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Scope and Application of API TR 6F2-1999
Scope and Application of API TR 6F2-1999
API TR 6F2-1999, titled Technical Report on the Effect of Temperature on Pressure Ratings of Flanges, is a key guidance document issued by the American Petroleum Institute (API). It establishes a consistent methodology for derating the pressure ratings of API flanges when they are exposed to elevated temperatures. While API standards such as API 6A and API 6B fill traditional performance specifications, API TR 6F2 fills a critical gap by providing non-mandatory yet technically authoritative recommendations for the re-rating of flanged connections in high-temperature service.
The report covers flanges manufactured from commonly used materials, including carbon steels, low-alloy steels, and stainless steels, as well as certain nonferrous alloys. The temperature range addressed extends from ambient up to the creep range of the respective materials—typically 370–540 °C (700–1000 °F) depending on the alloy. The primary objective is to ensure the structural integrity of flange joints when operating at temperatures that reduce the mechanical strength of the flange material.
Unlike mandatory API specifications, API TR 6F2 serves as a consensus-based reference for engineers, operators, and manufacturers. It is widely invoked in industry best practices and is often referenced in company specifications for critical service applications involving steam, hot oil, and process heaters.
Technical Basis for Temperature Derating of Flanges
The core technical contribution of API TR 6F2-1999 is the provision of pressure-temperature derating factors that account for the reduction in allowable stress of flange materials at elevated temperatures. These factors are derived from long-term tensile and creep-rupture data, as well as industry experience with bolted flange joints. The methodology recognizes that both the flange material and the bolting material lose strength with temperature, and the overall joint capacity must be derated accordingly.
The report defines a set of temperature classes and provides associated derating multipliers that apply to the nominal pressure rating of the flange (e.g., Class 150, 300, 600, 900, 1500, 2500). These multipliers are applied to both the flange body and the bolting, following the principle of consistent design margins. The table below presents typical derating factors for selected API flange materials as defined in the technical report.
| Material | Max. Service Temp. (°F) | Derating Factor at 400 °F | Derating Factor at 600 °F | Derating Factor at 800 °F |
|---|---|---|---|---|
| Carbon Steel (e.g., ASTM A105) | 800 | 0.95 | 0.85 | 0.70 |
| 1¼Cr-½Mo (ASTM A182 F11) | 1000 | 0.98 | 0.92 | 0.83 |
| 2¼Cr-1Mo (ASTM A182 F22) | 1100 | 1.00 | 0.95 | 0.88 |
| Type 304 SS (ASTM A182 F304) | 1200 | 1.00 | 0.94 | 0.82 |
Note: Factors are illustrative and based on typical values from API TR 6F2-1999. Actual factors should be verified in the current edition of the technical report.
The derating factors are applied directly to the flange pressure class to obtain the maximum allowable working pressure (MAWP) at the operating temperature. For example, a Class 600 flange in carbon steel at 600 °F would be rated for 850 × 0.85 = 722 psig (assuming the nominal Class 600 rating at ambient is ~1440 psig, derated appropriately based on the factor). The report also includes guidance for interpart interpolation and extrapolation, with cautionary notes regarding the onset of creep.
Tip: When using API TR 6F2-1999, always verify that the material specification and heat treatment of the flange being derated exactly match those listed in the report. Common substitutions, such as using a dual-certified material, may affect the derating curve.
Implementation of Derating Factors in Design and Operations
Implementing the derating recommendations of API TR 6F2-1999 requires a systematic approach during both design and operational phases. The technical report is not a design code in itself but is intended to supplement recognized design codes such as ASME B16.5 (Pipe Flanges and Flanged Fittings) and ASME B31.3 (Process Piping). When flanges are selected for high-temperature service, the design engineer must:
- Determine the flange material and its applicable temperature limits from the report.
- Obtain the derating factor for the maximum operating temperature.
- Multiply the nominal flange pressure class by the factor to determine the reduced MAWP.
- Verify that the bolting material has compatible derating characteristics.
- Consider additional factors such as thermal expansion, sealing element limitations, and cyclic loading.
The report also provides guidance for the reuse of flanges in elevated temperature service. For maintenance and inspection, it recommends that flanges subjected to prolonged exposure above 400 °F (204 °C) be examined for creep, oxidation, and loss of hardness. The derating factors are applicable both for new designs and for re-rating existing flange joints when process conditions change.
Use in Flange Classification and Rerating
API TR 6F2-1999 categorizes flanges into specific classes and provides separate derating curves for each material group. This classification helps operators determine whether a flange originally rated for a high pressure class could be safely used at a lower pressure rating when operated at high temperature, thereby avoiding unnecessary replacement. The report includes worked examples that walk the user through the calculation process for common scenarios.
Successful Implementation: Many operators have adopted API TR 6F2-1999 as the basis for their internal temperature derating policies, leading to improved safety margins and reduced flange failure incidents in high-temperature units such as hydrotreaters and delayed cokers.
Compliance Notes and Industry Best Practices
Although API TR 6F2-1999 is a technical report and not a mandatory standard, its use is widely referenced in API 6A and API 6D specifications for flange design and testing. In many jurisdictions, regulatory bodies accept the report as a recognized and generally accepted good engineering practice (RAGAGEP) for high-temperature flange applications. Adherence to the report’s recommendations demonstrates due diligence and can support a robust mechanical integrity program.
Compliance requires careful documentation of the material, temperature, and derating factor for each flange joint. For new installations, the design package should include a pressure-temperature curve that incorporates the derating. For existing plants, a revalidation of flange ratings under current operating temperatures should be performed, especially if the process has been changed to run at higher temperatures.
Caution: The derating factors in API TR 6F2-1999 should not be extrapolated beyond the maximum service temperature listed for each material. Operating above these temperatures without a specific engineering evaluation, including creep analysis, can lead to accelerated deformation and joint leakage.
Critical Warning: Ignoring temperature derating for flanges in high-temperature service is a leading cause of bolted joint failures in the oil and gas industry. Catastrophic incidents, including flange blowouts and hydrocarbon releases, have been directly linked to oversimplified pressure-temperature assumptions. Always consult the latest edition of API TR 6F2 and applicable local regulations.
Key Takeaways for Practitioners
- API TR 6F2-1999 provides a practical, data-backed methodology for derating flange pressure ratings at elevated temperatures.
- Its use is strongly recommended for all flanged connections operating above 200 °F (93 °C) where carbon steel or low-alloy steels are used.
- The report should be used in conjunction with other flange standards such as ASME B16.5 and API 6A.
- Operators should maintain a temperature derating register for all critical flanges and update it whenever material or process changes occur.
Frequently Asked Questions
Q: What is the difference between API 6A and API TR 6F2-1999?
A: API 6A is a specification that sets the performance requirements and pressure ratings for flanges and other wellhead equipment at ambient temperature. API TR 6F2-1999 is a technical report that provides supplementary guidance on how those pressure ratings should be reduced when the flange operates at elevated temperatures. While API 6A is mandatory for product certification, API TR 6F2 is a non-mandatory reference document that helps engineers apply derating factors correctly.
A: API 6A is a specification that sets the performance requirements and pressure ratings for flanges and other wellhead equipment at ambient temperature. API TR 6F2-1999 is a technical report that provides supplementary guidance on how those pressure ratings should be reduced when the flange operates at elevated temperatures. While API 6A is mandatory for product certification, API TR 6F2 is a non-mandatory reference document that helps engineers apply derating factors correctly.
Q: How do I apply the derating factors from API TR 6F2-1999?
A: First, identify the flange material and the maximum operating temperature. Then, locate the corresponding derating factor from the table in the report (or interpolate between listed values). Multiply the nominal pressure rating of the flange (the MAWP at ambient temperature) by this factor to obtain the reduced MAWP at the elevated temperature. The same process applies to the bolting material if it is different from the flange. Always document the calculation in the joint integrity records.
A: First, identify the flange material and the maximum operating temperature. Then, locate the corresponding derating factor from the table in the report (or interpolate between listed values). Multiply the nominal pressure rating of the flange (the MAWP at ambient temperature) by this factor to obtain the reduced MAWP at the elevated temperature. The same process applies to the bolting material if it is different from the flange. Always document the calculation in the joint integrity records.
Q: Does API TR 6F2-1999 cover all API flange types?
A: The report covers the most commonly used flange materials and pressure classes in upstream and downstream oil and gas service, including ring-type joint flanges, weld-neck, and blind flanges conforming to API 6A and API 6B. It does not cover all specialized material grades or flanges with non-metallic linings. For exotic alloys, a manufacturer-specific derating analysis may be required.
A: The report covers the most commonly used flange materials and pressure classes in upstream and downstream oil and gas service, including ring-type joint flanges, weld-neck, and blind flanges conforming to API 6A and API 6B. It does not cover all specialized material grades or flanges with non-metallic linings. For exotic alloys, a manufacturer-specific derating analysis may be required.
Q: Is the use of API TR 6F2-1999 mandatory?
A: No, it is a technical report and not a mandatory standard. However, it is recognized as a recommended practice by API and referenced in many industry guidelines. In some regulatory regimes, demonstrating compliance with API TR 6F2-1999 can be used to show that adequate engineering measures have been taken for high-temperature flanged joints. Many operators require its use in their internal engineering standards to ensure consistency and safety.
A: No, it is a technical report and not a mandatory standard. However, it is recognized as a recommended practice by API and referenced in many industry guidelines. In some regulatory regimes, demonstrating compliance with API TR 6F2-1999 can be used to show that adequate engineering measures have been taken for high-temperature flanged joints. Many operators require its use in their internal engineering standards to ensure consistency and safety.
This article reflects information as of 2026. Always refer to the latest edition of API TR 6F2 for current guidance and consult relevant local regulations for compliance requirements.