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(TP0A013) Liquid Measurement Station Design

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    James C. FiedlerlntroductionThe purpose of this paper isto review the criteria used inthe design of liquid measurement stations. After reviewing this paper, the reader should have a better understanding of the pivotal items to consider in this design,as well as an appreciation for the major componentswhich are a part of liquid metering stations.Design considerations can be separated into four majorareas: acCracy, space, pressure lss, and cost. Majrcomponents will be reviewed as they constitute the metering skid, the prover skid, and the instrumentation contrais. In most cases, the metering systems reviewed areused in high-accuracy, custody transfer applications.Typica app ications inc ude tanker and barge oadingand offloading, pipeline to storage measurement, LACT(Lease Automatic Custody Transfer) units, metering intoand out of refineries, ship bunkering, and iow-temperature applications such as LPG or Butane.

    ccuracy ConsiderationsSystems can be designed to have different degrees ofoverall accuracy. In some cases this is defined by localweights and measures requirements, the end user's standards, or the national standards to which the end usermust strictlv adhere. The overall svstem flow caoacitvmust be clarly defined, resulting in 'the proper selectioiof the number and sizes of meter runs. This will insurethat the meteis a e ope;ating not only within theii designcapacity, but also at or near their ideal flow rates. lfrequired, the meters can be supplied with premium linearity to ensure the highest degree of overall accuracy isachieved. The low flow rate also needs to be reviewed toensure the meters still maintain their required accuracyunder these low flow conditions. Flow circumstancesmay require that a smaller meter be included in thedesign i be uiilized during ihe siart-up and ipping-ffoperations. Viscosities also affect some meter srangeability and accuracy. This information is usuallylisted in the meter manufacturer's specification datasheets published by the manufacturer.Proving meihods and frequency f pmving will greailyimpact the system's accuracy. The standards associated with these are, again, normally set by local or national authorities. The majority of high-accuracy custodytransfer metering stations includes some type of a stationary volumetric prover. The exception to this would beLACT units, which are normally calibrated with a por-

    Liquid MeasurementStation DesignTechnical Paper

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    table prover. The different types of provers commonlysed foi these applications will be ;eviewed late i in thispaper. Some applications have required that a smallerprover be supplied along with the larger prover. Thesmaller prover is then used in combination with a master meter to confirm the accuracy of the main prover ona regularly scheduled basis. Frequency of proving isdetermined by local requirements, changing conditions,and possible wear due to corrosiva products. In alicases, pmvers are calibrated and waterdrawn at thefactory (usually to N.I.S.T.) and calibration certificatesshould be supplied with the final documentation package. Today's microprocessors allow for automatic, online proving; can calculate a meter factor; and generatethe required proving report.System designs can include different types of compensation, such as temperatura, pressure, and density.Temperatura compensation is the most common andthis can be accomplished through mechanical means(ATG) ar through today's microprocessor f ow computers. Microprocessors will have a much faster responsetime and allow the actual A.P.I. formulas to be loadedinto memory. Both of these capabilities result in a higheroverall degree of accuracy.Meter se ection type can a so affect the system accuracy. Most liquid meter systems are designed aroundeither positiva displacement (PD) or turbina meters.Operating conditions such as fiow rates, products, viscosity, temperatura, pressure, and type of service shouldali be considered in the final meter selection.Space Considerationslf space is very limited or expensive (such as on anoffshore platform), then it can become a prime factor inthe overall system design and selection of major components.Typically, a turbina meter system will require more spacethan a PD meter system, dueto the requirement for theupstream and downstream straightening sections. However, the application may still require that turbina metersbe utilized. The size and number of meters will alsoaffect the overall meter skid size.Not only the selection, but the design of the prover willhave an impact on the prover skid dimensions. Smallvolume provers would normally require the least space.Other orovers (bidirectional. unidirectional) can bed o u l e ~ l o o p e d for a more compact design'. Launchchambers can be at 90 degrees, 45 degrees, or straightthrough, ali varying the prover skid height and length.

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    The prover and meter skid can be ~ e s i g n ~ into oneintegral package to reduce space.Th1s des1gn may bemore costly (extra engineering, piping, and design support), but if space is the prime concern, this may be thesolution that you require for your application.

    Pressure Loss ConsiderationPressure ioss costs money in terms of wasted energyand capacity. lncreasing the size of the headers, meters,and valves will ali reduce the pressure drop through thesystem and prover. Supplying 100% full-port openingvalves versus 70% opening valves will reduce the pressure loss But is the extra associated cost for your appli-cation worth this change? Strainer bodies and the inletloutlet nozzles can also be increased to reduce pressuredrop. Strainer mesh size will likewise impact the pressure drop. Special launch chambers and bypasses onthe prover can be investigated for additional pressureloss reductions. The overall system pressure drop mustbe considered at the maximum flow rate of the systemwiih one metei in being pmved. This wHI ensue thatthe pressure drop through the prover is included in thiscalculation.

    ost ConsiderationsOverall system cost is usually the prime considerationby the en user in his esign. Tha finai s a i a c t i ~ n of ~ h ealternativas already reviewed will have a SIQnlflcant lmpact on the final price. Major cost items to considerinclude:Meter Selection TypePD's versus turbinas. Typically, on larger size metersturbinas will be less axpensive, but will they meet theother criteria for this application? There would also bethe trade-off for the larger meter skid if turbinas areselected.Prover Selection TypeBiirectionai provers are utiiize in the majority of appiications. Others to consider include small volume provers and unidirectional provers. Alternativas, includingthe use of a single-loop design or a double-loop design,also need to be investigated.CompensationThe degree of compensation (temperatura) V fill impactthe system cost. Mechanical o l l p e n ~ a t i o n will normallybe less expensive, however, w1th th1s method of comoensation it will orove difficult to achieve the accuracy

    a ~ a i i a b l e with today's electronics. The choice becomesaccuracy versus cost.AutomationNormally, the higher the degree of automation, thegreater the initial cost, however, the higher levei automation-type systems will probably save time (manpower,loading duration), improve the accuracy, improve safety,and provide management with more current and detailed reporting.

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    SafetySafety requirements - beginning with the alarm system through the construction cedes, offshore cedes,drio oans. and safetv relief valves- will ali impact costr r - - - - - - - - - - - - - - ~ShippingShipping costs can be extremeiy important, especiaiiy ifoverseas transportation is required. A few more dollarsspent in proper design may allow the system to bebroken down for shipment, greatly reducing the overallshipping cost.StandardizationStandardization (if you have more than one system ) onmeter sizes, valve sizes, and the type of instrumentation may allow you to reduce the quantity and value ofoperational spare parts requirements.Otherit may be more expensive to design grading, wa kways,and items such as lights and overhead cranes mto theinitial systems package. However, these particulars maysave a great deal ofmoney in the field during start-up,recalibration, or when maintenance on major compo-

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    Metering Skid omponents (See Figure 1The inlet and outlet header of the metering skid areused for the field connections to the inlet and outletpiping. The entire system flow capacity will travei throughthese headers, and therefore, they must be properlydesigned to handle this flow capacity. Velocity throughthese headers is normally held at 13 feet per second or

    ~ ~I ....... @ ) ( P ) ( i ) ~ , . . . . . . . , II II II I

    Figure

    Legend) ~ ( ] Block Vai ve

    t9 J Flow Contr. Valve@ Detector Switch@ Flow Meter Pressure lnstr.0 Temperatura lnstr.

    III

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    less. Headers can be extruded, tees, ar fabricated. Location of in-field connections needs to be identified. Typically, these can be at either end of the header ar in thecenter. The outlet header will also have a connection forthe prover return line. The most well-balanced system istypically to allow for the flow to enter at one end of thein et header and exit through the oppos te end of theoutlet header. This is sometimes referred to as the "Z"configuration.Coming off the inlet header are the flanged connectionsfor the individual meter runs. The system flow is splithere through t I.JO or more nieter runs. The componentsof each meter run will be identical.The first meter run component is normally an inlet valve.This valve is customarily a gate-type valve and its mainpurpose is meter run isolation for maintenance (clogged. strainer, equipment repairs, etc.).Since this valve is infrequentiy used, it is usually supplied with a manual operator. lt is not mandatory thatthis valve be of the high integrity, double-block-andbleed type necessary for proving.Deaerators (optional) may be required if there is thepossibility of entrained air (heavy crude oils) ar largebatches of air, as seen when completely off-loading aship ar a storage tank. Deaerators can be supplied withbaffle plates and a number of air release heads. Thearger diameter in the body of the deaerators reducesthe velocity of the product, allowing the air bubbles toaccumulate on the baffle plates and move to the top ofthe deaerator. From here, the air reiease heads wiiiallow the trapped ai r to escape.Strainers are supplied to act as a protective device. Aspreviously mentioned, the body sizes and flange connections can be altered to reduce pressure drop. lfdeaerators are not inciuded in the system, then an airrelease head would normally be mounted on the top ofthe strainer. Various types of quick-opening closuresare available for these strainers: This allows for quickaccess to clogged strainers or for the replacement of

    ,...' ',.,,..,.,.1 11 ..-.tr ... inl \r h , . , ~ l r l " l t ~ r \ i : " t . l t ~ O ~ u . 1 t , . . . h . o . ~ I '.O:.n ho. inU Q I I I O . Y C U L I Q I I I ' O ' I L IQ U n.V LW . L JV IL Q=I ..-:lYWILVI IVo;) VQ UV 111=cluded to check for clogged strainers. Drain valves arenormally supplied on the strainers, as this is a low pointof the system.The operating conditions and products will help with the,..._l,...,,..ti .., ,...f t hn unn f. 1 1 \l lti: t.r / n n ~ i t i u o r l i ~ n J " = ' " ' . o . , . , . , . a n t nr.: .ciCVLIUII VI U I O LYtJ v 't::av \ t J V W U i v v UI..;:JtJIClvvv n vturbina meters). lf turbina meters are being utilized, thenupstream and downstream straightening sections willalso be included. As a minimum, it is recommended thata backup counter and a transmitter be mounted on thetop of the meters. The entire system is built around themeter and proper care should be exercised in the selection of the type and size of the meters. The meter size(flow rate) will determine the size of the pove. tv1ltiplemeter runs are supplied to add flexibility, rangeability,and support the most economical cost alterative. Themajority of custody transfer systems is supplied with aspare meter run. The system can then operate at fullf ow capacity with a backup or standby meter in case ofdamage ar maintenance requirements. The spare meterrun should be rotated in and out of service with the othermeter runs.

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    Temperatura and pressure gauges and transmitters arelocated downstream of the meter. They are used for netvolume corrections and for determining the correct meterfactor when proving.Flow contrai valves (butterfly type) serve a number ofpurposes within the meter system. They will ensure thatihe meier is flowing wiihin its specified operating range.They will, likewise, ensure that the system flow is equallydispersed through the various on-line meter runs. Onlight products, such as LPG's, they will maintain backpressure on the meters to prevent flashing.Each meier rn has a connection to the prover inletheader. Only one meter product flows through the proverat one time; therefore, the prover header is designed tohandle a single meter run flow rate.Downstream of the flow contrai valve and on the inlet tothe prover header are located high-integration, doubleblock-and-bleed valves. The valve located on the meterrun serves two purposes. The first isto isolate this meterfor proving. The second is to isolate (working with themeter run inlet valve) the meter run for maintenance.This va ve is norma y supp ied with an e ectric or hydraulic actuator for remate operation, allowing for automatic proving. The integrity of these valves during proving can be verified through the doubie-blok-nd-bieedfeature. The double-block-and-bleed valve on the proverinlet ensures that during proving, no product from othermeter runs (those not -being proved) can bypass thisvalve and leak into the prover. lf this were to happen,the meter fa.ctor of the meter being proved wou d beincorrect and, most likely, you would not be able toobtain a repeatable and acceptable meter factor.Other items to consider in the meter skid design includethe support structure, electrical wiring, conduit, signalcab e, junction boxes, uti ities avai abi ity {power, air,hydraulics), thermal relief valves, walkways, grating drippans, and the drain system. Lifting eyes and spreaderbars can be suppiied for transporting and positioningthe system in the field. Anchor and bolt holes should beproperly located to match the field support structure.

    rover Skid omponents (See Figure 1)The prover is sized to handle the maximum flow throughone meter run. Provers can be mounted on the meterskid, but generally are loaded on their own skid foundation. These can be either fullv skid-mounted. or semiskid-- - - - - - - - - - I - - - - - - - - - ' - -mounted. Common to most provers are dete torswitches, a sphere or piston, exchange valves, and alibrted or certified measring section. Prover inletsand outlets would include pressure and temperaturainstrumentation for either manual or automatic proving.Most prover internais have some type of a coating (suchas an air-dried epoxy) for protection and a smooth flowof the sphere through the measuring section. n extremely severe conditions (product or temperatura), thiscoating may require a special material and to be bakedonto the internai surfaces. Vents and drains are usuiiysupplied at the high and low points of the provers. Toensure a stable temperatura during proving conditions,the prover can be heat-traced or insulated. lf this is

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    required, then usually ali piping from the meter downstream through the prover and back to the prover returnline will also be insulated or heat-traced. The four majortvoes of provers incorporated into custody transfer meterstations 'include: . . Bidirectional (Bidi) nidirectionai (ni) Low Temperature Piston Small VolumeBidi provers can be of the straight-ype design, singleloop, or double-loop. These provers incorporate a fourway vaive to reverse the fiuid fiow and to iaunch thesphere into the measuring section. An area of prerunlenath is included, precedina the detector switches, toensure the four-way valve is properly sealed prior to thesphere making contact with the first detector switch.Launch chambers are normally at a 90-degree angle tothe measuring section, but can be at 45 degrees orparallel to the measuring section. The design shouldrequire that one launch chamber be suppiied with aquick-opening closure for removal and inspection of thesohere. To ensure that no oroduct bvoasses the sohere.itis ; ~ r m a l l y sized a few percent lrger than the 'insidediameter of the measuring section pipe.Unidirectional provers operate under the same principieas bidi provers, except that the product flow is always inthe same direction. Uni orovers incoroorate an interchange valve to capture, 'seal, and lau'nch the sphere.Uni provers can reflect cost savings on larger metersizes.Low temperatura piston provers (-50 degrees F) areused to measure products such as LPG and Butane.The principie is the same as the straight-type bidi provers. However, they incorporate a piston (in place of thesphere) and special low tempeatre materiais for thepipe and fittings. On this type of prover, the launchchambers are in line with the measuring section andsized with the same pipe diameter.Small volume provers are being accepted and used incustody transfer applications. Some of thei advantagesinclude size, space, weight portability, full automation,and the ability, with a single prover, to calibrate numerous sizes of meters. Offshore platforms have provenideal for the use of small volume provers. These provers use a piston and optica or aser detector switches.lnstrumentation ontraislnstrumentation to contrai the metering system can beclassified into three major types: local, discrete, andfully-automated.Locai instrumentation woui be used on LACT-type systems. This equipment may include mechanical or electronic temperatura compensators, totalizers, transmitters (for proving, sampling) and a ticket printer. Thesewould normally be unmanned sites, operating 24 hoursn o r r i < : : ~ ~ \ JtJV Al A J

    Discrete instrumentation could be of the CMOS (Complimentary Metal Oxide Semiconductors) type whereone instrument is dedicated to each function. Theseinstruments would be housed in a contrai panellocatedin remote-contro ed environment. The pane wou d4

    probably include an alarm annunciator system and amimic panel. This type of system contrai is sometimesreferred to as "semiautomatic." Each instrument is dedicated to one function: one for totais, one for temperatura correction, one for batching, one for flow contrai,etc. Valves are normally opened and closed withpushbuttons ocated on the mimic pane .The vast majority of systems being supplied today areof the fully-automated type. With the enhancement ofmicroprocessors, these systems are capable of functions such as:ii Redundancy. Flow contrai. Automatic proving. Automatic meter factor calculation. Meter linearization. Customized reports and alarm systems. Graphics display of on-line activities. Interface to other computer systems. Batch contrai. Enhanced safety and security.Such systems are norma y operated from keyboardnd VOU. The panel-mounted instruments pose as abackup to this supervisory system. Two printers arenormaiiy requested: one for reports and one for event

    logging. When alarms occur, this system can be proarammed to shut the entire meter station down in asafe, secure sequence.GeneralOther equipment which is sometimes included in themeter system design includes samplers, BS&W, densitometers, pumps, and pump contrais. Ali systems shouldbe fully-tested at the manufacturer's facilities prior toshipment. This would include a hydrostatic test, as wellas fu -f o\J test lith the centro console. Documenta-tion should include "as-built" drawings, ali instrumentation and calibration certificates, and operating manuaisfor each piece of equipment in the system.

    onciusionAs seen above, the design of iquid measurement sys-tems is affected by a number of different factors. Whatis of key importance to one user may be a secondaryissue to another user. Even on systems with similar fiowrates, the number and sizes of meter runs may differ. ltis extremely important in the specification stage andearly in the' desgn stage to completely understand thealternatives and how they will affect the system designand the overa l performance of the measurement system. With this understanding in hand, you should havethe ability and knowledge to design a system ideallysuited to meet your specific requirements.cknowiegementThis paper was originally presented by the author at thelnternational School of Hydrocarbon Measurement(ISHM) at the University of Oklahoma on May 21-23,1991

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