7 3 pages fm Manual of Petroleum Measurement Standards Chapter 7 3 Temperature Determination—Fixed Automatic Tank Temperature Systems SECOND EDITION, OCTOBER 2011 Manual of Petroleum Measurement Stand[.]
Terms and Definitions
Terms used in this chapter are defined as follows:
An Automatic Temperature Transmitter (ATT) is a device designed to continuously measure and transmit the temperature of contents within tanks, vessels, or compartments without the need for manual thermometers These instruments often feature a local temperature display for easy monitoring.
3.1.1.1 spot ATT single-point ATT
A temperature instrument that measures the temperature at a particular point in the tank where the spot temperature sensor is located.
3.1.1.2 multiple-spot ATT multiple-point ATT multi-point ATT
A temperature instrument is composed of multiple individual spot temperature sensors, typically three or more, grouped together to form a temperature element This device is designed to accurately measure the temperature at specific liquid levels within a tank.
The display equipment for a multiple-spot averaging ATT is designed to calculate the average temperature of the liquid in the tank by averaging readings from submerged temperature element sensors Additionally, it can showcase the temperature profile within the tank.
The averaging ATT system utilizes multiple temperature sensors of different lengths, all positioned near the bottom of the tank It identifies the longest fully submerged sensor to accurately calculate the average temperature of the liquid within the tank.
An averaging Automatic Temperature Transmitter (ATT) selects one or more temperature sensors submerged in the tank liquid to calculate a representative average temperature from the sensor readings There are various types of averaging ATTs available.
— multiple-spot ATT: A component of the ATT system averages the reading of the submerged temperature sensors to compute the average temperature of the liquid in the tank.
— variable-length ATT: The ATT system selects the longest, completely submerged temperature sensor to determine the average temperature of the liquid in the tank.
An automatic tank thermometer system consists of multiple Automatic Tank Thermometers (ATTs) and devices that display temperatures, perform calculations, and generate alarms It also includes the necessary means to transmit data between the ATTs and the display devices.
A tube designed to enclose a temperature sensing device and protect it from the environment and process.
NOTE A protecting tube is not designed for pressure-tight attachment to a vessel.
Stilling wells are tubes within a tank that are used for gauging activities including sampling and the determination of level and temperature.
In this standard, the term "tank" encompasses storage tanks, compartments of ships or barges, and any other vessels that fall under the scope and introduction of this standard.
One or more sensors housed in a casing such as a metal probe or protecting tube that makes up a single temperature measuring unit for a thermometer.
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A temperature sensor is a crucial component of a thermometer that detects physical changes in temperature and translates these changes into measurable values on a scale, such as the markings on a liquid-in-glass thermometer or the digital readout of an electronic device.
A temperature measuring device that operates on the principle of a change in electrical resistance in wire as a function of temperature.
A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference.
A device that converts the temperature measured by the sensor(s) to electrical or electronic signal, and transmits the signal to a remote or local display.
A thermometer is a device that measures temperature through various principles It consists of two key components: a temperature sensor that undergoes a physical change based on temperature, and a mechanism for indicating or transmitting this change as a measurable value.
A pressure and liquid-tight receptacle is designed to accommodate a temperature sensing element, featuring external threads, flanges, or other mechanisms for secure pressure-tight attachment to a vessel The inclusion of a thermowell enables the easy removal and replacement of the temperature sensor without disrupting the ongoing process.
Acronyms and Abbreviations
The following acronyms are used in this publication.
API MPMS Manual of Petroleum Measurement Standards
ISGOTT International Safety Guide for Oil Tankers and Terminals
ISO International Organization for Standardization
NFPA National Fire Protection Association
OCIMF Oil Companies International Marine Forum
SOLAS Safety of Life at Sea
USCG United States Coast Guard
General
Safety considerations must be integrated into all equipment specifications, installation, and operation For guidance, consult API 500, API 551, NFPA 70, and API 2003 When handling liquids that may generate static charges, it is essential to follow the precautions outlined in the International Safety Guide for Oil Tankers and Terminals, as well as in API MPMS, Chapter 3.
When utilizing fixed ATT systems, it is essential to prioritize safety and ensure material compatibility Adhering to the manufacturer's guidelines for equipment use and installation is crucial Additionally, users must comply with all relevant codes, regulations, API standards, and the National Fire Protection Association (NFPA) 70, as well as the National Electric Code (NEC).
All marine ATTs should be specified and installed in accordance with the appropriate National and/or International (IMO, USCG, IEC, NEC, ISGOTT, ISO, etc.) marine electrical safety standards.
ATTs should be certified for use in the hazardous area classification appropriate to their installation.
Equipment Precautions
The following general precautions affect the accuracy and performance of all types of ATT systems These precautions should be observed where they are applicable.
All ATTs must endure the pressure, temperature, and environmental conditions of their intended service In corrosive environments, components exposed to liquids or vapors should be made from durable, corrosion-resistant materials to prevent product contamination and corrosion of the ATT Additionally, ATTs should be sealed to handle the vapor pressure of the tank's liquid For ATTs installed on marine vessels with an inert gas system (IGS), they must be designed to withstand the IGS operating pressure.
NOTE 1 This protection may require mounting the ATT sensor(s) in a thermowell.
ATT sensors play a crucial role in the automatic tank gauging (ATG) level sensor assembly However, certain integrated ATG/ATT designs may not be ideal for continuous operation For instance, float-operated ATGs often require the level and temperature sensor assembly to be elevated to a "store" position when not in use.
NOTE 3 Certain tank operations such as tank washing on marine vessels, may require removal of the ATT or special precautions be taken to avoid damage.
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The equipment manufacturer must clearly specify the operational range limits and the effects of ambient conditions on the measurement accuracy of all devices within a temperature measurement system.
The design and installation of Automatic Tank Gauges (ATTs) require approval from national measurement organizations and classification societies, which may provide general type approval for specific services This type approval is granted after the ATT undergoes a series of tests and is contingent upon proper installation.
Security—ATT systems should provide security to prevent unauthorized adjustment ATT systems used in fiscal/ custody transfer application should provide facilities to allow sealing for calibration adjustment.
Equipment and Apparatus
ATTs generally use one of the following types of temperature sensors:
Temperature measurement devices are typically encased in metal probes that fit into thermowells (for spot ATTs) or protection tubes (for multi-spot or variable length ATTs) For optimal heat transfer, it is essential that the tip-sensitive probes of spot ATTs are firmly positioned at the bottom of the thermowell It is advisable to use spring-loaded or adjustable-length probes for enhanced performance.
Proper wiring to the probe is essential due to the low signal levels of the devices For optimal accuracy, installation should follow the manufacturer's recommendations Linearization of these sensors is usually performed within the corresponding transmitter, and each sensor type necessitates a distinct circuit.
The selected sensor shall meet the requirements given in Table 4.
The resistance temperature detector (RTD) is primarily constructed from platinum, though copper can also be utilized, and is encased in protective material to ensure long-term stability The sensing element is typically housed in a stainless steel enclosure appropriate for the specific application and installation An appropriate electronic circuit measures the transducer's resistance using a small current to avoid self-heating, converting this resistance value into a corresponding temperature reading.
Platinum RTD temperature sensors adhere to standards like IEC 60751 and ASTM E1137, which outline accuracy and performance criteria These standards detail testing requirements and establish various classes or grades based on tolerance levels, along with mathematical expressions that describe the temperature-resistance relationship Sensors manufactured in accordance with these standards ensure a stable and clearly defined correlation between operating temperature and expected tolerances.
A characterized sensor refers to an individual RTD whose output has been evaluated at various temperatures against equipment traceable to a National Metrology Institute (NMI) This process yields a set of constants that are utilized in a mathematical expression to define the temperature-resistance relationship specific to that RTD, in compliance with established standards.
The use of classified or characterized sensors in compliance with such standards is optional, but for certain applications the added accuracy may be warranted
RTDs are ideal for custody transfer applications due to their high accuracy, quick responsiveness, and long lifespan, along with stable output characterization over time Unlike thermocouples, RTDs are less affected by electrical interference and errors from variations in lead-in wire properties.
When choosing a Resistance Temperature Detector (RTD), opt for three or four-wire configurations to effectively compensate for the temperature-dependent resistance of lead wires Additionally, ensure that the temperature-sensitive section of a spot ATT sensor does not exceed 100 mm (4 inches) in length.
Thermocouples are temperature-sensitive devices made from two different metals, generating an electromotive force (EMF) based on the temperature difference between their hot and reference junctions They come in various types, each with specific measurement ranges, capable of measuring temperatures from approximately –150 °C (–300 °F) to 1,300 °C (2,300 °F).
Electronically compensated single-junction thermocouples shall not be used for custody transfer measurement due to the following:
— they suffer from drift and corrosion as they age;
— the millivolt signal is quite low and subject to electrical interference;
— the length, composition and condition of the thermocouple lead wires affects accuracy.
Other thermocouple systems that meet the requirements of Section 8 may be used for custody transfer measurements.
Other types of temperature sensors (e.g thermistors, semiconductors, etc.) are also available and may be suitable for custody transfer purposes provided they meet the performance requirements of Section 8.
A spot ATT measures the temperature at a specific location within the tank, where the spot temperature sensor is installed However, since this measurement is taken at a single fixed point, it may not accurately reflect the average temperature of the entire tank Additionally, the position of the temperature measurement in relation to the liquid surface can change based on the tank's fill level.
An averaging temperature transmitter (ATT) utilizes one or more submerged temperature sensors to calculate a representative average temperature of the tank liquid These averaging ATTs are typically essential components of an automatic tank gauge (ATG) system, although some ATTs designed for level sensing can also perform averaging functions Averaging ATTs are available in various configurations, including multiple-spot and variable-length types.
Position of the sensors within a tank should be such that each sensor represents approximately the same cross sectional tank volume.
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To avoid ambient temperature effects placement of sensors within 1 m (3 ft) of the tank shell or tank bottom is not recommended For additional installation details see Section 6.
The recommended minimum number of sensors is given in Table 1 Additional sensors may be advisable if significant tank temperature stratification exists (see 5.3.2).
To determine sensor elevation in tanks with a regular cross-sectional profile, equations 1 through 4 can be utilized For multiple-spot Automatic Tank Gauges (ATTs), these equations calculate the height of the spot sensor from the tank bottom In the case of variable-length ATTs, the equations are used to ascertain the height of the variable-length sensor.
For tanks with irregular cross-sectional profiles, it is advisable to avoid using variable-length Average Temperature Transmitters (ATTs) due to the potential for inaccurate average readings caused by temperature stratification When utilizing multi-spot ATTs, it is essential to adhere to the manufacturer's guidelines.
S is the spacing between sensors, m;
H is the maximum tank fill height, m;
N is the number of sensors, see Table 1 for suggested minimum.
S is the spacing between sensors, ft;
H is the maximum tank fill height, ft;
N is the number of sensors, see Table 1 for suggested minimum.
Table 1—Minimum Number of Temperature Sensors Maximum Filling Height No of Sensors a, b
23 m (75 ft) to 30 m (100 ft) 7 a The number of temperature sensors and the locations shown are a suggested minimum. b Refer to 5.1.3.2 and 5.1.3.3 in case of tanks with large temp stratification.
Using the spacing determined in equation 1 or 2 the elevation of sensors 1 through N is calculated as follows:
En _ spot is the elevation from tank bottom for spot sensor, n (m or ft);
En _ variable is the height of variable-length sensor, n (m or ft); n is the sensor number, ranging from 1 to N;
B is the level of bottom sensor, 1 m (3 ft);
S is the spacing between sensors (m or ft).
Multiple-spot temperature sensors are installed (or encased in a flexible element housing) at approximate 2 m to 3 m
Temperature measurements in a tank are typically taken at intervals of 6 ft to 10 ft, with the lowest sensor positioned about 1 m (3 ft) from the tank's bottom Generally, a single transmitter is used to measure all temperatures for an Automatic Tank Temperature (ATT) element This ATT transmitter can be integrated into an Automatic Tank Gauge (ATG) system, which has the capability to generate a tank temperature profile or calculate an average temperature based on the submerged sensors In some instances, the averaging function may be handled directly by the ATT transmitter or system.
When operating a tank at levels below 1 m (3 ft), it is advisable to install an additional temperature sensor at the lowest practical level to ensure it remains submerged However, this sensor should not be averaged with other ATT sensors when the tank is at or above 1 m (3 ft) to prevent inaccuracies due to ambient ground temperature influences.
Multiple RTDs of different lengths are housed in a flexible sheath, extending from the tank's bottom The longest, fully submerged RTD is utilized to measure the average liquid temperature in the tank The appropriate RTD is usually chosen through a switching device in an ATT or ATG transmitter, or by software in the system, often via a human-machine interface (HMI) computer.
System Description
ATT systems vary from basic transmitters that send temperature signals to complex multi-tank systems used for inventory measurement and control in tank farms Usually, one ATT is installed for each tank, and they transmit signals, alarms, and diagnostic information via wired, fiber optic, or wireless systems to a computing system that manages and displays this data.
Temperature measurement is a vital component of an Automatic Tank Gauge (ATG) system, leading to the integration of the Automatic Tank Temperature (ATT) system with a unified supervisory setup and display Figure 3 illustrates a specific example of an ATT system as outlined in this standard, though it does not encompass all potential configurations.
Selection of an ATT System
The selection of a suitable ATT should be made based on recommendations in Table 3 and the following criteria:
— tank operating parameters (e.g temperature range);
— the minimum level in the tank at which temperature measurement is required;
— system computing, display, archiving and alarming requirements
Temperature stratification occurs in both vertical and horizontal directions within a tank In large tanks, specifically those with a capacity of 795 m³ (5,000 bbl) or more, stratification is typically observed unless the contents are adequately mixed.
Tank temperature stratification can be reduced by tank fill, empty and transfer procedures, the use of circulating pumps and by in-tank mixers.
In the vertical direction, temperature differences of as much as 3 °C (5 °F) are normal, and differences of 5 °C (9 °F) or more may occur.
Figure 3—Example of Automatic Tank Temperature System Diagram
(May be located at ground level and may also include avg calc.)
(May include avg., calc, local display)
ATT (tank) Data Acquisition and Human Machine Interface,
(e.g Control Room) Functionality may be within: y ATG supervisory computer y ATT supervisory computer y PLC y DCS Functionality may include: y Averaging, trending, calculations y Alarm Management y External communications y Etc.
ATT or ATT system may be connected to external computing system (i.e third-party, regulatory etc.)
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In low and medium viscosity petroleum liquids, horizontal temperature differences are usually under 0.5 °C (1 °F), while higher differences can occur in high viscosity liquids and large diameter tanks.
For horizontal stratification where temperature differences in excess of 0.5 °C (1 °F) are anticipated, an increased number of elements may be appropriate.
5.3.3 ATT Systems for Custody Transfer Applications
The implementation of an Automated Tank Gauge (ATT) system for custody transfer necessitates a mutual contractual agreement between the buyer and seller, and it may also be governed by various federal, state, or local regulations For accurate custody transfer measurements, it is recommended to utilize an averaging ATT, as detailed in Table 3 and outlined in section 5.3.1.
When designing an ATT system, it is essential to consider the number, type, and placement of sensors within the tank, as outlined in section 5.1.3.1 For temperature variations between 1 °C and 3 °C (2 °F to 5 °F) caused by vertical stratification, averaging ATTs are recommended In cases where temperature differences exceed 3 °C (5 °F), increasing the number of sensors per element, as suggested in sections 5.1.3.2 and 5.1.3.3, may be necessary.
Select a sensor type described above with suitable accuracy, longevity and stability for the application which meets the accuracy requirements in Table 4.
For increased reliability a redundant ATT element or secondary ATT device may be used.
Other ATT designs than those described herein may be in use and may be suitable for custody transfer applications provided Table 3, note 1 or note 2 applies.
5.3.4 ATT Systems for Inventory Control Applications
When selecting and designing an ATT system for inventory control applications, the performance requirements are generally less stringent compared to custody transfer It is essential to base the system's design on specific business needs to ensure that the temperature measurement accurately reflects the overall fluid temperature.
Table 3—Recommendations for the Use of ATTs
Spot No Yes See Note 1
Multiple-spot (non-averaging) No Yes See Note 2
Multiple-spot, averaging Yes Yes
Variable-length, averaging Yes Yes
Spot tank temperature measurement can be utilized for custody transfer or fiscal applications if certain conditions are satisfied These conditions include having tanks with a capacity of less than 795 m³ (5,000 bbl) or a level below 3 m (10 ft), and ensuring that the maximum vertical temperature variation is less than 1 °C (2 °F).
In all other cases, spot ATTs are not suitable for custody transfer or fiscal measurement.
NOTE 2 Can be used for custody transfer provided that the averaging principles apply, see 5.1.3 (averaging ATT).
Ambient Temperature
An Automatic Tank Gauge (ATT) measures the liquid temperature (T_L) for tank volume calculations as specified in API MPMS Chapter 12.1.1 Additionally, the ambient temperature (T_A) may be needed for tank shell corrections (C_TSh) Therefore, the design of the ATT system must take these factors into account.
— If ambient temperature will be live or manually input;
— If ambient air temperature will be transmitted as part of the ATT system.
Ambient temperature serves as a key atmospheric measure in tank farms, but its variability around storage tanks complicates accurate measurement This variability can lead to an uncertainty of ±2.5 °C (±5 °F) Notably, the ambient temperature accounts for only 1/8 of the total tank shell temperature (TSh), allowing for a lower accuracy compared to the more critical liquid temperature (T L).
See 7.3 about how and where to measure ambient temperature.
ATT Systems for Marine Applications
An ATT system installed on marine vessels or floating storage units transporting crude oil and refined products can be utilized for custody transfer temperature measurement, as long as it adheres to the accuracy standards outlined in Section 8.
The installation of the ATT system must adhere to all relevant codes and regulations outlined in Section 4 of this document, as well as comply with the manufacturer's instructions when applicable.
Location Requirements for ATTs in Custody Transfer Applications
The design of temperature element or sensor placement should ensure the following.
Tank operations remain uninterrupted, ensuring efficient gauging processes, safe filling and emptying of tanks, and secure access for equipment maintenance Additionally, there is no disruption to mixing or rotating equipment, nor to the movements of floating roofs.
To obtain accurate measurements of tank liquid temperatures, it is crucial to position temperature sensors away from inlets, outlets, tank floors, walls, mixers, heaters, sludge deposits, and water bottoms This ensures that the recorded temperatures truly reflect the overall conditions within the tank.
To minimize the risk of damage from liquid movement, it is essential to utilize thermowells and protective tubes while being mindful of roof movement, jetted streams used for cleaning, and the positioning of inlets and outlets.
Proximity of temperature sensors to various types of internal tank equipment or activities may result in nonrepresentative temperatures or failure of the device
Top entry installation is normally used for multiple-spot or variable-length ATTs Recommendations on the number of sensors and their elevation are given in 5.1.3.1.
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The element should be positioned at least 1 m (3 ft) from the tank shell, inlets and outlets.
For optimal maintenance, verification, and calibration, sensors should be installed within 1 meter (3 feet) of a gauging hatch, vapor lock valve, or another suitable access point Additionally, it is important to position sensors near a ladder, stairway, or accessible gauging platform to ensure easy access for these procedures.
To maintain the positional stability of the ATT, it is advisable to use a protective tube Additionally, it is essential to ensure that the ATT sensor remains fully extended at all times to achieve stable sensor positions in relation to the tank shell and liquid level.
To ensure the positional stability of the ATT when it is suspended in the air, it is essential to secure it using alternative methods This can be achieved by employing an anchor weight or other stabilization techniques to prevent any changes in sensor positions.
ATTs are typically designed to function without the need for extra mechanical protection, allowing the temperature element to be directly immersed in the fluid However, certain applications may necessitate additional mechanical safeguards for the ATT, which can be achieved by enclosing the temperature element within a protective tube, thermowell, or stilling well.
When installing a temperature sensor (ATT) in a stilling well, it is essential to incorporate holes or slots throughout the entire length of the well This design ensures that the fluid within the stilling well accurately reflects the temperature and level of the tank.
Side entry installation is typically utilized for spot ATTs, which usually necessitate a thermowell as outlined in section 5.1.6 While spot ATTs are not generally advised for custody transfer, guidance can be found in section 5.3.3 If spot ATTs are employed, it is recommended to install multiple units at varying levels within the tank.
The lowest sensor should be installed at least 1 meter (3 feet) above the tank floor If the tank operates below this level, a temperature sensor may be placed as low as feasible, but it should only be utilized to measure average tank temperatures at that fill level.
The sensor should extend at least 1 m (3 ft) from the tank shell (shorter insertion lengths may be suitable for insulated tanks) and at least 1 m (3 ft) from inlets and outlets.
Test thermowells should be installed adjacent to ATT thermowells for ease of verification and calibration.
Location Requirements for ATTs in Inventory Applications
With reference to company policies and procedures, less stringent requirements may apply for inventory applications compared to custody transfer applications, see further 5.3.4 for requirement on ATTs for inventory applications.
Lesser minimum heights above tank floor for thermowell installations may be appropriate for inventory applications with a recommended minimum height of 0.3 m (12 in.).
For tanks equipped with internal floating roofs and side entry installations, the thermowell and test thermowell must be positioned no higher than 0.15 m (6 in.) below any roof appurtenance that may potentially damage them, according to the low leg setting Alternatively, they should be situated approximately 0.15 m (6 in.) below the minimum operating level, which is typically determined by specific spacing criteria related to the tank level in relation to the center or top of the outlet pipe.
Other Mechanical Installation Arrangements
The following other installation arrangements may be of use.
A mid-level spot ATT is installed via a top entry, ensuring that the spot temperature is consistently located about halfway between the liquid surface and the tank's bottom.
Three spot ATTs are installed through a top entry to continuously measure the temperature at the upper, middle, and lower levels of the liquid The readings from these three spots can be either electrically combined or averaged to provide an overall average temperature.
ATT Systems for Marine Applications
The design and installation of the Automatic Tank Gauge (ATT) on cargo tanks linked to the vessel's Inert Gas System (IGS) must ensure that maintenance, verification, and replacement can be performed without the need to de-pressurize the IGS.
The installation of the spot and/or multiple-spot ATT should be positioned near a vapor lock valve, gauging hatch, or another appropriate gauging access point Common installation methods include placing the device in a vertical protecting tube that extends through the tank roof, allowing for the mounting of one or more temperature sensors Typically, three temperature sensors are suspended by individual metal cabling at varying depths: one in the upper third (70% to 80% of the tank height), one in the middle (40% to 50% of the tank height), and one in the lower third (15% to 20% of the tank height).
The placement of a protective tube in a ship's tank is limited to being "close to the bulkhead" to ensure mechanical supports can endure the ship's vibrations Additionally, these tubes are integrated with Automatic Tank Gauges (ATGs) that feature level sensors in direct contact with the liquid, with the height of each temperature sensor varying based on the ATG's installation Typically, single spot Automatic Tank Temperature sensors (ATTs) on marine vessels and barges are equipped with one to three sensors strategically positioned in the upper, middle, and lower sections of the tank to accurately monitor temperature.
Operators should have easy access to the ullage data corresponding to the depth of each temperature sensor for every tank, along with other information from the ATG/ATT system.
Data Acquisition, Data Transmission, and Receiving Equipment
The data acquisition, data transmission and receiving equipment should be designed and installed in accordance with the requirements in 5.3.3.
Normal practice is to transmit temperature information using the wiring network provided for the remote reading ATG level transmitters.
Additional aspects to consider when constructing the data acquisition and transmission system are:
— signal path length between ATT and remote display;
— interference from AC power wiring;
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In regions prone to frequent lightning strikes, especially where tanks are dispersed over a large area away from central display equipment, it is essential to implement additional lightning protection measures to ensure safety and reliability.
Configuration
After the mechanical and electrical installation of the ATT, software configuration may be necessary If the ATT features temperature average calculation, it is important to refer to the tank drawings or ATT documentation for specific considerations.
— the associated level gauge (ATG);
— the sensor positions relative to the tank references (datum plate);
— ensure that the sensors included in the average calculation are submerged;
— specific configuration recommended by the ATT manufacturer.
Timing of Temperature Measurement
Temperatures must be measured and recorded concurrently or as closely as possible to the liquid level measurement The same procedures should be applied to measure tank temperature both before the product transfer (opening gauge) and after the transfer (closing gauge).
Reporting of Temperatures
Fixed temperature elements of ATTs must be read and recorded at their maximum display resolution, usually 0.1 °C or 0.1 °F In cases where the ATT display equipment does not average temperatures, average tank temperatures should be computed from multiple readings following the averaging principles outlined in section 5.1.2.
Ambient Temperature
To accurately measure temperature, it is recommended to use a temperature device brought by the gauger into the tank area just before gauging, ensuring at least one reading is taken in a shaded location and averaging multiple readings if necessary Additionally, shaded external thermometers that are permanently installed in the tank farm area and local on-site weather stations can also be utilized for reliable temperature measurements.
Temperature readings are to be taken at least 1 m (3 ft) from any obstructions or the ground Additionally, allow sufficient time for temperature reading to stabilize.
For reporting purposes, round ambient temperature to the nearest whole degree.
NOTE Measurement of ambient temperature is not applicable for temperature measurements on a marine vessel.
Requirements for ATTs
The ATT must be calibrated and verified to ensure it meets the required accuracy for its intended service Table 4 outlines the maximum tolerances that the ATT must achieve during various verification activities, considering the uncertainty contributions from both the sensor and the conversion electronics.
The accuracies specified in Table 4 are for temperature range –40 °C to +70 °C (–40 °F to +160 °F).
Requirements for Test Equipment
Calibration and verification standards must be traceable to a national metrology institute (NMI) like NIST through a valid calibration certificate Test equipment should be verified daily or before use, whichever is less frequent, to ensure proper functionality For specific requirements related to petroleum measurement, refer to API MPMS Chapter 7-2001, Section 5.2, or the forthcoming API MPMS Chapter 7.2 Additionally, Table 5 outlines the requirements for test equipment.
Table 4—ATT Tolerances at Different Verification Stages
Factory calibration (FAT) ±0.2 ºC (±0.4 ºF) ±0.15 ºC (±0.3 ºF) ±0.25 ºC (±0.5 ºF)
Initial Field Verification (SAT) ±0.4 °C See note 1 ±0.25 °C See note 1 ±0.5 ºC (±1 ºF)
See note 1 ±0.25 °C See note 1 ± 0.5 ºC (±1 ºF)
Factory calibration (FAT) ±0.2 ºC (±0.4 ºF) ±0.15 ºC (±0.3 ºF) ±0.25 ºC (±0.5 ºF)
Initial Field Verification (SAT) ±0.4 °C See note 1 ±0.25 °C See note 1 ±0.5 ºC (±1 ºF)
See note 1 ±0.5 °C See note 1 ±1.0 ºC (±2 ºF)
Verification by component is impractical for multiple-spot or variable-length ATTs due to the challenges of field sensor verification, which is typically conducted at the system level If system verification fails, the transmitter component can be verified For inventory applications, the tolerances provided are recommendations, as business or regulatory requirements may necessitate different criteria.
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9 Inspection, Verification, and Calibration Requirements
General
All ATTs shall be inspected, verified and if needed calibrated periodically to assure proper performance The verification and calibration should be performed against reference standards.
Verification
Verification involves comparing the Actual Temperature Thermometer (ATT) in use against a certified reference standard thermometer This process determines if the ATT operates within the tolerances specified in Table 4 or if it needs calibration, repair, or replacement.
Verification of the ATT is performed in different stages prior to being put in service as well as during operation; these are:
Verification of an ATT system can be performed either as a complete system or by individual components, with the complete system approach being the standard practice in the field It is essential to utilize test equipment that complies with the specifications outlined in section 8.2.
Before being put into service, temperature sensors shall be verified by one or more measurements, preferably at several points throughout the range of the device.
The complete system as a whole shall be verified either at the FAT, SAT, or both before being put into service.
Table 5—Requirements for Test Equipment Test Equipment Resolution Accuracy Requirements 1 Frequency of Calibration
(liquid-in-glass) 0.1 3 times better than the device being tested
RTD Simulator N/A 3 times better than the device being tested Yearly
Thermal Blocks and Baths N/A 3 times better than the device being tested Yearly
NOTE 1 Test equipment accuracy expectations do not normally differ for custody transfer or inventory applications For inventory applications, lesser accuracies may be appropriate.
NOTE 2 Requirements given in API MPMS Chapter 7-2001, Section 5.3, or when published, API MPMS Ch 7.1 (see the Foreword), if different, should be taken as the requirement.
NOTE 3 Requirements given in API MPMS Chapter 7-2001, Section 5.2, or when published, API MPMS Ch 7.2 (see the Foreword), if different, should be taken as the requirement.
A FAT includes the following checks:
— Verify accuracy (see 9.2.1.1 or 9.2.1.2) throughout the operating range of the device, which is critical because of the difficulty in varying temperature in an operating tank.
— Verify ATT type, model and design to ensure the intended use as well as conformity documentation such as test delivery certificates (see also Section 5 and Section 6).
The verification of the ATT system involves connecting a small segment of the system to ensure that temperature measurements on both local and remote displays fall within an acceptable range.
The ATT system is validated by immersing the temperature elements in constant temperature baths at three or more temperatures within the operational range The temperature difference between the ATT system and the reference thermometers must not exceed the tolerance specified in Table 4 Each sensor, or the average sensor in the case of a variable-length ATT, should undergo this verification process.
Alternately, the components of the ATT are separately verified.
To ensure accurate readings, measure the output of the temperature sensor in the bath at three or more temperatures within the operating range The difference between the bath temperature and the sensor output must not exceed the tolerance specified in Table 4.
The ATT system can utilize industry-standard sensors designed for specific accuracies and testing methods, potentially eliminating the need for additional tests during the Factory Acceptance Test (FAT).
To ensure accurate temperature readings, utilize reference standard test equipment like precision resistors or a thermal calibrator that is traceable to a National Measurement Institute (NMI) for simulating temperature inputs to the transmitter It is crucial that the difference between the simulated input temperature and the output temperature from the ATT transmitter remains within the tolerance limits specified in Table 4.
All components of the ATT system, including those used for signal conversion and data acquisition between the ATT transmitter and the human-machine interface (HMI), must undergo testing If data acquisition system components are not included with the ATT system, they should be tested at the Site Acceptance Test (SAT) The total tolerance for the ATT transmitter and any associated conversion or data acquisition components must not exceed the limits specified in Table 4 for the transmitter It is anticipated that signals transmitted digitally will be accurately reproduced at the HMI.
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Prior to putting the ATT in service, the following should be checked:
— ensure no damage occurred during transport;
— verify ATT type and model to ensure the intended use (see also Section 5 and Section 6);
— mechanical installation (correct sensor length, properly mounted etc.);
— electrical installation (grounding, wire connections in junction boxes etc.);
— temperature values can be read on local and/or remote displays;
— verify accuracy (see 9.2.2.1, 9.2.2.2, or 9.2.2.3 depending on ATT type).
Due to potential challenges in positioning the thermometer near the temperature element and the possibility of slight horizontal temperature stratification, thermometer measurements may not always align perfectly Generally, for fluid temperatures near ambient, calibration using a Portable Electronic Thermometer (PET) is deemed acceptable if the PET sensor is placed within 1 meter (3 feet) of the spot ATT sensor.
To ensure accurate initial field verification, lower the PET to the level of a fully submerged ATT spot sensor and wait for the temperature reading to stabilize Compare the temperature displayed by the ATT for that specific spot sensor with the PET measurement, ensuring it falls within the tolerance limits specified in Table 4 for the intended application This verification procedure should be followed for each ATT spot sensor.
For heated tanks, where uneven heating by heating coils is often encountered, it is recommended that the heating process be stopped temporarily prior to verification.
To ensure accurate temperature measurements, take ten evenly spaced readings every 0.7 m (2 ft) for tank levels below 7.7 m (22 ft), covering the entire liquid level The average temperature from the PET should be compared to the average temperature from all ATT temperature sensors submerged in the liquid The difference between these two averages must fall within the tolerance specified in Table 4.
NOTE An “Upper-Middle-Lower” ATT system, which automatically adjusts according to the liquid level, does not require the tank to be full.
This procedure verifies variable-length averaging ATT systems that automatically identify the longest, fully submerged sensor for calculating the average tank temperature, utilizing a PET for validation.
For accurate temperature measurement, ensure the tank is nearly full with all sensors submerged Take ten evenly spaced temperature readings every 0.7 m (2 ft) for tanks under 7.7 m (22 ft) to cover the entire liquid level Manually select each temperature sensor using a software or hardware switch Compare the average temperature from the PET readings with the average temperature shown on the ATT display, ensuring both are within the tolerance limits specified in Table 4 for initial field verification.
Establishing a regular verification program for fiscal, custody transfer, and inventory control ATT systems is crucial It is essential to check all components of the ATT system installation according to the manufacturer's instructions Each ATT system must undergo inspection and calibration verification using the procedures outlined in section 9.2.2 to ensure that the instruments operate within the tolerances specified in Table 4.
To maintain the accuracy of automated sensor selection for tank temperature, it is essential to periodically verify the Automated Temperature Transmitter (ATT) at randomly selected fill levels within the standard opening and closing gauge readings of the tank.
Calibration
For the purpose of this standard, the term “calibration” refers to the process of verifying and then adjusting if necessary.
Temperature devices often need checks or calibrations using a temperature-controlled reference source However, utilizing such a source can be impractical in field settings It is more effectively done in a controlled laboratory, shop, or test facility, following the manufacturer's recommendations and adhering to ASTM E77 standards.
Certification
In a few countries, national regulations may require the ATT or ATT system to be pattern approved and certified by an NMI, to be used in custody transfer applications.
Inspection and Maintenance
Inspection is normally performed in connection with a SAT or at subsequent verifications See 9.2.1.2.3 and 9.2.3 respectively.
Requirements for other regular maintenance are limited and the need for maintenance will normally be established through a failed verification Consult the manufacturer's documentation for any other expectations.
Accuracy Limitations of Tank Temperature Measurements
Tank temperature measurements using marine ATTs face several inherent limitations Firstly, the cargo temperature can change due to the loading temperature Additionally, shortly after loading, a sharp vertical temperature gradient may develop in cargo holds that are in contact with ballast tanks or seawater, primarily influenced by differing heat exchange rates above and below the waterline This is particularly relevant when the cargo temperature exceeds that of the seawater, as strong convection currents are established below the waterline While horizontal temperature differences within the cargo are minimal due to convection, significant variations can occur between wing and center tanks due to heat exchange dynamics Furthermore, seawater temperature can also affect cargo temperature, and temperature discrepancies may arise from tank bulkheads in contact with the ocean Other factors include temperature changes from adjacent cargo tanks, cargo heating, and thermal offsets and time delays resulting from thermowell design and properties.
The limitations listed above may have significant impact on the overall accuracy of temperature measurement by all types of marine automatic tank temperature systems.
[1] API MPMS Chapter 3 (all sections), Tank Gauging
[2] API MPMS Chapter 4 (all sections), Proving Systems
[3] API Recommended Practice 500, Recommended Practice for Classification of Locations for Electrical
Installations at Petroleum Facilities Classified as Class I Division 1 and Division 2
[4] API Recommended Practice 551, Process Measurement Instrumentation
[5] API Recommended Practice 2003, Protection Against Ignitions Arising Out of Static, Lightening, and Stray
[6] ASTM E77 2 , Standard Test Method for Inspection and Verification of Thermometers
[7] ASTM E344, Terminology Relating to Thermometry and Hydrometry
[8] ASTM E1137, Standard Specification for Industrial Platinum Resistance Thermometers
[9] IEC 60751 3 , Inudstrial Platinum Resistance Thermometers and Platinum Temperature Sensors
[11] IMO 5 , Safety of Life at Sea (SOLAS)
[12] OCIMF 6 , International Safety Guide for Oil Tankers and Terminals (ISGOTT)
2 American Society for Testing and Materials, 100 Barr Harbor Drive, West Conchocken, Pennsylvania 19428, USA.
3 International Electrotechnical Commission, 3, rue de Varembé, P.O Box 131, CH-1211, Geneva 20, Switzerland, www.iec.ch.
4 National Fire Protection Association, 1 Batterymarch Park, Quincy, Massachusetts 02269, USA.
5 International Maritime Organization, 4 Albert Embankment, London SE1 7SR, UK.
6 Oil Companies International Marine Forum, 6 th Floor, Portland House, Stag Place, London, SW1E 5BH, UK.
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