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API MPMS 4.9.3:2010 (R2015) — Fixed Automatic Tank Thermometers for Hydrocarbon Temperature Measurement
A comprehensive guide to the Manual of Petroleum Measurement Standards Chapter 4.9.3 for accurate temperature determination in storage tanks
Scope and Field of Application
API Manual of Petroleum Measurement Standards (MPMS) Chapter 4.9.3, originally published in 2010 and reaffirmed in 2015 (R2015), provides the minimum requirements for fixed automatic tank thermometers (FATTs) used to determine the temperature of hydrocarbon liquids in atmospheric storage tanks. The standard applies to FATTs installed on fixed-roof, floating-roof, and other tank configurations commonly encountered in the petroleum industry. It covers both contact-type sensors (e.g., resistance temperature detectors, thermocouples) and non-contact infrared systems, as long as they are permanently installed and intended for continuous temperature monitoring.
The standard is part of the broader MPMS framework and is intended to be used in conjunction with other API MPMS chapters, particularly Chapter 4.9 (Temperature Determination – General Requirements) and Chapter 3 (Tank Gauging).
Technical Requirements
Temperature Sensor Accuracy and Classes
The standard defines three accuracy classes for FATT systems, depending on the measurement uncertainty required for the application. Table 1 summarizes these classes.
| Accuracy Class | Maximum Permissible Error | Temperature Range | Typical Application |
|---|---|---|---|
| Class A | ±0.3 °C | –40 °C to +85 °C | Custody transfer, fiscal metering |
| Class B | ±0.5 °C | –40 °C to +100 °C | Inventory control |
| Class C | ±1.0 °C | –20 °C to +150 °C | Process monitoring, non-custody transfer |
Tip: When selecting a Class A FATT for custody transfer, ensure the entire measurement chain from sensor to display meets the ±0.3 °C tolerance.
Design and Construction
The FATT system comprises temperature sensors, thermowells (if used), interconnecting wiring, and the electronic indicating/transmitting unit. The standard mandates that all materials in contact with the hydrocarbon or vapor spaces be compatible with the stored product and meet applicable electrical area classification requirements (e.g., Class I, Division 1 or Zone 0). Temperature elements must be of the platinum RTD type (100 Ω or 1000 Ω) conforming to IEC 60751 or equivalent, or thermocouples with appropriate cold-junction compensation. The electronic unit must be capable of displaying temperature to a resolution of at least 0.1 °C (or 0.1 °F) and shall provide a digital or analog output for integration with tank gauging or supervisory control systems.
Installation and Positioning
Correct positioning of the temperature sensor in the tank is essential for representative temperature measurement. The standard specifies a minimum probe immersion depth of 300 mm (12 in) unless otherwise justified by thermal modeling. Sensors must be located away from tank shell, heating coils, mixing impellers, and other sources of local temperature distortion. For floating-roof tanks, the sensor should be installed in the liquid column at a height that provides a representative average temperature of the product volume. Where multiple temperature sensors are used to obtain an average temperature, the standard provides guidelines on spacing and weighting.
Warning: Inadequate immersion depth or proximity to heat sources can cause measurement errors that significantly exceed the stated accuracy class.
Implementation Highlights
Successful implementation of API MPMS 4.9.3 involves several key stages:
- Selection: Choose an FATT system with the required accuracy class, temperature range, and hazardous area certification.
- Installation: Follow the manufacturer’s installation manual and the positioning requirements of the standard. Perform a baseline accuracy verification using a traceable temperature standard at the time of installation.
- Calibration: The standard recommends initial calibration of the complete FATT loop and periodic recalibration at intervals not exceeding 12 months. Calibration must be performed at a minimum of three temperature points spanning the expected operating range.
- Documentation: Maintain records of type approval certificates, calibration reports, installation reports, and maintenance logs for compliance audits.
Success: A well-implemented FATT system reduces measurement uncertainty to less than 0.3 °C, aligning with the requirements of API MPMS Chapter 12 for custody transfer.
Compliance Notes
Compliance with API MPMS 4.9.3 is typically a contractual or regulatory requirement for hydrocarbon custody transfer operations. During an audit, assessors will check:
- Evidence that the FATT system meets the applicable accuracy class for the metering application.
- Calibration certificates traceable to national standards with documented uncertainty budgets.
- Installation records that confirm probe immersion depth and location comply with the standard.
- Software validation for any averaging or linearization algorithms used in the electronic unit.
- Operator training records demonstrating competence in FATT operation and maintenance.
Danger: Non-compliance with API MPMS 4.9.3 can lead to rejection of metering reports in custody transfer disputes and potentially large financial adjustments.
It is important to note that API MPMS 4.9.3 does not replace national or local regulations. Users must verify that their FATT installation also meets any mandatory legal metrology requirements in the jurisdiction of operation.
Frequently Asked Questions
Q: Is API MPMS 4.9.3 applicable to liquefied gas (LPG/NGL) storage?
A: The standard primarily addresses hydrocarbon liquids stored at atmospheric or low pressures. For LPG/NGL tanks under pressure, refer to API MPMS Chapter 4.9.4 or other applicable standards. However, the general principles of sensor accuracy and installation may be adapted with careful engineering judgment.
A: The standard primarily addresses hydrocarbon liquids stored at atmospheric or low pressures. For LPG/NGL tanks under pressure, refer to API MPMS Chapter 4.9.4 or other applicable standards. However, the general principles of sensor accuracy and installation may be adapted with careful engineering judgment.
Q: How is the weighted average temperature calculated using multiple FATTs?
A: The standard does not prescribe a specific algorithm but requires that the system calculate the volumetric average temperature based on the tank’s geometry and liquid level. Typically, this involves dividing the liquid column into zones and weighting each zone’s temperature measurement by its volume fraction.
A: The standard does not prescribe a specific algorithm but requires that the system calculate the volumetric average temperature based on the tank’s geometry and liquid level. Typically, this involves dividing the liquid column into zones and weighting each zone’s temperature measurement by its volume fraction.
Q: Can a single FATT probe be used for both temperature and level measurement?
A: API MPMS 4.9.3 only addresses temperature determination. If a probe serves dual functions, the temperature measurement portion must still meet all requirements of this standard. The level measurement should comply with API MPMS Chapter 3. Verify that any interference between the two measurements is minimized.
A: API MPMS 4.9.3 only addresses temperature determination. If a probe serves dual functions, the temperature measurement portion must still meet all requirements of this standard. The level measurement should comply with API MPMS Chapter 3. Verify that any interference between the two measurements is minimized.
Q: What is the recommended recalibration frequency if the FATT system is continuously in service?
A: The standard recommends calibration at intervals not exceeding 12 months. For critical custody transfer applications, a more frequent schedule (e.g., every 6 months) may be justified based on historical drift data and risk assessment.
A: The standard recommends calibration at intervals not exceeding 12 months. For critical custody transfer applications, a more frequent schedule (e.g., every 6 months) may be justified based on historical drift data and risk assessment.
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