Chapter 7 Manual of Petroleum Measurement Standards Chapter 7—Temperature Determination FIRST EDITION, JUNE 2001 REAFFIRMED, FEBRUARY 2012 Copyright American Petroleum Institute Provided by IHS under[.]
Scope
This chapter describes the methods, equipment, and proce- dures for determining the temperature of petroleum and petroleum products under both static and dynamic conditions.
This chapter outlines the general requirements for temperature measurement in custody transfer, inventory control, and marine applications The specific methods and equipment used for determining temperature are subject to the agreement of the involved parties.
The temperature of hydrocarbon liquids in static conditions can be accurately measured at specific locations within various static vessels, such as storage tanks, field gathering tanks, ships, barges, tank cars, tank provers, and test measures For custody transfer, there are three methods to determine the average static tank temperatures: an automatic method utilizing fixed electronic temperature sensors, a manual method employing portable electronic thermometers, and a manual method using mercury-in-glass thermometers.
The automatic method covers the determination of temper- ature using ịxed automatic tank temperature (ATT) systems for hydrocarbons having a Reid Vapor Pressure at or below
101 kPa (15 pounds per square inch absolute) ATT systems include precision temperature sensors, ịeld-mounted trans- mitters for electronic signal transmission, and readout equip- ment.
The manual method applies to nonpressurized tanks and marine vessels, as well as blanketed tanks and marine vessels It is also suitable for tanks and marine vessels that have been rendered inert and are maintained at pressures below 21 kPa (3 pounds per square inch gauge).
It does not cover hydrocarbons under pressures in excess of
21 kPa (3 pounds per square inch gauge) or cryogenic tem- perature measurement, unless the tank is equipped with a thermowell.
The temperature of hydrocarbon liquids in motion can be measured by monitoring the liquid's temperature as it flows through a pipe This dynamic temperature can be assessed either automatically or manually using electronic temperature devices or mercury-in-glass thermometers To ensure accurate measurements, thermowells may be necessary to separate the liquid from the temperature sensor during dynamic assessments.
The requirements of this chapter are based on practices for crude oils and petroleum products covered by API MPMS
Chapter 11.1 (ASTM D 1250) Requirements in this chapter may be used for other òuids and other applications However,other applications may require different performance and installation speciịcations.
Safety
Safety considerations must be included in all equipment speciịcations, installation and operation Refer to API RP
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 API MPMS, Chapter 3, in accordance with API RP 551 and NFPA 70 guidelines.
Manual of Petroleum Measurement Standards
Chapter 1 ÒVocabularyÓ Chapter 2 ÒUpright Cylindrical TanksÓ Chapter 3 ÒTank GaugingÓ
Chapter 4 ÒProving SystemsÓ Chapter 5 ÒMetering SystemsÓ Chapter 6 ÒMetering AssembliesÓ Chapter 11 ÒPhysical Properties DataÓ Chapter 12 ÒCalculations of Petroleum QuantitiesÓ Chapter 15 ÒGuidelines for Use of the International
System of Units (SI) in the Petroleum and Allied IndustriesÓ
Chapter 21 ÒFlow Measurement Using Electronic
RP 500 Recommended Practice for Classiịcation of Locations for Electrical Installations at Petroleum Facilities Classiịed as Class I Division 1 and Division 2
RP 2003 Protection Against Ignitions Arising Out of
Static, Lightening, and Stray Currents
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D 1250 Standard Guide for Petroleum Measure- ment Tables
E 77 Standard Test Method for Inspection and
E 344 Terminology Relating to Thermometry and
International Safety Guide for Oil Tankers and Terminals
Safety of Life at Sea (SOLAS)
Terms used in this chapter are deịned as follows:
Automatic tank thermometers (ATTs) are essential instruments for continuously measuring temperature in storage tanks An ATT, also referred to as an automatic tank temperature system, typically comprises precision temperature sensors, field-mounted transmitters for electronic signal transmission, and receiving or readout devices There are different types of ATTs, including single-point (spot) ATTs, which measure temperature at a specific location within the tank, and multiple-spot ATTs, which utilize three or more spot temperature elements to assess temperatures at various liquid levels The readout equipment for multiple-spot ATTs averages the temperatures from the submerged elements to determine the overall liquid temperature and may also provide a temperature profile of the tank Additionally, averaging ATTs can come in various forms to suit different measurement needs.
1 A multiple-spot ATT 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.
A variable length Automatic Temperature Transmitter (ATT) features multiple temperature elements of different lengths, all extending upward from near the bottom of the tank The readout equipment identifies the longest fully submerged temperature element to calculate the average temperature of the liquid within the tank.
The Celsius scale (°C), also known as the centigrade scale, defines the freezing point of water at 0°C and its boiling point at 100°C The relationship between Celsius and Fahrenheit is given by the formula °C = 5/9 (°F - 32).
A complete-immersion thermometer is specifically designed to accurately measure temperatures when fully submerged in the environment being assessed However, it is important to note that no ASTM thermometer is intended for complete immersion use.
3.4 discrimination: The ability to sense and record the actual temperature of a liquid to the speciịed temperature increments.
3.5 Fahrenheit scale: A temperature scale on which the freezing point of water is 32¡ and the boiling point 212¡, both at standard pressure [¡F = 9/5¡C + 32].
3.6 field standard test measure: A portable certiịed vessel which is primarily used for the purpose of prover water draw calibrations.
3.7 lightning or surge: A high-energy, fast-rising volt- age pulse that temporarily causes an increase in line voltage over the operating tolerances normally permitted.
3.8 partial-immersion thermometer: A thermometer designed to indicate temperatures correctly when the bulb and a speciịed part of the stem are exposed to the tempera- ture being measured.
3.9 resistance temperature detector (RTD): An elec- trical temperature-sensing element in common use to mea- sure the temperature of the contents of a storage tank or the contents of a pipeline.
3.10 temperature measurement device: Consists of a sensor, transmission medium, and readout equipment in an operating conịguration used to determine the temperature of a liquid for measurement purposes.
The 3.11 temperature sensor comprises a sensing element and its housing, if applicable, and is defined as the component of a temperature device that is immersed in a liquid whose temperature is being measured.
The 3.12 temperature transmitter is a device that supplies electrical power to temperature elements, converting the measured temperature into an electrical or electronic signal for transmission to a remote readout It may also include a local readout feature Frequently, the temperature transmitter's function is integrated into the level transmitter of an automatic tank gauge (ATG).
1American Society for Testing and Materials, 100 Barr Harbor
Drive, West Conshocken, Pennsylvania 19428, USA.
2National Fire Protection Association, 1 Batterymarch Park, Quincy,
3Oil Companies International Marine Forum, 6th Floor, Portland
House, Stag Place, London SW1E 5BH, UK.
4International Maritime Organization, 4 Albert Embankment,
The 3.13 total-immersion thermometer is a liquid-in-glass device specifically designed to accurately measure temperatures when only the portion containing the liquid is exposed to the temperature being assessed.
3.14 transient: As used in this standard, refers to high- voltage, fast-rising, lower-energy pulses The disturbances caused by transients usually have duration of 0.2 seconds.
Temperature significantly impacts the accurate measurement of liquid quantities during custody transfer and inventory control Therefore, utilizing the most precise methods for temperature determination is essential for these applications.
The average temperature of a liquid is required to calculate its volume at a standard temperature, so it is imperative that temperatures be determined accurately.
For custody transfer, the means of temperature determina- tion should be agreed to among the parties involved.
This standard accommodates both Metric (SI) and US Customary units for user convenience, though the provided units may not represent exact conversions The choice of units is generally dictated by contracts, regulatory requirements, manufacturers, or the user's calibration program Once a specific system of units is selected for an application, this standard does not permit arbitrary changes between units.
When selecting a temperature device, it is essential to match it to the specific application requirements Key factors to consider include the temperature range, scale (Celsius or Fahrenheit), response time, accuracy, discrimination, repeatability, and the ambient temperature and atmospheric conditions of the installation site Additionally, the intended use of the temperature data should be taken into account.
When selecting a temperature device for measuring a metered stream or meter prover, it is essential to consider accuracy requirements, mechanical and operating limits, ambient conditions, and individual preferences Additionally, the device's capability to sense and record the actual temperature of a liquid with specified increments, known as temperature discrimination, must be assessed For guidance on temperature discrimination requirements for various measurements and calculations, refer to API MPMS Chapter 12.
Using a temperature device that exceeds the discrimination requirements outlined in Chapter 12 is both acceptable and preferred It is essential that the selection, installation, maintenance, operation, and calibration of this equipment are sufficient to guarantee its performance meets the agreed-upon standards by all parties involved.
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 following equipment and apparatus are used in tem- perature determination:
Fixed Automatic Tank Thermometers (ATTs)
An ATT system serves both custody transfer and inventory control functions For custody transfer, it typically necessitates a mutual contractual agreement between the buyer and seller, and it may also be governed by various federal, state, or local regulations.
An ATT system typically consists of temperature ele- ment(s), ịxed thermowell(s), and telemetry and readout equipment.
Aboveground bulk storage tanks typically feature a local direct-reading thermometer installed in a fixed thermowell However, this thermometer is not part of the Automatic Tank Gauging (ATT) system and is not advisable for determining custody transfer temperatures.
For custody transfer temperature measurement, it is advisable to avoid using local direct-reading thermometers Instead, resistance temperature detectors (RTDs) with copper or platinum temperature element bulbs are typically employed for accurate results.
Choosing between a single-point, mid-level, multiple-point, or averaging Automatic Tank Gauge (ATT) depends on the anticipated temperature stratification within the tank and the necessary accuracy for applications such as custody transfer or inventory control.
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 Electric Code (NEC).
When selecting an appropriate Automatic Temperature Transmitter (ATT), it is essential to consider several key criteria: the required accuracy, the operating conditions that may influence this accuracy such as expected tank temperature stratification, and the minimum tank level for temperature measurement Additionally, environmental conditions, the number, type, and size of the tanks, as well as the need for local and remote readout, signal transmission, and cabling must also be taken into account.
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To ensure the accuracy and performance of all types of ATT systems, it is essential to adhere to specific general precautions These precautions include ensuring that all ATTs are designed to withstand the pressure, temperature, and environmental conditions expected in their intended service.
When installing an ATT in corrosive environments, it is crucial that all components exposed to liquids or vapors are made from durable, corrosion-resistant materials to prevent product contamination and corrosion of the ATT itself Additionally, ATTs must be properly sealed to endure the vapor pressure of the tank's liquid For ATTs on marine vessels equipped with an inert gas system (IGS), they should be engineered to handle the IGS's operating pressure Furthermore, to ensure compatibility and prevent contamination and corrosion, all ATT components in contact with the product must be suitable for the specific product and designed to meet the required operating conditions.
Note 1: This protection may require mounting the ATT sen- sor(s) in a thermowell.
ATT sensors play a crucial role in the automatic tank gauging (ATG) system, particularly within the level sensor assembly, such as float and tape or pole designs In certain configurations, like the float and tape, it may be necessary to elevate the level and temperature sensor assembly to a "store" position when not in use.
Marine Automatic Tank Gauges (ATTs) must not be utilized during tank washing operations on vessels It is essential that all marine ATTs are specified and installed in compliance with relevant national and international marine electrical safety standards, such as those set by IMO, USCG, IEC, NEC, ISGOTT, and ISO Additionally, ATTs should be certified for use in the appropriate hazardous area classification for their installation All external metal components of ATTs mounted on tanks must be securely connected to an electrical earth, which, in the case of marine ATTs, is the ship's hull Furthermore, it is crucial to maintain all ATT equipment in a safe operating condition, adhering to the manufacturers' maintenance instructions.
The design and installation of Automatic Tank Gauges (ATTs) require approval from national measurement organizations and classification societies, which may issue type approvals after rigorous testing These tests typically assess visual inspection, performance, vibration, humidity, dry heat, inclination, power supply fluctuations, insulation, resistance, electromagnetic compatibility, and high voltage Tank levels must be measured concurrently with tank temperature readings, and temperatures should be recorded immediately unless the ATT system automatically logs them Consistent procedures should be followed for measuring tank temperatures before and after product transfers Additionally, temperature sensors should be positioned to avoid measuring sludge deposits or water bottoms in the tank Finally, ATT systems must ensure security against unauthorized adjustments, particularly in fiscal or custody transfer applications, and should allow for sealing during calibration adjustments.
5.1.3 Electronic Temperature Elements in ATTs
Copper or platinum resistance temperature detectors (RTDs) are preferred for custody transfer temperature measurement due to their exceptional accuracy and stability These RTDs can be constructed as resistance wire wound on a nonconductive core, in a thin film configuration, or other designs It is essential for the sensing element to be securely housed in a stainless steel enclosure, and the electronic circuits must meet intrinsic safety requirements Additionally, the temperature-sensitive section of a single-point element should not exceed 100 millimeters (4 inches) in length.
Other types of temperature elements (e.g., thermocouples, thermistors, semiconductors, etc.) are also available and may be suitable for custody transfer purposes.
ATTs should be installed in accordance with the ATT and Automatic Tank Gauge (ATG) manufacturersÕ instructions.
Single-point temperature elements must be positioned near gauging hatches, vapor lock valves, or other appropriate access points Common installation methods include placing them in a metal thermowell that extends through the tank shell or deck for marine applications.
The installation requires a height of at least 900 millimeters (36 inches) from the tank bottom, with the element positioned approximately 900 millimeters (36 inches) into the tank It should be suspended from the tank roof using a suitable metallic or nonmetallic tube or hose, secured to the tank bottom or stabilized with anchor weights.
C HAPTER 7—T EMPERATURE D ETERMINATION 5 shell and the low point at an elevation of approximately 900 millimeters (36 inches) from the tank bottom.
To ensure proper installation, it is essential to maintain sufficient clearance between the sensor assembly and the thermowell To avoid measurement inaccuracies caused by thermal convection in the gap, the thermowell should be filled with a heat-conductive fluid, and provisions for the thermal expansion of this fluid must be considered The temperature element can be installed by either connecting it to the flexible elbow of the swing suction line or by suspending it from a pulley system attached to the floating roof.
5.1.4.2 Multiple Spot and Averaging Temperature
Portable Electronic Thermometers (PETs)
Portable electronic thermometers used for custody transfer shall meet the accuracy requirements of Table 3 and shall come to equilibrium within the immersion time requirements of Table 6.
The temperature probe or sensor head of a PET contains the temperature-sensing element, which is electrically con- nected to electronic circuits contained in the readout device.
To ensure the thermometer meets specified accuracy, it must have internal adjustment means for calibration, as outlined in Table 3 Access to these adjustments should be restricted to trained personnel equipped with proper calibration tools Additionally, users may supply paper seals or similar devices to confirm that calibration adjustments remain untampered.
Each unit shall include a test system or switches to indicate low battery voltage Each unit shall include provision for attaching an earth ground cable.
All components, including the probe and cable, must be certified by an appropriate agency to ensure they are safe for use in flammable environments and with liquids that may accumulate static charges.
The display should be capable of being read to the nearest 0.1¡C or 0.1¡F.
The specifications outlined in this table indicate the minimum acceptable accuracy for portable electronic thermometers utilized in custody transfer It is important to note that thermometers with enhanced accuracy are also available and can be specified through mutual agreement.
Note 2: PETs shall be provided with displays that provide a resolu- tion of 0.1¡C or 0.1¡F or better.
The portable electronic thermometer must ensure specified accuracy and provide a clear display that is easily readable within the expected ambient and operational temperature ranges at the point of use.
Glass Thermometers
Glass reference thermometers include complete-immer- sion thermometers, partial-immersion thermometers, and total-immersion thermometers (see Figure 4 and refer to
ASTM E 344) These thermometers should conform to
ASTM E 1 speciịcations for thermometers or to National
Institute of Standards and Technology 5 (NIST) speciịcations.
Calibration must be traceable to NIST-certiịed instruments.
CAUTION: No ASTM thermometer is designed to be used at complete immersion See Figure 4.
ASTM E 1 glass thermometers, designed for meter prover calibration and temperature device checks, are typically total-immersion types They are calibrated for immersion to the scale level that matches the liquid's temperature, featuring scale graduations of 0.05°C (0.1°F) or 0.10°C (0.2°F) with a tolerance of 0.10°C (0.2°F) Using these thermometers outside of total immersion can lead to inaccuracies due to the differential expansion of the glass and the liquid column in the stem.
5NIST, 100 Bureau Drive, Stop 3460, Gaithersburg, Maryland
Figure 4—Types of Glass Thermometers and Their Use
Full or complete immersion (see Note 2)
2 No ASTM thermometer is designed to be used at complete immersion.
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For accurate meter prover calibration and temperature device checks, it is essential to analyze potential scale errors and apply necessary corrections Stem corrections are typically minor unless the average stem temperature significantly deviates from the liquid temperature by more than 8°C (15°F) Each case should be evaluated individually regarding stem correction, with detailed methods outlined in Appendix A.
Permanently installed glass thermometers must be securely mounted in a thermowell and safeguarded against breakage with a protective housing They should feature a high-resolution scale with the same graduation interval and tolerance as glass reference thermometers Regular calibration and verification using reference thermometers, as outlined in section 8.3, are essential for accurate measurements.
Tank thermometers shall be totally immersible and shall be made in accordance with the speciịcations in ASTM E 1.
Each thermometer must be a mercury-in-glass type, featuring an inert gas like nitrogen above the mercury column, with permanently etched graduation marks on the glass stem Angle-stem thermometers are required to comply with ASTM E 1 specifications for partial-immersion thermometers, although they may exceed total length specifications and utilize a separate graduated scale as outlined in section 5.3.3.3.
The thermometers listed in Table 4 shall be used.
The cup-case assembly depicted in Figure 5 can be constructed from varnished hardwood or a non-sparking, corrosion-resistant material It is essential that the cup has a minimum capacity of 100 milliliters (6.1 cubic inches) and dimensions that ensure the bulb's side measures at least 9.5 millimeters.
( 3 /8 inch) from the nearest wall and the bottom of the bulb will be 25.4 millimeters ± 5.0 millimeters (1 ± 3 /16 inch) above the bottom of the cup
The armored-case assembly shown in Figure 6 shall be made of non-sparking, corrosion-resistant tubing that does not exceed 13 millimeters ( 1 /2 inch) in outside diameter.
The angle-stem thermometer, as shown in Figure 7, is designed for installation in a standard metal-separable well or socket within a tank For vertical tanks exceeding 5000 barrels in capacity, the thermometer's glass stem must measure at least 0.9 meters (3 feet) in length, not including the graduated section, and should be safeguarded by a light metal tube.
The thermometer for measuring 5000 barrels must have a stem length of 0.3 meters (1 foot), excluding the graduated section, and should be safeguarded with a lightweight metal tube Additionally, the sensitive part of the thermometer must not exceed 60 millimeters (2.5 inches), and the stem can be angled at 90 degrees or more to fit the tank shell's contour.
The assembly must be securely connected to the well using a threaded coupling A thermometer featuring a separate graduated scale is permissible, provided that the scale markings are permanently engraved and that temperature lines are etched on the glass stem at approximately 27°C (80°F) intervals to align with the corresponding scale markings.
In addition to applications discussed in this section, angle- stem thermometers can be used in pipeline metering and prover applications to measure the temperature of the proving medium
ASTM Thermometer Range Length (inches) Graduation Accuracy
Angle-stem ẹ Suitable range 12 a 1ĂF ± 1.0ĂF
Tank thermometer b ẹ 20ĂF to 220ĂF 12 1ĂF ± 0.5ĂF
All thermometers listed in the table, except for the angle-stem thermometer, are of the total-immersion type The length of the graduated portion is specified, and while one thermometer lacks an ASTM designation, it is widely utilized for specific heated materials.
Figure 5—Typical Cup-Case Assembly
100-ml cup (corrosion- resistant metal)
Figure 6—Typical Armored-Case Assembly
Figure 7—Typical Angle-Stem Thermometer
Note: The etched reference line on the glass must be aligned with zero on the scale.
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Filled bulb systems consist of a temperature sensor bulb connected via capillary tubing to a pressure sensitive trans- ducer Three types of ịlled systems are in common use: Class
System selection for temperature control involves three classes: Class I utilizes liquid expansion, Class II relies on vapor pressure, and Class III operates on gas pressure The choice of system is influenced by the specific application and maintenance philosophy Proper installation and usage are crucial to avoid potential damage.
(crimping or puncture) to the ịlled bulb system.
Electronic Temperature Devices
Electronic temperature devices for measurement generally use one of the following temperature sensors: a Thermistor. b Thermocouple. c Resistance temperature detector (RTD).
Temperature sensors are typically housed in metal probes that fit into thermowells, and for optimal heat transfer, these tip-sensitive probes must be securely positioned at the bottom of the thermowell It is advisable to use spring-loaded or adjustable-length probes and to apply a suitable heat-conducting material between the temperature sensor and the thermowell wall Proper wiring is essential due to the low signal levels of these devices, and installation should follow the manufacturer's guidelines to ensure maximum accuracy Additionally, linearization of the transducers is usually performed within the associated transmitter, with each probe type requiring a specific circuit.
Safety must also be included in the equipment speciịca- tions The equipment and transducers should be installed in accordance with API RP 500 and RP 551 and with NFPA 70,
National Electrical Code (NEC) hazardous area speciịcations
All electronic temperature devices should be provided with displays that provide a resolution of 0.1¡C or 0.1¡F or better.
Thermistors are compact ceramic resistors known for their high resistance coefficients, making them highly sensitive to minor temperature fluctuations However, due to their sensitivity, they are not suitable for custody transfer applications unless subjected to frequent calibration and verification testing.
Due to aging, these sensors experience long-term drift, resulting in lower accuracy and ambient temperature compensation compared to conventional temperature sensors They are also less stable and exhibit nonlinearity, making them suitable primarily for less precise temperature control and switching within a range of approximately -100°C to 500°C (-200°F to 900°F).
Thermocouples are temperature-sensitive devices made of two dissimilar metals, generating an electromotive force (EMF) based on the temperature difference between their hot and reference junctions They can measure temperatures ranging from approximately -150°C (-300°F) to 1300°C (2300°F) However, electronically compensated single-junction thermocouples are not suitable for custody transfer measurements due to issues such as drift, corrosion, low millivolt signals prone to noise, and the impact of lead wire length and condition on accuracy Alternative thermocouple systems that comply with Section 8 requirements can be utilized for custody transfer measurements.
A resistance temperature detector (RTD) is a highly accurate temperature sensor that operates based on the principle that its electrical resistance changes with temperature Typically made from a small coil of platinum wire, RTDs provide reliable temperature signals to various readouts and equipment when paired with suitable circuits They outperform thermocouples and most other temperature sensors in terms of accuracy and stability over time Additionally, RTDs have a higher current flow, making them less susceptible to noise and errors from lead-in wires These sensors are ideal for precise temperature measurements in applications such as custody transfer service, narrow span measurements below 40°C (100°F), temperature difference measurements, and critical control processes.
Note: Three or four wire RTDs are recommended to compensate for lead length resistance.
A temperature transmitter is an essential device that transforms signals from temperature sensors into a format that allows for effective transmission of temperature data from the measurement site to its intended location Typically, this temperature signal is converted into either a current or serial digital format for optimal data propagation.
A temperature sensor may or may not be part of the transmit- ter Sensor linearization can be typically provided by the transmitter, and the proper linearization option must be selected.
Electronic, digital (ÒsmartÓ) transmitters may have the fol- lowing beneịts over the conventional analog transmitters: ¥ Wider rangeability ¥ Calibration procedures ¥ Improved performance ¥ Lower drift rate
C HAPTER 7—T EMPERATURE D ETERMINATION 13 ¥ Elimination of loop errors (analog drift, analog conver- sions, etc.)
It is important to read the speciịcations for a transmitter carefully.
Thermowells
The use of thermowells may be required in dynamic and static temperature measurement to isolate the liquid material from the temperature sensor.
Thermowells are classified into two main types: test wells and sensor wells Test wells are designed for occasional temperature checks and should be capped when not in use to prevent foreign material from entering the well bore, which can lead to measurement errors and potential damage to thermometers It is advisable to periodically inspect and clean thermowells to ensure accurate readings In contrast, sensor wells are intended for permanent temperature sensors and must be appropriately matched to the specific sensor being used.
Thermowells should be selected based on the application criteria described below Thermowells should be designed to resist òow-induced vibration.
The thermowell selected shall comply with design codes for the operating pressures and temperatures of the system.
When selecting a thermowell, it is essential to adhere to relevant codes and installation practices Thermowells can be threaded, welded, or flange mounted, and their immersion length must be adequate to position the sensor element within the center one-third of the pipe's diameter or at least 0.3 meters (12 inches), unless restricted by fluid velocity For optimal performance, the thermowell should be installed as vertically as possible to facilitate the use of an appropriate heat-conducting material.
When selecting a thermowell, it is essential to choose a material that is compatible with the liquid it will encounter, ensuring corrosion resistance on all surfaces Typically, Type 304 or 316 stainless steel is recommended for optimal performance.
To enhance the temperature sensor's response time, it is essential to fill the gap between the sensor and the thermowell wall with a suitable heat-conducting material This ensures better heat conduction, but avoid using materials that may freeze under typical operating and atmospheric conditions.
Some applications may warrant special design to provide the fastest possible thermal response (for example, truck loading meters).
Data Collection, Data Transmission, and Receiving Equipment
Data collection, transmission, and reception requirements differ based on the type and model of the temperature measurement system For instance, an Automatic Tank Gauging (ATG) system typically includes an Automatic Temperature Transmitter (ATT) It is essential to adhere to the manufacturer's guidelines, and additional measures may be needed to ensure the security and protection of the collected data Furthermore, installations must comply with all relevant codes and regulations.
The remote readout of a temperature device can be utilized for custody transfer, provided that the entire system, including the remote readout, adheres to the calibration tolerances specified in this standard.
Note: Some readout equipment can be programmed to alarm on high or low temperatures.
Data collection and transmission equipment must be designed and installed according to API RP 500 and RP 2003 standards to ensure accurate measurements The temperature difference between the remote receiving unit and the temperature transmitter should not exceed ± 0.1°C (0.2°F) Additionally, the resolution of the measurement output signal must be maintained, and proper security measures should be implemented to protect the integrity of the data Furthermore, the system should provide adequate speed to meet the required update time while ensuring electromagnetic immunity.
Electronic temperature readout equipment and wiring practices utilized for refinery process unit temperatures are also applicable for tank temperature readouts However, in extensive tank farms, the expense of directly wiring the elements to the central control house can be prohibitive Therefore, it is common to transmit temperature data through the existing wiring network designated for remote reading Automatic Tank Gauging (ATG) level transmitters.
Much of the commonly used ATG equipment provides ịeld converters to convert RTD resistances into data trans- mission codes of various formats These ịeld converters per-
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14 API M ANUAL OF P ETROLEUM M EASUREMENT S TANDARDS mit the ATG ịeld multiplexers or ịeld selectors to transmit both level and temperature information.
The temperature elements, ịeld converters, ịeld multi- plexer or selectors, and ATG readout equipment are all pro- prietary to the ATG systems.
Commonly used modern ATG equipment provides readout equipment to display and log both levels and temperatures.
This readout equipment can determine average temperatures either by selecting the appropriate longest, fully submerged variable length RTD or by averaging the appropriate sub- merged spot elements.
This read-out equipment is capable of being programmed to trigger alarms for both high and low temperatures Additionally, it can reference the tank capacity table, utilize the relevant expansion coefficients, and compute the standard volumes.
A data collection unit gathers measured data such as temperature, level, and flow from various sources like tanks and pipes This unit can either be integrated into the transmitter or exist as a separate microprocessor-based field unit The collected data is then transmitted, ideally in digital format, to a remote receiving unit, which is typically a host computer.
Data collection units must be designed to comply with the electrical classification standards of their location Given that these units are typically situated outdoors, it is essential to ensure they are housed in weatherproof or rain-tight enclosures, such as junction boxes, as a minimum requirement.
5.6.2.1 Interference from the AC Power Wiring
All AC power wiring should be run with at least 1 meter
(3 feet) of separation distance from the signal wiring Most systems do not require electromagnetic shielding of the power wiring if the current is less than 10 amperes.
To prevent radio frequency (RF) interference, it is crucial to implement effective cable shielding and routing strategies Additionally, filtering may be necessary at the inputs of equipment to ensure optimal performance.
Signals are transmitted using pairs of twisted, shielded conductors within insulated multipair cables, which are often installed in conduits or buried It is essential to calculate the line impedance to remain within the maximum specifications set by the equipment manufacturer For long-distance transmission or high impedance scenarios, digital signals are preferred over analog signals.
Alternatively, signals may be transmitted via other media (for example, ịber optics, or coaxial cables) as recommended by the equipment manufacturer.
Proper grounding is important to protect equipment from damage due to transients or surges, which can result in loss of measurement data.
Grounding requirements vary by type and make of the equipment Therefore, the manufacturerÕs recommendations should be explicitly followed.
In aerial installations, it is essential to bond the supporting messenger to ground For buried cable installations, only cables designed for direct burial must be utilized Additionally, in conduit systems, it is crucial to maintain continuity of ground through conduit joints, which can be achieved by ensuring proper makeup of joints or by implementing bonding connections around each joint.
Wiring shields can be made of copper, aluminum, or steel, depending on the manufacturer's specifications It is essential to bond the overall shielding at all junction boxes and ensure it is grounded at only one end, either to a power line grounded neutral or a driven ground rod.
The requirements covered in 5.6 should be followed to pro- vide immunity to noise pickup.
The receiving unit is a crucial component of a temperature monitoring system, which can be configured locally at the tanks or remotely in a control house A remote temperature readout unit must effectively scan all monitored tanks in compliance with data acquisition standards, provide real-time temperature displays, conduct data validity checks to alert operators of any errors, and show alarms for conditions such as high or low temperatures.
To ensure the safety of level and temperature transmitters, it is essential to implement protection against transients, which secures the transmission of measurement data Proper grounding and shielding, as outlined in sections 5.6.2.4 and 5.6.2.5, are effective measures for achieving this protection.
C HAPTER 7—T EMPERATURE D ETERMINATION 15 quate protection However, the manufacturerÕs recommenda- tions should be followed if they are more restrictive.
In regions with frequent lightning strikes, especially where tanks are dispersed far from central monitoring equipment, it is essential to implement enhanced lightning protection measures A reliable lightning protection system must be in place to effectively absorb surge energy from lightning in both signal and power lines.
Surge protection must ensure that normal equipment operation remains unaffected, preventing damaging surges from entering the system Lightning protection devices should be self-restoring and require no maintenance It is essential to select and install protective devices according to the equipment manufacturers' recommendations.
Ambient Temperature
Tanks experience expansion and contraction as a result of changes in ambient and product temperatures The volume changes in the tank can be calculated once the temperature of the tank shell is established Proper calibration of tanks should follow the guidelines set forth in API MPMS Chapter 2, "Upright Cylindrical."
Tanks have capacity tables that are based on a specific tank shell temperature If the actual tank shell temperature varies from the temperature listed in the capacity table, the volumes obtained from the table must be adjusted accordingly.
To do this the tank shell temperature must be determined.
The tank shell temperature for noninsulated tanks is a func- tion of the liquid temperature and the ambient temperature.
Non-insulated storage tanks are exposed to ambient air temperature, which must be taken into account alongside the liquid temperature This consideration is essential for calculating a correction factor that assesses the impact of temperature on the steel shell of the tank, as outlined in API MPMS Chapter 12.1.
Ambient temperature refers to the atmospheric temperature near a tank farm, which can vary significantly This variability makes it challenging to identify the optimal location for measurement Consequently, the uncertainty associated with this measurement can be as much as ± 2.5°C (5°F).
The ambient temperature (T a) contributes only 1/8 of the total tank shell temperature (T s), indicating that the precision needed for measuring ambient temperature is less critical than that for liquid temperature (T L).
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 assessments.
Temperature readings are to be taken at least 1 meter (3 feet) from any obstructions or the ground Additionally, allow sufịcient time for temperature reading to stabilize.
Thermometers used for this purpose shall have an accuracy (maximum permissible error) of 1¡C (2¡F) or better, that should be veriịed every three months.
For reporting purposes, round ambient temperature to the nearest whole degree.
Timing of Temperature Measurement
Temperatures shall be measured immediately before or after the liquid level is measured.
Fixed Automatic Tank Thermometers
For optimal temperature determination in custody transfer, tanks should ideally be equipped with multiple-point Automatic Temperature Transmitters (ATT), unless they have a capacity below 1000 m³ (5000 barrels), a level lower than 3 meters (10 feet), experience a maximum vertical temperature variation of less than 1°C (2°F), or rely on manual average temperature measurement for custody transfer.
Single-point or spot tank temperature measurement is appropriate when the liquid temperature in the cargo tank is uniform or when any temperature stratification is minimal and acceptable However, these types of Automatic Temperature Transmitters (ATTs) are unsuitable for custody transfer or fiscal measurement in other scenarios.
Fixed temperature elements of ATTs must be read and recorded individually at the highest display resolution, usually 0.1°C or 0.1°F In cases where the ATT readout equipment does not average temperatures, it is essential to calculate the average tank temperatures from the multiple readings taken.
6.3.3 Single-Point Tank Temperature Measurement
Single-point tank temperature measurement is suitable only when the liquid temperature in the tank is uniform or when temperature stratification is minimal This method is acceptable for small tanks (under 5000 barrels), those containing materials with consistent temperatures, and tanks equipped with effective mixing systems, as they exhibit reduced temperature stratification Therefore, single-point temperature measurement can be used for custody transfer in these specific scenarios.
Temperatures in large tanks (5000 barrels or larger) are normally stratiịed unless the tank contents are thoroughly mixed Vertical temperature differences of as much as 3¡C or
5¡F are normal, and differences of 5¡C or 10¡F are common.
In low and medium viscosity petroleum liquids, horizontal temperature differences are generally under 0.5°C (1°F), while higher viscosity liquids may exhibit slightly greater variations.
For custody transfer measurement, it is essential to utilize an average temperature rather than a single-point temperature when employing automatic tank gauges (ATGs) that measure level, such as float-operated, servo-operated, or radar ATGs.
In tanks exhibiting vertical temperature stratification, the temperature gradient is often non-linear For custody transfer purposes, an accurate average temperature is essential Relying solely on the mid-level temperature of the tank contents may not provide a true representation of the average temperature.
When hydrostatic tank gauges (HTGs) are used, which compute standard volume using pressure sensors, a single temperature sensor, located about halfway between the lower pressure sensors, may be adequate.
The average temperature of a liquid can be determined using a single-point temperature sensor at the tank outlet, allowing for the calculation of a volume-weighted average temperature during loading or discharging This method is particularly useful for loading marine vessels from bulk storage tanks.
Portable Electronic Thermometers
This section describes the proper use of portable electronic thermometers Thermometers used for custody transfer shall be calibrated against a reference standard (see 8.2).
6.4.1 Reading and Reporting Temperatures or
All temperatures should be read and recorded to the nearest
0.1¡C or 0.1¡F The temperature (or average of multiple tem- peratures) shall be reported to the nearest 0.1¡C or 0.1¡F.
Portable electronic thermometers are precise measurement instruments They should be transported and used carefully.
The following procedure is recommended for measuring temperatures with a portable electronic thermometer:
1 Attach an electrical ground between the thermometer and the tank before opening the hatch (Check that the ground is securely attached to the thermometer).
2 Verify the condition of the battery before and after each use.
3 Set the temperature range selector as appropriate.
4 Lower the sensing probe to the predetermined level.
5 Raise and lower the probe 0.3 meter (1 foot) above and below the predetermined level to allow rapid stabilization. (See 6.4.3.1).
6 After the temperature has stabilized, read and record the temperature at the depth measured.
7 Repeat steps 4, 5, and 6 at each level if multiple tem- peratures are required (see 6.4.4).
9 Round off the average temperature and report the tem- perature to the nearest 0.1¡C or 0.1¡F.
10 After use, clean all the parts of the thermometer assembly with a suitable solvent and dry it with a cloth to prevent the formation of an insulating ịlm.
To ensure accurate temperature readings, thermometers must be stabilized at the liquid temperature before use This can be achieved by moving the probe up and down approximately 0.3 meters (1 foot) around the desired measurement depth, which helps reach stability more quickly Stationary probes can lead to inaccurate low readings due to colder convection currents in the oil, especially when the bob weight above the probe has a higher thermal capacity and is at a lower temperature A thermometer is considered stable when the readout fluctuates by no more than 0.1°C (0.2°F) over a 30-second period.
Liquid temperatures in a storage tank often vary signiị- cantly with depth When custody transfer measurements are being performed, temperatures at multiple levels are required to calculate an average temperature.
For liquid levels exceeding 3 meters (10 feet), it is essential to measure the temperatures at the centers of the top third, middle third, and bottom third of the liquid height.
When the temperature range surpasses 1°C (2°F) or when custody transfer measurements necessitate averaging at multiple levels, it is essential to measure temperatures at intermediate levels and calculate their average.
All measurements will be documented and averaged, with the final average reported to the nearest 0.1 degree For liquid levels below 3 meters (10 feet) or tanks under 5000 barrels, a single measurement taken at the midpoint of the liquid can be utilized, provided there is mutual agreement among all parties involved.
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18 API M ANUAL OF P ETROLEUM M EASUREMENT S TANDARDS
Temperatures should be taken in all tanks or compartments as described in 6.4.1 through 6.4.3.
Upper, middle, and lower temperatures should be taken.
By mutual agreement, it is permissible to take more than three temperature measurements to determine an average temperature For barge compartments with a capacity of less than 5000 barrels, at least two temperature readings are necessary: one from the middle of the bottom half and another from the middle of the top half of the liquid.
During a loaded voyage, fluctuations in marine cargo temperature can lead to non-uniform temperatures at discharge, complicating the determination of the average cargo temperature Automatic averaging of PETs provides a straightforward solution for calculating the average temperature For PETs lacking this feature, the most effective method involves averaging the upper, middle, and lower temperatures for each tank Additionally, it is essential to adjust the total ship volume to standard temperature on a tank-by-tank basis, utilizing the average temperature calculated for each tank.
In cases where the on-board quantity (OBQ) or remaining-on-board (ROB) is insufficient for temperature measurement, it can be assumed that the material is at standard temperature For significant amounts of OBQ or ROB, such as slops, the temperature should be measured at the midlevel of the oil or oily layer.
Temperatures must be measured in the middle of the liquid in every tank car However, if multiple tank cars are loaded with non-heated oil from a single source, temperatures can be assessed from at least 10% of the cars, which equates to a minimum of three randomly selected tank cars, provided they are of the same nominal size and insulation type When heating a car to aid in discharge during cold weather, the temperature of each car's contents should be recorded simultaneously with the gauging process.
Mercury-in-Glass Thermometers
To measure the temperature in a non-pressure tank, lower an ASTM or IP tank thermometer with the appropriate range and a suitable sampling apparatus, such as a cup-case, through the gauge hatch to the specified liquid level After immersing the thermometer for a predetermined time, quickly withdraw it and read the temperature This method is also applicable to low-pressure tanks with a gauge hatch or perforated standpipe, but it is not suitable for pressurized or inerted tanks equipped with vapor control valves.
For a tank equipped with a thermowell, a temperature is obtained by reading a thermometer placed in the well with its bulb at the desired liquid level.
Table 6—Comparison of Recommended Immersion Times for PETs and Woodback Cup-Case Assemblies
Recommended Immersion Times (minutes) a Electronic
Thermometer Woodback Cup-Case Assembly
When Temperature Differential 50 30 seconds 5 minutes 10 minutes 5 minutes 10 minutes
40 to 49 30 seconds 5 minutes 15 minutes 5 minutes 15 minutes
30 to 39 45 seconds 12 minutes 25 minutes 12 minutes 20 minutes
20 to 29 45 seconds 20 minutes 45 minutes 20 minutes 35 minutes
The immersion times of 20, 75 seconds, 45 minutes, 80 minutes, 35 minutes, and 60 minutes were determined according to the test procedure in Appendix D Adhering to these recommended times is crucial, as failure to do so may lead to inaccurate temperature readings.
When measuring temperature, it is essential to keep the sensor probe in motion within the fluid by raising and lowering it 0.3 meters (1 foot) above and below the target depth.
The woodback cup-case assembly is versatile, functioning effectively in both in-motion and stationary modes In-motion refers to the process of repeatedly raising and lowering the assembly by 0.3 meters (1 foot) above and below the desired depth.
Cup-case assemblies constructed from alternative materials will require varying immersion times It is essential to determine these times through testing, with consensus among all parties involved regarding the established immersion durations (refer to Appendix D).
When additional mass is added to the woodback cup-case assembly to ensure it sinks in the liquid near the thermometer, the immersion time will exceed the durations specified in the table It is essential to determine the appropriate immersion times through testing, with consensus among all parties involved (refer to Appendix D).
Note 5: The etched reference line on the glass of the woodback cup-case assembly must be aligned with zero on the scale.
In all cases where more than one temperature is deter- mined, the average temperature of the liquid is calculated from all observed temperatures.
A thermometer of suitable range for the temperature to be measured shall be selected from Table 4, and the appropriate assembly for the various types of tanks and cargo carriers (see
Table 7) shall be used After the thermometer with its appro- priate assembly has been selected and veriịed according to the procedure provided in 8.3, then the procedure outlined in
For custody transfer of a tank with a capacity of less than 5000 barrels, the cup-case assembly must be lowered through the gauge hatch or pressure lock to the designated level It should then be gently moved within a range of ± 0.3 meters (1 foot) of this level for the specified duration to obtain an accurate temperature reading.
Table 6 outlines the recommended immersion times for woodback cup-case assembly, while Appendix D offers a procedure for determining times not specified To optimize the immersion time, the assembly should be continuously raised and lowered When taking a reading, the assembly must be withdrawn, and the thermometer should be read with the cup sheltered below the hatch edge to reduce the impact of wind or atmospheric temperature changes It is essential to keep the cup full during the reading, and the temperature must be recorded immediately For efficient operations and accurate temperature stratification in tanks of 5000 barrels or more, using a portable electronic thermometer significantly enhances response time.
The armored-case assembly is essential for pressurized tanks with vertical thermowells filled with an appropriate heat transfer medium In small pressure tanks, the thermometer assembly should be positioned in the middle of the liquid, while for larger tanks, it must be lowered to the specified level as indicated in the accompanying table.
5 Usually a minimum of 5 minutes is required for the ther- mometer to reach a stabilized temperature; however, tests should be conducted on each given application to determine the required immersion time.
To accurately read the temperature, the assembly must be withdrawn while keeping the perforated end in the well to reduce the risk of altering the thermometer's indication The thermometer should be read and the temperature recorded immediately to ensure precise results Delays in withdrawal and reading can lead to inaccuracies.
Table 7—Thermometer Assemblies and Temperature Levels for Tanks and Cargo Carriers Tanks
Fixed-roof Roof hatch Cup-case See note
Floating-roof Gauge hatch Cup-case See note
Variable vapor space Gauge hatch Cup-case See note
Vertical thermowells Armored See note
Pressure lock Cup-case See note
Horizontal separable wells Angle Stem Three for tank heights less than
10 meters (30 feet); four for tank heights greater than 10 meters (30 feet) Horizontal-cylindrical
Nonpressure Gauge hatch Cup-case See note
Pressure Vertical thermowells Armored See note
Horizontal separable wells Angle-stem Two, one at middle of tank and one
0.3 meter (1 foot), above bottom Tank Cars and Tank Trucks
Nonpressure Dome hatch Cup-case Middle of liquid a
Pressure Vertical thermowells Armored Middle of liquid a
Nonpressure Deck hatch Cup-case See note
Pressure Vertical thermowells Armored Middle of liquid a
In certain situations, it may be beneficial to measure temperatures at multiple levels and compute a weighted average temperature For the minimum number of measurement levels, refer to Table 5.
See Table 5, Note 2, for the correct method of averaging multiple temperatures.
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20 API M ANUAL OF P ETROLEUM M EASUREMENT S TANDARDS the thermometer can result in erroneous readings due to tem- perature gradients within the tank and ambient temperature.
Angle-stem thermometers are designed for permanent installation in separable sockets or thermowells, with removal only necessary for calibration or replacement It is crucial to install these thermometers in the same orientation as during calibration Additionally, if a separate graduation scale is utilized, ensure that the etched reference line on the thermometer aligns with the zero mark on the scale for accurate readings.
6.5.5 Non or Low Pressure Tanks and Cargo
Accurate temperature measurement in liquid custody transfer tanks is essential for calculating volume at standard temperature Follow company procedures for inventory control, ensuring temperatures are taken at the correct measurement level after allowing the thermometer assembly sufficient time to stabilize For tanks over 5000 barrels, record temperatures to the nearest 0.5°C (1.0°F), with the option to report less precise readings if agreed upon by all parties and compliant with legal standards Averages of multiple temperature readings should be reported as specified in Note 2 of Table 5.
The cylindrical core of a storage tank offers the most accurate temperature readings, as it is the region least influenced by external factors that can affect the tank's shell.
Large storage tanks equipped with external floating roofs typically feature multiple gauge hatches located around the tank's perimeter, and they may also include a central hatch on the roof area.