ASME STP-PT-024-2008 — Development of Basic Time-Dependent Allowable Stresses for Creep Regime in Section VIII Division 1

⚖️ ASME STP-PT-024-2008 is a technical report prepared by Becht Engineering Co. (Charles Becht IV, Greg Hollinger, and Charles Becht V) for the ASME Pressure Technology Codes & Standards committee under project 07-08 (BPVC#7). The report recommends two classes of design rules for the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1: rules for very short-term loads for which creep should not be a design consideration (termed “Occasional Loads,” such as earthquake and extreme wind), and rules for loads for which creep is a design consideration (termed “Time Dependent Design Considering Creep”), enabling designers to design explicitly for a specified life (e.g., 300,000 hours) instead of relying on the implicit 100,000-hour creep property basis of current rules.

1. Background and the Need for Two Rule Sets 📌

The report identifies three problems with current rules:

• Over-conservatism for very short events: earthquakes last only moments; creep rupture is not a relevant failure mode. For overload failure, the relevant properties are yield and tensile strength, not creep properties.
• Inability to differentiate variable conditions: for example, long-term operation at lower temperature combined with short-term regeneration at higher temperature — if load duration could be considered explicitly, the short-term condition could be designed at a higher allowable stress than the long-term code value.
• No explicit life design: users may wish to design for a specific life (e.g., 300,000 hours), whereas current rules produce an indeterminate life from combined margins.

2. The Occasional Loads Rule 🔧

Section VIII, Division 1 currently permits the allowable stress for conditions including wind and earthquake to be 1.2 times the basic allowable stress. In the creep regime, however, the basic allowable stress is governed by 100,000-hour creep properties, while creep rupture is not relevant during an earthquake. The report recommends that allowable stresses for these very short events be based on tensile properties:

• The recommended basic allowable stress for sustained-plus-occasional load combinations is 1.2 × (2/3) × Sy = 0.8 Sy for non-austenitic materials, providing the same margin relative to yield as existing rules.
• For austenitic stainless steel and materials with similar stress-strain characteristics, take Sy (slightly more conservative relative to yield than the existing 1.2 × 0.9 Sy = 1.08 Sy).
• Rationale: materials operating in the creep range are ductile, and the failure mechanisms of concern are low-cycle fatigue and ratchet rather than rupture.

3. Time-Dependent Design Considering Creep ⏱️

The report proposes a framework that explicitly introduces load duration (life) in the creep regime, centered on time-dependent allowable stress parameters. Table 1 lists the proposed allowable stress parameters; Figure 2 compares existing and proposed allowable stresses versus temperature; Figure 3 presents the maximum time at load.

Appendix A provides ten example calculations for time-dependent design considering creep, and Appendix B summarizes the material data used in examples 1–10, covering typical creep-regime materials so that the calculation method can be applied directly in engineering.

4. Engineering Practice Essentials 🛠️

• 🔍 Classify loads first: determine whether a load is truly short-term (earthquake, hurricane-class wind) before deciding between the occasional-load rule and time-dependent creep design; do not design earthquake loads to long-term creep properties.
• 🔧 Explicit-life design: for equipment with a defined life target (e.g., 300,000 hours), use the Appendix A method to compute time-dependent allowable stress explicitly and avoid the indeterminate life from stacked margins.
• 📊 Regeneration / variable conditions: for “long low-temperature + short high-temperature regeneration” processes, check the short high-temperature segment by its actual duration to exploit short-term material capability.
• 📋 Data and margins: time-dependent design depends heavily on creep/rupture data beyond 100,000 hours; verify the data source and failure criterion (average versus minimum rupture strength) before use.

5. Frequently Asked Questions ❓