2. Manual of Petroleum Measurement Standards Chapter 2-TankCalibration Section 2B-Calibration of Upright Cylindrical TanksUsingthe Optical Reference Line MethodMeasurementCoordinationDepartment FIRST EDITION, MARCH 1989REAFFIRMED,MAY 1996 American Petroleum Institute HelpingYou Get TheJob Done Right? 3. SPECIAL NOTES 1. APT PUBLICATIONS NECESSARILYADDRESS PROBLEMS OF A GENERAL NATURE. WITH RESPECT TO PARTICULARCIRCUM- STANCES, LOCAL, STATE, AND FEDERAL LAWS AND REGULATIONSSHOULD BE REVIEWED. 2. API IS NOT UNDERTAKING TO MEET THE DUTIESOFEMPLOYERS, MANUFACTURERS, OR SUPPLIERS TO WARN AND PROPERLY TRAINAND EQUIP THEIR EMPLOYEES, AND OTHERS EXPOSED, CON- CERNING HEALTHAND SAFETY RISKS AND PRECAUTIONS, NOR UNDERTAKING THEIR OBLIGATIONSUNDER LOCAL, STATE, OR FEDERAL LAWS. 3. 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THIS PUBLICATION WILL NO LONGERBEINEFFECTFIVEYEARSAFTER ITSPUBLICATIONDATEAS AN OPERATIVEAPISTANDARDOR, WHEREANEXTNSIONHASBEEN GRANTED, UPON REPUBLICATION.STATUS OF THE PUBLICATION CAN BE ASCERTAINED FROM THE API AUTHORINGDEPARTMENT MATERIALS IS PUBLISHED ANNUALLY AND UPDATED QUARTERLY BYAPI, i220 L STREET, N.W., WASHINGTON, D.C. 20005. SOMETIMESAONE-TIME EXTENSION OF UP TO TWO YEARS WILL BE [TELEPHONE (202)682-SWIl. A CATALOG OF API PUBLICATIONS AND Copyright O 1989American Petroleum institute 4. FOREWORD Chapter 2.2B of the Manualof Petroleum Measurement Srandards should be used in conjunctionwith API Standard 2550. Measuremenr and Calibration of UprightCylindrical Tanks. Standard 2550 is currently being revised andwill eventually be published asChapter 2.2A of the Manual ofPetroleum Measurement Standards. API publications may be used byanyone desiring to do so. Every effort has been made by theInstitute to assure the accuracy and reliability of the datacontained in them; however, the Institute makes no representation.warranty, or guarantee in connection with this publication andhereby expressly disclaims any liability or responsibility for lossor damage resulting from its use or for the violation of anyfederal, state, or municipal regulation with which this publicationmay conflict. Suggested revisions are invited and should besubmitted to the director of the Measurement CoordinationDepartment, Amencan Petroleum Institute. 1220 L Street, N.W.,Washington. D.C. 20005. iii 5. CONTENTS SECTION 2B-CALIBRATION OFUPRIGHT CYLINDRICAL TANKS USING THE OPTICAL REFERENCE LINE METHOD2.2B. 1Introduction.................................................. 12.2B.2 Scope....................................................... i 2.2B.3Definitions................................................... 12.2B.4 Referenced Publications....................................... 1 2.2B.5Significance.................................................. 12.2B.6 Equipment................................................... 1 2.2B.7Optical Calibration Procedure .................................. 22.2B.7.1 Tank Status Before Calibration............................. 2 2.2B.7.2Preparation............................................... 22.2B.7.3 Instrument Verification.................................... 2 2.2B.9 Development ofCapacity Table ................................ 5 2.2B.7.4Calibration Procedures..................................... 32.2B.8 Calculation Procedure......................................... 5 Figures I-Tank Plan(Shown for Eight Horizontai Stations).................... 3 2-TankElevation.................................................. 4-tical Reference Line Method: Determination of Internal Radius(External Measurement).......................................... 64-Optical Reference Line Method: Determination of Internal Radius(Internal Measurement) .......................................... 72 Table ]-Tank Diameter Versus Minimum Number of HorizontalStations...... V 6. Chapter 2-Tank CalibrationSECTION2B-CALIBRATION OFUPRIGHTCYLINDRICALTANKS USINGTHE OPTICALREFERENCE LINE METHOD 2.28.1 Introduction The optical referenceline method (ORLM) is an altemative method to the manual tankstrapping method (MTSM) for determining tank diameter. The primarydifference between the ORLM and the MTSM is the procedure fordeterminingtank diameterat shell courses other than the bottomcourse. The ORLM provides for measuring a reference diameter on thebottom course by manual strapping and measuring deviations in tankdiameter at other predetermined horizontal and vertical stations byusing an optical device. The other required special measurements,procedures. methods. and analytical tools for the development of atank capacity table are identical to those stated in API Standard2550. 2.28.2 Scope This standard describes measurement and caicula-tion procedures for determining the diameters of u p right welded(laphutt) cylindrical tanks, or vertical Cylindrical tanks, with asmooth outside surface and either floating or ked roofs. Thisstandard should be used in conjunction with API Standard 2550. Forexternal application of the ORLM on insulated tanks, the insulationmust be removed. The ORLM may be used on the interior of the tank;however, when the reference circumference on the bottom course isestablished externally, the insdation at the bottom ringmust beremoved.Thismeasurementshall conform to the procedures described inAPI Standard 2550. Abnormally deformed tanks (that is, tanks thatare dented or have other visible signs of damage) should berepaired before any calibration is undertaken. 2.28.3 DefinitionsThe definitionslisted in Chapter i and API Standard 2550 areapplicable to this chapter. In addition, the definitions given in2.2B.3.1 through 2.2B.3.7 apply to the ORLM. 2.28.3.1 A horizontalstation is a preestablished location in the horizontal plane atground level along the tank circumference. 2.28.3.2 A verticalstation is a preestablished loca- tion in the vertical plane alongthe tank shell, corre- sponding to a given horizontal station.2.28.3.3 The ofiset is the measurement observed through an opticaldevice on the horizontal scale of a traversing magnetic trolley ateach vertical station. 2.283.4 The reference offset is themeasurement observed through an optical device on the horizontalscale of a traversing magnetic trolley at the vertical station onthe bottom plate. 2.28.3.5 The reference circumference is thecircum- ference of a tank measured by the MTSM on the bottom ring.2.28.3.6 The reference radius is the reference cir- cumferencedivided by 272, where P = 3.141593. 2.28.3.7 An optical device isan optical plummet or a theodolite equipped with a precision level.2.28.4 Referenced Publications The following publications arereferenced in this chapter: API Std 2550 Measurement andCalibration of Upright Cylindrical Tanks (ANSVASTM D 1220) Manualof Petroleum Measurement Standards Chapter i , Vocabulary 2.28.5Significance Accurate tank circumference/radius measurements arecritical in determinations of liquid volume. These measurements areinfluenced by both random errors associated with physicalmeasurements and systematic errors associated with the particulardevice andlor procedure used. The combination of random andsystematic errors has an impact on the overall preci- sion, oraccuracy, of any measurement. The ORLM is as precise as the MTSMand can be used as an alternative method of determiningtankcircumferences or diameters (radii). 2.28.6 Equipment Thefollowingequipment is required for the ORLM: a. The equipment andapparatus described in API Standard 2550. 1 7. 2 CHAPTER 2 f A N KCALBRATtON b. An optical device mounted on a tripod or anequivalent supporting device. The device should have aperpendicular line of vision (aW e g r e eline of sight) and shouldhave leveling fixtures along three different axes.Theperpendicularity ofthe opticalray (orvertical reference line) shallbe within i in 20,000over the tank height. c. A traversing magnetictrolley with a horizontal graduated scale (0.01-foot orI-millimeter increments) used to measurethe offset at differentvertical stations. The magnetic trolley shall maintain its contactwith the tank shell during the entire vertical traverse. Thetrolley shall be in good working order. and the zero on the scaleshall be located closest to the tank shell. The numencal valuesetched on the scale shall be readable through the optical device.The scale's ac- curacy shall be verified against a master tape. Thecombined resolution of the scaleand the optical device shall enablethe operator to read the offset measure- ment to the nearest 0.005foot (or 1 millimeter) at any given vertical station. (Since theminimum resolution of the metric scale is i millimeter, this systemhas a better overall resolution.) 2.28.7 Opticai CaiibrationProcedure The step-by-step procedure described in 2.2B.7. i through2.2B.7.4 should be followed to measure de- viations in the radiusof a vertical storage tank by means of the ORLM. 2.28.7.1 TANKSTATUS BEFORE CALIBRATION Before calibration, the tank shall havebeen filled at least once at its present location with liquid atleast as dense as the liquid it is expected to contain. The usualhydrostatic test (for a period of approximately 24 hours) willusually satisfy this requirement. Any hydrostatic test should beperformed in accordance with applicable construction and operatingstandards. Wherepossible, the liquid in the tank shouldbe allowedto stand still for approximately 24 hours before cali- bration isperformed. Other requirements for tank status are specified in APIStandard 2550. 2.28.7.2 PREPARATION The minimum number ofhorizontal stations shall be selected in accordance with Table i.Additional stations, beyond the minimum specified in Table 1, maybe selected, but an even number of stations shall always be chosen.The stations shall be spaced as nearly equidistantly around thetank circumference as Table 1-Tank Diameter Versus Minimum Numberof Horizontal Stations Minimum Number of Stations Tank DiameterFeet Meters IS u) 50 70 85 100 u0 8 u 16 U) 24 30 36 ispossible.Figure i showsan exampleof the horizontal spacing. Horizontalstations shall be chosen to ensure that the vertical traverse ofthe trolley along the shell at any given station is at least i2inches (300millimeters) away from the vertical weid seam. Thetrolley shall be traversed at each horizontal station to ensurethat there is no interference with the vertical weld seam. If thereis interference, the horizontal station shall be relocated toeliminate the interference. For the bottom ring, the preferredvertical station is (a) at 20 percent of the course height belowthe upper horizontal weld seam and (b) within the focal range ofthe optical instrument. If Criteria a and b conflict, the locationwill be dictated by the focal range of the equipment. For any givenhorizontal station, at least two vertical stations shall beestablished for each ring, except for the bottom ring, as shown inFigure 2. 22B.7.3 INSTRUMENT VERIFICATION The optical device shouldbe leveled along three axes at each horizontal station. Theperpendicularity of the optical ray or verticai reference lineshall be verified as follows: a. The magnetic trolley shall bemoved all the way to the top of the tank, and the reading on thehorizontal scale attached to the trolley shall be noted. Theoptical device shall not be permitted to move during thismeasurement. b. The optical device shall be rotated 180 degreesabout the vertical axis, and the reading on the hori- zontal scaleshall again be noted. The optical device shall not be permitted tomove during this measure- ment. The difference between the twomeasurements shall be within I in 20.000 over the total height ofthe tank. For example. if the tank height is 60 feet, thedifference between the two measurements should be less than 0.003foot. However, .since the minimum resolution of the scale is 0.005foot (or i millimeter), the acceptable difference between the twomeasure- ments is 0.005 foot (or i millimeter). 8. SECTION 2 M W BR A T I O N USING THE OPTICAL REFERENCE LINE METHOD 3 / /I / Figure1-Tank Plan (Shown for Eight HorizontalStations) c. Steps a and bshall be repeated randomly at three horizontal stations (includingthe first station) during the course of the calibration. 2.28.7.4CALIBRATION PROCEDURES The calibration measurements may be takenwith any fill height of liquid in the tank. No transfer or mixingof the liquid shall occur during the calibration. Thetemperatureandthe relative density or API gravity of the liquid and the height ofthe liquid shall be recorded prior to the calibration procedure.Measure- ments shall proceed in the following manner: a. Thereference circumference shall be measured using a master tape. Thereference circumferenceshall be measured at or very near the heightof the first vertical stationon the bottom ring (wherethe referenceoffset shall be measured as shown in Figure 2). The proceduresgiven in API Standard 2550 shall be fol- lowed for the referencecircumference measurement. b. The optical device shall be verifiedas being in a stable position. At any given horizontal station, thedevice should not move or be disturbed during the traverse of themagnetic trolley from the bottom ring to the top. c.Theperpendicularity shall be verified(see2.2B.7.3). and thereference offset (for example, Distance a in Figures 3 and 4) shallbe measured from the first horizontal station. d. The trolley shallbe vertically traversed to the next predetermined vertical stationin each ring, and the offset (for example, Distance m in Figures 3and 4) shall be read. This procedure shall be repeated se-quentially at each of the vertical stations. e. After the offset inthe topmost course is read, the trolley shall again be lowered tothe bottom ring and the reference offset measurement shall berepeated. The initial and finalreference offsets at each horizontalstation shall be within 0.005 foot (or i millimeter) of each other.If the initial and final reference offsets for a horizontal stationdo not agree within 0.005 foot (or 1 millimeter), the procedureshould be reinitiated, beginning with Step c. f. Steps b-e shall berepeated for each horizontal station. g. After the opticalmeasurements around the tank are completed, the referencecircumference measurement (Step a) shall be repeated using the samemaster tape. The initial and final readings shall agree within0.005 foot (or 1 millimeter). If they do not agree, the 9. 4CHAPTER '-TANK CALIBRATION I I I I l pticaireferenceline +yMagnetictrolley Gradua!&, 4horizontal scale :-opticaireferenceline [email protected] I 12"(300mm) i-I I I h,kRing height 1 l 1 20% h,[email protected]+k' Horizontalstation Figure 2-Tank Elevation 10. S E C z Al J B R A T I O NUSING THE OPTICAL REFERENCELINE METHOD 5measurements shall be repreated from the beginning of theprocedure. When the windgirder is located below the tanks maximumfill height, the tank circumference measure- ments above the windgirder shall be performed by the MTSM. 2.2B.8 Calculation ProcedureThe calculation procedure to determine the radius of each ring fromthe offset measurements and the reference circumference measurementis presented in Figures 3 and 4. Once the ring radii aredetermined, the capacity table shall be developed in accordancewith the procedures given in API Standard 2550. 2.28.9 Developmentof Capacity Table The ring circumferencedradii determined by theORLM shall be used to develop the strapping table in accordancewiththe proceduresgiven in API Standard 2550. AI1 correction factors(for example, temperature and tilt) shall be considered. inaccordance with API Standard 2550. 11. 6 CHAPTER ;-TANK CALIBRATIONCENTERUNE FLUSH optical4 I reference : line OUTSIDEFLUSH Note:Using external measurement. the calculation lo determine internalradius is performed as follows: R',+ m, = R + a R', = R + a - m, =R - (m, - - i,] + IZ(rn, - a ) h For n horizontal stations.Similarly. R, = [(CRn)- i,] + IX(m2 - a)l/n Where: C = referenceouter circumference. R = reference outer radius (bottom ring) t, =thickness of bottom ring. R,= reference internai radius(bottomring) u = referenceoffset. m,. rn, = individual ringoffsets. = C/%. = R - i l . R,,= internal radius (bottom of secondring). R, = internal radius (top of second ring). A similarcalculation shall be performed for all other rings. CENTERUNE FLUSHreference Tank kntedine OTSlDE FLUSH INSIDE FLUSH Figure 4-OpticalReference Line Method: Determination of Internat Radius (InternalMeasurement) 13. 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