Vertical Cylindrical Storage Tank Calibration Technologies ... · PDF fileVertical Cylindrical Storage Tank Calibration Technologies and Application Srini Sivaraman SK Japan . March

Vertical Cylindrical Storage Tank Calibration Technologies and Application

Srini Sivaraman SK Japan

March 2012 API Conference & Expo Singapore 2012

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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

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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

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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

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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

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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

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7

FR in Operating Position FR in Maintenance Position

Roof Legs

Floating Roof CZ

CZ

Note: Critical Zone Position varies with FR position

Floating Roof Position 7 Figure 1

Calibration by Manual Method • Manual Method

– API Chapter 2.2A • 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

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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 =

• Plate Thickness – Measured ultrasonically all around in each course (8 to 12 data points) and

averaged for each course • Diameter

– Computed from the measured circumference and the thickness

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* ( )100( )

D ftft

π

10

A

B

C

AB, BC, CA : Three straps (An example) A to A : Single Strap (heavy)

48 to

64

ft

Multiple Straps easier to handle

Manual Calibration : API Chapter 2.2A

Note: Ideally scaffolding fixed or portable needed to maintain tape in contact with shell

10 Figure 2

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

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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

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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

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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

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Corrections – 1 FR Buoyancy Correction

– 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 100

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Corrections -2 • Hydrostatic Head correction

– 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

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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

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Corrections - 4 • New Shell Temperature Equation • 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

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7*8

L AS

L

A

T TT

T Liquid TemperatureT Ambient Temperature

+=

==

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

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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 .

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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.

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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

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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

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25 25

A

B

C

D

E

F

G

H

HORIZONTAL STATIONSOPTICAL REFERENCE LINE METHOD

NOTE: Plan view shown for 8 stations

Optical Device

•

Optical Reference Line

Reference Diameter

Reference Offset

Vertical Station:Typical

h/5

h/5

Course Height ‘h’

Magnetic Trolley

Scale

Optical Device

Weld Seam

AB

A , B ….Horizontal Stations

300 mm

Optical Reference Line

Optical Reference Line Method (ORLM) Figure 4

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

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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 ( T1 T2 ) • Diameters are computed using mathematical computational procedures

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28 28

• •

•

D

A(x, y)

T D

A1*

A2*AN*

X

Y

A1, A2….AN Horizontal Stations

α β

T, L = Theodolites

A(X,Y) : Coordinates

D = Reference Distance

α, β : Coordinate Angles

T1

T2

+

+

T1 , T2 …. For External Calibration

T…D, For Internal Calibration

A1

h

h/5

h/5

A2 ANRing 1

Ring 3

Ring 2

Target Points(A1…..AN)

Vertical Stations

OTM: Internal and external Figure 5

OTM • Important Considerations

– 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

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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

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31 31

++ + + +

Target Points

αβ

r

+Ref. Target

Optical Device

r, α, β : Spherical Coordinates

α : Vertical angle

β : Horizontal angle to reference target

Electro Optical Distance Ranging Method : EODR Figure 6

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

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Liquid Calibration • Reference Standard: API Standard 2555 • Level Vs Volume is established directly • Volume Q1 is metered ( volume through meter that is calibrated prior to start of the tank

calibration) and corresponding level L1 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

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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

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