API TR 6AF2-2013: Temperature Derating for Flanges and Components – Technical Overview

API Technical Report 6AF2-2013, titled Temperature Derating for Flanges and Other Components, provides critical guidance for engineers and designers working with flanged connections under elevated temperature conditions. This technical report, published by the American Petroleum Institute (API), defines derating factors that adjust the pressure-temperature ratings of API flanges and components to account for reduced material strength at high temperatures, especially when additional mechanical loads (e.g., bending, axial forces) are present. Understanding and correctly applying API TR 6AF2-2013 is essential for ensuring safe and reliable operation of pressure-containing equipment in the oil and gas industry.

Scope and Applicability

API TR 6AF2-2013 applies to flanges, valve bodies, fittings, and other components manufactured in accordance with API 6A and API 6D. It specifically addresses the derating of pressure-temperature ratings when components are subjected to a combination of internal pressure, external loads (such as bending moments, axial tension, or compression), and elevated temperatures. The report covers commonly used materials, including carbon steels, low-alloy steels, and stainless steels, with derating factors derived from standard material property data (e.g., yield strength reduction at temperature).

The key objective of API TR 6AF2 is to provide a consistent methodology for reducing the allowable working pressure of flanged connections at elevated temperatures, ensuring that the combined effects of pressure and external loads do not exceed the component’s load-carrying capability. The scope includes both primary and secondary loads, but excludes cyclic or dynamic loads.

Technical Requirements and Derating Factors

Derating Factors Methodology

The core of API TR 6AF2 lies in the temperature derating factors (K_T). These factors are applied to the rated pressure of the flange or component at room temperature to obtain the reduced rating at elevated temperature. For a given material, the derating factor is defined as the ratio of the material’s allowable stress at temperature to its allowable stress at ambient (38 °C / 100 °F).

Additionally, the report prescribes how to account for concurrent external loads. The combined loading is evaluated using a three-stage limit analysis approach: primary stress check, hydrostatic end-load effect, and flange rotation consideration. The derated pressure combined with external loads must satisfy established acceptance criteria.

Combined Load Evaluation

For flanges subjected to combined loads, API TR 6AF2 introduces the concept of “equivalent pressure,” which incorporates the effect of external bending moments and axial forces. The equivalent pressure (P_eq) is calculated based on the geometry of the flange and the applied external loads. This equivalent pressure is then used together with the temperature derated pressure to ensure that the total equivalent stress remains within allowable limits.

The report provides detailed calculation procedures and examples for different load scenarios, including end loads that are tensile or compressive, and bending moments about one or two axes.

Implementation Highlights

Proper implementation of API TR 6AF2-2013 requires careful attention to material certification, design documentation, and the specific load conditions that the component will experience in service.

• Material Confirmation: The material of construction must match one of the material groups listed in the report. The actual mechanical properties at elevated temperature should be based on authoritative data (e.g., ASME BPVC Section II, Part D).
• Load Analysis: In addition to internal pressure, all external loads that will be sustained during normal operation must be quantified, including thermal expansion loads, pipe weight, valve actuation forces, and wind/seismic loads.
• Design Basis: The derated pressure-temperature rating should be clearly stated in the design documentation. For flanges operating near the limit of the derated rating, additional verifications (e.g., finite element analysis) may be justified.

Compliance Notes and Recommendations

API TR 6AF2-2013 is a technical report and not a consensus standard; however, it is widely referenced in purchase specifications and design codes. Compliance with its recommendations is often required when API 6A or 6D equipment must operate above the standard temperature limits.

Documentation Requirements

Users of API TR 6AF2 should maintain a design file that includes:

• Material data sheets showing the specified material and its allowable stresses at relevant temperatures.
• Load calculations demonstrating the applied external loads and the computed equivalent pressure.
• Comparison of the total operating pressure (derated) plus equivalent pressure against the flange’s allowable capacity.
• Any test reports verifying flange integrity at elevated temperature (e.g., hydrostatic tests at reduced pressure).

Common Pitfalls

• Ignoring external loads when establishing the allowable pressure at elevated temperature. The derating factors alone are not sufficient if significant bending or axial loads exist.
• Applying derating factors from other codes (e.g., ASME B16.5) which have a different basis and may not be consistent with API 6A/6D designs.
• Neglecting creep effects for long-term exposure at very high temperatures (above 350–400 °C for most steels). API TR 6AF2 focuses on short-term derating; for creep regime, additional analysis per API 579 or ASME BPVC is recommended.

Adoption of API TR 6AF2-2013 as part of the design basis promotes consistent safety margins across different material types and operating conditions. Facilities that already comply with API 6A/6D requirements will find the integration straightforward. For new construction, API TR 6AF2-2013 continues to serve as the primary reference for temperature derating of flanges and components in the upstream and midstream oil and gas sectors.

This article provides a technical summary of API TR 6AF2-2013 and is intended for educational purposes. For full compliance, reference the official publication from the American Petroleum Institute. | © 2026