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API MPMS 2.8A 1991 (2007): Temperature Measurement of Liquid Hydrocarbons in Storage Tanks
A Comprehensive Guide to Automatic Tank Temperature Measurement in the Petroleum Industry
The API Manual of Petroleum Measurement Standards (MPMS) provides the petroleum industry with authoritative guidelines for accurate and consistent measurement of hydrocarbons. Chapter 2.8A, originally published in 1991 and reaffirmed in 2007, specifically addresses the temperature measurement of liquid hydrocarbons in stationary storage tanks using automatic tank gauging (ATG) systems. Precise temperature determination is essential for volume correction to standard conditions, inventory control, and custody transfer. This article reviews the scope, technical requirements, implementation highlights, and compliance notes of API MPMS 2.8A 1991 (2007).
Scope and Application
Applicability
API MPMS 2.8A applies to the measurement of temperature of liquid hydrocarbons in atmospheric and low-pressure storage tanks through permanently installed automatic temperature sensors. It covers the selection, installation, verification, and maintenance of temperature probes and associated thermowells used in fixed-roof, floating-roof, and pressurized tanks. The standard is intended for use by operators, engineers, and inspectors engaged in custody-transfer and custody-transfer–related operations. It complements other MPMS chapters, such as Chapter 3 (Tank Gauging) and Chapter 11 (Temperature and Pressure Volume Correction).
Out of Scope
This standard does not cover manual temperature measurement (Chapter 3.1A), portable electronic thermometers, or temperature measurement of gaseous hydrocarbons. It also does not address the calibration of secondary electronic components (e.g., signal converters) beyond the sensor loop.
Technical Requirements
The standard establishes minimum performance and installation criteria to ensure that the measured temperature is representative of the average bulk liquid temperature in the tank. The key requirements are grouped below.
Temperature Sensing Elements
The standard mandates the use of platinum resistance temperature detectors (RTDs) with a nominal resistance of 100 Ω at 0 °C (Pt100). The RTD must have a temperature coefficient of 0.00385 Ω/Ω/°C and must meet the accuracy classes specified in the standard. Thermocouples are not permitted for custody-transfer temperature measurement due to their lower accuracy and stability.
Thermowell Specifications
Thermowells are required to protect the sensor and to facilitate replacement without draining the tank. Key specifications include:
- Material: 316 stainless steel or equivalent corrosion-resistant alloy.
- Immersion length: Minimum 150 mm (6 in.) into the liquid to avoid thermal stratification effects at the tank wall.
- Thermal conduction: The thermowell wall thickness and fill material (if any) must not degrade the response time below 30 seconds for a 90 % step change.
- Sealing: The thermowell assembly must be rated for the maximum static head and vapour pressure in the tank. O-rings or gaskets must be compatible with the stored product.
Accuracy and Calibration
The overall system accuracy (sensor and transmitter) must be within ±0.1 °F (±0.05 °C) over the operating range of –40 °F to 180 °F (–40 °C to 82 °C). Calibration must be verified at least annually using a certified reference thermometer traceable to national standards. The calibration setup must cover at least three points spanning the expected operational range.
| Parameter | Requirement |
|---|---|
| Sensor type | Pt100 RTD (α = 0.00385) |
| System accuracy (sensor + transmitter) | ±0.1 °F (±0.05 °C) from –40 °F to 180 °F |
| Thermowell material | 316 SS or equivalent |
| Minimum immersion length | 150 mm (6 in.) into the product |
| Response time (τ90) | ≤ 30 seconds |
| Calibration interval | 12 months |
Implementation Highlights
Installation Best Practices
Tip: Ensure the thermowell tip is fully immersed in the liquid and located at least 300 mm from any tank wall, heat source, or agitator to avoid local hot zones that can skew the measurement.
When installing new temperature probes, the thermowell should be inserted through a tank nozzle at a height that corresponds to the average liquid level when the tank is partially full. For tanks with large height-to-diameter ratios, multiple sensors (e.g., top, middle, bottom) are recommended, and the standard references API MPMS Chapter 7 for averaging methods. All electrical connections (sensor to junction box, junction box to ATG controller) must be shielded to avoid electromagnetic interference, and the cable shield must be grounded only at the controller end.
Integration with Automatic Tank Gauging
The temperature signal is fed into the ATG system, which applies it to the measured volume (via tank strapping tables) to compute the volumes at standard temperature (60 °F per API MPMS Chapter 11.1). The ATG software must validate the temperature against the liquid level—an alarm should be raised if the temperature deviates from expected values by more than 5 °F (2.8 °C) unless a valid cause (e.g., product mixing) is identified.
Training and Documentation
Personnel performing installation and calibration must be trained on the specific probe models and on the standard’s requirements. The standard recommends maintaining a calibration log that includes the date, reference thermometer reading, as-found and as-left deviations, and technician signature. This log is critical for audits and for demonstrating compliance.
Compliance Notes
Warning: Continuous operation with a temperature error of just 0.2 °F (0.1 °C) can result in volume corrections that differ by 0.02–0.04 % for typical hydrocarbons, which may translate into significant fiscal losses over large throughput volumes.
Regulatory bodies and terminal operators often require demonstration of compliance with API MPMS 2.8A for custody-transfer metering. Non-conformities commonly found include:
- Use of non-compliant RTDs (e.g., thermistor-based sensors) without proper justification.
- Insufficient thermowell immersion, especially after tank modifications or product level changes.
- Overdue calibration (exceeding 12‑month interval).
- Missing or poorly maintained calibration records.
To ensure ongoing compliance, facilities should implement a periodic audit schedule that includes physical inspection of sensor installations (verify immersion length, check for fouling, inspect seals) and a comparison of temperature readings between the ATG and a manually deployed reference thermometer at the same tank opening. Any discrepancy greater than 0.15 °F should trigger an investigation.
Note: Many operators treat API MPMS 2.8A as a de facto minimum requirement, and some jurisdictions adopt it directly into regulation. Maintaining conformance with this standard helps satisfy both internal quality policies and external audit requirements.
Frequently Asked Questions
Q: Can I use a thermocouple instead of an RTD for temperature measurement under API MPMS 2.8A?
A: No, the standard explicitly requires a Pt100 RTD sensor for custody-transfer applications. Thermocouples do not provide the long-term stability and accuracy needed. Thermocouples may be used for monitoring purposes only, but not for volume correction.
A: No, the standard explicitly requires a Pt100 RTD sensor for custody-transfer applications. Thermocouples do not provide the long-term stability and accuracy needed. Thermocouples may be used for monitoring purposes only, but not for volume correction.
Q: How often must the temperature probe be calibrated?
A: API MPMS 2.8A recommends an annual calibration interval. However, if the probe experiences frequent thermal cycling, vibration, or is installed in a corrosive environment, more frequent calibration (every six months) is advisable.
A: API MPMS 2.8A recommends an annual calibration interval. However, if the probe experiences frequent thermal cycling, vibration, or is installed in a corrosive environment, more frequent calibration (every six months) is advisable.
Q: Is a single temperature sensor sufficient for a large tank?
A: For tanks with a height greater than 10 m (33 ft) or where significant temperature stratification is expected, a single sensor may not represent the average bulk temperature. The standard suggests using multiple sensors (top, middle, bottom) and averaging the readings according to API MPMS Chapter 7. For smaller tanks (< 5 m tall) with well-mixed product, one sensor is usually adequate.
A: For tanks with a height greater than 10 m (33 ft) or where significant temperature stratification is expected, a single sensor may not represent the average bulk temperature. The standard suggests using multiple sensors (top, middle, bottom) and averaging the readings according to API MPMS Chapter 7. For smaller tanks (< 5 m tall) with well-mixed product, one sensor is usually adequate.