PROCEDURE GUIDE
FOR THE
CALIBRATION AND
CERTIFICATION OF
STORAGE TANK
FOR CUSTODY
TRANSFER
BY
CHIFE B. K. J. OJI
1
This procedure is used for all equipment and
instrument used at custody transfer points.
The equipment and instruments are:
 Thermometers
 Hydrometers
 Steel Tapes
 Storage Tanks (upright cylindrical types)
 Storage Tanks (horizontal cylindrical types)
 Barges
 Trailer Mounted Tanks (Tank Cars)
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These equipments are to be calibrated by an
approved or accredited corporate body.
This corporate body shall be prequalified by
the Director of Petroleum Resources
(Government Agency).
 This validity period for each equipment shall
be stated.
 Calibration shall be local except when the
facilities are not available and approval must
be given by D.P.R.
 Corporate Bodies and Agencies accredition for
each equipment type must be set out and
stand the reference.

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This shall vary from job to job and nominated
corporate body must formally apply for
permission from the Department of Petroleum
Resources (Government Agency).


Approval shall be guided by previous job
A D.P.R. Staff must be present for the following
exercise:


Strapping exercise
Calibration chart compilation
The following shall be added to the chart
 Full name and address of the Company.
 The tank number
 Name of Operator
 Location of tank number
 Other parameters indicated in the format at paragraph (2)
below. Finally a column must be provided for the approval of
the Director of D.P.R.
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That is the process involved in measurement
such as:
For a vertical storage tank:
 Circumferences at specific locations
 Depth
 Plate Thickness
 Weights
 Volume of internal and external fixtures
 Bottom plate alignment with the purpose of
drawing up a calibration chart for the tank to
show quantity of oil at any given depth.
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New Tank
 Five years operational life (Tank bottom must
be clean and checked)
 Maintenance involving replacement of plate
sections or alteraction of the “Deadwood”

Any modification affecting tank volume to
0.015%
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This shall be the API Standard 2550 or ASTMD1220-85 under the title
“Method for Measurement and Calibration of
Upright Cylindrical Tanks”
As stated in the reference standards i.e. The
essential equipment shall be the same with
those for taking measurement of tank, shall be
above ground and shall always be maintained in
good condition.
Both the tape for height measurement and that for
circumference measurement should themselves
have been calibrated in accordance with DPR guide
or
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 Current
certificate of recent calibration at
680F in accordance with the National Bureau
of Standard Test Fee Schedule 202.404.
 Accessory equipment shall be provided.
 Equipment for determining temperature and
gravity:



Sample Can: For withdrawing samples in the
process of tank temperature measurement, shall
be the thief type as specified in ASTM-D4057.
Thermometer: Shall be the total immersion type
of range -5 to 215 deg F and shall conform with
specification E1 described in ASTM book of
Standard Part 18.
Hydrometer: For gravity measurement shall be
the thermohydrometer type calibrated in deg API
and with range 0 to 100 deg API conforming to
the specification E100 for ASTM Book of
Standards.
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Calibration Procedure:
Tank clean and degassed.
 Tank be filled completely with water prior to the
time of strapping.
Tanks with the capacity above 500 bbl could be
strapped at any condition of fill. (No discharge no
fill)
For critical measurement the following must be
noted.
 Expansion and contraction of the tank shell
under liquid head and temperature.
 Effect of tilt on the tank shell.
 Alignment of the tank bottom.

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Float Roof Factor:
Floating roof tank shall be provided with a
table or formula for computing roof effect.
 The weight of the roof and all its
appurtenances shall be separately
determined and reflected in the tank table.
 Critical zones must be calibrated with liquid
in accordance with API 2555 for ASTM-D1406.
 A separate calibration chart for the critical
zone shall be prepared for each tank.
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Field Data: All data collected during strapping as
illustrated in the form.
CALIBRATION OF HORIZONTAL STORAGE TANKS
(a) Nominated company shall be accredited by DPR.
The procedure for accreditation as enumerated
above (apparatus).
(b) Principle: For horizontal Tanks completion of its
incremental volume from geometric dimensions
of the cylindrical main body of the tank and those
of the two heads are obtained.
Other factor:
(1) Deadwood
(2) Elasticity of the Shell
(3) Inclination of the tank
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(c) Frequency of calibration of Horizontal
Tanks.
(1)
(2)
(3)
(4)
(5)
All new tanks of this type to be used for custody
transfer shell be calibrated before it could be
approved for service and mandatorily after every
five years of their operational life.
A tank shall also be automatically due for
recalibration, if the circumferential reading show a
difference of 0.015% and above at a reference point,
over reading of the last calibration of the tank.
Whenever the deadwood is changed in configuration
or arrangement.
On relocating the tank to another position.
After any major repair or maintenance of the tank.
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Applicable Standard: This shall be the API
Standard 2551 or ASTM-D-1410 captioned “Standard
Method for Measurement and Calibration of
Horizontal Tanks.”
(e) Calibration Equipment: This shall comply in all
respects with the specifications stipulated in the
standard.
(f) Calibration Procedure: The tank shall be clean,
degassed and completely filled with water.
The procedure outlined in the reference standard
is followed to obtain the necessary parameters. A
calibration chart for the tanks critical parameters
to be obtained are:
(d)
(1)
(2)
(3)
(4)
(5)
(6)
Shell plate thickness
Circumferential parameter
Geometric dimension
Elasticity
Deadwood dimension
Inclination of the tank
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Field Data
(1) A diagrammatic representation
(2) Other data are liquid type and its level in the
tank during strapping, cylinder shell
thickness, the head and the horizontal
inclination of the tank
Tank calibration shall include:
(1) Calculation of the volume of the main
cylinder.
(2) The spread diameter and the partial.


The appropriate K factor of the circular segments of the
heads.
Shall be obtained from the standard tables of the
reference standard to produce the required calibration
chart for the tank to be presented in the format below
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