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API TR 6AF-2008 Technical Report: Temperature Derating of API Flanges Under Combination Loading
Comprehensive Guide to Temperature Derating Factors and Compliance for High-Pressure Flanged Connections
1. Scope and Purpose
API Technical Report 6AF-2008 (second edition, August 2008) addresses the temperature derating of API flanges when subjected to simultaneous internal pressure and external bending moments – a condition commonly referred to as combination loading. The report applies to flanges conforming to API 6A (both 6B and 6BX types) and provides derating factors that reduce the allowable working pressure at elevated temperatures. It supersedes the 1996 edition and incorporates updated empirical data and refined calculation methods.
The primary purpose of API TR 6AF-2008 is to offer a rational method for establishing temperature limits and pressure reductions for flanged connections that must sustain both pressure and significant external loads (e.g., from piping thermal expansion, valve actuation, or environmental forces). This technical report is not a mandatory specification but a recommended practice that supports safe design and operation in accordance with API 6A.
Intended Audience: Design engineers, materials specialists, and compliance managers involved in the specification of flanges for high-temperature oil and gas applications, particularly in subsea, offshore, and refining environments.
2. Technical Requirements
2.1 Derating Factor Definition
API TR 6AF-2008 defines a temperature derating factor (TDF) that multiplies the rated working pressure of a flange at ambient temperature to obtain the allowable pressure at elevated temperatures under combined loading. The general form is:
Pallowable(T) = Prated × TDF
where TDF is a function of both temperature and the severity of external loads (expressed as a fraction of the flange’s moment capacity).
2.2 Derating Factor Tables
The report provides derating factors for standard API temperature classes (e.g., 200°F, 300°F, 400°F, 500°F, 550°F) and for various moment loading ratios (M/Mmax). Table 1 below summarizes the key derating factors for a typical 6BX flange at two representative loading conditions.
| Temperature (°F) | Derating Factor (M/Mmax = 0) | Derating Factor (M/Mmax = 0.5) |
|---|---|---|
| 200 | 1.00 | 0.90 |
| 300 | 0.95 | 0.82 |
| 400 | 0.87 | 0.72 |
| 500 | 0.78 | 0.61 |
| 550 | 0.72 | 0.55 |
Note: Values shown are illustrative and may vary based on flange size, material group, and specific loading ratios. Always consult the latest edition of the report for exact factors.
2.3 Combined Loading Methodology
API TR 6AF-2008 uses a stress-based approach that accounts for the combined effects of internal pressure, axial tension/compression, and bending moment. The derating factor is derived from the ratio of the reduced allowable stress at temperature to the ambient allowable stress, normalized by the load combination. The report includes detailed calculation procedures and examples for flange sizes from 7-1/16 in. through 21-1/4 in. (180 mm to 540 mm).
Important: Derating factors are not intended for flanges operating below 200°F (93°C) or for purely pressure-only loads at ambient conditions. They apply exclusively to temperature-sensitive combination loading scenarios.
3. Implementation Highlights
Engineers implementing API TR 6AF-2008 should follow these key steps:
- Identify flange type and material: Confirm that the flange conforms to API 6A types 6B or 6BX and that the material is listed in the report (e.g., carbon steel, low-alloy steel, stainless steel).
- Determine design temperature and loads: Establish the maximum temperature and the external loads (pressure, bending moment, axial force) under combined conditions.
- Calculate the moment ratio: M/Mmax, where Mmax is the rated bending moment capacity at ambient temperature for that flange size and pressure class.
- Select derating factor: From the appropriate table or interpolation, obtain the TDF corresponding to the temperature and moment ratio.
- Verify design: Ensure that Pallowable(T) is greater than or equal to the maximum operating pressure at that temperature.
The report also provides guidance on the use of alternative materials (such as Inconel) where standard derating factors may not apply. In such cases, a material-specific engineering analysis is required.
Tip: API TR 6AF-2008 includes worked examples for a 13-5/8 in. 10,000 psi flange. These serve as excellent validation cases when setting up your own calculation spreadsheet or simulation.
4. Compliance Notes
Although API TR 6AF-2008 is a Technical Report (not a standard), it is referenced by API 6A (21st edition and later) as a recommended method for temperature derating under combined loading. Regulators, operators, and certifying bodies may require its use as part of design verification. Key compliance considerations include:
- Jurisdictional acceptance: Some regions (e.g., NORSOK, EU PED) may accept API TR 6AF as part of a recognized design code. Verify with the applicable statutory body.
- Material traceability: Derating factors assume that the flange material has been tested for elevated-temperature yield strength in accordance with ASTM A370 or equivalent. Ensure material test reports (MTRs) support the temperature class used.
- Documentation: The design report should cite API TR 6AF-2008 and list all input parameters (temperature, loads, flange dimensions) and the derived TDF.
- Limitations: The report does not cover cyclic loading (fatigue) or corrosive environments. Separate assessments (e.g., per API 6X or ISO 10423) may be needed.
Caution: Do not confuse API TR 6AF with API 6AF2 (Fire Resistance Test for API Flanges). TR 6AF deals solely with mechanical load derating, not fire performance.
Frequently Asked Questions
Q: Does API TR 6AF-2008 apply to all API 6A flanges?
A: It applies to standard API 6A flanges (types 6B and 6BX) of sizes listed in the report. Compact or proprietary flanges require special analysis.
A: It applies to standard API 6A flanges (types 6B and 6BX) of sizes listed in the report. Compact or proprietary flanges require special analysis.
Q: Can the derating factors be used for fire case scenarios?
A: No. The report addresses steady-state elevated temperatures (up to 550°F / 288°C). Fire conditions involve transient thermal gradients and should be evaluated using dedicated standards (e.g., API 6AF2).
A: No. The report addresses steady-state elevated temperatures (up to 550°F / 288°C). Fire conditions involve transient thermal gradients and should be evaluated using dedicated standards (e.g., API 6AF2).
Q: Is the 2008 edition still current?
A: Yes, as of 2026 the 2008 edition remains the latest published version. API periodically reviews technical reports, but no update has been issued to date.
A: Yes, as of 2026 the 2008 edition remains the latest published version. API periodically reviews technical reports, but no update has been issued to date.
— Published for general information purposes — 2026 —