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Consider the following scenario: in a particular process at your refinery or plant, you experience multiple ruptures or a leak in a piece of equipment.
The ruptures or leaks could be caused by holes or cracks in piping, coiled tubing, or worse, a pressure vessel. You review the records of the asset and see that it has been in service for 30 plus years, and make the assumption, “It’s old. Let’s just replace in kind.”
Failure analysis and prevention – Why is it important?
Figure 1. As-received tubing coil experiencing multiple fractures.
Within 18 months of replacement, the same thing happens. You begin to wonder, “The last asset lasted 30 years. Why did this one only last 18 months?” You complete another replacement in kind and in less than a year the same failure occurs again. The next thought going through your mind is, “What is going on? Why did I get 30 years out of my first asset, and now I can’t even get two years out of my replacements?” An example of such a failure is shown in Figure 1.
The above example highlights the importance of conducting a failure analysis of equipment that has experienced a leak, breach, or rupture. While it may be commonplace or tempting to write off a failure or breach that occurred in an aging asset by saying “It’s old,” it may not always prevent a future failure. Evaluation of the equipment through metallurgical examination and failure analysis can provide insight into damage mechanisms present and identify how one or more damage mechanisms could have caused the failure. Understanding the defects at hand prior to replacement is critical to keep the event, or worse, from recurring. The financial impact alone from repeated “replacements in kind,” not to mention a far worse event taking place, should warrant a deeper look into the “why” of a failure. This article discusses the critical information that can be gathered during a failure analysis, and what questions can be answered to work towards the prevention component of failure analysis and prevention (FAP).
A failure analysis can often reveal the failure mechanism and identify if there was a corrosion mechanism, mechanical mechanism such as overload or fatigue, material or weld defect, or any number of other types of failure mechanisms. A failure analysis may start in the field with examination and evaluation of the environment, determination of what is attached to the asset that failed, and assessment of operating conditions. Essential elements when performing a failure analysis is knowing the background and the usage of the component, the material of construction, and the operating conditions, such as temperature, pressures and vibrations. Factors that could have caused or contributed to the failure, as well as operating history, are important aspects to consider. This may include asking the following:
- Were there any changes in the process preceding the failure?
- Were there any process upsets?
- What was connected to the component that failed and could that have contributed to the failure, e.g., vibrations?
- Can details of the operating history be reviewed?
Following this, often a review of acquired process data can reveal information that is essential for a failure analysis.
After a review of field and process conditions, removal of the failed component and submission to a materials laboratory for a systematic failure analysis in a controlled environment is often the next prudent step. Evaluation of the failed component includes both nondestructive and destructive methods. In many cases the failure analysis will require cutting and sectioning of the component to evaluate materials characteristics and composition to determine what role, if any, this may have played in the failure.
Laboratory analysis and testing methods
Methods of nondestructive evaluation may include:
- Visual examination and dimensioning
- Radiography (X-ray)
- Magnetic particle testing
- Dye penetrant or wet fluorescent dye penetrant
- Ultrasonic testing
- Phased array ultrasonic testing
- Magnetic resonance imaging
These are common examination methods, but the above list is not all encompassing. Following nondestructive examination, often the failure component may require destructive methods for further analysis and testing to determine the cause of the failure.
Methods of destructive evaluation may include:
- Disassembly of an assembly
- Sectioning or cutting near or at failure locations
- Microscopic examination of sectioning items, e.g., fracture surfaces that have been separated via fractography using a stereomicroscope or scanning electron microscopy
- Spot chemical analysis and evaluation of corrosion products using energy dispersive X-ray spectroscopy and possibly X-ray diffraction
- Sample preparation for chemical analysis and mechanical testing, e.g., tensile testing, impact testing or hardness testing
- Metallographic examination requiring examination of sectioned, mounted, polished and etched cross-sections
The aforementioned methods are some of the most commonly utilized when conducting a laboratory failure analysis. The purpose of these methods can generally answer the following questions:
• Where did the failure originate?
• Was there an anomaly or stress concentration at the origin?
• Can a corrosion failure mechanism be ruled in or out?
• If there was a corrosion failure mechanism, can we differentiate which specific one?
○ If stress corrosion cracking is observed in an asset, can the cause be determined, or can the problem be identified? Through analysis of corrosion products using the above methods the corrosion mechanism and problem elemental species can be identified and can be used to differentiate between different mechanisms that may include chloride stress corrosion cracking in stainless steel, polythionic cracking or caustic embrittlement (caustic stress corrosion cracking).
○ If there is not a corrosion mechanism identified, what other type of damage mechanisms may be present, e.g., mechanical, thermal, fatigue, and could there be a manufacturing defect?
Findings and discussion of collected information
In addition to the questions above, there are numerous other questions that can be answered during a failure analysis depending on the circumstances of the failure and the application. Figure 2 highlights some of the data that can be collected from analysis of corrosion products to aid in determining a specific failure mechanism.
Failure analysis and prevention – Why is it important?
Figure 2. As-received coil sample cracking
Review and comparison of design, manufacturing and fabrication methods may also provide insight into the susceptibility and cause of failure. Looking back to the initial example, an environmental contaminant was eventually identified as one cause of the equipment’s failure.
As a secondary determination resulting from the analysis, the manufacturing methods indicated that the tubing that lasted 30 years was stress relieved, while the replacement tubes were not. This difference, in combination with the environmental contaminant, indicated why the subsequent replacement tubing failed. Not identifying and recognizing the need for stress relief to prevent environmentally assisted damage was the reason for multiple subsequent failures. A failure analysis and investigation could have identified these issues, and a proper mitigation and prevention strategy could have been devised to mitigate the two subsequent failures.
Failure analysis is an essential part of a solid mitigation and prevention strategy. Utilization of field investigative work as well as laboratory analytical techniques and testing methods can lead to technically sound conclusions, and an understanding of the required path forward. It can also help to prevent costly downtime, and worse, a catastrophic event. BakerRisk is ready to help your team perform a detailed failure analysis and provide mitigation solutions to protect your people and assets.
