fundamentals of liquid measurement iii

8/8/2019 Fundamentals of Liquid Measurement III

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F U N D A M E N T A L S O F L IQ U I D M E A S U R E M E N T IIICLASS 2180Peter W KosewiczHydrocarbon Measurement Consultants, Inc.P.O. Box 590347Houston, Texas 77259

I N T R O D U C T I O N C U S T O D Y T R A N S F E R

We've learned when measuring crude oi l that l iquidsexpand and contract with increases and decreasesin temperature. The l iquid volume also decreaseswhen pressure is appl ied. Al l these effects are partof the physical properties of l iquid petroleum fluids.We learned in Fundamentals of Liquid MeasurementI ho w these ph ysical prop erties effect themeasurement of l iquid hydrocarbons. The object iveof either stat ic measurement or dynamicmeasurements is to determine the quant i ty andqual ity of hydrocarbons transferred. Ho we ver thesemeasurements are rarely performed at the standardconditions discussed in Fundamentals I, thereforenot only must temperature be measured, but alsodensity, sediment and water, vapor pressure,pressure and viscosity must be measured. Withthese measurements correct ion factors such asVolu me Correct ion Factors (VCF) can be determinedto al low volum es determined at operat ing conditionsto be expressed at standard reference conditions.The means of measuring hydrocarbon l iquids fal linto one of two methods:

Static measureme ntDynamic measurementStat ic measurement is performed when thehydrocarbon l iquid is at rest and contained within acontainer such as a tank, hence i t is commonlyreferred to as Tank Gauging. On the other handwhen hydrocarbon l iquids are measured while inmotion, this is referred too as dyn am ic measurementor M etering.Ano ther wh y to think of the difference inmeasurement techniques is to think of stat icmeasurement as measur ing the volume in acontainer at a point in t ime and dynamicmeasurement as m easuring the volume in acontainer over t ime.This paper wi l l examine the various types of meters,their accessories and the devices to veri fy themeter's performance.

In simple terms, "custody transfer" is the transfer ofownership or responsibi l i ty for a l iquid hydrocarbonfrom one party to another. Since ownership is beingtransferred, either immediately or eventually, it isessent ial that accurate account ing be used so thatall parties involved in the transaction receive th eirfair measure. With the prices for hydrocarbons thesedays i t is obvious how important accurateaccount ing---hence, accurate measurementbecomes. The words "custody transfer" havebecome synonymous with accurate measurement.How ever the terms "measurement (volume)accuracy" and "m eter accuracy" are not the same.Mea suremen t accuracy as applied to volumes is theabsolute accuracy of the volume measured,whereas "meter accuracy" is the accuracy of themeter relat ive to a reference standard, such as aprover.The term's repeatabi l i ty and l inear '~ are commonlyused to def ine meter accuracy. Repeatabi l i ty is thevariat ion in the meter's performance under constantoperating conditions, i.e. constant f low rate,tempe rature, den sity, etc. W he re-a s linearity is thevariat ion in the meter's performance over a range off low, com mo nly referred to as turndown rat io.Therefore measurem ent accuracy o f volumes isinfluenced by the following factors:

Me ter repeatability Liquid density corrections du e tovarying temperature andpressure Me ter cal ibrat ion o r proving,including procedures Variations in operatingcondit ions an their ef fect on ameters performance Calibration of prover, i.e.waterdraw.

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M E T E R S E L E C T IO NI n g e n e r a l t w o t y p e s o f m e t e r s a r e u s e d i n c u s t o d yt r a n s f e r m e a s u r e m e n t o f l i q u i d h y d r o c a r b o n s . T h e ya re pos i t i ve d i sp l acem en t (P D) m e te rs and tu rb i nem ete rs . T he se l ec t i on o f a spec i f i c m e te r t ypedep end s on i t s app l i ca t ion . I n som e app l ica t i ons aspec i f i c t ype i s p re fe r red , wh i l e i n o the rs e i t he r m e te rt ype w ou l d pe r fo rm sa t is fac to ri ly .A l l m e te r i ns ta l l a t i ons m us t m ee t ce r ta i nf u n d a m e n t a l r e q u i r e m e n t s . T h e s e i n c l u d e a c c u r a t eprov ing fac i l i t i es ; adequate pro tec t i ve dev ices , suchas s t ra ine rs , re l ie f va lves , and a i r o r vap o re l i m i na to rs ; and dependab l e p ressu re and f l owcon t ro l s . A dd i t i ona l l y accu ra te i ns t rum en ta t i on f o rm easur i ng t he phys i ca l p rope r t i es o f t he f l ow i ngf lu i d , such a s tem pera tu re , dens i t y , e t c . A f u r t he rfundam en ta l i s t ha t phys i ca l cond i t i on 's du r i ngn o r m a l m e t e r in g o p e r a t io n s a n d p r o v in g o f t h e m e t e rm us t be i den t i ca l . T he fo l l ow i ng shou l d becons i de red when se l ec t i ng a m e te r and i t s aux i l i a r ye q u i p m e n t .

F l o w r a n g e a n d w h e t h e r f l o w isi n te rm i t ten t o r con t i nuou s P r e s s u r o m a x i m u m o p e r a t in g p r e s s u reand m ax i m um pe rm i ss i b l e d i f f e ren t i a lp ressu re T yp e o f l iqu i d and it s cha rac te r i s t ic s T e m p e r a t u r e ra n g e a n d a c c u r a c y o ft e m p e r a t u r e c o m p e n s a t i o n T yp e o f vo l um e reg i s tra t ion dev i ce A ccu racy requ i red T y p e a n d m e t h o d o f p r o v in g r e q u ir e d A pp l i cab i l it y o f aux i l ia r y m e te rreg i s t ra t i on equ i pm en t M a i n t e n a n c e r e q u i re m e n t s F o re i gn m a t te r i n f lu i d s t ream s Ins ta l la t ion spa ce ava i lab le

T Y P E S O F M E T E R SP o s i t i v e D i s p l a c e m e n t ( P D ) M e t e r - - a pos i t i ved i sp l acem en t m e te r i s a dev i ce i ns ta l led i n a p i p ingsys tem i n wh i ch f l ow i ng l iqu i d i s cons tan t l y andm echan i ca l l y i so l a ted i n to segm en ts o f knownv o l u m e s . T h e s e s e g m e n t s o f l i q u i d a r e c o u n t e d a sthey a re d i sp l aced , and t he i r accum u l a ted t o ta lcon t i nuous l y and i ns tan taneous l y i nd i ca ted i n un i t so f l iqu i d quan t i t y by a m e te r reg i s te r , pu l se g ene ra to ro r f l o w c o m p u t e r . T h e t y p e o f m e c h a n i s m e m p l o y e dto i so l a te t he l i qu i d segm en ts , i . e . by t he na tu re o ft he i r m easu r i ng e l em en t , gene ra l l y d i f f e ren t i a tesp o s i ti ve d i s p la c e m e n t m e t e r s. T h e t e r m s u s e d t o

d e s c r i b e t h e m o s t c o m m o n t y p e s o f m e a s u r i n ge l e m e n t s a r e : Nu ta t i ng d i sc Rec i p roca t i ng p i s ton S l i d i ng -vane t ype ro ta ry B uck e t - t ype ro ta ry Lobed ro ta ry Hel i ca l ro tary C e r t a in c o m b i n a t i o n s o f t h e a b o v e

P o s i t i v e d i s p l a c e m e n t m e t e r s a r e c o m m o n l y u s e dfo r a l l t ypes o f l i qu i d m easurem en t , espec i a l l y f o rv i scous l i qu i ds such as c rude o i l . However , t he i r usei n l i gh t hyd roca rbon se rv i ces such as p ropane ,bu tane , e t c . , m ay resu l t i n h i gh m a i n tenance due tothe l ack o f l ub r i ca t i on . T hey a re f requen t l y used i np roduc t se rv i ces such as l oad i ng rack app l i ca t i onse v e n t h o u g h a tu r b i n e m e t e r m a y b e m o r e d e s i r a b lefo r t he l iqu i d be i ng m easu red . T he reas ons be i ngtha t l oad i ng racks a re i n te rm i t t en t se rv i ce . . . a se rv i cebe t te r su i ted t o P D m e te rs .P os i t i ve d i sp l acem en t m e te rs a re genera l l ycons i de red t o have a 511 tu rndown , i . e . , m ax i m umf l ow to m i n i m um f low . T he y a re t yp i ca ll y se l ec ted t ol i m i t t he i r ope ra t i ng range to be tween 40 pe rcen ta n d 8 0 p e r c e n t o f t h e i r s t a t e d f l o w r a n g e d u e t oi n a c c u ra c i e s a t th e l o w e n d a n d r a p id m e t e r w e a r a tt h e h i g h e n d . H o w e v e r , i f p r o v e d w h e n o p e r a t i n ga b o v e 8 0 p e r c e n t a n d b e l o w 4 0 p e r c e n t a n d t h a tp rov i ng m ee ts t he rep ea tab i l it y requ i rem en ts , t henthey can be used success fu l l y .P o s i t i v e d i s p l a c e m e n t m e t e r s h a v e a n a d v a n t a g eove r t u rb i ne m e te rs i n t ha t t hey a re no t a ff ec ted byv i scos i t y and t h ey requ i re no f l ow cond i t ion i ng , i .e . ,s t ra i g h t e n in g v a n e s u p s t r e a m o f t h e m e t e r . T h e y a r es u b j e c t t o w e a r w h i c h m e a n s t h a t e r o s iv e f l u idapp l i ca t i ons requ i re spec i a l ca re i n f i l t e r i ng andf requen t m e te r p rov i ng t o m a i n ta i n t he i r accu racy .P o s i t i v e d i s p l a c e m e n t m e t e r s a r e u s e dp r e d o m i n a n t l y o n L A C T ( L e a s e A u t o m a t i c C u s t o d yT rans fe r ) un i t s and on c rude o i l A CT (A u tom at i cCus tody T rans fe r ) un i t s .

T u r b i n e M e t e r - - t h e l i q u i d t u r b i n e m e t e r i s a ni n fe ren t i a l t ype o f vo l um e t r ic m eas urem en t dev i cei ns ta ll ed d i rec t l y in t he p i pe o f a f l ow i ng sys tem . T h etu rb i ne m e te r hous i ng has an i n te rna l ro to r t ha tro ta tes w i t h respec t t o t he l i nea r ve l oc i t y o f t he f l u i dpass i ng t h rough the c ross -sec t i ona l a rea o f t hem e t e r h o u s i n g . A s t h e f l u i d p a s s e s t h r o u g h t h em ete r hous i ng , t he angu l a r ve l oc i t y ( rpm ) i m par tedto t he t u rb i ne ro to r i s p ropo r t i ona l t o t he l i nea r

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velocity of the flowing fluid. T he volumetric flow ratecan then be determined by the rotor spccd ( rpm).The volumetr ic f low rate (Q) is assumed to beproport ional to this measured f low veloci ty (V) byassuming a constant f low area (A).Q = ( V ) ( A )( f t % e c ) = ( f U s e c ) ( f t2 )

The accuracy of the f low measurement can beaffected i f the a ssumption that the f low are a remainsconstant does not. Factors contributing to thechanging f low area include: Dep osits (such as paraffin) Boundary layer thickness Cavitation Debrisq, Ope rating conditions (i.e., tempe rature,pressure, density, etc)

Additionally the assumption that the rotor velocity isdirectly proportional to the axial velocity through theturbine m eter can be affected by the following: Bea ring friction Viscous ct ion Ro tor blade configuration Flow conditioning

Turbine meters are used primarily for refined productand l ight hydrocarbon measurement. Al though theyreportedly ha ve a 1011 turndown rang e, care shouldbe taken to select a m eter size to op erate within 50percent to 100 percent of i ts stated flow range.Large inaccuracies may result when operating belowthe 50 pe rcent flow level.Turbine meters are generally used in trash-freeflowing applications since any possibil i ty of build-upon the rotor wil l impede its rpm---hence measuringabil i ty. Early installations of turbine meters sufferedhigh failures due to frequent rotor bearing failures.The use of tungsten carbide bearings has greatlyreduced the failures and inaccuracies due to bearingfriction.When using turbine meters in l ight hydrocarbonl iquid mixtures such a s ethane-propane, N GL's careshould be exercised, in that the l inear operatingrange is raised, and th e me ter should be selected tooperate over the 80 percent to 125 percent range ofthe stated f low rang e. Th e l ighter f luid impinging o nthe rotor blades has less force at lower capacitiescausing erratic rotation and accompanyinginaccuracies.

Turbine meters register volume uti l izing a pulsegenerator called a pick-off coil and a pre-am p. Asthe blade t ip or paramagnetic button p asses throughthe magnetic field generated by the pick-off coil apulse is gene rated. Th is pulse is collected orcounted by an electronic register, such as a f lowcomputer. Typically each turbine meter has a K-factor assigned, which is a nominal number ofpulses per uni t volume. Volume is determined bydividing the accumulated pulses by the K-factor asshown:Volum e = (Pulses) + (K-factor)

Turbine meters being velocity or inference typedevices require flow stream conditioning for theiraccurate performance. T he detai led requirements fo rf low condit ioning can be found in API MPMSCha pter 5, section 3--:'Measurem ent of LiquidHydrocarbons by Turbine Meters". Typical flowcondit ioning consists of upstream and downstreamstraightening sections. The upstream section usuallycontains a tube bundle, which al lows the upstreamsection to be reduced in length. This tube bundleserves to el iminate an y "sw ir l " in the f low streambefore reaching the meter presenting a Symme tricalvelocity profi le to the turbine rotor.

M E T E R A C C E S S O R I E SCustody transfer meter installations require properaccessories i rrespective of meter type. Commonaccessories include strainers or fi l ters, pressuregages, thermometers and thermowells, registeringdevices, temperature and pressure transmitters,pulse generating devices, pulsation dampeningequipment, prover connections, air el iminators,pressure and flow controllers, and straighteningvanes. A discussion of some of these devices is asfollows:

1. Straine rsmb oth PD and Turbine metersrequire this equipment upstream of themeter to protect the internals. Foreignobjects can cause meter damage,excessive w ea r, or inaccuratemeasurement. Housing need to becompatible with appl icable pipingspecifications and pressure ratings.2 . P r e s s u r e g a g e s o r t r a n s m i t t e r s - - a naccurate pressure determination isrequired since the measured l iquid isalways metered at a pressure higherthan atmospheric and is in acompressed form. A compressibil i ty

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factor must be establ ished and appl iedto the m etered volume.3 . T e m p e r a t u r e t r a n s m i t t e r s a n ddevices-- l iquid is general ly measuredat some temperature other than base60F, in order to correct the volume to anet 60F volume, a volume correctionmust be made. This can be doneman ually, in a com puter (flow) or directlyby the me ter. An addit ional thermowel lis normally installed adjacent to thetransmitter to allow checks to be madeperiodically.4 . P u l s e g e n e r a t i n g d e v i c e c= pulsingdevice connected directly to the me ter isrequired for transmitting the meteredvolume and for proving the accuracy ofthe meter. The pulse rate varies withmeter size and type and typical ly maybe 1000 or 8400 pulses/Bbl for PDmeters and 500, 1000 pulses/Bbl for

turbine meters.5. Re gis ter ing dev ice s-- a local regis tershould be provided to show grossreadings. T his device is typically eith er amechanical or electronic totalizer.Increasingly Measurement RowComputers are uti l ized to accumulatenot only the pulses but also thetemperature and pressure variables a ndin some systems the f lowing real t imedensity.6. Pulsa t ion damp ers-- -meters located inl ine with positive displacement pumpsshould use a dampening device toeliminate the pulsating effects. Thetypical device usually consists of anappendage to the pipeline, which has aninflatable bag inside and is prechargedwith nitrogen.7 . A i r e l im ina tors - -a i r o r vaporeliminators should be installedpreceding the meter since largecompressed volumes wi l l tend toexpand through the meter causingoverspeed and serious damage. An aireliminator is usually an enlarged sectionof pipe or vessel that is vented andprovided with a level control device.8 . B ac k p r es s u r e c on t r o l - - s om einstallations require a method ofcontrol l ing backpressure on the metersufficiently high to prevent vaporizationacross the meter. API MPM S Chapter 5,section 3 recommends that thebackpressure be equal to or greaterthan twice the pressure drop across the

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meter at maximum flowing condit ionsplus 1.25 t imes the equi l ibr ium vaporpressure at flowing temperature.S t r a i g h t e n i n g vanes- - turb ine meterspreferably should have straighteningvanes upstream o f the meter. The vanescondition the l iquid flow profi le for t h eturbine meter. If straightening vanes arenot u s e d , suffic ient straight runupstream an d downstream piping mustbe provided to condition the flowingstream.

M E T E R P E R F O R M A N C EThe meter 's operating performance should beverified by proving it periodically. The frequency ofproving is guided by the type of operations, i .e.batched systems, regulatory requirements,agreements between part ies and operationalexperience. Therefore the prover is an integral partof any metering system. For the meter to registeraccurately its registration needs to be compared to adevice with a known volume. All provers' function inthe same manner , they measure the volume of f lu idpassing through the meter. The two measurements,the prover and meter, are compared in order todetermine a m eter factor as follows:

Meter Factor = Prover Volume / Meter VolumeThis meter factor is appl ied to the meter 'sregistration to correct i t.Th e types o f provers include the following:

Cal ibrated Prover Tan kPipe Prover Bi-directional Uni-directional Reduced volume Sm all volume

Master meters are uti l ized from time to t ime inspecific app lications.Meters that are used to mea sure l iquids of di f ferentdensities, viscosity's or other characteristics, whichmay affect meter sl ippage or volume corrections,should have meter factors developed for each typeof fluid. Additionally, meters subjected to varyingrates of f low should b e proved a t a suff ic ient numb erof points to al low preparation of a performance curveand an appropriate meter factor selected from thiscurve. For al l practical purposes, a change of 15

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percent f rom the base rate is considered to besufficient change to warrant multiple proving's.A l though there are a number of prover types andproving techniques vary in complexity they al lfunct ion in the same basic manner. The basicconcept of al l pipe provers is the same; accumulatethe registrat ion of the meter during a t ime when thedisplacer through a pipe of known volume. Thisknown volum e is called the cal ibrated sect ion and isprecisely def ined by the detector switches. As thedisplacer passes the detector switch, it activates theprover counter to commence col lect ing the pulsesgenerated by the meter and when the displaceract ivates the second detector, the counter stops i tsaccumulation of pulses. Adjusting the total pulses bythe meters K-factor yields the meter's indicatedvolume, which can then be compared to the proversknown volume. This rat io yields the meter factor forthe given m eter.Prove r Cal ibrat ion-- - the known volume of theprovers calibrated section must be preciselydetermined. T his cal ibrat ion process for pipe proversi s known as a waterdra w calibrat ion. Tank proversare calibrated util izing a similar process known aswaterfil l method. Using precisely calibrated TestMeasures the prover is cal ibrated in a mannersimilar to proving a meter. As water is circulatedthrough the prover, i t moves th e displacer. Whe n thedisplacer activates the first detector, a solenoidvalve is typically activated and the fluid is displacedfrom the prover into the Test Measure, stoppingwhen the second detector is act ivated. The processis repeated until a repeatability criterion is met. Thevolume recorded from these cal ibrat ion runs iscorrected to base condit ions and then related to thevolume of the cal ibrated sect ion. This volume atbase condit ions is cal led the Prover Base Volume,and becomes the bas is for the meter prov ingprocess. The process of cal ibration, the '~Vaterdraw"is covered in the API MPMS standard Chapter 4,sect ion 7 "Field Standard Te st Measures" and a n ewstandard API MPMS Chapter 4, sect ion 9 "ProverCalibration"

SAMPL INGWhile some of the physical propert ies needed tocomplete the measurement t ransact ion, such astemperature and pressure are measured usingelectronic transmitters. A means needs to be util izedto determine the density of the transaction. Th is canbe accomplished on-l ine using a vibrat ing elementdensitometer, as is done typically in refined product,l ight hydrocarbon liquids and NGL's or off-l ine using

a sampling system to capture the sample foranalysis.Regardless o f which test method is used for analysisone mu st f i rst start with a sample o f the hydrocarbonl iquid to be analyzed. Samples can be obtainedthrough tw o principle means:

Manual Sampling Autom atic Sampling

Manual sampling procedures and equipment areaddressed in ASTM D 4057 (API Ch 8.1) =ManualSampling of Petroleum and Petroleum Products".Automat ic sampling is covered in ASTM D 4177 (AP!Ch 8.2) "Automatic Sampling of Petroleum andPetroleum Products". The successful and accurateanalysis of any sample depends on the appropriatehandl ing and m ixing o f that sample from point ofextraction to its placement into the analyticalapparatus. These procedures are covered in ASTMD 5854 (API Ch 8.3) "Pract ice for Mixing andHandling of Liquid Samples of Petroleum andPetroleum Products".While both sampling methods can be used withmetering systems, depe nding on the qual i typarameter, most metering systems use theAutomatic Sampling method. From the samplereceiver m ultiple samples can be extracted fo r thevadous analytical tests.

SUMMARYA dynamic metering system al lows a degree offlexibil ity, that static measurement does not, inoperat ions, wh i le meet ing the exact ing requirementsfor Custody Trans fer Measurement. A typical systemis composed of mult iple individual components thatinteract to provide the information, conditioning orcontrol, w hich results in accurate measureme nt.The qual i ty of those individual components, theirinstallation, maintenance and calibration has a directbearing on accurate measurement. As we movefrom the mechanical systems to the electronicenvironment a nd increased au tomation, theserequirements will not decrease, but substantiallyincrease. Th e increasing use of Vibrating Elementdensitometer's for real-t ime density determination,especial ly for hydrocarbon stream with varyingcompositions, requires increased verif ication and

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proving of the primary device. Th e miniaturization ofelectronics has resulted in Measurement FlowComputers with the robustness and processingcapabi l i t ies, which used to reside on a mainframecomputer. However, this requires that the f lowcomputer be "cal ibrated" through the use of audittrails for both input data and calculation processes.As we ut i l ize new metering technology such ashelical turbine meters and integrate it into existingsystems we must search for measures forverif ication of its performance. Similarverif ication/proving concern s exist as the industryexplores the use of other metering technology suchas Coriolis Meters and Liquid Ultrasonic Meters.Both of these m eters are inferential m eters similar toturbine meters. However, they require signalprocessing integrated o ver time in order to generatea pulse output signal f rom their electronicspackages. The Coriolis m eter measures thedeflection force genera ted b y the fluid m ovingthrough the sensors. Th e Ultrasonic me ter measuresthe t ime i t takes for a pulse to travel f rom o ne sensorto another. The signals of both are processed and avolum e is imputed from the signal.The key to accurate measurement in a dynamicmo de is the requirement that all measurements mu stbe verif ied against a "master" device, which istraceable to NIST (National Institute for Standardsand Technology). T his veri fication must b eperformed on an ongoing basis.

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fundamentals of liquid measurement iii

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