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API Bull 97-2013: External Axial Force Limits for Flanged Joints – Technical Overview
Understanding the Allowable Thrust Loads and Design Criteria for Pressure-Containing Flanges in Piping Systems
API Bulletin 97, first edition published in July 2013, provides engineering guidance on the allowable external axial force (thrust) that can be applied to standard flanged joints used in pressure-containing piping and equipment. This bulletin fills a critical gap in the ASME B16.5 and B16.47 flange standards, which primarily address internal pressure ratings but do not explicitly define limits for external axial loads. Engineers performing system flexibility analyses, thermal expansion studies, or evaluating nozzle loads benefit from the methodology presented in API Bull 97-2013 to ensure flanged connections remain leak-tight and structurally sound.
Scope of API Bull 97-2013
API Bull 97-2013 applies to standard ring-type joint (RTJ) and raised-face flanges conforming to ASME B16.5 (Classes 150 through 2500) and ASME B16.47 Series A and B (Classes 150 through 900). The bulletin covers steel flanges with facings that rely on an annular gasket (e.g., spiral-wound, ring-joint, or flexible graphite) and assumes bolting is tightened to the recommended seating stress per ASME PCC-1 or manufacturer instructions. The scope specifically excludes:
- Flanges subjected to external axial forces combined with large bending moments or torsion beyond typical service limits.
- Non-metallic gaskets that experience significant creep at elevated temperatures.
- Face-to-face flanges designs that do not match standard dimensions.
The bulletin is intended as a general design aid; final verification of joint integrity must be performed by the responsible engineer considering actual operating conditions and material strength.
Technical Requirements
Thrust Limit Calculation
The core of API Bull 97 is the formula for the maximum allowable external axial force Fa,allow :
Fa,allow = (Ab · Sb · Nb - Fp) / (K + K′)
where:
- Ab – root area of one bolt.
- Sb – allowable bolt stress at design temperature.
- Nb – number of bolts.
- Fp – gasket seating force required to maintain a seal.
- K and K′ – factors accounting for joint stiffness, pressure thrust, and gasket flexibility.
The bulletin provides tables of K and K′ for common flange sizes, classes, and gasket types. Users input design pressure, bolt preload, and desired minimum gasket stress at operating condition.
Flange Class and Material Factors
Allowable thrust is heavily dependent on flange class and material yield strength at temperature. API Bull 97 lists correction factors for carbon steel, stainless steel, and alloy steel flanges. For example, Class 150 carbon steel flanges typically have lower permissible thrust than Class 600 of the same size due to thinner hub sections and fewer bolts.
Gasket and Bolt Interaction
The bulletin emphasizes that gasket relaxation over time can reduce residual compression and thus lower the tolerable external axial force. It recommends using bolt-load retention factors (BLRF) derived from ASME PCC-1 Appendix O or similar methods. Factors affecting joint tightness include:
- Gasket type and thickness
- Surface finish of flange faces
- Bolt material creep relaxation at elevated temperature
| Flange Size (NPS) | Class 150 (lbf) | Class 300 (lbf) | Class 600 (lbf) | Class 900 (lbf) |
|---|---|---|---|---|
| 4 | 2,450 | 4,730 | 8,120 | 12,400 |
| 6 | 3,880 | 7,560 | 13,600 | 21,100 |
| 8 | 5,300 | 10,400 | 19,200 | 29,700 |
Implementation Highlights
Design Integration
When performing stress analysis of a piping system (e.g., using Caesar II or AutoPIPE), engineers extract nodal forces at flanged connections and compare them against the allowable thrust from API Bull 97. The bulletin allows separate evaluation of axial tension and compression limits, noting that compression often provides higher allowable loads due to additional bearing support.
Implementation Tip: Always apply the gasket seating stress (minimum of 2,000 psi or per manufacturer) during bolt-up and verify that the operating condition does not drop below this value after applying external forces. Use the bulletin’s iterative method to account for preload relaxation.
Operating Conditions
For elevated temperature services (e.g., above 400°F), the allowable bolt stress Sb must be reduced per ASME Section II Part D. Additionally, if the flanged joint cycles frequently, fatigue limits may govern rather than static thrust capacity. API Bull 97 provides a note on low-cycle fatigue but directs users to API 579 / ASME FFS-1 for detailed fitness-for-service assessment.
Design Warning: Do not substitute API Bull 97 for a full flange design when the external axial force exceeds 50% of the allowable bolt yield capacity. At higher ratios, nonlinear effects such as prying and flange rotation significantly reduce accuracy.
Documentation
Record the following in the design report:
- Flange size, class, facing, and material.
- Bolt material, size, number of bolts, and installed preload (torque or tension).
- Gasket type, thickness, and required seating stress.
- Calculated allowable thrust (tension and compression) for design and operating conditions.
- Comparison with applied loads from piping flexibility analysis.
Compliance Notes
API Bull 97 is not a mandatory standard but is widely referenced by industry as good engineering practice. Regulatory bodies (e.g., BSEE for offshore, PHMSA for onshore pipelines) may require documented compliance in a technical report. Users should be aware of the following:
Compliance Checklist:
- Confirm flange and gasket are within the scope of the bulletin.
- Use bolt stress values from API Bull 97 Table 1 or verified manufacturer data.
- Apply temperature derating factors per ASME BPVC Section II.
- Document the minimum residual gasket stress at all credible load cases.
Critical Note: The bulletin does not cover flanges that experience cyclic loading with significant plastic deformation. For repeated thermal or mechanical cycling, perform a detailed fatigue evaluation (e.g., ASME BPVC Section VIII Div. 2 §5.5). API Bull 97 values may be non-conservative in such scenarios.
Because the bulletin was published in 2013, some gasket and bolt technologies have evolved. It is the user’s responsibility to validate that the empirical factors K and K′ remain appropriate for modern gasket materials (e.g., expanded PTFE, camprofile). Always compare the bulletin’s allowable thrust with the flange manufacturer’s own ratings when available.
Related Standards
- ASME B16.5 – Pipe Flanges and Flanged Fittings (pressure-temperature ratings).
- API 660 – Shell-and-Tube Heat Exchangers (often applies nozzle load limits).
- ASME PCC-1 – Guidelines for Pressure Boundary Bolted Flange Joint Assembly.
- ISO 7483 – Gaskets for flanges (dimensional specifications).
Frequently Asked Questions
Q: Is API Bull 97-2013 still current or has it been superseded?
A: As of 2026, API Bull 97 remains in effect. No newer edition has been published. Users should check the API website for reaffirmation or revision status. For specific applications, contact the responsible engineer.
A: As of 2026, API Bull 97 remains in effect. No newer edition has been published. Users should check the API website for reaffirmation or revision status. For specific applications, contact the responsible engineer.
Q: Can API Bull 97 be used for non-metallic flanges or lined piping?
A: The bulletin was developed for steel flanges. It does not cover FRP, PVC, or plastic-lined flanges. For such materials, consult the flange manufacturer or relevant ASTM/ISO standards for axial load limits.
A: The bulletin was developed for steel flanges. It does not cover FRP, PVC, or plastic-lined flanges. For such materials, consult the flange manufacturer or relevant ASTM/ISO standards for axial load limits.
Q: How do I handle combined axial and bending loads?
A: API Bull 97 provides a combined loading interaction check (section 5.3). If the bending moment is significant (i.e., more than 10% of the axial load limit), you must perform a full interaction analysis using the bolt load distribution method described in the bulletin.
A: API Bull 97 provides a combined loading interaction check (section 5.3). If the bending moment is significant (i.e., more than 10% of the axial load limit), you must perform a full interaction analysis using the bolt load distribution method described in the bulletin.
Q: Do I need special software to implement API Bull 97?
A: The bulletin is designed for hand calculation or spreadsheet use. However, many commercial piping stress software packages include an automated compliance check with API Bull 97 if you input the flange properties.
A: The bulletin is designed for hand calculation or spreadsheet use. However, many commercial piping stress software packages include an automated compliance check with API Bull 97 if you input the flange properties.
© 2026 This article is for informational purposes and does not substitute official standard documents. Always refer to the latest version of API Bull 97 and consult a licensed professional engineer.