ISO 4266-4:2023

INTERNATIONAL ISO
STANDARD 4266-4
Second edition
2023-07
Petroleum and liquid petroleum
products — Measurement of level
and temperature in storage tanks by
automatic methods —
Part 4:
Measurement of temperature in
atmospheric tanks
Pétrole et produits pétroliers liquides — Mesurage du niveau et
de la température dans les réservoirs de stockage par méthodes
automatiques —
Partie 4: Mesurage de la température dans les réservoirs à pression
atmosphérique
Reference number
ISO 4266-4:2023(E)
© ISO 2023

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ISO 4266-4:2023(E)
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© ISO 2023
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Published in Switzerland
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ISO 4266-4:2023(E)
Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Precautions . 2
4.1 Safety precautions . 2
4.2 Equipment precautions . 2
4.3 General precautions . 3
5 Accuracy . 3
5.1 General . 3
5.2 Intrinsic error of ATTs . 3
5.3 Calibration prior to installation . 4
5.3.1 General . 4
5.3.2 ATT calibrated as a system . 4
5.3.3 ATT calibrated by components . 4
5.3.4 Multiple-point ATTs . 4
5.3.5 Variable-length ATTs . 4
5.3.6 Uncertainty of the reference . 4
5.4 Error caused by installation and operating conditions . 4
5.5 Overall accuracy . . 5
5.5.1 General . 5
5.5.2 Use of ATT for fiscal/custody transfer purposes . 5
6 Selection of ATTs . 5
6.1 General . 5
6.2 ATTs for fiscal/custody transfer purposes. 6
7 Description of ATT equipment .6
7.1 General . 6
7.2 Electrical temperature elements. 6
7.2.1 Resistance temperature detectors . 6
7.2.2 Other temperature elements . 7
8 Installation of ATTs . 7
8.1 General . 7
8.2 Single-point (spot) temperature elements . 7
8.3 Averaging temperature elements . 7
8.3.1 General . 7
8.3.2 Upper, middle and lower temperature elements . 7
8.3.3 Multiple-point temperature elements . 7
8.3.4 Variable-length RTD temperature elements . 8
8.3.5 Mid-level temperature element . 8
8.3.6 Moveable spot temperature element . 8
8.4 Thermowells for electronic temperature elements . 9
9 Calibration and field verification of ATTs .11
9.1 General . 11
9.2 Calibration of single-point, or mid-level temperature element ATTs for fiscal/
custody transfer purposes . 11
9.2.1 Calibration prior to installation . 11
9.2.2 Initial field verification . 11
9.3 Calibration of upper, middle and lower or multiple-point ATTs for fiscal/custody
transfer purposes.12
9.3.1 Calibration prior to installation .12
9.3.2 Initial field verification .12
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ISO 4266-4:2023(E)
9.4 Calibration of variable-length ATTs for fiscal/custody transfer purposes .13
9.4.1 Calibration prior to installation . 13
9.4.2 Initial field verification .13
9.5 Subsequent verification of ATTs . 13
9.5.1 General .13
9.5.2 Frequency of subsequent verification . 13
9.5.3 Record keeping . 14
10 Data communication and receiving .14
Bibliography .15
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ISO 4266-4:2023(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO document should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use
of (a) patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed
patent rights in respect thereof. As of the date of publication of this document, ISO had not received
notice of (a) patent(s) which may be required to implement this document. However, implementers are
cautioned that this may not represent the latest information, which may be obtained from the patent
database available at www.iso.org/patents. ISO shall not be held responsible for identifying any or all
such patent rights.
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constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 28, Petroleum and related products, fuels
and lubricants from natural or synthetic sources, Subcommittee SC 2, Measurement of petroleum and
related products.
This second edition cancels and replaces the first edition (ISO 4266-4:2002), which has been technically
revised.
The main changes are as follows:
— in 4.3.2¸ it has been clarified that the tank level should be measured and recorded simultaneously
with the temperature;
— in 9.2, it has been clarified that individual component verification is optional, but verification as
a whole system is required.
A list of all parts in the ISO 4266 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
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INTERNATIONAL STANDARD ISO 4266-4:2023(E)
Petroleum and liquid petroleum products — Measurement
of level and temperature in storage tanks by automatic
methods —
Part 4:
Measurement of temperature in atmospheric tanks
1 Scope
This document gives requirements and guidance on the selection, accuracy, installation, commissioning,
calibration and verification of automatic tank thermometers (ATTs) in fiscal/custody transfer
applications.
The ATT is used for measuring the temperature of petroleum and liquid petroleum products having a
Reid vapour pressure less than 100 kPa, stored in atmospheric storage tanks.
This document is not applicable to the measurement of temperature in caverns or in refrigerated
storage tanks.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 1998 (all parts), Petroleum industry — Terminology
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 1998 (all parts) and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
automatic tank thermometer
ATT
instrument that continuously measures temperature in storage tanks
Note 1 to entry: An ATT, which can also be known as an automatic tank temperature system, typically includes
precision temperature sensors, field-mounted transmitters for electronic signal transmission, and receiving/
readout device(s).
3.2
resistance temperature detector
RTD
electrical temperature-sensing element in common use to measure the temperature of the contents of
a storage tank
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ISO 4266-4:2023(E)
3.3
single-point automatic tank thermometer
spot automatic tank thermometer
automatic tank thermometer (3.1) that measures the temperature at a particular point in a tank by the
spot temperature element
3.4
multiple-point automatic tank thermometer
automatic tank thermometer (3.1) consisting of multiple (usually three or more) spot temperature
elements to measure the temperature(s) at selected liquid level(s)
Note 1 to entry: The readout equipment can average the readings from the submerged temperature elements to
compute the average temperature of the liquid in the tank and can also display the temperature profile in the
tank.
3.5
multiple-point automatic tank thermometer
averaging automatic tank thermometer (3.1) where the readout equipment selects the individual, spot
temperature element(s) that are submerged in the liquid to determine the average temperature of the
liquid in the tank
3.6
variable-length averaging automatic tank thermometer
averaging automatic tank thermometer (3.1) consisting of several temperature elements of varying
length, with all the elements extending upwards from a position close to the bottom of the tank, and
where the readout equipment selects the longest, completely submerged temperature element to
determine the average temperature of the liquid in the tank
3.7
temperature transmitter
instrument that typically provides electrical power to the temperature element(s), converts the
temperature measured by the element(s) to an electrical or electronic signal, and transmits the signal
to a remote readout
Note 1 to entry: A local readout may be provided. Often, the function of the temperature transmitter is provided
by the level transmitter of the automatic level gauge (ALG).
4 Precautions
4.1 Safety precautions
International standards and government regulations on safety and material-compatibility precautions
can apply when using ATT equipment. In addition, the manufacturer’s recommendations on the use and
installation of the equipment should be followed. It is presupposed that all regulations covering entry
into hazardous areas are observed.
4.2 Equipment precautions
4.2.1 All of the ATT equipment should be capable of withstanding the pressure, temperature,
operating and environmental conditions likely to be encountered in service.
4.2.2 ATTs should be certified for use in the hazardous-area classification appropriate to their
installation.
4.2.3 Measures should be taken to ensure that all exposed metal parts of the ATT have the same
electrical potential as the tank.
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ISO 4266-4:2023(E)
4.2.4 All parts of the ATT in contact with the product or its vapour should be chemically compatible
with the product, to avoid both product contamination and corrosion of the ATT.
4.2.5 All ATT equipment should be maintained in safe operating condition and the manufacturer’s
maintenance instructions should be complied with.
4.2.6 The temperature elements should be located in an area where it can be ensured that the
temperature of any sediment deposits or free water bottoms that can be present in the tank are not
measured.
4.3 General precautions
4.3.1 The general precautions given in 4.3.2 to 4.3.6 apply to all types of ATTs and should be observed
where they are applicable.
4.3.2 Tank levels should be measured and recorded simultaneously, or as close as practical, as the
tank temperature is measured. For more details on level measurement, refer to ISO 4266-1.
4.3.3 Temperatures measured for bulk transfer should be recorded when they are taken, unless the
remote readout equipment of the ATT automatically records the temperatures periodically.
4.3.4 The same general procedures should be used to measure a tank temperature before product
transfer (opening gauge) and after product transfer (closing gauge).
4.3.5 ATTs should provide security to prevent unauthorized adjustment or tampering. ATTs used in
fiscal/custody transfer applications should provide facilities to allow sealing for calibration adjustment.
4.3.6 The design and installation of ATTs can be subject to the approval of the national measurement
organization, who normally have issued a type or pattern approval (“Type Approval”) for the design
of the ATT for the particular service for which it is to be employed. Type approval is normally issued
after an ATT has been subjected to a specific series of tests and is subject to the ATT being installed in
an approved manner. Type approval tests may include the following: visual inspection, performance,
vibration, humidity, dry heat, inclination, fluctuations in power supplies, insulation, resistance,
electromagnetic compatibility and high voltage.
5 Accuracy
5.1 General
The accuracy of petroleum temperatures taken by the ATTs should be consistent with the accuracy
of the levels taken by the automatic level gauging system so that the overall accuracy of the standard
volume measurement is not seriously degraded. Observance of the accuracy requirements for level and
temperature gauging systems given in ISO 4266-1 and in this document will ensure overall accuracy of
the standard volume measurement.
5.2 Intrinsic error of ATTs
The intrinsic error of ATTs, i.e. the accuracy of the ATTs when tested under controlled conditions
as specified by the manufacturers, can be a major component of the uncertainty of the temperature
measurement of the ATT, as installed. It is presupposed that the calibration reference device used to
calibrate the ATT is traceable to appropriate national standards.
NOTE The temperature elements and field transmitters used for fixed, automatic tank temperature
measurement are calibrated prior to installation. The transmitters normally do not provide field calibration
adjustments.
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ISO 4266-4:2023(E)
5.3 Calibration prior to installation
5.3.1 General
ATTs to be used in fiscal/custody transfer applications can be calibrated/verified either as a system, or
by components.
5.3.2 ATT calibrated as a system
If verified as a system, the temperature reading of the ATT readout should agree with that of the
thermostatically controlled reference bath or oven temperature within 0,25 °C, at a minimum of three
test temperatures spanning the anticipated working range of the ATT.
5.3.3 ATT calibrated by components
If the ATT is verified by components:
a) the temperature equivalent of the measured resistance should agree with the reference bath
temperature within 0,20 °C at each temperature;
b) the temperature transmitter/converter and the ATT readout should be checked using precision
resistors or a recently calibrated thermal calibrator. The ATT readout should agree with the
temperature equivalent of the resistors or calibrator within 0,15 °C at each temperature.
5.3.4 Multiple-point ATTs
The required accuracy for each spot temperature sensor should be as given in 5.3.2 or 5.3.3, depending
on the method used.
5.3.5 Variable-length ATTs
The required accuracy for each temperature element should be as given in 5.3.2 or 5.3.3, depending on
the method used.
5.3.6 Uncertainty of the reference
The uncertainty of the reference should not exceed ±0,05 °C.
5.4 Error caused by installation and operating conditions
The total error of the ATT in fiscal/custody transfer applications can be affected by the installation and
by variations in the operating conditions.
The accuracy of an ATT depends on the following:
— the number of the temperature-sensing elements;
— the location of the temperature-sensing elements.
The tank content’s temperature can be subject to stratification which varies with:
— tank mixing;
— multiple sources of supply;
— viscosity or density of the liquid in the tanks, and
— tank insulation.
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ISO 4266-4:2023(E)
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NOTE 1 Temperatures in large tanks (i.e. 750 m or larger) are often vertically stratified unless the contents
are thoroughly mixed. Larger stratification can be expected in high-viscosity or high-density petroleum liquids.
NOTE 2 With some mass measurement technologies (e.g. hydrostatic tank gauges which are pressure-based),
a single spot temperature can suffice.
5.5 Overall accuracy
5.5.1 General
The overall accuracy of temperature measurement by the ATT, as installed, is limited by the intrinsic
error of the ATT equipment (temperature-sensing element, transmitter and readout), the effect of
installation methods, and the effect of the operating conditions.
5.5.2 Use of ATT for fiscal/custody transfer purposes
An ATT system should be considered suitable for fiscal/custody transfer services if the ATT system
meets the field verification tolerances given in 9.2.2, 9.3.2 and 9.4.2.
The ATT should meet the calibration tolerances prior to installation (see 5.3).
The ATT should meet the field verification tolerance (see 9.2.2, 9.3.2 and 9.4.2), as well as the effects of
installation methods and changes in operating conditions.
The remote readout, if used, should meet the recommendations of Clause 10.
6 Selection of ATTs
6.1 General
Copper or platinum temperature element bulbs, i.e. resistance temperature detectors (RTDs), are
normally used for this application. Three types of ATT elements are widely used:
— single-point (spot) ATT;
— multiple-point ATT;
— variable-length averaging ATT.
Other types of ATT elements, which provide comparable performance, may be used.
The selection of a suitable ATT should be made based on the following criteria:
a) the accuracy required;
b) the operating conditions which can affect the accuracy (e.g. expected product temperature
stratification);
c) the minimum level in the tank at which temperature measurement is required;
d) environmental conditions;
e) type and size of the tank;
f) available tank entries for new or existing tanks;
g) requirements for local and remote readout, signal transmission, and cabling.
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ISO 4266-4:2023(E)
6.2 ATTs for fiscal/custody transfer purposes
Tanks using an automatic method to determine temperature in fiscal/custody transfer should be fitted
with average temperature equipment, except when:
— the tanks are equipped with operational mixer(s), or an effective recirculation system,
— the maximum vertical temperature variation is less than 1 °C,
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— the tanks have a capacity of less than 159 m (1 000 barrels) or the level is less than 3 m.
Single-point or spot tank temperature measurement may be used when the temperature of the liquid in
the tank has been demonstrated to be uniform, or when any temperature stratification in the tank has
been shown to be small and acceptable (see ISO 4268).
It is possible that the mid-level temperature of the tank contents does not give an accurate average
temperature.
Small tanks, tanks storing a uniform temperature material, or tanks with adequate mixing equipment
have less temperature stratification. Therefore, a single-point temperature measurement can be
sufficiently representative.
Heated tanks rarely have uniform temperatures.
Tanks with multiple sources of supply rarely have a uniform temperature. Single-point temperature
elements should not be considered in these circumstances.
NOTE It is possible to determine the average temperature of a tank transfer quantity by using a single-point
(spot) temperature element located on the inlet or outlet of a tank, using the ALG to calculate a volume-weighted
average temperature of a parcel being loaded into or discharged out of the tank.
7 Description of ATT equipment
7.1 General
Most above-ground bulk storage tanks are equipped with at least one local direct-reading thermometer
mounted in a fixed thermowell. This local thermometer is not considered as part of the ATT and should
not be used for fiscal/custody transfer temperature determination, unless it has been demonstrated
that its readings are representative of the temperatures of the tank contents under typical operating
conditions (see ISO 4268).
7.2 Electrical temperature elements
7.2.1 Resistance temperature detectors
Tempera
...