Vertical Cylindrical Storage Tank Calibration Technologies and Application. Srini Sivaraman SK Japan March 2012 API Conference & Expo Singapore 2012

Transcription

1 Vertical Cylindrical Storage Tank Calibration Technologies and Application Srini Sivaraman SK Japan March 2012 API Conference & Expo Singapore

2 Calibration Overview - 1 Process by which the volume in a tank in relation to the liquid height (up to maximum fill height) is established The diameter of the courses is determined by field measurements using following technologies Reference Standards: API Chapter 2.2 A: Manual API Chapter 2.2 B: Optical Reference Line Method (ORLM) API Chapter 2.2 C: Optical Triangulation Method (OTM) API Chapter 2.2 D: Electro Optical Distance Ranging Method (EODR) API Standard 2555: Liquid Calibration 2

3 Calibration Overview - 2 All new tanks must undergo calibration Have access to internal for accurate deadwood determination Datum plate (reference plate at the bottom) flatness and level check and correction as necessary Calibration after successful hydrostatic test All tanks in service must undergo recalibration or re computation Recalibration at set frequency or after repair Either set by customs or local regulations General informative guidelines per API Chapter 2.2 A Re computation only under certain conditions 3

4 Frequently Asked Questions Do you have to empty the tanks for re calibration NO Can you calibrate the tank at any liquid level Yes Can you calibrate the tank when the tank is moving (receiving or discharging) No, you cannot calibrate the tank while the tank is in motion. The tank level must be steady and no movement in and out of the tanks Does product temperature impact volume Yes. It results in expansion of tank shell wall and the additional volume can be significant Can calibration be undertaken over the insulation No, not for custody transfers and inventory 4

5 Frequently Asked Questions Can you calibrate the tank when the tank is full of water for hydro test Yes. Once the test is completed you can calibrate the tank full of water, de-stress the tank to zero stress condition and re-stress the tank for the actual gravity of the product What is the impact of gravity in tank calibration For a given liquid level the hydrostatic pressure is a function of gravity and this results in tank expansion. If not accounted for it could impact the tank volume significantly depending on diameter and thickness of shell Also FR must be compensated for buoyancy that is function of gravity Gravity of course is needed for VCF Do you need traceability for working tape calibration Working tape is calibrated by master tape and master tape is calibrated per National Standards 5

6 Calibration Process Parameters Following operational parameters must be supplied by the tank owners to the calibration contractor Product Temperature Product Gravity Roof Leg Position for FR ( Critical Zone: Figure 1) Zone needed for FR to float fully from rest position (no custody gauging in this zone) Ambient Temperature Maximum Fill Height ( depends on safety rules) These must not be decided by or assumed by the calibration contractor 6

7 Note: Critical Zone Position varies with FR position CZ Roof Legs Floating Roof CZ FR in Maintenance Position FR in Operating Position Floating Roof Position Figure 1 7

8 Manual Method API Chapter 2.2A Calibration by Manual Method Often is referred to as the Referee Standard (basis for all other methods) Circumference measured with a working tape at various courses Working tape is Calibrated against a master tape and applicable tension determined for actual application Master tape readings generally are at 68 deg F Tape is maintained physically in perfect contact with shell Tape is maintained in horizontal plane Stroke the straps two or thee times to ensure perfect contact with the shell Single strap or multiple straps may be used Multiple straps with a smaller length tape are preferred as they are easy to handle They are easy to maintain control, contact with shell and horizontality All circumference measurements are external Tank shell surface must be clean 8

9 Field Measurements-1 Calibration in the field involves following physical measurements Circumference measurement of each course (Figure 2) Using working tape calibrated with appropriate tension Multiple straps or single strap at each course Typical tape length of 100 ft may be used Number of Straps required = π * D( ft) Plate Thickness Measured ultrasonically all around in each course (8 to 12 data points) and averaged for each course Diameter 100( ft) Computed from the measured circumference and the thickness 9

10 Note: Ideally scaffolding fixed or portable needed to maintain tape in contact with shell C A 48 to 64 ft B AB, BC, CA : Three straps (An example) A to A : Single Strap (heavy) Multiple Straps easier to handle Manual Calibration : API Chapter 2.2A Figure 2 10

11 Field Measurements-2 Reference height and reference gauge point (Figure 3) Critical component of calibration For new tanks easily established For old tanks the bottom access to datum plate may not be possible as bottom maybe filled with solid sludge or other foreign materials If access is not available one should not try and measure RH but use the RH from previous calibration table Gauge point is the point from which gauging should be undertaken The gauge point should be clearly marked on the stilling well Critical Zone (Figure 1) On empty tanks roof leg position can be verified physically On tanks in service, information may be taken from the last tank calibration table Typically this is in the range of 6 to 12 in but could be as high as 18 in depending on FR design 11

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13 Field Measurements-2 Deadwood All internal piping and other structures inside are physically measured and their volumes distributed vertically from the datum plate This is necessary to subtract the volume of the deadwood as tank table is developed (volume Vs height) This is possible only when entry is permitted into the tank, if not it should be taken from the most recent calibration data FR Weight During calibration FR weight is collected either from old table data or physically measured and computed. But computation could potentially carry large uncertainty Number of welding rods that are used in the FR fabrication must be taken into account or else could understate the FR deadweight Best obtained from the fabricator and documentation on file maintained for all future calibration 13

14 Field Measurements-3 Maximum Shell Height This height is measured and documented as part of the development of the tank table Measured externally from the base Maximum fill Height Depends on local conditions Earthquake zones ; 4 to 6 feet below the top rim Others limited by FR height Bottom Calibration Tank bottom could be flat, cone up or cone down Tank bottoms are measured by physical survey when entry is allowed Tank bottoms may also be calibrated with liquid (water) When in service the zero gauge volume is copied from old tables Zero gauge volume is the volume below the datum plate Tilt Measured optically or manually by plumb line 14

15 Capacity Table Development The capacity table is simply a table that gives the volume of the tank at any given height In the development of the table following corrections should be applied FR buoyancy correction Tank tilt correction Hydrostatic correction Shell temperature correction Master tape correction Working tape correction Other correction such as tape rise 15

16 FR Buoyancy Correction Corrections 1 Correction is based on gravity of the product and FR weight FR correction (volume units) must be subtracted from the total volume at any given level as long as FR is fully floating In critical zone the FR correction is distributed over the range of the zone Below the critical zone FR correction is zero Tank table carries the base FR correction for a given gravity and incremental correction for variations in base gravity Tilt Correction No correction needed when tilt is less than 1 in 70 Tilt correction is requires when tilt exceeds the above value Maximum tilt should be less than 2.4 in

17 Hydrostatic Head correction Corrections -2 Hydrostatic head (liquid pressure) causes expansion of the tank shell Additional volume from pressure expansion may be as high as 0.08% depending on plate thickness Expansion function of plate thickness and gravity for a given tank API 2.2 A provides detailed procedure for calculating the incremental volume and the total volume for pressure correction The additional incremental and total volume is generally included in the capacity table for a given gravity of the product Variation in gravity of +/- 5 deg API will have negligible impact on the computed volume If the gravity changes by more than 10 deg API the table must be rechecked 17

18 Corrections - 3 Shell Temperature Expansion Correction Shell expands due the combined effect of product and ambient temperature The impact on total volume could be 0.05% and higher The shell temperature determination equation has been modified from the old API Standard 2550 It is no longer the mean of ambient and product temperature In the new equation product temperature dominates Tanks which are insulated, the shell temperature equals product temperature The temperature expansion factor may be included in the main capacity table for a give product and ambient condition or The capacity table may be established at 60 deg F or 15 deg C and the shell temperature expansion factor may be applied externally for each batch received or discharged from the tank with actual field temperatures The capacity table may also be accompanied by a temperature expansion factor table when the capacity table is at 60 deg F 18

19 Corrections - 4 New Shell Temperature Equation T T T S L A 7* TL + TA = 8 = Liquid Temperature = Ambient Temperature Master Tape Correction Tape carries calibration to 68 deg F Measured lengths should be corrected to 60 deg F Other Corrections Tape rise correction, if needed, should be applied 19

20 Recalibration Frequency Informative Appendix in API Chapter 2.2 A provides guidelines Recalibration is required on all tanks if internals are modified Recalibration may also be mandated by local regulations or customs Recalibration is required if the tank bottom repair work is undertaken 5/15 rule for tanks in Custody Service Bottom course verified once every 5 years for diameter, thickness and tilt Variations in D, t, and tilt (from previous calibration) are computed and impact on volume determined If variation in volume is in excess of 0.02% recalibration is recommended If variation in volume is within 0.02%, 5 year verification is continued until 15 years when total recalibration is recommended 20

21 Working Tape Re calibration Working Tape should undergo re calibration after application on 20 tanks, once every 20 tanks Working tape should undergo re calibration if it is to be used on a tank or tanks whose circumference(s) vary by more than 20% the circumference of the tank on which the tape was originally calibrated Master tape should be re certified once every two years. 21

22 Re Computation Re computation/verification of table required when gravity changes by 10 deg API or higher Diameters from the last calibration may be used to compute the new volumes for gravity changes Re computation required when average product temperature has changed by 20 deg F or higher (if the temperature correction is built into the table) Capacity table revised to reflect New RH if the stilling well is extended on top with a nozzle for alternate gauging devices. 22

23 Capacity Table and Raw Field Data All raw data collated in the field should be made available to the tank owner along with main capacity table Capacity table should generally contain following information at the very minimum: Product ID, RH, Nominal Diameter Product Gravity, Product temperature Critical Zone FR total and incremental correction Shell Temperature correction table if capacity table at 60 deg F Appropriate foot note if corrections are already built into the table RH and Reference gauge point location Method of calibration and date of calibration Certificate of calibration of working tape and master tape Signature of the certifying authority API Standard number (e.g. 2.2A) used in the calibration 23

24 Optical Reference Line Method (ORLM) Reference Standard: API Chapter 2.2B This method establishes diameters of the courses by optical method The method can be applied internally or externally (external easier) Procedure (Figure ) Tank divided into horizontal and vertical stations Number of stations horizontally vary from 8 to 36 depending on diameter Magnetic trolley with graduated scale moved vertically Reference circumference of bottom course by manual method (API Chapter 2.2A) Reference offset is measured optically at the same height where the reference circumference is measured At each horizontal station, course offsets are measured (Two per course) optically Deviations in course offsets from the reference offset are averaged for each course Using the reference circumference and deviations the course diameters are established 24

25 E F D Optical Device G C H B A Optical Reference Line A, B.Horizontal Stations HORIZONTAL STATIONS OPTICAL REFERENCE LINE METHOD A Vertical Station:Typical B Optical Reference Line NOTE: Plan view shown for 8 stations h/5 h/5 Course Height h Weld Seam Magnetic Trolley Scale Reference Offset 300 mm Reference Diameter Optical Device Optical Reference Line Method (ORLM) Figure 4 25

26 ORLM Important Considerations Optical device stability is critical Device must be in level in all directions The optical ray must be vertical throughout the height of the tank (within limits) At each station reference offset is rechecked b after the full vertical traverse The optical device is checked randomly at three locations for perpendicularity by rotating the device 360 deg In extreme windy condition, when it is difficult to maintain the trolley in contact with the shell, calibration should not ne undertaken Other Measurements Identical to manual method API Chapter 2.2 A Development of the Capacity table Per API Chapter 2.2A Advantages Much easier, no scaffolding and reference circumference is easier to control being at the base 26

27 Optical Triangulation Method (OTM) Reference Standard : API Chapter 2.2 C This method establishes diameters of the courses by optical method The method can be applied internally or externally (internal easier) Procedure (Figure 5) Tank is divided into horizontal and vertical stations for both internal and external methods Tank profile is established by triangle at each target point and hence the name OTM For internal method reference distance D is established optically using temperature compensated Stadia typically 2 m long Subsequently tank coordinates A(x, y) are measured optically using two theodolites For external method the tangential angles are measured along with the distance between the two theodolites ( T 1 T 2 ) Diameters are computed using mathematical computational procedures 27

28 X T, L = Theodolites A(X,Y) : Coordinates A(x, y) T 1 + T α D β Y D T 2 + * AN * A2 Target Points (A1..AN) T 1, T 2. For External Calibration T D, For Internal Calibration * A1 A1, A2.AN Horizontal Stations h Ring 3 D = Reference Distance α, β : Coordinate Angles Ring 2 h/5 h/5 A1 A2 Vertical Stations AN Ring 1 OTM: Internal and external Figure 5 28

29 Important Considerations OTM Optical devices stability is critical Devices must be in level in all directions Distance D for internal method should be measured again at the end Other Measurements Identical to manual method API Chapter 2.2 A Development of the Capacity table Per API Chapter 2.2A 29

30 Electro Optical Distance Ranging Method (EODR) Reference Standard: API Chapter 2.2 D This method is for Internal application only Like ORLM and OTM the method establishes diameters of all courses Procedure (Figure 6) Establish a reference target on the bottom course and note the reference distance and reference angle Spherical coordinates are measured using distance ranging device (r, α, β) for each target point Tank profile is thus established from bottom to top The reference distance of the target and the reference angle of the target at the end are rechecked Using standard mathematical procedures, diameter of courses is computed With an on line computer, diameters can be determined instantaneously 30

31 r, α, β : Spherical Coordinates α : Vertical angle β : Horizontal angle to reference target Target Points α r β Ref. Target + Optical Device Electro Optical Distance Ranging Method : EODR Figure 6 31

32 EODR Important Considerations Optical device stability is critical Device must be in level in all directions The measurements at the reference target at the end of the tank traverse should be repeatable Other Measurements Identical to manual method API Chapter 2.2 A Development of the Capacity table Per API Chapter 2.2A 32

33 Reference Standard: API Standard 2555 Level Vs Volume is established directly Liquid Calibration Volume Q 1 is metered ( volume through meter that is calibrated prior to start of the tank calibration) and corresponding level L 1 is measured Increments will depend on tank diameter and generally should be 6 in to a foot Recalibration of the meter at the conclusion is required In liquid calibration hydrostatic correction is not necessary as at each level the tank is already in an expanded state Alsothe deadwood correction is not necessary RH must be measured per API Chapter 2.2A Liquid used: Product to be stored in tanks or water If water is used, then adjustments to the volume by courses is necessary due to the gravity variation between water and the product Time consuming and may take as much as two days This standard is an old standard and will be revised in future 33

34 Conclusions Tank calibration is a must for custody transfers, mass balance in refineries and volume balances in tank farms and pipeline terminals Tank fabrication drawings should not be used for determination of tank diameter. Recalibration at set frequency is equally important Any of the methods presented herein may be used to establish tank diameters Tank calibration should never be undertaken over insulation in insulated tanks For Insulated tanks, internal calibration or liquid calibration may be used if insulation cannot be removed If insulation can be removed, external calibration may be used Shell expansion due to hydrostatic pressure and expansion due to temperature are not negligible and must be included in the development of the capacity table 34