The Emergent Threat of Brittle Fracture
The petroleum industry faces a significant, high-consequence risk from the brittle fracture of aging assets, especially those in Liquefied Petroleum Gas (LPG) service. This application provides an interactive exploration of this hazard, guiding you through the underlying science, assessment frameworks, real-world case studies, and mitigation strategies. This is not a new science, but a new industry awareness of a latent threat in equipment designed before 1987, which often did not account for the severe low temperatures caused by LPG auto-refrigeration during process upsets.
Original Report Access
This page summarizes and visualizes the content of the accompanying report. You can view or download the original files below for full details.
The Hazard
Understand the dangerous intersection of low temperatures and material properties.
The Assessment
Learn how to quantify the risk using industry-standard frameworks like API 579.
The Solution
Explore engineering and operational controls to ensure asset integrity.
The Confluence of Hazards
The risk arises from the dangerous intersection of two phenomena: the inherent nature of steel to become brittle at low temperatures, and the thermodynamic process of auto-refrigeration that creates those temperatures. A brittle fracture is a sudden, catastrophic failure governed by the three conditions below. Interact with the diagram to learn more.
Sufficient Stress
Critical Flaw
Low Temperature
The Brittle Fracture Triangle
When these three conditions—sufficient stress, a critical flaw, and a susceptible material at low temperature—are met simultaneously, the potential for a catastrophic brittle fracture exists. Hover over or tap a side of the triangle to explore each condition.
Framework for Reassessment
A formal engineering assessment is required to determine if equipment is "fit for service." This involves calculating the lowest possible temperature (CET) and comparing it to the material's capability (MAT) using the API 579-1/ASME FFS-1 standard.
Step 1: Calculate Critical Exposure Temperature (CET)
There are several methods to determine the CET, ranging from simple, conservative estimates to complex simulations. The choice involves a trade-off between analysis cost and the potential cost of unnecessarily modifying equipment.
Step 2: Assess Fitness-For-Service (FFS) via API 579
API 579 provides a tiered approach to determine if the equipment's Minimum Allowable Temperature (MAT) is lower than the CET. The fundamental acceptance criterion is: $CET \ge MAT$.
Interactive Safe Operating Envelope
A key output of a Level 2 FFS assessment is a MAT vs. Pressure curve, which defines the equipment's safe operating envelope. It shows that material can withstand colder temperatures if the stress (pressure) is reduced.
At this pressure, the Minimum Allowable Temperature (MAT) is –20 °C. Operation is acceptable if the predicted metal temperature (CET) is warmer than this value. Safe region is above the curve.
- Curve is illustrative. Replace with your Level 2 FFS results for your specific asset.
- Acceptance criterion: CET ≥ MAT at the operating pressure.
- Reducing pressure lowers required MAT (allows colder operation).
Case Studies: Lessons from Experience
Real-world incidents provide invaluable lessons. These case studies highlight the severe consequences of failure and the benefits of proactive assessment.
Catastrophic Failure: Husky Superior Refinery Explosion (2018)
Narrative:
An explosion in the FCC unit caused the catastrophic brittle fracture of a large pressure vessel, which shattered and punctured a nearby asphalt tank, leading to a major fire.
Analysis:
The US CSB found the vessel was made of an older steel with poor fracture toughness. A more modern, tougher steel would likely have ruptured without shattering, reducing consequential damage.
Key Takeaway:
Inherently Safer Design through material selection is a fundamental layer of protection that can mitigate the consequences of even unforeseen, extreme events.
Mitigation and Modernization
A realistic approach to managing risk in older refineries relies on a "defense-in-depth" strategy, combining engineering solutions and robust operational controls.
- Material Upgrades: The most effective solution. Specify normalized carbon steels (e.g., SA-516) or, for more severe service, low-alloy nickel steels or austenitic stainless steels (e.g., 304/316) which do not become brittle.
- Post-Weld Heat Treatment (PWHT): A critical remediation technique for welded components. Heating the weldment to 600-675°C relieves residual stresses and tempers brittle microstructures, significantly improving toughness.
- Mechanical Re-rating: If an FFS assessment shows a vessel is unsafe at its current design pressure, it can be formally re-rated to a lower, safer Maximum Allowable Working Pressure (MAWP).
- Develop Safe Operating Envelopes (IOWs): Use the MAT vs. pressure curve from the FFS assessment to establish formal Integrity Operating Windows, defining the permissible boundaries of pressure and temperature.
- Process Controls and Interlocks: Implement low-temperature alarms and automated Safety Instrumented Systems (SIS) to alert operators or automatically intervene to prevent temperatures from dropping below the MAT.
- Strict Procedural Controls: Develop detailed, written procedures for startup, shutdown, and emergency depressurization that explicitly account for brittle fracture risks.
- Targeted Inspection (RBI): Use the FFS analysis to focus Risk-Based Inspection (RBI) efforts on the highest-risk equipment and locations (e.g., nozzle welds), increasing the probability of finding critical flaws.
- Operator Training: Rigorous, recurring training is essential. Operators must understand the "why" behind the procedures and IOWs to ensure compliance. This is one of the most cost-effective risk reduction measures available.
Conclusion & FAQ
Effective management of brittle fracture risk requires a proactive, multi-layered approach. The following section clarifies key technical concepts to aid understanding and provides strategic recommendations for refinery management.
Clarifying Key Technical Concepts (FAQ)
The CET is not an intrinsic property of the metal or the liquid. Instead, it is a property of the process or environment, representing the lowest anticipated temperature that the metal will be subjected to during any credible scenario, including normal operation, process upsets, or auto-refrigeration. It is the "demand" placed upon the equipment.
The MAT is the "capability" of the equipment's material, representing its inherent resistance to brittle fracture at a given pressure. It is determined using the tiered assessment procedures in Part 3 of the API 579-1/ASME FFS-1 standard. A Level 1 assessment uses code-based exemption curves to find a single, conservative MAT value. A Level 2 assessment provides a more detailed MAT vs. pressure curve. A Level 3 assessment uses rigorous fracture mechanics for the most precise determination.
For older assets (pre-1987), the MDMT and MAT should not be assumed to be equal. The MDMT is a design parameter stamped on the nameplate, which under old code rules, was often based on normal conditions and did not account for upsets like auto-refrigeration. The MAT is an in-service parameter calculated using modern standards that reflects the material's actual toughness. Due to this historical design gap, the calculated MAT for a legacy asset is often significantly warmer (less safe) than its nameplate MDMT.
For newer equipment, the MDMT is a much more reliable value. Modern design codes require that the MDMT account for all credible low-temperature scenarios. In a properly designed new vessel, the MDMT is set to be equal to or warmer than the material's MAT at the time of construction. However, the distinction remains important, as the MDMT is a static design value, while the MAT can change over the equipment's life due to in-service degradation mechanisms, which would require a reassessment.
Strategic Recommendations
- Mandate a Proactive Screening Program: Commission a systematic review of all at-risk assets (pre-1987) using API 579 Level 1/2 assessments to prioritize mitigation efforts.
- Treat FFS as an Asset Optimization Tool: Frame FFS investment not as a cost, but as a strategic opportunity to optimize operations and avoid unnecessary capital expenditure by replacing assumptions with data.
- Establish an "Evergreen" FFS Management System: Implement a formal process (e.g., MOC) to periodically review past assessments against the latest API 579 standard, ensuring the facility's safety basis remains current.
- Prioritize a "Defense-in-Depth" Mitigation Strategy: For at-risk assets, implement a documented combination of engineering controls (e.g., PWHT, re-rating) and administrative controls (e.g., IOWs, procedures, targeted RBI).
- Invest in High-Fidelity Operator Training: Fund recurring, in-depth training on auto-refrigeration hazards and procedures. This is one of the most cost-effective risk reduction measures available.