vertical cylindrical storage tank calibration technologies and application

8/11/2019 Vertical Cylindrical Storage Tank Calibration Technologies and Application

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Vertical Cylindrical Storage Tank Calibration

Technologies and Application

Srini Sivaraman

SK Japan

March 2012API 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 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 traceabilityfor 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 PositionFR in Maintenance Position

Roof Legs

Floating RoofCZ

CZ

Note: Critical Zone Position varies with FR position

Floating Roof Position7Figure 1


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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

9

* ( )

100( )

D ft

ft


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10

A

B

C

AB, BC, CA : Three straps (An example)

A to A : Single Strap (heavy)

48t

o

64f

t

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

10Figure 2


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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 bottommaybe filled with solid sludge or other foreign materials

If access is not available one should not try and measure RH but use theRH 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 calibrationtable

Typically this is in the range of 6 to 12 in but could be as high as 18 independing on FR design

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Field Measurements-2 Deadwood

All internal piping and other structures inside are physically measured andtheir volumes distributed vertically from the datum plate

This is necessary to subtract the volume of the deadwood as tank table isdeveloped (volume Vs height)

This is possible only when entry is permitted into the tank, if not it should betaken from the most recent calibration data

FR Weight

During calibration FR weight is collected either from old table data orphysically measured and computed. But computation could potentially carrylarge uncertainty

Number of welding rods that are used in the FR fabrication must be taken intoaccount or else could understate the FR deadweight

Best obtained from the fabricator and documentation on file maintained forall 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 1FR 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 - 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 Temperature

T Ambient Temperature

+=

=

=


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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 bymore than 20%the circumferenceof 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 ontop 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 referencecircumference 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 areestablished

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2525

A

B

C

D

E

F

G

H

HORIZONTAL STATIONS

OPTICAL 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


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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 (withinlimits)

At each station reference offset is rechecked b after the full vertical traverse

The optical device is checked randomly at three locations for perpendicularityby rotating the device 360 deg

In extreme windy condition , when it is difficult to maintain the trolley incontact 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 controlbeing 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 externalmethods

Tank profile is established by triangle at each target point and hence the nameOTM

For internal method reference distance D is established optically usingtemperature compensated Stadia typically 2 m long

Subsequently tank coordinates A(x, y) are measured optically using twotheodolites

For external method the tangential angles are measured along with the distancebetween the two theodolites ( T1 T2)

Diameters are computed using mathematical computational procedures

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2828

D

A(x, y)

T D

A1*

A2*AN*

X

Y

A1, A2.AN Horizontal Stations

T, L = Theodolites

A(X,Y) : Coordinates

D = Reference Distance

, : Coordinate Angles

T1

T2

+

+

T1 , T2 . For External Calibration

TD, For Internal Calibration

A1

h

h/5

h/5

A2 AN

Ring 1

Ring 3

Ring 2

Target Points

(A1..AN)

Vertical Stations

OTM: Internal and external

Figure 5


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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, , ) 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 theend 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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3131

++ + + +

Target Points

r

+Ref. Target

Optical Device

r, , : Spherical Coordinates

: Vertical angle

: Horizontal angle to reference target

Electro Optical Distance Ranging Method : EODR

Figure 6


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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 Q1is 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 thegravity 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 ifinsulation 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 notnegligible and must be included in the development of the capacity table

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vertical cylindrical storage tank calibration technologies and application

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