Proactive integrity management of aging plants
Next-generation technologies to manage aging plants more proactively are discussed. An analysis of advantages and disadvantages of current (reactive) technologies is provided, and then some of the latest technologies are discussed with a focus on measuring material properties in-situ. These type of measurements, combined with continuous monitoring approaches, hold potential to dramatically improve the effectiveness and accuracy of integrity management programs.
Extending the lifetime of plants
Across the country aging plants are operating at, or beyond, their design life and are expected to continue for decades more. Economic pressure to operate as cost-effectively as possible, with minimal downtime, reduced shutdown lengths, and longer service lives, require new approaches to integrity management that are more proactive to accomplish these goals.
Health, safety, and environmental concerns also act as drivers for improvement, and to top it all, the increase in public scrutiny makes the implications of failure more serious than ever. As the plant or site manager, the continuing struggle is to know how long the current plant infrastructure can safely operate without having to replace or repair it. The safety and integrity of aging plant infrastructures are predicted by utilizing mathematical models, corrosion models, and tools that measure the degradation in mechanical properties and the effects of corrosion damage over time. The most common tools utilized to monitor the degradation of mechanical properties and effects of corrosion damage include:
(1) field metallographic replication to determine condition of the aged microstructure,
(2) hardness testing to have a basis for mechanical properties,
(3) non-destructive evaluation (NDE) sensors for wall loss and wall thickness measurements, and
(4) non-destructive evaluation sensors for crack and defect detection.
Understanding the value (and drawbacks) of the information provided by each of these tools is critically important for accurately assessing the integrity of components.
Detecting the changing microstructure of metals
The microstructure of metal dictates the metal properties and performance, and in many applications the microstructure will change over time. Field metallographic replication (FMR) provides the only current method to non-destructively assess the microstructure of metallic components in the field. Having a means to assess the microstructure is extremely valuable; however, it is extremely important to note the limitations of any technology.
For example, FMR techniques provide information about the microstructure, but the microstructure is only observed at the surface of the steel component being examined. The microstructure and mechanical properties are often very different in the bulk than at the surface due to carburization, scale formation, and many other reasons. Hardness measurements have the same limitation as FMR in that hardness only provides a perspective of the mechanical properties of a component at the surface. Monitoring the properties of just the outer diameter (OD) or inner diameter (ID) surface (that does not provide through-thickness detection) of a metal component can provide a false sense of security and integrity.
A variety of non-destructive sensors are currently being utilized in aging plants to measure wall thickness/wall loss due to corrosion and to examine for cracks and defects. These measurements are essential in predicting when parts should be replaced and repaired and for predicting remaining service life of components. Understanding the type of information provided by these tools and their limitations is essential to gaining accurate information for prediction of material integrity and fitness for service. For example, wall loss measurements are best used to determine corrosion rates of components and thus evaluate when parts should be replaced, but with a big caveat: wall loss measurements are only valuable for examining for uniform corrosion and wall loss.
Any localized corrosion is unlikely to be detected and would be averaged into the entire footprint of the probe, which is often several inches in size. Unfortunately, though, localized corrosion poses the largest threats for aging plants considering that a corrosion pit could penetrate through steel in days or weeks, while it could take years to completely uniformly corrode a plate of steel. Wall loss measurements must be used with caution if localized corrosion cannot be evaluated. NDE techniques currently utilized to locate and evaluate cracks and defects can provide the size and shape of cracks and defects and that information is utilized in mathematical models for assessing the effects of cracks/ defects on the overall integrity of the component being examined. To be useful, these require components to be thoroughly (and often fully) inspected and even after a complete component inspection, there are still cracks, defects, and wall loss that can go undetected. Even when a crack or other indication is detected, the pertinent details (including the critical question: is it still propagating or not) about the crack are still unknown.
Early detection of high temperature hydrogen attack. Proactive electromagnetic non-destructive sensors were used here to detect early stages of HTHA in the form of decarburization and fi ssuring through the thickness of a steel plate removed from hydrogen service. The presence of HTHA damage is confirmed through metallography.
Computer generated models can try to model the stresses associated with the cracking mechanisms, however
the models do not and cannot account for all of the variables associated with metals in service because the effect of all the variables are not measured with current technologies. For example, critical information like stress and hydrogen content are not available, and must be assumed. If hydrogen atoms migrated to a crack tip, the stress state at the crack tip is very different than if the hydrogen were not present. The influence of hydrogen migrating to a crack tip is very high and could lead to catastrophic failure.
The lack of ability to accurately account for hydrogen and many other variables associated with cracks and defects is the limiting factor for more accurate integrity predictions. These tools described above are monitoring the effects or consequences of material property degradation and corrosion damage due to aging. Monitoring the consequences of aging provides a reaction type of approach to integrity management. Plants across the world have been reacting to the consequences of materials aging in many of the same ways for over half of a century. Searching for cracks is being reactive.
If there is a crack present in a component, there is already a problem. The future of integrity management programs will move to a more proactive approach by using tools that measure the real-time bulk material properties, such as strength and toughness, of aged components which will allow the aging plants to be proactive about repairs and replacements as opposed to being forced to be reactive because of unforeseen cracks, defects, and failures.
A proactive integrity management approach?
A proactive integrity management approach is only now becoming possible because of the advances in non-destructive testing capabilities. The latest generation of non-destructive tools are moving beyond the reactive type of sensors for crack, defect, wall thickness detection, and towards proactive systems that provide real-time throughthickness material property and condition assessment. The proactive measurements can broadly be split between material properties (such as residual stress, microstructure, and mechanical properties) and advanced corrosion evaluation processes. Furthermore, within proactive integrity management, the properties (not shape and size, but residual stress measurements) associated with existing cracks and defects will be measured to assess
their criticality (by measuring the stress state at the crack tip) and whether the cracks/defects have an actual effect on the integrity and performance of the material.
Proactive integrity management can be utilized to monitor the properties of materials during all stages o the material’s life where the life of a materials consists of birth (the casting, HIP, or other production processes), to growing up (welding, forging, heat treating, etc.), to maturity (optimizing the final product), to death (failure). A thorough proactive integrity management program would utilize the next generation of NDE sensors to monitor the properties and integrity of a material throughout these stages because the original material at birth can exhibit very different properties before the material has even reached maturity. Knowing how the micro structure and properties are and have changed over the material lifetime will allow for better prediction of material performance through the stages of life.
New monitoring capabilities will allow better prediction of material failure so parts can be removed from service and replaced before failure. In parallel with more sophisticated analytical technologies is the dramatic growth in the capability of sensors to be used for continuous online monitoring. As the price per sensor drops and data delivered becomes more valuable, it is likely that plants will incorporate large amounts of real-time data that provides a more complete view of the plant integrity.
For example, residual stress analysis sensors previously demonstrated by G2MT can be used to provide continuous monitoring of in-situ cracks to ensure they are not growing until a replacement could be found. Similarly, other mechanisms such as creep, HTHA, fatigue, and other forms of cracks can be monitored during service to prevent them from reaching dangerous levels. It is essential to continue using reactive integrity management in aging
plants, but the value and use of proactive integrity is rising and will eventually eclipse reactive maintenance.
Real-time determination of material properties such as stress and hydrogen content will provide a powerful method for safely extending service life. Furthermore, the wealth of data produced will lead to new insights into the science behind the fracture mechanics, metallurgy, and physics of industrial systems. The use (and evolution over time) of the Nelson curve in high-temperature hydrogen applications is an early example of the way that data and experience lead to new science and engineering insights. These new proactive methods will enable aging plants to continue to safely operate well beyond their designed service lives as well as to operate under more demanding conditions than ever before.
About the Author:
Dr. Angelique Lasseigne is the Chief Technology Officer of Generation 2 Materials Technology (G2MT), LLC and G2MT Laboratories, LLC in Houston, Texas. Angelique received her BSc., MSc., and PhD degrees in Metallurgical and Materials Engineering from Colorado School of Mines in Golden, Colorado. She has published more than 100 papers and coauthored three books in the fields of metallurgy, corrosion, welding, and non-destructive testing for materials characterization. Angelique has spent her career providing metallurgical consulting and failure analysis to aging plants around the world as well as working towards developing the next generation of advanced non-destructive sensors that will provide aging plants with more accurate information about aging material properties and predictions for fitness for service and remaining life.