Orifice Fittings and Meter Tubes - Measurement Solutions - FMC ...

once the clamping bar is removed. The orifice platebore is precisely positioned in the body bore by meansof precision machining of the carrier and the use of alocation dowel pin. Such positioning is positive, accurate,fast and assures compliance with measurementstandards such as API 14.3 which will be discussedlater in this paper.Single Chamber Orifice Fittings are used if frequentorifice plate changes are required and if the flow to theOrifice Fitting can be shut down, or by-passed and theline depressurized without costly interruption to the pipelineor process. Orifice fittings of this type do not requirethe removal of flange bolts or spreading of flanges toremove the orifice plate. These type fittings also avoidthe loss of liquid or gas from the pipeline, which occurswhen flanges are separated.Advantages• Simple to operate••Positive alignment of plate to line boreShort down time compared to OFUDisadvantages• Higher cost than Orifice Flange Union• Shut down or by-pass of line is required to changethe plate• Loss of liquid or gas in blow down processDual Chamber Orifice Fitting (Meter)Dual Chamber OrificeFittings are used whereit is necessary or desirableto remove theorifice plate from the fittingwithout interruptingthe flow of liquid or gasin the pipeline. Theyare commonly used formeasurement of naturalgas, custody transfer,transmission pipelinesor any installation wherethe pipeline can not beshut down.The Dual Chamber OrificeFitting has all theattributes of the SingleChamber Fitting plusthe advantage of orificeplate inspectionor change while underflowing conditions andline pressure. DualChamber Fittings areavailable in common sizes from 2” to 24” with larger sizesup to 48” having been manufactured and available onrequest. Available pressure classes range from ANSIClass 150 to ANSI Class 1500. Typical cast carbon steelbodies are configured in Flange by Weld as the mostcommon (Figure 6) and Flange by Flange (Figure 7).The body includes the same two sets of pressure tapsas described above.The Dual Chamber terminology evolved from the fittingsdesign as there are two chambers (dual) in the body. Alower chamber and an upper chamber isolated fromeach by a sealing mechanism. Under flowing conditionsboth chambers are under line pressure. To remove orinspect the orifice plate referring to Figure 8 the platecarrier assembly is retracted into the upper chamber bymeans of a non-rising elevator screw. A dry seal plugvalve in this design is rotated 90° sealing or isolating thedual chambers. While flow continues in the lowerchamber the upper chamber pressure is releasedthrough a bleed valve bringing it to atmosphericpressure. The cover plate clamp and sealing bar canthen be removed exposing the upper chamber.Continuing to rotate the elevator screw will lift the carrierplate assembly out of the fitting. The orifice plate canthen easily be removed inspected or replaced all whilethe pipeline remains in operation. The reverse of thisprocedure will again lower the carrier assembly into itsresting location in the lower chamber. Older designs ofthis style required the use of grease to lubricate the plugvalve. The current design requires no lubrication. Retrofitplug valve kits are available to upgrade the fitting fromthe old grease style to the dry seal type should themaintenance operation desire to do so.As with a Single Chamber Fitting the Dual Chamber designalso has a plate carrier. The plate carrier preciselylocates the orifice plate bore concentric to the body boreby means of three locator pins. Two pins locate the platehorizontally and one pin in the bottom of the body locatesthe plate vertically.Advantages••••••No shut down or by-pass of line requiredPositive alignment of plate to line boreMinimal down timeSimple to operateEase of maintenance and plate changeNo lost liquid or gas due to line blow down processDisadvantages• Highest price Orifice FittingDesign Criteria to Assure MeasurementAccuracyMost flow meters require flow calibration to verify accuracy.However, a properly designed Orifice Fitting willnot require flow calibration if designed and manufacturedin accordance with acceptable design practices. Sincethe beginning of Orifice Measurement numerous studiesand research along with field and laboratory testing hasbeen conducted to determine what physical hydraulicchanges take place in a pipeline when the liquid or gasapproaches and leaves the orifice plate. Conclusionsof all this work has determined that when a orifice plateis properly installed and maintained and the Orifice Fittingis properly designed and manufactured the OrificeFitting can provide an accuracy in the range of 0.5 – 1%.However, orifice measurement is sensitive to flowingconditions, piping installation conditions, precision ma-Issue/Rev. 0.0 (9/06)TPOR001 • Page

Figure 6 Figure 7 Figure 8chining of the fitting itself and its attachment to straightrun pipe plus the accumulative error of the secondaryinstrumentation. Overall accuracy of the meteringinstallation under average operating conditions couldbe in the range of 2 – 5%. If not properly maintainedand inspected periodically this accuracy could be in theorder of 10 – 25%.To increase the confidence level of obtaining the highestdegree of accuracy in orifice measurement the AmericanPetroleum Institute published installation guidelines andmanufacturing design parameters for the gas measurementindustry to follow. These recommendations are setforth in what has become known as API 14.3. The fulltitle is Manual of Petroleum Measurement StandardsChapter 14 – Natural Gas Fluids Measurement Section 3– Concentric, Square-Edged Orifice Meters Fourth Edition,April 2000. The European community has adoptedISO-5167 which is similar but not identical to API 14.3.The updating and evolution of these standards is a resultof all the research and testing that has taken place toformulate an industry best practice publication.If one wants to be assured of the highest degree of accuracywhen purchasing Orifice Fittings you must ask foran API 14.3 compliant fitting. There are Orifice Fittingsavailable that are not API 14.3 compliant and of courseare less expensive than those that are compliant. Toassure compliance one should ask the manufacturer forthe fittings calibration record on which specific measurementsand tests are performed and recorded to assurethe fitting is within the manufacturing design parametersrecommended in API 14.3.Some of the parameters are as follows:Tap HolesThe differential pressure sense points are thru the tapholes. The holes are to be drilled and taped radially thruthe fitting and perpendicular to the centerline of the fittingbore in the same plane. The inside edge of the hole isto be without any burrs. The hole diameter should bemeasured and recorded on the fitting calibration record.For 4” and larger fittings the holes are to be 0.5” diameter+/-0.016” and for 2” and 3” fittings the holes are to be0.375” +/-0.016”.Page • TPOR001The location of the holes is critical and their measurementshould also be measured and recorded on thefitting calibration record. Their location is 1” upstreamand 1” downstream from the orifice plate face. Theirlocation tolerance is fitting size and b ratio (beta ratio;ratio of orifice plate bore diameter to orifice fitting borediameter) dependent. The tightest tolerance is for a0.75 b design. Sizes 4” and larger it is 1” +/-0.035”, for2” and 3” it is 1” +/-0.015”.EccentricityThe orifice plate bore must be concentric with the boreof the fitting. The eccentricity maximum tolerance is sizeand b (beta ratio) dependent. When measured in a planeparallel to the tap holes both upstream and downstreamof the orifice plate, for a 0.75 b design and 2” size themaximum tolerance is 0.006” for 3” it is 0.009”. Whenmeasured perpendicular to the tap hole axis the allowabletolerance may be 4 times greater e.g. 2” is 0.024”and 3” is 0.036”. Refer to API 14.3 paragraph 2.6.2.1 forallowable tolerances. These as built dimensions shouldbe recorded on the fitting calibration record.Fitting HydrotestThe fitting should be pressure tested in accordancewith the relevant piping design code. This test is usuallyconducted at a pressure of 1.5 times greater than thefittings maximum allowable working pressure. Such atest will assure the integrity of the fitting body and sealsin preventing leaks when placed in normal operation.The pressure and time parameters should be recordedand made available for review.Plate Seal TestThere exists the possibility that the flowing liquid orgas may by-pass the orifice plate leading to inaccuratemeasurement. The fitting manufacturer should performa plate seal or bubble test to assure no leakage takesplace around the plate. A blind orifice plate (one with nohole) is installed in the fitting. The fitting is pressurizedupstream of the plate while a detection liquid is placedon the downstream side of the plate. Any evidence ofbubbles in the liquid is an indication of by-pass. Thetest parameters and results should be recorded on thefittings calibration record.Issue/Rev. 0.0 (9/06)

Tap Hole By-pass TestAs most fittings are cast carbon steel there exists thepossibility of casting porosity which may lead to pressureleakage in the tap holes. A vacuum is drawn on thetested hole. If no loss of vacuum pressure is witnessedthe next tap hole is tested etc. until all holes have beentested and verified no leaks exist. The test parametersand results should be recorded on the fitting calibrationrecord.Bore Surface RoughnessRoughness of the inside diameter of the fitting bore aswell as the straight run piping impacts the velocity profileof the liquid or gas as it approaches the orifice plate dueto the viscous drag along the pipe wall. In an attempt toduplicate all the research testing and maintain consistencyin fittings and piping a maximum recommendedsurface roughness is provided and should be checked.As with the other tests it is size and b (beta ratio) dependent.For a 0.75 b design and sizes 12” and smallerthe bore surface roughness should be no greater than250 μ inches (micro-inches). For sizes larger than 12”the bore surface roughness should be no greater than500 μ inches and in all sizes no less than 34 μ inches.Measurements should be taken in a plane 1” upstreamand downstream of the orifice plate. All measurementsshould be recorded on the fitting calibration record.Bore DiameterOne of the necessary flow rate computation componentsis the inside diameter of the bore. The bore should bemeasured in four axial planes (four measurements).The first axial measurement is in the same plane as thetap holes, 1” upstream from the face of the orifice plate.These four axial measurements are averaged and theresult is defined as the measured meter tube internaldiameter. Two additional upstream check measurementsare made. One of these is made in a region at least 2pipe diameters from the face of the orifice plate. Thelast check measurement location is undefined but shouldbe in a region greater than 2 pipe diameters from theface of the orifice plate. Check measurements are alsomade downstream of the orifice plate. The first is in thesame plane as the tap holes, 1” downstream from theface of the orifice plate. Two additional checks are madedownstream at unspecified locations.If all the above measurements are made, recorded andfall within the tolerances specified, the Orifice Fittingis said to be compliant to API 14.3 and should provideaccurate measurement results. The purpose of thispresentation was to address the Orifice Fitting andMeter Tube. In most cases the Orifice Fitting is suppliedwithout the orifice plate which may be added by ameter run fabricator, OEM, service company or end userto name a few. API 14.3 also recommends in similarfashion as above the design parameters, measurementsand tolerances specific to just the orifice plate for whichthere are numerous supply sources. Besides the OrificeFitting being in compliance it is just as critical to havepurchased orifice plates in compliance to assure measurementaccuracy.Meter TubesDue to the many varying installation conditions, researchhas shown that piping upstream and downstream of theOrifice Fitting must be consistent if any degree of accuracyis to be expected. The velocity profile of the liquidor gas as it approaches the orifice plate is important toassure accurate measurement. Pipe elbows in planeand 90° out of plane, partially open valves, thermowells,probes etc. may and most often will distort the velocityprofile. To preclude these effects, API 14.3 recommendsa minimum length of straight pipe be installedupstream and downstream of the Orifice Fitting. Thislength is again dependent on the installation b ratio plusthe type of piping configuration existing upstream. Theminimum recommended length of unobstructed straightpipe varies from 13 to as much as 145 pipe diameters(internal pipe diameters) upstream of the orifice plateand 4.5 pipe diameters downstream of the orifice plate(Figure 9).Such pipe length requirements are usually not practicalat most installations. To minimize liquid or gasswirl caused by piping configurations and reducethese lengths, Flow Conditioners were developed.The first conditioner applied was the 19-Tube BundleFlow Straightner. This device is a cluster of 19 smalldiameter tubes that fits in the piping bore upstream ofthe Orifice Fitting. The straightner will aid in the removalof any swirl as the liquid or gas passes throughthe tubes. Thus, the velocity profile is conditionedprior to entering the Orifice Fitting. With the use ofsuch a conditioner, the minimum upstream length maybe reduced to 29 pipe diameters with the downstreamlength being 4.5 pipe diameters at a max b ratio of0.67 (Figure 10).Additional research has been conducted to improve flowconditioning and thus reduce installation cost. FlowConditioner Plates have evolved and may now be usedprovided they pass a series of tests defined in the APIstandard. Most all conditioners available today havepassed the necessary testing with documented results.Having done so, API recognizes such devices and theiruse. With the installation of such a conditioner in lieu ofthe 19-Tube Bundle, the upstream pipe length may bereduced even more lending to a reduced space requirementand reduced cost. The minimum upstream lengthmay now be 17 pipe diameters with the downstreamlength the same as above (Figure 11).Requirements of pipe lengths with or with out flow conditioningrequires the purchasing of more than just theOrifice Fitting. End users will purchase a Meter Runconsisting of the Orifice Fitting, usually a flow conditioneralong with the necessary lengths of pipe all fullyassembled and pressure tested (Figure 12). OrificeFittings are attached to the pipe either by welding (mostcommon) using a Flange x Weld Orifice Fitting or witha flange connection using a Flange x Flange OrificeFitting. The welding approach is most often preferredas it provides assurance that the pipe bore is smoothimmediately upstream of the orifice plate. The finishedIssue/Rev. 0.0 (9/06)TPOR001 • Page

weld is machined, honed or ground smooth. If a flangeconnection is selected care must be exercised duringinstallation to assure the fitting bore and the pipe boreare concentric and the same diameter and any flangegaskets do not protrude into the bore.Pipe connections to accommodate thermometers, pressuregauges or transmitters, sample probes or blowdownare not permitted within the described lengthsabove. Most of these connections are placed in thedownstream section. The first connection can be nocloser than 4.5 pipe diameters from the downstreamface of the orifice plate. Additional connections areusually spaced every 6” from the first connection. Thedownstream section length then becomes dependent onthe number of connections plus the minimum 4.5 pipediameters (Figure 12).To maintain consistency of manufacture and assurethe highest degree of measurement accuracy, measurements(similar to those conducted on the OrificeFitting) are recommended in the API 14.3 standard.Pipe lengths should be measured and recorded as built.Pipe bore diameter measurement checks are made inthe weld region and further into the upstream tube aspreviously described as well as measurement checksmade in the downstream section as described. Surfaceroughness checks should also be made on the pipebore in the upstream and downstream sections whichfollow the requirements of the Orifice Fitting describedpreviously. Additionally, piping roughness upstream ofthe section should not exceed 600 μ inches.13 - 145D 4.5DFigure 913D29D4.5DFigure 10Page • TPOR001Issue/Rev. 0.0 (9/06)

FLOW CONDITIONERPLATE7.5D17D4.5DFigure 11FLOW CONDITIONERPLATECOUPLINGS4.5D 6"Figure 12ConclusionWith the price of natural gas on the rise reaching alltime highs the need for measurement accuracy is everincreasing. Compliance with the new Sarbanes-Oxleylaw which tightens the control of financial reporting, directlyaffects a companies operating procedures relatedto the measurement of production volumes and thusaccurately recording revenue and reporting financialresults. Tightening measurement procedures, OrificeFitting inspections and assurance that Orifice Fittingsmeet the highest possible standards are steps towardreporting correct revenue and improving company profitability.Insisting on procurement of API 14.3 compliantOrifice Fittings and Meter Tubes is a first step.Issue/Rev. 0.0 (9/06)TPOG001 • Page

ReferencesThe following references are among those used in the preparation of this paper.1. American Petroleum Institute (API), Manual of petroleum Measurement Standards (MPMS), Chapter 14– Natural Gas Fluids Measurement, Section 3 – Concentric Square Edged Orifice Meters, Part 2 – Specification andInstallation Requirements, Fourth Edition, April 2000.2. Nesler, Tim, “Gas Measurement Has Key Role In Sarbanes-Oxley Law Compliance,” Pipeline & Gas Journal,September 2005, page 44.Note: This paper was presented at the ISHM meeting held in the spring of 2006.The specifications contained herein are subject to change without notice and any user of said specifications should verify from the manufacturer that the specifications are currentlyin effect. Otherwise, the manufacturer assumes no responsibility for the use of specifications which may have been changed and are no longer in effect.Headquarters:500 North Sam Houston Parkway West, Suite 100 Houston, TX 77067 USA, Phone: +1 (281) 260-2190, Fax: +1 (281) 260-2191Gas Measurement Products:Houston, TX USA +1 (281) 260-2190Thetford, England +44 (1842) 82-2900Kongsberg, Norway +47 (32) 286-700Buenos Aires, Argentina +54 (11) 4312-4736Integrated Measurement Systems:Corpus Christi, TX USA +1 (361) 289-3400Kongsberg, Norway +47 (32) 286-700San Juan, Puerto Rico +1809 (787) 274-3760United Arab Emirates, Dubai +971 (4) 331-3646Liquid Measurement Products:Erie, PA USA +1 (814) 898-5000Los Angeles, CA USA +1 (310) 328-1236Slough, England +44 (1753) 57-1515Ellerbek, Germany +49 (4101) 304-0Barcelona, Spain +34 (93) 201-0989Moscow, Russia +7 (495) 564-8705Melbourne, Australia +61(3) 9807-2818Visit our website at www.fmctechnologies.com/measurementsolutionsPrinted in U.S.A. © 9/06 FMC Technologies Measurement Solutions, Inc. All rights reserved. TPOR001 Issue/Rev. 0.0 (9/06)Beijing, China +86 (10) 6500-2251Singapore +65 6861-3011Chennai, India +91 (44) 450-4400