Improving Measurement Uncertainty of Differential Pressures at High Line Pressures & the Potential Impact on the Global Economy & Environment.

Transcription

1 Improving Measuremen Uncerainy of Differenial Pressures a igh Line Pressures & he Poenial Impac on he Global Economy & Environmen. Speaker/uhor: Mike Collins Fluke Calibraion 5 urricane Way Norwich. NR6 6JB, UK Tel: mike.collins@flukecal.com Co uhors: Kenneh Kolb (formerly) Neculai Moisoi GE Infrasrucure Sensing GE Infrasrucure Sensing 03 Wespark Drive Fir Tree Lane ouson, Texas US 7704 Groby, Leiceser, LE6 0F, UK Tel: Tel: kenneh.kolb@ge.com neculai.moisoi@ge.com bsrac Typically he calibraion of igh Line Differenial Pressure Transmiers is performed wih he low side of he device open o amosphere. Such measuremens make no accoun of he changes ha occur o he weed pars when hese devices are subjeced o he high line pressures ypical of heir normal working environmen. This pracice can inroduce significan measuremen errors o he final oucome of a process wheher i be fiscal monioring and cusody ransfer of naural resources such as oil and gas, he urbine efficiency of je engines used on aircraf and in oher areas of power generaion, o uiliies delivering gas, waer and elecriciy o indusry and he associaed indusrial processes, o domesic end users and even possibly o more recen aemps a carbon sequesraion and he accurae accounabiliy for carbon credis. This paper describes a novel approach ha enables he calibraion of high line differenial pressure ransmiers a elevaed line pressures. Developmen of his echnology began in response o increased demand from indusry for an accurae and easy o use differenial pressure sandard. This new insrumen subsanially reduces he calibraion ime by eliminaing he iniial cross-floa required when using a winpos pison gauge or a pressure divider. Calibraions a elevaed line pressures are furher simplified hrough acive conrol of differenial pressures, hus reducing he dependence on operaor echnique and experience. The line pressure range of his insrumen is from 0 o 0 MPa wih a differenial pressure range of 0 o 00 kpa.

2 . Inroducion I is a well-known fac ha he performance of mos differenial pressure ransmiers is dependen on line pressure. owever, mos are calibraed only wih an amospheric reference. The main reasons hese devices are no calibraed a elevaed line pressure are he high cos and he complexiy of operaion of he radiional calibraion sandards, which are ypically a win-pos pison gauge or a pressure divider sysem, linked o a single-pos deadweigh pison gauge. This paper describes he key elemens of a new design of differenial pison gauge, including he novel riple-pison assembly and load cell arrangemen, as well as he calibraion mehod used o characerize he effecive area of he specialized pison assembly. This calibraion process demonsraes ha he effecive area of he riple-pison assembly is no affeced by he line pressure. The lack of pressure dependence simplifies operaion of he insrumen by minimizing he compuaions needed for accurae resuls, and significanly conribues o a reduced uncerainy in oupu pressure. The line pressure range for his insrumen is from 0 o 0 MPa wih a differenial pressure range of 0 o 00 kpa. The main elemen of he new differenial pressure sandard is an inegraed riple-pison assembly. This unique assembly eliminaes he dependence on he second deadweigh pison and allows direc idenificaion of he differenial pressure as i is direcly proporional o he applied mass load. Oher key elemens of he new design include a load cell, acually a precision digial force balance, and an acive differenial pressure conrol sysem. The digial balance allows he sysem o esablish differenial pressures well below he normal minimum by offseing he force of he are componens. The digial balance also provides feedback o he acive conrol sysem o mainain he differenial pressure produced when changing he mass load. The calibraion of he new differenial pison gauge sandard was performed in wo pars. The masses are calibraed using radiional means, bu he effecive area of he differenial pison was deermined using a variaion of he radiional win-pos differenial calibraion mehod. Basic design and operaion The schemaic of he insrumen in Figure shows he main sysem componens. The hree pisons a he hear of he sysem, he riple-pison assembly, are shown as iems, and 3. The riple-pison assembly is consruced wih sub-micromere axial symmery o ensure precision operaion wih minimal leakage. During he iniial operaion a a given line pressure, he pressure is applied simulaneously o boh ends of he differenial diameer pison (). The weigh of he floaing elemens is suppored by he digial balance (4) hrough a spring-loaded coupling sysem (9). This coupling sysem reduces he effecs of ransiens resuling from moorized roaion of he pison assembly, and prevens overloading he digial balance mechanism. Once he line

3 pressure has been esablished he digial balance is ared and an exernal mass load is applied corresponding o he desired differenial pressure. The pressure applied o he high-pressure side of he differenial pison (he lower caviy) is increased unil he digial balance indicaes zero, designaing ha he downward force is he same as before he masses were loaded, hereby indicaing ha he exernal mass load is fully offse by he differenial pressure across he large-diameer pison. 7 L.P..P PCB,,3 - Pison ssemblies 4 - Mass Balance 5 - Prined Circui Board 6 - Temperaure Conrol Elemen 7 - Weigh Carrier 8 - Mechanical Sop 9 - Mechanical Connecion 0 - Safey Valves - Inpu Valves - djusable Fee 5. Figure. Schemaic drawing of he new differenial pison gauge Thermal generaors inegraed ino he sysem mainain he differenial pressure according o feedback from he digial balance. s emperaure or oher influences cause he differenial pressure o drif from he desired level he hermal generaors adjus he emperaure of a porion of he sysem liquid o mainain he force measured by he digial balance, hereby mainaining he differenial pressure. The signal from he digial balance is ransmied o he compuer conrol program which can be configured o graph he aciviy of he acive conrol sysem and o show small deviaions from he arge differenial pressure. In a liquid-only calibraion sysem he differenial pressure is esablished simply by changing he mass load. The large diameer differenial pison produces mos of he differenial pressure direcly, and he auomaic conrol sysem makes up he remainder, auomaically. Calibraion mehod Various calibraion mehods were considered for deermining he effecive area of his new differenial pison assembly. The iniial mehod considered he exisence of wo effecive areas, one for he low-pressure side and anoher for he high-pressure side. nd furher, ha hese wo areas would no be idenical, in ha here could be a significan offse relaed o line pressure. This innovaive mehod proved o be accurae bu was also complicaed and very ime consuming. less complicaed mehod was been developed ha eliminaes he need for discreely deermining he differen effecive areas and as well, he variable effec of line pressure.

4 Figure. Crossfloa Calibraion Sysem The final calibraion of he differenial pison was accomplished using a variaion of he radiional win-pos differenial calibraion mehod. In he convenional win-pos differenial mehod he saic pison is adjused o generae he same pressure as he sandard pison. This esablishes zero differenial a he line pressure defined by he sandard pison. Small load changes are hen made o he sandard pison o generae he desired differenial pressures. The mehod used for he calibraion of he new differenial pison is essenially he same excep for he iniial balancing operaion. Normal operaion he new differenial pison sysem includes a digial balance o offse he weigh of he are componens, allowing he sysem o produce near-zero differenial pressures. During he calibraion of he differenial pison he force balance is disabled o reduce all unnecessary influences on he characerizaion of he pison area. s a resul, he sandard win-pos crossfloa sysem had o make up his difference. Whereas in he convenional operaion of he win-pos sysem he saic and sandard pisons are brough o he same pressure for he iniial reading, in he differenial pison calibraion he win-pos pisons were offse o equal he are pressure of he differenial pison, hereby replacing he effec of he digial balance. The balance poin was considered as he zero poin o which all subsequen readings would be compared. This approach eliminaes he need for discree deerminaion of he weigh of he differenial pison are componens and he deerminaion of how much he are pressure changes wih line pressure. Replacing he offse force of he digial balance wih an offse in he win-pos sandard seup resuls in direc proporionaliy beween he differenial pressure and he exernal weigh load, and he calibraion sysem beer represens he normal operaing mode of he differenial pison sysem.

5 Mahemaical model of calibraion The effecive area of he insrumen pison-cylinder assembly is deermined from he calibraion equaion: p = () p s The pressure generaed by he win-pos pressure balance [] in his modified differenial mode is given by: p s = 0 m gb [ + λ ( p + p) ][ + α ( 0) ] 0 s p L0 + p + α L + α L ( + λ p 0[ + α ( 0 0) ]) 0 [ + λ ( p 0 + p) ][ + α ( 0) ] ( L0 0) ( 0) L () where: m s - mass of he weighs applied o generae he differenial pressure; g - acceleraion due o graviy; b - air buoyancy correcion facor; 0 - effecive area of he high-pressure pison-cylinder assembly; λ - pressure-dependen erm of he high-pressure pison-cylinder assembly; p 0 - pressure generaed by he high-pressure pison-cylinder assembly; p L0 - pressure generaed by he low-pressure pison-cylinder assembly; p - nominal differenial pressure; α - hermal coefficien of he high-pressure pison-cylinder assembly; - emperaure of he high-pressure pison-cylinder assembly during he generaion of p s ; α L - hermal coefficien of he low-pressure pison-cylinder assembly; L - emperaure of he low-pressure pison-cylinder assembly during he generaion of p s 0 - emperaure of he high-pressure pison-cylinder assembly during he iniial cross-floa; L0 - emperaure of he low-pressure pison-cylinder assembly during he iniial cross-floa; Iniially he pressure generaed by he high-line differenial pison gauge [3] can be wrien as follows: p = mgb [ + α( 0) ] ( ρ ρ a ) gh (3) where: m - mass of he weighs applied o generae differenial pressure; g - acceleraion due o graviy b - air buoyancy correcion facor; - effecive area of he high-pressure side of he pison-cylinder assembly;

6 α - hermal coefficien of he pison-cylinder assembly; - emperaure of he pison-cylinder assembly during he generaion of p ρ - densiy of he working fluid; ρ a - densiy of he ambien air ; h - disance beween he wo acive faces of he large diameer pison. The mehod used eliminaes he influence of he fluid head [4] and he equaion (3) can be rewrien: p = mgb [ + α( 0) ] (4) which simplifies more he equaion for he effecive area: = mgb [ + α( 0) ] ps (5) The above equaion has been used for deermining he effecive area of he differenial pison and for he evaluaion of he associaed expanded uncerainy (in which case equaion (5) was wrien explicily). Resuls Calibraions of several unis have been performed. These calibraions include observaions a several pressures covering he differenial operaing range. These differenial observaions are made a each of several line pressures covering he full line pressure range. In all cases i was observed ha here was no significan dependence of he differenial pison area on line pressure. Furher, he predominan influence o scaer in he residuals of he effecive area fi is aribuable o he resoluion limi of he win-pos sandard. s shown in Figure 3, he calibraion residuals exhibi significan scaer, which ends o increase wih line pressure and wih smaller differenial pressures.

7 Figure 3. Calibraion Residuals as PPM of rea Considering he sandard used in his calibraion, his is exacly he resul ha should be expeced. The calibraion sandard sysem has a repeaabiliy limi of abou par per million. This ppm hreshold impacs he differenial pressure calibraion as ppm of he oal pressure being generaed, or essenially in his case, he line pressure. One ppm of oal pressure is be a progressively larger porion of differenial pressure as he differenial pressure decreases, and as he line pressure increases. When he area fi residuals are graphed as a fracion of line pressure, he residuals follow he expeced paern. s shown in Figure 4, he residuals are all wihin he par per million repeaabiliy hreshold of he sandard.

8 Figure 4. Calibraion Residuals as PPM of Line Pressure I is apparen in Figure 4, ha wihin he PPM resoluion limi of he sandard, here is no correlaion beween he area of he differenial pison and pressure, eiher line pressure or differenial pressure. Furher, since he resoluion limi of he sandard is he dominan uncerainy influence, is considered o have normal disribuion, and affecs all observaions equally, he measured area values for he differenial pison are considered independen. These independen values allow expressing he differenial pison area as a weighed average [5] wih a weighed sandard uncerainy: 0 i= i ( dp, lp) ( ) i= U i = 0 (6) U ( ) i= ( ) i U U = 0 (7) ( ) i This resuls in a ypical differenial pison area of: =64.6 mm (8)

9 Wih an expanded uncerainy (coverage facor of ) of: U ( ) = 0.00 mm (9) Conclusions new design of differenial pressure sandard has been developed o address he need in indusry for an accurae and easy-o-use sandard for boh pneumaic and hydraulic high-line differenial pressure calibraions. The design of his insrumen eliminaes he complexiy of he iniial cross-floa operaion of he win-pos and divider mehods by using a precision digial balance o offse he pison are. The digial balance also allows elecronic feedback o he conrol sysem o faciliae auomaed mainenance of he differenial pressure. The high performance of he differenial pison and he calibraion mehod seleced during he developmen of his new sandard keep he uncerainy in he pison area o a minimum. The large area of he differenial pison, he low hermal coefficien and high sabiliy raing of he maerial seleced for he pison/cylinder componens, and he very low uncerainy capabiliy demonsraed in he calibraion process make he insrumen suiable for use as a primary sandard o help improve measuremen uncerainy of differenial pressures a high line pressures leading o beer accounabiliy in fiscal monioring and cusody ransfer of naural resources, reduce oxic emissions from power plan and associaed processes and provide a poenial ool o aid in he process of carbon sequesraion and accounabiliy for carbon credis. References [] N. Moisoi, M. Bryan, S. Page, S. Corrie, The new Pressuremens differenial deadweigh eser model 4XX, Inernaional Merology Conference, Buchares, Sepember 00 [] M. ay, D. Simpson, Developmen of he high line differenial pressure sandards, NPL- Repor, Teddingon, MSO, 999 [3] N. Moisoi, D. Godfrey, New igh Line Differenial Pison Gauge, Conference Proceeding, Inernaional Symposium on Pressure and Vacuum, Sepember 4, 003, Beijing, China [4] S.L. Lewis, G.N. Peggs, The Pressure Balance: pracical guide o is use, Second Ediion, London, MSO, 99 [5] M.G. Cox, The evaluaion of key comparison daa, Merologia 00, 39,

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