(tp0v001) theory and application of pulse lnterpolation to prover systems

8/12/2019 (TP0V001) Theory and Application of Pulse Lnterpolation to Prover Systems

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FMC mith eter IncAn FMC Corporation subsidiaryPeter P Jakubenas

This paper discusses a fractional pulse counting technique, which in its application to flow measurementproving), has made small volume provers a practicalreaiity.HistoryIn May 1978, the API Manual of Petroleum Measurement Standards was revised to allow bidirectional provers to e sized on the basis of a minimum number ofpulses 10,000) per half trip of the displacer from themeter being proved.Prior to 1978 the API Standard for bidirectional proversrecommended a calibrated provervolum f 0 5o f thmaximum hourly flow rate. A 2,500 BPH prover wasrequired to have a volume of 12.5 barreis or 525 gallons.

Theory and pplication of ulselnterpolation to Prover SystemsTechnical Paper 126

Bulletin TPOV001For certain sized meter/prover combinations, the calibrated volume could be made smaller; thus, a smallerprover could be used with the associated lower cost.Due to the change in the requirements, various techniques have been developed to obtain additional pulsesper unit of voiume for meters used for petroieum custody transfer measurement, primarily positive displacement and turbina meters.How are Meter Pulses Produced?In arder to fully understand the pulse interpolation discussion which follows, we need to know somethingabout the source of meter pulses used for proving. Thepulses obviously are produced by various deviceswithin or attached to the meters themselves.

Seal Monitor Sensor On-Board Microprocessor

Piston ReturnActuator

Bypass SeallntegrityPressure Sense HoseBypasfvalve /

lnlet FlangeBypass Flange

Bypass Valve Actuator

Figure 1 - Typical Small Volume Prover

Vertical LiftStop

HydraulicReservoir

Pressure Sensing Hoseto Balance Valve


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Turbine meter pulses are usually produced by theaction of a hi-mu button set into a rotor rim or bymagnetic rotor blades themselves. This button, rotatingpasta reluctance type pickup coil, produces a sine waveoutput which can be made into a square wave or otherconfiguration using electronic circuits. Up to fourpulses per button or blade can be produced. In usingthese multiple pulses for proving, it is important thatthey represent true roto r motion, which is proportionai

I o l l , :: ooiduod > O ~ I s remwill not operate. Fluid flow is illustrated withbypass valve ful ly open, the piston downstreamand the selected meter run on-li ne with uninterrupted flow.(2) ldle Sll teThe piston is fully downstream atter completing the prove cycle. The bypass valve remainsclosed until another proving cycle is commanded. The unit automatically remains "poweredup" for ten minutes unless manual shutdown ormultiple proving cycles are selected.

(3) Retum StateThe bypass valve is opened momentarily, thepiston is commanded to return to its upstreamposition. Fluid flow in the piston chamberbriefly changes direction in response to pistonmovement.

(4) Pre-Prove StateWhen proximity sensing finds the piston fu yextended, and the bypass valve fully closed, alitlow s directed through the measurement chamber.

(5) Proving StateWhen bypass valve seal integrity is achieved,the piston is launched into the measurementchambe. As the piston assembly and instiment rod retract downstream, optical detectionis triggered when the instrument rod flagpasses each detector. Simultaneously, meterpulses and proving data are transmitted. Dynamic seal integrity is monitored while the piston passes through the measurement chamberand transmitted to the analog device atthe endof piston travei.

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to volume through the meter. Usually the rotor buttonsmust be approximately equal in size to the spacebetween them, in order for multiple pulses per button torepresent equal increments of rotor motion.On positive displacement meters, optical transmitters,in combination with gears, are normally used to givepulse outputs even at zero velocity. Using various combinations of gears and opticai discs, a variety of puises

r :Figure 2 - Typical Small Volume Prover - Sequence of Operation

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per unit volume are usually available; however, there area number of practical limitations on the maximumpulses available per unit volume. These pulses represent equal volume increments, only if the associatedgear trains and optical disks are made with greatprecision.In order to gain even more enhanced resolution fromeither type of meter pulse, interpolation techniques areused.What is Pulse lnterpolation?Most of the techniques used to achieve additionalpulses are dependent on resolving o r interpolating the. fractional difference between a) the start of the calibrated proving volume and the first meter pulse, and b)the last pulse before the end of the calibrated provingvolume and the end of the proving volume. This thenallows counting ofthe full pulses, plus the two fractionalamounts, and adding of the two fractional amounts tothe whole pulses, alternatively, the fractional pulsebetween the end of the proving volume, and the pulseafter the proving volume can be subtracted from the fullpulses counted. The sum of the two fractional amountscan vary from slightly more than zero to just less thantwo comolete oulses. The orecise resolvina of thesetractio-riaptllse S is-teirmed .pulseinterpolation. fhe ability to accurately and economically perform this interpolation, paved the way for provers with relatively smallvolumes.Why Resolve Fractional Pulses?Resolving fractional pulses into a large number of partsallows the use of considerably smaller volumes between detectors. The use of smaller volumes presentsthe following advantages:a. Proving time is considerably shorter using a smallvolume prover. A proving which previously required35 seconds, now only requires one second.b. A smaller amount of fluid is required to pass through

the prover to accomplish the proving. This is significant when proving, for example, a load rack meter,where the fluid used must pass into a relatively smalltank truck compartment. A 525 gallon prover can bereduced to 15 gallons.c. Due to short proving time and compact size, onesmall volume prover can serve a large number ofmeters, eitherthrough permanent piping orthrougha portable arrangement for meters equipped withquick connection devices.d. For equivalent maximum flow rates, a small volumeprover is less costly than a bidirectional or unidirectional prover.e. Applications with restricted space requirements,such as offshore platforms, favor use of small volumeprovers.f. Small volume provers offer rangeability of the arderof 1,000 to 1; thus, one prover can serve a largervariety of meter sizes.Figure 4shows a typicai smaii voiume prover an Figure2 shows the sequence of operations.What Techniques are Used to Resolvethe Pulses?Various techniques can be used to perform pulse interpolation. They involve multipie timers andior highspeed electronic devices. Fortunately modern microprocessors are able to be programmed to perform thehigh speed computations required.

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Four Timer TechniqueOne technique is the low frequency pulse technique o rthe four timer technique, said to be developed by theJapanese (see Figure 3). Note that TA, TB, TC, and TDare each timers with start and stop representing displacement detectors. TB and TD represent time forcomplete pulses immediately adjacent to those to beresolved. TA represents a time proportional to the fraction of the puise after the start to be ae to N, thewhole numberof pulses, and TC represents a fraction ofa whole pulse beyond the stop which must be subtracted from N. Thus, the exact number of pulsesbetween start and stop is:N' = N + TA _ TCTB TDNote that this method has the advantage that TB and TDare measured adjacent to the fractional pulses and areproportional to the flow rate at the beginning and theend of the proving run; thus, it can compensate for flowrate variations during proving.Phase Locked Loop TechniqueThe second method, which is attributed to the British, isthe phase locked loop technique (see Figure 4). A voltage controlled oscillator is sed to podce highfrequency pulse stream at a factor, X, higher than theinput pulse stream. This pulse stream is continuouslydivided by X and compared with the incoming streamby a phase comparator. The phase comparator outputis routed via a filter to the contrai input of the voltagecontrai output. The effect of the comparator/filter com- : ~ : : . . . . . . . . . . ; . . . . . 1 ~ H h,.., \ I r ' r\ ...,. - l,l"''t.. "''on VUII ICI .L IUI I LU tJI C \.tl>)t:;;ly OUJUO:.L LI IV V VULtJUL LV nt - nan integral number through the use of a precise dividecircuit, 1/X. This electronic flywheel compensates forchanges of flow rate pulse by pulse. The output is Ntimes X. The phase locked loop method can reliablyinterpolate only symmetrical meter pulses.Dual Pulse ChronometryThe third techniq ue, developed by the French, is termeddual pulse chronometry and is illustrated in Figure 5.High resoiution crystai controiied ciock puises (approximately 150,000 Hz) are gated into counter T1, by integral meter pulses beginning with the first meter pulseafter the first detector switch and ending with the firstmeter pulse after the second detector switch. Theseclock pulses are termed T1. N is the number of wholemeter pulses which occurred in the time interval represented by T1. T1 /N is equal to the number o clockpulses equivalent to a whole meter pulse.A second precision counter, T2, is capable of accmulating the same high resolution clock pulses from thesame source as CounterT1. Counter T2 starts accumulating clock pulses when the first detector switch isactivated and stops after the second detector is activated. T2 then represents the number of clock pulsesequivalent to the precisely calibrated prover volume.By dividing T2 by the pulses per whole meter pulseT1/N, we can then determine the magnitude of thefractional meter pulses which occur between the firstdetector and the first meter pulse and the last detectorand the next to the last meter pulse. This formula reducesto N (T2/T1) and is equal to N' the exact number ofwhole and fractional meter pulses between detectors.Ali of the above methods have been used for pulseinterpolation with acceptable results.


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n T_B_FirstDetector

2

TA Telnterpolated Pulses N = N TB ToExample: N = 6,100TA= 4Ts= 45Te= 1

NSecondDetector

To= 5o 40 1N1 = 6,100 45 50 = 6,100 0.888-.02 = 6,100.888

FirstDetector

igure 3 our Timer or Low requency Pulse Technique

SecondDetector

A

B

Phaseeomparator

igure 4 Phase Locked Loop Method

4

VoltageeontrolledOscillator

OutputN 1 = N Times Xlnterpolated Pulses


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Application to Small Volume Proverslndependent ofthe method of pulse interpolation, smallvolume provers must be constructed with detectorswitches that are repeatable to better than 0.02 or 2parts in 10,000. A typical small volume prover opticaldetector is, in fact, repeatable to 0.001 inch in 27.4inches or 1 part in 27,400. Detectors which determineth prclse volume mst be monted on a mateiialwhich does not change the length between switchesdue to temperatura changes. This is usually an invartype of material.In addition to the pulse interpolation electronics, smallvolume provers are typical ly equipped with electronicsto provida control and data collection functions.What Other Components are Necessary forUse of the Technique with SmallVolume ProversA small volume prover performs the proving function bydisplacing a volume of fluid through a cylinder with apiston. The piston is propelled through the cylinder bythe flow of the fluid itself. When the prover is idle, thepiston is removed from the flow stream. Normally, anrn\ inn l ' n n ~ t i h 1toc: tho rlic::.nl::al ' inn n f fh lirl rl rinn rnn.. ... ,,, :::11 .... .. ........ '.""'"'"'"' :::ll ................ ' ' ' ' ~tion of the piston in one direction. The prover is cockedor prepared for a proving stroke by moving the pistonupstream using an externai pneumatic, hydraulic, ormechanical means. When the piston is being movedupstream, the flow is diverted through a bypass path inorder to not shut off the flowing stream. The bypass isthen c osed anda f ow is through the ca ibrated measuring section. The piston is mechanically, hydraulically,

Turbine Meter

or pneumatically pushed or launched into the flowstream and carried to the opposite end o the measuringsection by the flow stream itself. The displacement ofthe piston is communicated out of the cylinder by adevice attached to the piston. This device carries a flagto activate the displacement sensing equipment. Positiva sealing of both the main piston and the bypassarrangement is essential to proper small volume proveroperation.Piston seals are normally of the dual cup type and aredesigned to positively seal from one side of the piston tothe other. In order to prove the integrity of the seal,monitoring of the cavity between the seals is essential.This can be done using various pressure or differentialpressure sensors.In order to properly seal, and not cause excessiva sealwear and provi de accuracy through the displacer stroke,smooth surface finish, roundness, and concentrici ty ofthe measuring section cylinder bore is essential. Cylinder bores are normally honed and plated and have athick wall for dimensional stability. Seallife and cylinderbore life can be greatly increased when using dirtyproducts if the measuring section is mounted vertically,so particulate matter can not collect in the bore. lt is alsodesi rable to have the seals leave the bore on each strokeso they may be flushed by the fluid stream.During the upstream stroke of the main piston, equipment must be provided to divert flow. This is usuallydone with some sort of bypass valve. lt is desirable toconstruct the bypass arrangement so that seal integritycan be monitored. This can be done using double sealswith pressure monitors in between. Norma y; thebypass is an integral part of the prover.

N1 = Number oi pulses withenhanced resolution.N1 = N (T2/T1)Example:N1 = 900 (140507/140443) = 900.41013

(1st Detector)Flag

2nd Optical Detector1st Optical Detecto

Reset~ ~ - - ~ - - - - - - - - - - - - ~

Figure 5 - Dual Pulse Chronometry Pulse lnterpolation5


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The electronics package for the small volume provermust not only receive the meter and the detector switchsignals and do the pulse interpolation calculation aspreviously explained, it must also control the entiresequence of operations in order to repeatedly operatethe small volume prover. Microprocessor-based equipment is well suited to the application and is normallyused. The control system must withstand the ambienttemperatura variations in the area used and be suitablefor hazardous areas as defined in the National ElectricCode.What Are Code Requirements Applicable toSmall Volume Provers?Meter proving equipment usually must meet a numberof national and local codes. The measurement accuracyrequirements are specified in the API Manual of Petroleum Measurement Standards and National 8ureau ofStandards Handbooks.Design, fabrication, and inspection of the pressureretaining parts are specified in ASME/ANSI Standards831.3 for Refinery Applications and 831.4 for PipelineAppiications. These standards reterence the ASME-8oi ler and Pressure Vessel Code Sections l iA, IIC, V,VIII, and IX, and API Standard 1104.

Electrical safety requirements for hazardous areas arecovered by the National Electric Co de and National F reProtection Association Publications.For applications overseas the corresonding nationalcodes would apply. For ship-mounted equipment theAmerican 8ureau of Shipping, U.S. Coast Guard, DetNorske Veritas, Lloyds, 8ureau Veritas, or other standards wou d be app icab e.ConclusionPulse interpolation techniques combined with presentelectronic, optical, mechanical, and hydraulic technology have allowed small volume provers to be technically and practically feasible. Ali requirements of theAPI Manual of Petroleum Measurement StandardsSpecifications can be met. Small volume prover performance is equal to, o r superior to, that of conventionaluni or bidirectional provers, and they offer a largenumber o f advantages over their predecessors beca useof compact size and speed of operation. As with anyother precision measurement device, the small volumeprover must be properly specified and applied.AcknowledgementTh s pape r was origi nally presented by the author at thelnternational School of Hydrocarbon Measurement(ISHM), University of Oklahoma, 1986.

Ths spsc_ifications c o n t a i ~ e d hansin ie Sbjeei t change wlihu1 f i i l ~ a nd any usar ai sai_ s p e c i f i c a ~ i o n s shouid verify from the manufacturer that the specifications arecurrently n effed_ Otherw1se, the manufactur er assumes no responsibilrty forthe use of spec1flcat1ons wh1ch may have been changed andare no longer in effectHeadquartors 1602 Wagner Ave . P.O. Box 10428, Erie, PA 16514-0428, Phone 814/898-5000, Telex: 19-9902, Fax: 814/899-8927Smith Systoms Oper . 737 North Padre lsland Dr., P.O. Box 4658, Corpus Chrrst1. TX 78469. Phona: 5121289-1100, Telex 650-369-12t4, Fax: 512/289-11 t 5E. Homisphe ro Oper . Regent Strassa, 2087 Ellerbak, P O Box 6t 0246. 2000- Hamburg 6t Germany. Phona: (49) 4 tot -3040, Telex: 17410134. Fax (49) 4101-304133Sales Off1ces:Hous ton t467 Brittmoora, Bldg. 2, Houston. TX 77043. Phona: 713/464-2526, Telex: 697581 o. Fax: 713/464-3024New York P.O Box 246, Roselle, NJ 07203, Phona: 908/24 t-6073, Fax: 908/298-9025London Ambassador Housa 181 Farnham Road Slough SL 1 4XP, B a r k s h ~ r a . England, Phona (44) 753-571515 Telex 846765, Fax (44) 753-529966Barcelona V1a Augusta t 25 Desp 1 ~ 7 a , E-? l_D1?6-Bar :elona,l>_e_ lln_ l l ~ n e @43) 201-0989, Telex 98375, Fax (343) 201-0576 ar 200-0534, Telex 98375B ~ t i i a i n P O Box Si 20, Mnama, ;::,tate m wnra.m, -'nane ~ / 3 ) 116-949, t-ax (973) 713-597S1ngaporo Shaw Centre 1 Scotts Rd Hex 26-02, S1ngapora 0922. Phona (65) 732-3822, Telex. RS23460 AOSING, Fax: (65) 732-5784Pnntad in U.SA 4193 Smith Matar Inc. Ali rights reserved.lssua/Rav. O.O(t/86)

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(tp0v001) theory and application of pulse lnterpolation to prover systems

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