calibration. 5 3. Theory The method of transfer proving consists of passing a known gas or gas mixture, such as air or pipeline quality natural gas, through a MUT and a transfer standard (master meter) in series under steady-state conditions of flow rate, pr
25、essure, temperature and gas composition (Figure 3-1). Under these steady-state conditions, assuming no leakage into or out of the system, the mass rate of flow through the master meter and the MUT is the same. PT= Pressure measurementTT= Temperature measurementAT= Gas Quality analyzer(s)MUTPT TTATMA
26、STERPT TTPT TTCONNECTING VOLUMEFigure 3-1: MUT and master meter in series with connecting volume. Flow can be in either direction. If unsteady flow occurs because of the configuration between the MUT and the master meter or because of changing operating and flowing conditions upstream and downstream
27、 of the meters during the period over which performance of the MUT is established, the line pack (mass contained in the connecting volume) between the MUT and the master meter can affect the results of the calibration. The larger the connecting volume or longer the distance between the MUT and the m
28、aster meter, the greater the potential effect on the calibration of the MUT. In addition, the differences in pressure and temperature at the MUT and the master meter may result in different compressibility factors at the two meters. These effects can lead to additional calibration uncertainty that s
29、hould be considered during meter proving. See Appendix C for equations to be used where the effect of the connecting volume on the measurement uncertainty for the in-situ calibration is to be considered. Examples in Appendix D are given, assuming that the effect of connecting volume for the in-situ
30、calibration is not considered. The relationship between the flow recorded by the MUT and the transfer standard is an indication of MUT accuracy relative to the transfer standard. When expressed as a ratio it is referred to as “meter proof at a specified rate of flow.” The proof equations presented i
31、n this document are not applicable to unsteady flows. During unsteady flows, the effect of mass storage in the piping between the master meter and the MUT must be considered. 6 Meter proof may vary from flow rate to flow rate. Other factors, such as pressure, temperature, gas density and composition
32、 may also influence the meter proof and should be held as constant as is practical. In some cases an operator will calculate a single flow-weighted proof for a MUT and will use that value to develop a single adjustment factor to be applied to all data throughout the meters range of operation. These
33、data often are used to either adjust meter output in a flow computer, electronic corrector or to mechanically adjust meter output by changing gear ratios within the meter itself. A series of proofs may be performed at several points across the MUTs range of flow. The data may be used to create a sin
34、gle point adjustment, an adjustment table, a series of linear adjustments, or an adjustment curve that will bring the MUT into tolerance across the MUTs entire range of operation. A meter proof in the field may differ from a prior proof measured in a laboratory. Each proof is the result of the overa
35、ll uncertainties associated with the respective tests. Typical laboratory uncertainties are smaller than those that can be obtained in field proofs. Differences may be due to many factors, including installation effects, contamination, worn components, and differences in gas composition. The differe
36、nces may also be due to faulty test methods or test equipment. Field proofs should not be used to replace the initial calibration factor(s) of the MUT obtained in a qualified laboratory unless the reason for the differences can be determined. The master meter should have a lower uncertainty than the
37、 uncertainty required for the MUT. The uncertainty requirements for the transfer standard should be determined by performing an uncertainty analysis of the overall proving process as described in Appendix C. Under steady-state conditions all necessary parameters, such as pressure, temperature, and m
38、eter output such as frequency (or pulses and time), are recorded. Sufficient pulses must be accumulated to achieve the desired uncertainty. In order to perform uncertainty calculations for the proof, multiple data points will be required at each flow rate. For transfer standards and MUTs that indica
39、te flow in units of actual volume, a proof requires conversion to mass units or standard volume units at a common pressure and temperature base condition. Using the master meter performance data, measured parameters and fluid properties, the mass rate of flow through the transfer standard and the MU
40、T may be calculated. By using the indicated flow rate from the transfer standard, MUT measured parameters and fluid properties for the MUT, the meter factor(s) for the MUT may be calculated. It has been shown that with some flow meters, the meter factor at elevated operating pressures may be signifi
41、cantly different than a meter factor ascertained by proving at or near atmospheric pressure. Therefore, it is recommended that the MUT be proved at typical operating conditions whenever possible. Master meters, piping and associated piping components should be sized to ensure similar flow dynamics t
42、hrough the master meter and the MUT. Using much smaller valves and/or piping, and using master meters that may test only a small portion of a MUTs operating range will increase the uncertainty of the resulting test and is not recommended. 7 Flow meters, including both the master meter and the MUT, c
43、an be affected by installation issues such as asymmetric velocity profile, swirl, and pulsation. These effects may be minimized by following the appropriate AGA Reports and other relevant industry standards. During field proving, operators should be aware of the potential influence of any installati
44、on effects on both the MUT and the master meter. By proving the MUT in its normal operational installation, these effects may be more readily accounted for in the field proving results. The interval between meter proofs should be determined by meter history, operating conditions, regulatory requirem
45、ents or contractual requirements, or by performing a risk analysis. If from one proving to the next, the meter factor shifts beyond the meters repeatability requirement, remedial measures should be taken to prevent such shifting and a shorter interval should be considered for the next proving. When
46、selecting a master meter, consideration should be given to the following attributes: Quality Repeatability Reproducibility Traceability Known uncertainty Well characterized dependence on process conditions Rangeability Installation requirements Output interface requirements Pressure loss Diagnostic
47、capability or the ability to verify the measurement integrity Appendix A includes a discussion of these attributes as related to different potential master meter types. Regardless of the meter used as the master meter, it is critical that all the flow that passes through the MUT also passes through
48、the master meter. Therefore, any valves that could allow gas to enter or exit between the master meter and the MUT or that could allow gas to bypass either of the meters should be capable of being verified. Double block and bleed valves are one solution because they possess dual seats integral to th
49、e valve with the capability to bleed the space between the seats and verify that the valve is sealing properly. 8 4. Proving Meters Using Critical Flow Devices 4.1. Introduction Critical flow venturi nozzles (CFVN) are devices that accelerate the flowing gas to the maximum possible velocity by restricting the flow. This restriction is usually in the form of a converging section called the “inlet,” followed by a section of minimum area called the “throat,” and optionally followed by a section called the “diffuser,” where the area is expanded and
