A Novel Approach to Improve Diverter Performance in Liquid Flow Calibration Facilities

Transcription

1 A Novel Approach o Improve Diverer Performance in Liquid Flow Calibraion Faciliies R. Engel Physikalisch-Technische Bundesansal (PTB) Braunschweig, Germany U. Klages Universiy of Applied Sciences a Wolfenbüel, Germany Absrac: A divering device (diverer) is an essenial and error deermining componen par of a liquid flow calibraion faciliy based upon a saic gravimeric sysem wih flying sar and finish. The diverer iming error can be considerably reduced by he use of an angular encoding ransducer in combinaion wih an appropriae elecronic and sorage device. Furher benefis in diverer performance can be obained by an elecric diverer acuaor. Keywords: Liquid flow calibraion, Fluid diverer, Time error correcion, Elecric diverer acuaion 1 INTRODUCTION High accuracy liquid flow calibraion faciliies are generally based upon saic weighing gravimeric sysems wih flying sar and finish [1]. Such a saic weighing calibraion faciliy generally comprises 3 fundamenal componen pars (see Figure 1): - flow generaion sysem: comprising sorage ank, speed conrolled pumping sysem, flow conrol sysem, which acuaes he conrol valve, and opionally a consan head ank; - calibraion lane: piping sysem implemening defined flow condiions and incorporaing he meer under es (MUT), i.e. he meer o be calibraed; - reference sysem: precision balance wih weighing ank and flow divering device. Consan head ank Diverer Conrol valve Balance Meer under es Pump Sorage ank Figure 1. Schemaic of a gravimeric liquid flow calibraion faciliy The flow generaion sysem causes he liquid o pass hrough he calibraion lane a a given and consan flow rae a which he meer under es is o be calibraed. The liquid flow rae Q M is sabilized by a flow conrol sysem acuaing a conrol valve, and a speed conrolled pump sysem. As an addiional means a consan head ank can be used as an opion o provide a consan flow rae over he ime hrough he measuring lane. The consan head ank improves he flow rae ime consancy hrough he measuring lane where he meer under calibraion is insered ino he fluid pah.

2 Time consancy of he liquid flow rae during calibraion is a srong requiremen in order o improve diverer performance, i.e. o reduce he uncerainy of measuremen of he respecive calibraion faciliy. The measuring lane represens ha par of he insallaion where he meer under calibraion is insalled wihin he pipe sysem, and whose main ask is o implemen opimum flow condiions for flowmeer operaion, i.e. no swirls, a given and consan velociy profile of he sreaming liquid. Therefore, a flow condiioner is ofen insalled upsream of he flowmeer o avoid any flow disurbances ha migh affec he meer performance. The reference sysem represens ha par of he calibraion faciliy in which, during he ime period beween 10 and 40 (see Figure 2), i.e. he collecion ime, a sample of he sreaming liquid is direced ino a ank whose conens can generally be deermined in erms of volumeric or gravimeric unis. In he case of a saic weighing calibraion faciliy his par of he sysem is realized by a weighing ank on a balance sysem. In such a calibraion assembly he divering device represens a highly accuracy deermining componen par whose funcion is o direc he liquid flow, alernaively, eiher o he sorage ank or o he weighing ank wihou disurbing he flow rae hrough he flowmeer under calibraion. s ' B E q(s) αe α B Widh w Q 30 M ' E B Lengh l Balance readou m() Flow o weighing ank Q M m 0 Timer acuaion START q () W m() 10 T M 30 Timer acuaion STOP 12 Time 40 Time Figure 2. Principle of flow diversion 2 DIVERTER IMPACT ON CALIBRATION ERROR The respecive flow rae hrough a flowmeer can be considered as: - volumeric flow rae and oalized volume hrough volume flowmeer: V! and V = V!()d (1) or: - mass flow rae and oalized mass hrough mass flowmeer: m! and m = m! ()d (2) The majoriy of flowmeers are volumeric flow rae meering devices (excepions are rue mass flow meers, such as Coriolis-ype unis). Calibraing such a ype of flowmeer requires mass-o-volume conversion by addiionally using a densiy meer coninuously fed from he calibraion liquid sream. This conversion mus be aken ino consideraion in he definiion of he measuring uncerainy budge of he calibraion sysem. For a oalizing volume flowmeer (he subjec of he sudy described in his paper) he meer readou, when he calibraion has been finished, is given as follows: V = TM M V! ( ) d (3) 0 In ypical echnical applicaions he procedure of ime inegraion is represened by a pulse coun from he flowmeer signal oupu accumulaed by an elecronic couner during he measuring ime T M.

3 This measuring ime is derived from dedicaed posiions of he divering edge ha are marked by posiion swiches (see Figure 2) generaing he gaing signals for he elecronic couner measuring T M. This meer readou will be compared wih he reference volume V REF deermined via mass o volume conversion from he balance readou m 0 by applicaion of he liquid densiy ρ : V REF = m 0 ρ In order o minimize he diverer impac on he whole calibraion sysem measuring uncerainy he duraion of eiher raverses (deermined by ime differences 20 minus 10 and 40 minus 30, respecively) should be as shor as possible, heir absolue values being limied by he effecive measuring ime T M of a calibraion procedure. The maximum velociy of raverse is limied o values above which he divering edge would disor he liquid je a he nozzle oule, so ha all geomerical assumpions concerning cross secional flow area and he respecive relaions describing he divering moion (which are being generally applied, and so in his paper) would no longer be valid. Pracical values for he divering imes may range from some 50 ms o some 100 ms, depending on diverer size and acual flow rae. Examples showing how he single componen pars (ne mass deerminaion, liquid densiy meering, collecion ime, and ohers) of a high accuracy liquid flow calibraion faciliy deermine he oal uncerainy of measuremen of he calibraion sysem are given in [2], [3], and[4]. Wih respec o he conribuion of he diverer o he oal uncerainy of measuremen he error affecing facors of he divering device in a liquid flow calibraion faciliy can be grouped as follows: - Flow relaed facors of influence, such as shape of he cross secional flow area of he liquid je a he nozzle oule, non-consancy of local flow velociy along he pah which he divering edge is passing, splashing, and ohers [5]. - Facor of influence ha is deermined by he non-consancy of he diverer ravel velociy when is edge is passing across he liquid sream in eiher direcion. - Accuracy, adjusabiliy, and sabiliy of he swiching condiion of he "proximiy" swich ha derives sar and finish condiions of iming couners in flowmeer calibraion. The objecive of his paper is o show appropriae measures o reduce funcion and performance degrading effecs caused by facors of influence 2 and 3. Flow relaed facors of influence are reaed in paper [5]. A PTB Braunschweig, Germany, a new high accuracy waer flow es faciliy is presenly under consrucion. This faciliy is inended o represen he naional sandard for mass and volumeric flow rae and flow relaed quaniies like mass and volume of sreaming waer in he fuure. Design work and firs pracical resuls, described here and in [5], were realized in he projec aimed a designing, erecing, and esablishing his new faciliy. Special provisions were made, among oher deails, o implemen high performance divering devices wih minimum impac on measuremen uncerainy; firs experimenal resuls can be presened here. 3 MEASURES AND MEANS TO REDUCE DIVERTER TIMING ERROR 3.1 Improved Diverer Drive and Transiion Moion Conrol In PTB's new high accuracy waer flow es faciliy, which is being realized a presen ime, special provisions were made ha he diverer movemen of raverse follows a given rajecory (see Figure 9a) mos exacly, and ha he imer acuaion is obained wih maximum accuracy. These wo goals were accomplished by applying an elecric diverer acuaor and an angular encoder aached o he diverer pivo. Figure 3 shows he schemaic side view of a divering device developed as a prooype diverer for he new PTB waer es faciliy (Figure 4). Flow relaed aspecs of he diverer design (flow condiioning by appropriae piping design and shaping of he cross secional flow area along he diverer feeding pipe and a he nozzle oule, avoiding je splashing) were discussed in deail in [5]. The local fluid velociy in he nozzle oule sream may no exceed a maximum value o provide opimum flow condiions and o guaranee ha he waer je has a recangular-shaped cross secional flow area in he region where he divering edge is crossing. For his purpose he mechanical design of (4)

4 he diverer provides means o vary he widh w of he cross secional flow area (see Figures 2 and 3) by an elecric drive acuaor. The nozzle widh can hus be adaped o he respecive liquid flow rae Q M so ha he local flow velociy q(s) does no exceed a given value, say 4 m/s. Figure 3. Diverer drive and moion conrol (schemaic) To achieve an improved divering performance he new diverer design incorporaes an elecric drive acuaor whose funcions are conrolled by a dedicaed programmable logic conroller (PLC) supervised by he process conrol sysem of he calibraion plan. Figure 4. Diverer prooyp equipped wih elecric drive and PLC based moion conrol The diverer raverse is hus an exacly posiion and velociy conrolled and moniored moion, he whole raverse can be imagined as a series of a huge number of very precise single posiion seps exacly locaed a a given series of se-poin values. This series of angular posiion se-poins is he predefined rajecory of he diverer edge during fluid diversion. Parameers of his rajecory (phase of acceleraion, velociy, deceleraion, and ohers) can be adaped o achieve an opimum response behavior, for insance in he case of a non-ideal local fluid velociy disribuion along he pah of he divering edge across he liquid je (as indicaed in Figure 2). This ousanding diverer performance can be observed in Figures 9 a and b. 3.2 Measuremen Subsysem o Acquire Diverer Transiion Daa and o Generae High Precision Timer Acuaion As shown in he schemaic of Figure 3, here is a high precision angular encoder aached o he diverer pivo. On he basis of he digial encoder oupu signals a special embedded processor conrol and daa acquisiion uni provides high accuracy imer acuaion and daa acquisiion funcions. The

5 acquired ransiion daa serve for an improved approach o correc he diverer iming error hrough a special ype of rerospecive analysis of he diverer posiion-ime rajecory during a measuremen cycle of he calibraion faciliy. Figure 5. Programmable angular posiion acquisiion sysem The posiion-ime rajecory is capured during eiher ransiion of he diverer and is analyzed in an off-line operaing mode, using sandard sofware ools such as spreadshee programs. By an absolue angular encoder (Figure 5) a ime series of angular posiions is capured wih a ime resoluion of 50 µs in a ransien recorder. Afer a measuremen cycle he daa sored in he memory of he ransien recorder is ransferred o a hos compuer over a sandard inerface (e.g. parallel por). As an addiional benefi he ransien recorder sysem has exended feaures o provide a number (e.g. eigh) of programmable posiion swiches, which may be compared wih he convenional proximiy swiches as o heir funcion, bu wih a numerically definable posiion. The angular encoder is linked o he ransien recorder sysem via serial link using a proprieary proocol by he encoder manufacurer Heidenhain, Germany. The encoder can deliver a maximum of 40,000 angular posiion daa ses per second, is operaional speed is slowed down o 20,000 daa ses per second in he sysem presened here. The encoder angular resoluion equals 19 bis for one full urn, which delivers 524,288 unique posiion poins. Due o he limied angular movabiliy of he diverer he usable resoluion will be beween 12,000 o 24,000 posiion incremens, his will be abou 14 bis. The ransien recorder iself will only be acive during he raverses of he diverer. Sar and sop commands o capure he angular daa can be generaed inernally by seing angular hresholds o inegraed programmable posiion swiches marking he beginning and he end of he diverer movemen. One series of daa ses is hus capured during he opening phase of he diverer (see Figure 9: BALANCE posiion) and anoher series during he closing phase (BY-PASS posiion). An exernal rigger signal can be used if a special experimen requires differen capure phases (Figure 6). All daa of he programmable swiches, especially he angular hresholds, is ransferred from he hos compuer o he ransien recorder sysem via he hos inerface. The sysem conroller - a dedicaed microprocessor - programs he hresholds o he regisers of he programmable posiion swiches, conrols he iming of he ransien recorder for sar and sop rigger signals and ransfers he acquired daa o he hos compuer for analysis purposes.

6 Time measuremen is performed by soring ime samps a he beginning and he end of boh of he opening and closing phase. These ime samps are generaed inernally by a free running clock wih a resoluion of 50 µs (he sysem clock). Sysem iming is conrolled by a crysal oscillaor (maser clock) from which all iming signals are derived. Figure 6. Funcion block diagram of a processor conrolled posiion daa recorder Afer he measuremen cycle, he wo capured ime series (diverer opening and closing, see Figures 9 a and b) of he angular posiion daa ses are ransferred ogeher wih he ime samps o he hos compuer. A firs approach o calculae he ime during which he liquid was direced owards he weighing ank will be a search for he angular posiion values equal o he mechanical swich poin of he diverer during opening and closing. The relaive ime o he beginning of he posiion daa capure may hus be calculaed. Togeher wih he ime samps a he beginning of he wo capure phases for opening and closing, he period of ime T M during which liquid has flown o he weighing ank is calculaed. These ime values are independen of he inaccuracy of proximiy swiches, of heir posiioning errors during se-up and of non-consancy of he diverer ravelling velociy. The only errors come from iming deviaions of he maser clock (a crysal) and angular posiion deviaions of he angular encoder and he bearing of he diverer. 4 DIVERTER TIMING ERROR CORRECTION 4.1 General Soluion In [2] an exac soluion was given for he evaluaion of he iming error of he diverer in a saic weighing liquid flow calibraion faciliy. This iming error resuls from improper alignmen of he posiion swiches dedicaed o saring and sopping he liquid collecion ime inerval. Small quaniy facors in he soluion equaion in [2] were negleced for pracical purposes o lead o a simplified formula ha has been applied o ISO Sandard 4185 ( in [6], Mehod 1): T M TM = N Q Q N m i i 0 i= 1 i= N m0 TM The iming error correcion is performed as follows: N T Under saic flow condiions, i.e. consan flow rae, consan fluid pressure and emperaure, he weighing ank is filled during a single burs of flow (called sandard run [6]) o full scale m 0 (he balance oupu having been zeroed before); he meered ime of flow is T M. (6)

7 Afer empying he weighing ank is filled again, now in a series of N consan flow burss. As a resul we now obain a series of N incremens m i in balance readou, as well as he corresponding series of T i, which represen he individual measuremen periods of each single burs. Applying hese acquired quaniies o Equaion (6) deermines he fracional ime correcion T M. The facor Q 0 /Q N akes ino accoun any variaion of flow rae. Q 0 represens he average flow rae during he single-burs filling of he weighing ank, Q N being he average flow rae during he N burss while he m i are colleced and accumulaed in he weighing ank. The resul of his iming error correcion is only valid for a given flow rae a which he above procedure has been performed. The procedure mus be applied o all flow raes requiring calibraion. The usual pracice is ha he deermined fracional iming correcion T M is simply added o T M wihou performing any mechanical alignmen of he posiion swiches if i does no exceed a cerain value. 4.2 Novel Approach for Minimizing Diverer Timing Error In he classic approach for iming error correcion [6] a single correcion procedure delivers a single fracional ime inerval T M due o he fac ha he only ransiion daa informaion comprises couner/imer readou for T M. Any mechanical alignmen work wih he imer acuaing swiches, Diverer posiion s() [mm] s SW i+1 s SW i s SW i-1 Single-burs filling T STORAGE T 0 STORAGE 1 Series of N burss filling T STORAGE2 T STORAGE N Mass in weighing ank m() [kg] m 0 N-1 m k k=1 m 1+m 2 m 1 10 s11 20 i-1 i i+1 TMø (SW i+1) TMø(SW i) TMø(SW i-1) 10 N m k=1 k s12 40 j j+1 s s22 41 i-1 i i+1 j-1 j j+1 TM1(SW i+1) T M1 (SW i ) TM1(SW i-1) j N sn1 2N 3N sn2 i-1 i i+1 j-1 TMN(SW i+1) T MN (SW i ) TMN(SW i-1) N Time [ms] j j+1 2N 3N 4N 4N Figure 7. Diverer iming error correcion: diverer ransiion responses and balance readous generally, implies a grea uncerainy as o wha he corresponding change of he ime inerval is wih respec o a given local shif of he proximiy swich. All mechanical alignmen work requires ime consuming verificaion of correcness and error-minimizaion, respecively. If we use an angular encoder whose posiion oupu signals are fed o a ransien recorder, as described above, o acquire he ransien responses of he fluid diverer, we have a huge amoun of informaion concerning he diverer s saic and dynamic sysem behavior afer a single correcion procedure.

8 The diagram in Figure 7 represens all daa obained from a single correcion procedure: diverer response daa sored in he ransien recorder and balance readou. When saring such an elecronic swich adjusmen we refer o iniial condiions, represened by indices i, respecively j (see Figure 7): - swiching hreshold s SW i - sored ransien daa (diverer ransien posiions) ) T s, s ) - STORAGE k, period of ime in which he diverer is a res a he BALANCE posiion; his ime is deermined by a couner/imer and is sored ogeher wih he ransiion daa - effecive measuremen ime inerval during he respecive burs numbered by k : T s = ) + T + ( (7) M k ( ) ) SW i Now one may apply ( s ) M k SW i ( i ( j ( 2k S k1 STORAGE k S k 2 3k T in Equaion (7) o Equaion (6) o compue he fracional iming error correcion of he diverer wih s being adjused a an appropriae iniial value. A good SW i saring posiion of s may be half he way from BY-PASS posiion o BALANCE posiion. On SW i varying s SW i as an independen variable in he modified funcion of Equaion (7) he imer acuaion adjusmen is simply reduced o solving a numerical problem, i.e. a minimum search saring from iniial posiion: T M = f ( s ) MINIMUM! SW i The resoluion of T M amouns o 50 µs, as his ime-discree soluion is based upon ransiion daa ha were scanned a a rae of 1 daa poin per 50 µs. To improve accuracy, he above approach can also be applied o he exac iming error equaion given in [2]. Applying measured value inerpolaion in he given daa ses acquired during calibraion (Figure 7), he iming resoluion achievable for T M could be decreased o a magniude as low as 10 µs. Even a sysemaic deviaion of he flow characerisic of he diverer can be compensaed for by applying off-line inerpreaion and analysis of he acquired ime series daa of he diverer s raverse. As an essenial benefi he approach presened here reveals: - Ease of diverer ime error correcion by avoiding successive series of repeaed seps: a single balance full scale run followed by a burs of a number of runs whose oal mass equals he full scale run. In he "classic" way of ime error correcion he correcness of he swich alignmen has o be verified by repeaing he above procedure. High-precision flow rae conrol mus be implemened in a high-accuracy flow calibraion faciliy o assure sabiliy and repeaabiliy, and so ha Q 0 and Q N may be considered o be pracically equal, and he respecive quoien in Equaion (6) becomes uniy. Wih PTB s new waer flow es faciliy, special emphasis was laid upon high precision flow rae conrol [4] as one design goal o make ime consancy of flow rae a facor of influence ha may generally be negleced wih respec o Equaion (6). 5 EXPERIMENTAL RESULTS 5.1 Small-Size Diverer Prooype for a Down-Scaled Experimenal Flow Calibraion Faciliy In order o prove he feasibiliy of he novel approach o fluid diverer iming error reducion, elecrical drive acuaion and angular encoder based ransiion daa acquisiion was implemened and esed wih a down-scaled, small-size diverer prooype, as he waer flow insallaion of he new PTB high flow rae calibraion faciliy (up o 2,300 m 3 /h) is no ye available. This small-size calibraion faciliy provides a maximum flow rae up o 10 m 3 /h. The fluid diverer in use represens a rerofi design of an older diverer design in use wih PTB for several years. (8)

9 Improvemens were gained by providing (besides several oher design improvemens) elecric drive diverer acuaion and addiion of an angular encoder o he diverer se-up (see Figure 7). Figure 8. Smaller size diverer prooype ( Q max = 10 m 3 /h ) The ess of he new design componens have shown excellen funcion performance, laer proven by he real-sized diverer prooype for PTB's new es faciliy. Design deails follow he funcional principle shown in Figure 3. The excellen ime responses achieved are comparable o hose in Figures 9 a and b and he diverer iming error correcion based upon he above described approach has been proven o be feasible. 5.2 Real-Sized Diverer Prooype for New PTB Waer Flow Tes Faciliy The new PTB waer es faciliy will comprise wo differen-sized calibraion lanes (DN 150 and DN 400) which can, alernaively, feed a calibraion flow o 3 weighing sysems (30 ons, 3 ons, and 300 kg). Each lane is equipped wih an appropriae fluid divering device. The choice of he appropriaely sized weighing sysem hus provides opimum mass measuremen a a given flow rae, he measuremen ime being opimized for a low diverer iming error. Figure 9. Diverer ransiion response a) Velociy response b) Posiion error when comparing wo succeeding diverer ravels (oal pah lengh amouns o incremens, represening 54.1 mm) The medium-sized diverer (wih he 3 ons balance) was implemened as a firs prooype (see Figure 4) for sudy purposes. Besides oher aspecs (e.g. opimum flow condiioning in he diverer piping sysem, and oher flow relaed problems [5]), a special design goal was o implemen an elecric

10 acuaor, providing driving power which maches he greaer forces of ineria of a real-sized diverer as well as he impac forces of a waer je a a flow rae of 100 m 3 /h and greaer. Repeaed experimens have shown (Figure 9 b) ha over he diverer raverse here was a deviaion no exceeding 3 incremens in a ime sequence of succeeding diversions. The oal pah lengh of a single diversion comprised some 11,500 incremens a an operaing flow rae up o around 100 m 3 /h. So hese experiences acquired wih he small-sized as well as wih he real-sized diverers are essenial precondiions o exrapolae o a larger-size diverer design ha is par of PTB s new waer calibraion faciliy a flow raes up o 2,100 m 3 /h [4]. 6 CONCLUSIONS AND FINAL REMARKS The provision of an angular posiion encoding device in combinaion wih an appropriae elecronic circuiry ha allows he diverer swiching posiions (hreshold of START or STOP pulse generaion) o be adjused elecronically enhances accuracy and swiching hreshold sabiliy, avoiding mechanical alignmen work and oher drawbacks. By he use of an encoder and a combined ransien recorder and digial comparaor assembly he iming error correcion problem is simply reduced o a non-linear algebraic equaion solving problem, based upon a single se of measuremen daa. The resul of his equaion solving process is a number or (digial) value ha represens he swiching hreshold of he START and STOP deecors of he diverer. This numerical value can be easily ransferred o and sored in an comparaor regiser sorage of he diverer elecronic uni. The correcion of he diverer iming error has hus been accomplished wihou requiring furher mechanical calibraion and alignmen work or addiional verificaion measuremens. Finally, i is worh menioning ha he performance of older diverer designs in exising calibraion faciliies can be remendously enhanced wih respec o heir divering and meering properies by rero-fiing an elecric acuaor and an angular encoding sysem. REFERENCES [1] M. R. Shafer,F.W. Ruegg, Liquid-Flowmeer Calibraion Techniques, Transacions of he ASME, Ocober 1958, pp [2] G. E. Maingly, Volume Flow Measuremen, Chaper 6 in "Fluid Mechanics Measuremen" by R. J. Goldsein, pp , ISBN Hemisphere Publishing Corporaion, Washingon, New York, London, [3] G. E. Maingly, Flow Merology: Sandards, Calibraions, and Traceabiliies, Chaper 24 in "Flow Measuremen: pracical guides for measuremen and conrol" by D.W. Spizer, edior, pp , ISBN Insrumen Sociey of America, Washingon, Research Triangle Park, NC 27709, [4] W. Poeschel, R. Engel, The Concep of a New Primary Sandard for Liquid Flow Measuremen a PTB Braunschweig, The 9h Inernaional Conference on Flow Measuremen FLOMEKO '98, Lund, Sweden, June 15-17, [5] W. Poeschel e al, A Unique Diverer Design for Waer Flow Calibraion Faciliies, Paper, FLOMEKO 2000 Conference, Salvador, Brazil, June [6] -, Measuremen of liquid flow in closed conduis - Weighing mehod, Inernaional Sandard, ISO 4185, 1s ediion, 1980, pp [7] J. Krahn, J. Schaper, Acquiring a diverer's ime response in PTB's new waer calibraion es facily by an angular encoding device (wrien in German), Diploma paper for receiving he degree of a Diplom-Ingenieur, Universiy of Applied Sciences, Wolfenbueel, Germany, [8] W. Poeschel, R. Engel, Funcional descripion of PTB's planned new waer flow calibraion faciliy, unpublished inernal repor, Laboraory for Liquid Meers, PTB, Braunschweig, Germany, AUTHORS: Dr. Rainer Engel, Physikalisch-Technische Bundesansal (PTB), Laboraory for Liquid Meers, Braunschweig, Germany, phone: , [email protected], Prof. Dr. Ulrich Klages, Universiy of Applied Sciences Wolfenbueel, Germany, phone: , [email protected]