ASME B31.3 Process Piping Code: Special Considerations for Designing with Non-Standard Alloy Compositions

ASME B31.3 Process Piping Code: Special Considerations for Designing with Non-Standard Alloy Compositions

Understanding "Non-Standard" in the B31.3 Context

Within ASME B31.3, a "non-standard" alloy composition typically refers to a metallic material that does not conform to a specification listed in the code's Table A-1 (Approved Piping Materials) or whose chemical/mechanical properties fall outside the established ranges of listed specifications. This includes:

• Proprietary or "trademarked" alloys (e.g., many nickel-based superalloys)

• Modified versions of standard grades (e.g., "316L Plus" with enhanced nitrogen)

• New alloys not yet incorporated into ASTM/ASME material specifications

• Materials governed by non-ASME standards (e.g., EN, JIS, GB standards) without established equivalency

The Compliance Pathway: Engineering and Documentation Requirements

When standard specifications don't apply, B31.3 provides a structured but rigorous path to compliance under Paragraph 323 (Materials) and related sections.

1. Establishing Allowable Stresses (Para. 302.3 & Appendix A)

For unlisted materials, you must determine allowable stresses per Appendix A. This requires:

• Creep Regime (> 815°F/435°C for most alloys): Stress values based on 100,000-hour rupture strength criteria.

• Non-Creep Regime: The lowest of:

  • 1/3 of specified minimum tensile strength at temperature

  • 2/3 of specified minimum yield strength at temperature

  • 100% of average stress for a creep rate of 0.01%/1,000 hours

  • 67% of average stress for rupture at end of 100,000 hours

Practical Challenge: This necessitates comprehensive, certified elevated-temperature test data from the material manufacturer—often the most significant hurdle.

2. Essential Material Documentation

Robust documentation is non-negotiable and must include:

• Certified Material Test Reports (CMTRs) with full chemistry and mechanical properties

• Heat-specific data for tensile, yield, elongation, and hardness

• Corrosion test data relevant to the process environment

• Welding procedure qualification records using the actual non-standard alloy

Key Design Considerations and Adjustments

1. Pressure Design (Para. 304)

The basic wall thickness formula $t=PD\mathrm{/}(2(SE+PY))$ applies, but with critical inputs:

• S (Allowable Stress): Derived as above, not from published B31.3 tables.

• E (Quality Factor): Typically 1.0 for seamless/welded with 100% radiography, but must be justified.

• Design Life: Must be explicitly stated, as allowable stresses for non-standard materials are life-dependent.

2. Flexibility and Sustained Load Analysis (Para. 319 & 320)

• Modulus of Elasticity (E) and Thermal Expansion (α): Obtain manufacturer's certified values at all operating temperatures. Do not assume they match standard alloys.

• Stress Intensification Factors (i-Factors): For non-standard fittings/branches, you may need to use the more conservative default i-factor of 2.0 or justify alternative values through testing/analysis.

3. Material-Specific Fabrication Requirements

• Welding (Para. 328): PQR/WPQ qualifications become critical. Expect to perform:

  • Hot-cracking tests (e.g., Varestraint)

  • Corrosion testing of weldments (e.g., ASTM G48 for pitting resistance)

  • Post-weld heat treatment (PWHT) trials to verify no detrimental phase formation

• Forming & Bending (Para. 332): Establish minimum bend radii and heat-treatment requirements through trial bends, as non-standard alloys may have limited ductility or work-hardening characteristics.

4. Impact Testing (Para. 323)

The exemption curves in Figure 323.2.2A/B do not automatically apply. You must perform Charpy impact testing if:

• Design temperature is below -29°C (-20°F)

• OR if the material's known behavior or service history suggests brittleness risk

• Testing must simulate the most severe condition (e.g., after PWHT, cold forming)

The Critical Role of Corrosion and Metallurgical Analysis

For non-standard alloys, the standard corrosion allowances (Para. 323.2.1) may be insufficient or unnecessary.

1. Establish a Project-Specific Corrosion Allowance (CA):

   • Based on coupon testing in actual or simulated process fluid

   • Must consider all operational phases (start-up, upset, cleaning)

   • Document the technical basis explicitly in the design file

2. Metallurgical Stability Review:

   • Identify risks of sigma phase, chi phase, or Laves phase formation in nickel/chromium-rich alloys during fabrication or service

   • Specify control measures in procurement and fabrication specifications (e.g., maximum heat input, cooling rates)

Recommended Project Execution Workflow

Phase 1: Feasibility & Specification

• Engage a Materials Engineer early. Define the Technical Inquiry to the alloy supplier requesting all necessary design data.

• Draft a Comprehensive Material Specification covering chemistry ranges, heat treatment, testing, marking, and documentation.

• Initiate Welding Consumable Qualification in parallel with material procurement.

Phase 2: Design & Analysis

• Perform "Paper Design" using conservative, assumed properties.

• Upon receiving certified data, update calculations and issue Design Package with:

  • Material Datasheet with approved property values

  • Corrosion Justification Memorandum

  • Fabrication & Examination Special Requirements

Phase 3: Procurement & Fabrication Surveillance

• Review Mill Certifications against your project specification—not just ASTM standards.

• Witness Critical Tests (e.g., heat treatment, positive material identification).

• Maintain Chain of Custody for traceability of all non-standard material.

Phase 4: Documentation & Compliance Package

Assemble the final Engineering Package for Owner acceptance and potential regulatory review, including:

1. Stress Calculation Basis Memorandum

2. Certified Material Test Reports with heat-specific data

3. Welding Procedure Specifications and Performance Qualifications

4. Impact Testing Reports (if required)

5. Corrosion Test Data and Allowance Justification

6. Design Calculations referencing the above

Common Pitfalls and Mitigation Strategies

Conclusion: A Philosophy of Justification

Designing with non-standard alloy compositions under B31.3 shifts the paradigm from prescriptive compliance to performance demonstration. Success hinges on:

1. Early Engagement of materials engineering expertise

2. Comprehensive Data Collection from qualified sources

3. Conservative, Documented Analysis that connects material properties to design decisions

4. Rigorous Fabrication Control that preserves the material's intended characteristics

The code provides the framework, but the engineering team provides the justification. By methodically addressing each requirement and documenting every assumption, you can safely leverage advanced materials to solve unique process challenges while maintaining full compliance with B31.3's intent: the safe design and construction of process piping systems.