Important considerations regarding calibration of ultrasonic gas flow meters

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1 Imprtant cnsideratins regarding calibratin f ultrasnic gas flw meters Fr NFOGM Nytt, June 2013 Mrten Marstein, FMC Kngsberg Metering AS Intrductin Multipath ultrasnic gas flw meters are nrmally flw calibrated at an accredited flw labratry befre installatin in rder t verify the flw meter s perfrmance and t crrect fr flw meter ffset. The cmmn practice is t install gas flw meters withut in situ calibratin arrangement. Many f these flw meters are never recalibrated. It is therefre imprtant t ensure that the flw meters are calibrated prperly with the highest degree f cnfidence t the invlved parties. This article will highlight sme imprtant aspects regarding preparatins and executin f gas USM flw calibratin. Sme imprvement issues are als discussed. Figure 1. Typical ultrasnic gas flw meter installatin Preparatins The upstream pipe which will be installed with the flw meter, r an identical pipe spl with the same length and inner diameter, must be used fr the flw calibratin in rder t prvide similar flw prfile during calibratin as will be expected at the final installatin site. If a flw cnditiner is part f the final installatin, this flw cnditiner must als be used during calibratin. Page 1 f 7

2 The calibratin labratries have limited test runs available with flange type and rating designed fr the maximum pressure at the facility. Adapter spls are therefre ften required t match the line size and flange type f the flw meter. The calibratin labratries use their wn pressure and temperature transmitters in rder t cnfrm t their accredited prcedures. The flw meter r the adjacent pipe spls f the same line size shuld have a pressure tapping t allw fr cnnectin f the labratry s pressure sensr. This is particularly imprtant if the labratry lines size is different than the flw meter size. Fr instance, if a 6 inch flw meter is installed in a 12 inch test pipe, the pressure in the 6 inch flw meter will be lwer than the pressure in the 12 inch pipe due t higher gas velcity in the flw meter. It is als imprtant t supply the blts and gaskets required fr the flw meter, upstream pipe and adapter spls. Mst calibratin labratries will require a pressure test certificate befre installing the flw meter and test spls int their labratry. Chice f calibratin labratry The chice f calibratin labratry will be determined by the flw capacity and the gegraphical lcatin as mst custmers will select the nearest labratry that cmplies with the maximum and minimum calibratin flw rates. It is wrth checking that the labratry is accredited t ISO fr the chsen flw range. Althugh mst labratries are accredited fr the cmplete flw range that is ffered, this may nt always be the case. Als, the type f test gas as well as pressure and temperature levels ffered varies between the different calibratin labratries. An verview f the mst cmmnly used flw calibratin labratries fr gas is listed belw. Name Lcatin Gas type Range (m3/hr) Pressure (barg) Temperature (C) Uncertainty (k=2) Pigsar Drsten, Germany Natural Gas N/A 0.16% Frce Technlgy Vejen, Denmark Natural Gas % grid eurpe.cm/cps/rde/xchg/sid F2210DCD 89EB588A/pen grid eurpeinternet/hs.xsl/2788.htm 40D6 4E1A E0BFB69805F/3713/24846en.pdf GL Nble Dentn Durham, England Natural Gas % nbledentn.cm/en/cnsulting/flwmetercalibratin.php Page 2 f 7

3 NMi Eurlp Rtterdam, The Netherlands Natural Gas N/A 0.15% CEESI Iwa, USA Natural Gas N/A 0.23% TransCanada Calibratins Manitba, Canada Natural Gas N/A 0.19% Table 1. Overview f gas flw calibratin labratries. Check pints during flw calibratin At the flw calibratin labratry there are sme imprtant issues t be addressed. I have listed them belw in a chrnlgical sequence. Befre calibratin start Agree n the calibratin prcedure and acceptance requirements. Supplier, client and labratry persnnel shuld meet t g thrugh the prcedure with respect t flw range, number f repeats, verificatin pints and ther prject specific requirement. Inspect the flw meter installatin. Check that the flw cnditiner and upstream pipe is installed crrectly with respect t flw directin and rientatin. It is a gd practice t take pictures, if allwed, t dcument the physical installatin arrangement. Check that the same utput signal is used during calibratin as will be used in the final installatin fr traceability purpse. It is recmmended t use pulse utput since this is a simple and straight frward representatin f the flw rate. Check the parameter setup f the flw meter t be calibrated. Check that the nminal K factr (pulses/m 3 ) in the flw meter is used by the flw labratry. Check that inner diameter, path lengths and transducer parameters are entered in accrdance with supplier certificates. Check that crrect linear thermal expansin cefficient and mdulus f elasticity (Yung s mdulus) is entered int the flw meter fr crrect temperature and pressure diameter cmpensatin. Typical values fr linear thermal expansin cefficient are 1.10E 05 m/m/c fr carbn steel, 1.60 E 05 m/m/c fr SS316 grade steel and 1.30E 05 m/m/c fr duplex material. Typical value fr mdulus f elasticity is 2.0E+06 bar fr carbn and stainless steel. Page 3 f 7

4 Check that the prcess pressure and temperature is entered manually int the flw meter fr crrect diameter cmpensatin. Print the parameter setup reprt t dcument the flw meter cnfiguratin during flw calibratin. During calibratin Check that the prcess cnditins such as pressure, temperature and flw rate are stable during the flw calibratin. Drifting prcess cnditins will mst likely affect the flw measurement repeatability. Check that there are n alarms generated in the flw meter by its internal diagnstics system. Check fr nrmal values f autmatic gain, percentage f accepted ultrasnic pulses, path velcities and velcity f sund (VOS) n each transducer pair. The spread in VOS shuld exceed 0.5 m/s between all transducer pairs during nrmal flw. After calibratin Check the repeatability and linearity twards the calibratin prcedure. If the flw crrectin shall be entered int the flw meter, check that crrect parameters are entered. The crrectin is entered either as flw weighted mean average, a plynmial r as a flw rate/crrectin factr matrix with interplated crrectin. A verificatin pint may be repeated after entering the flw meter crrectin in rder t dcument crrect entering f the flw crrectin. Print the parameter setup reprt t dcument the flw meter cnfiguratin after flw calibratin. 0,20 Average deviatin frm reference (%) 0, ,20-0,40-0,60-0,80-1,00 Velcity (m/s) Figure 2. Typical linearity plts fr multipath ultrasnic gas flw meters Page 4 f 7

5 Re calibratin r cnditin mnitring Sme authrities r peratrs have requirement fr recalibratin f fiscal multipath ultrasnic flw meter like. Fiscal ultrasnic gas flw meters n the UK side are typically flw calibratin peridically with a maximum interval f five years. On the Nrwegian sectr the peratrs will nrmally mnitr cnditin parameters t determine if flw calibratin is required. The cmmn cnditin parameters are Autmatic Gain Cntrl (AGC), acustic signal acceptance, velcity prfile and velcity f sund. The AGC is measured per transducer and is dependent n the gas density. This means that as lng as the gas density r gas pressure remains stable, the AGC value fr each transducer shuld remain unchanged. The acustic signal acceptance is typically displayed as a percentage value indicating hw large prtin f the acustic signals received by each transducer are recgnized by the signal prcessing unit in the flw meter. A drp in the acceptance value can indicate a deterirated transducer. The nrmalized velcity rati (velcity measured by each path cmpared t the verall flw velcity) is an indicatin f the gas flw prfile in the flw meter. The flw prfile will remain fixed fr a large flw range. A majr change in the indicated velcity prfile indicates a prblem with ne transducer pair. The velcity f sund (VOS) is the mst imprtant parameter t be mnitred. The VOS is measured by each path. Regardless f changes in the prcess cnditins, all paths shuld measure the same VOS as lng as there is flw thrugh the flw meter. Maximum difference between the highest and lwest VOS reading will nrmally stay with 0.5 m/s. The average VOS (acrss all paths) shuld be cmpared t AGA 10 calculated VOS frm nline gas chrmatgraphy. Drifting discrepancy f measured (USM) VOS cmpared t calculated (GC) VOS indicates a drift in either the USM r the gas chrmatgraph. The cnditin mnitring pprtunities in the multipath ultrasnic gas flw meters shuld be used actively t evaluate the need fr recalibratin. Figure 3. Typical diagnstics data fr multipath ultrasnic gas flw meters Page 5 f 7

6 Suggestins fr imprvement Better availability A flw calibratin jb must be rdered typically several mnths in advance. The flw calibratin labratries are staying busy. Mre labratries wuld increase the availability fr flw calibratin as the number f flw meters in peratin cntinuus t increase. Lwer uncertainty The uncertainty f the flw measurement at the calibratin labratries range frm 0.16 t 0.30%. The USM manufacturers claim uncertainties fr their multipath gas flw meters in sme cases less than 0.50% as delivered frm the factry. The resulting linearity and repeatability values fr a meter under test is therefre largely affected by the uncertainty f the calibratin labratry. The uncertainty f the labratries shuld be imprved in rder t maintain lwer uncertainty than the test meters by a cnsiderable margin. Fr calibratin f ther instruments the reference instrument typically is five r even ten times mre accurate than the test instrument. Fr USM flw calibratin the reference instrumentatin is nly tw r three times better. Higher flw capacity A simple calculatin f the flw meter capacities is shwn in the table belw assuming nminal diameter and maximum 25 m/s gas velcity. Size (Inches) ID (mm) Vel (m/s) Flw rate (m3/h) Table 2. Typical USM maximum range fr different size f meters We see frm the table that fr large flw meters f size 24 inch and higher the number f test sites is very limited. There are tw test sites in Nrth America that can handle the full flw range. There is a need fr calibratin labratries within Eurpe that can flw calibrate these size meters. Higher pressure and temperature The flw calibratin labratries perate n gas pressures up t 50 barg, sme up t 70 barg. Gas temperatures are typically in the range f 5 t 30 C at the labratries. The ultrasnic transit time measurements in ultrasnic gas flw meters must be crrected fr delays in the transducers. The transducer delay is dependent n pressure and temperature. The extent f transducer delay variatin ver pressure and temperature depends n the design f the ultrasnic transducer, sme Page 6 f 7

7 factrs being stability f piez electric sensr, munting f sensr and type f matching layer. The manufacturers f the ultrasnic gas flw meters use different methds fr determining the transducer delay, and fr the pressure and temperature cmpensatin f the transducer delay, either thrugh factry calibratins r thrugh empirical estimates. Pressure and temperature will als affect the inner diameter f the flw meter. Since ultrasnic flw meters measure the flw velcity, vlumetric flw rate is prprtinal t the square f the inner diameter. The diameter crrectin is dne either inside the flw meter r in the system flw cmputer, nrmally by standard pipe expansin algrithm which may nt be fully applicable fr the flw meter bdy. Many ultrasnic gas flw meters in pipeline applicatins perate at pressures between 100 and 200 barg. Operating temperature between 50 and 100 C are nt uncmmn. An added uncertainty fr installatin effect must be applied if these flw meters are calibrated at cnsiderably lwer pressures and temperatures. The added uncertainty fr installatin effect is typically estimated t 0.2 t 0.5% in uncertainty budgets fr metering systems. If the flw meters are calibrated at pressures and temperatures clse t final perating cnditins, the cntributin frm installatin effects can be reduced. Page 7 f 7