EPRI Is Developing Technically Rigorous, Customized Methods to Answer this Question
Operators of fossil power plants around the world face a growing dilemma: they are finding more plant damage. Two factors are driving this trend. First, aging plants increasingly operating in flexible modes for which they were not designed have the potential for cracks and other damage to develop in components at an accelerating rate. Second, operators are pursuing more comprehensive strategies to manage the health of systems and components, which includes more targeted, thorough inspections. As more damage is discovered, it may eventually be impractical to repair or replace every affected component.
“Historically, if plant operators found a crack in a component, they repaired or replaced it,” said John Siefert, who manages the Materials and Repair Program in EPRI’s Generation sector. “If we assume in today’s challenging environment that operators cannot afford to repair or replace everything, then for each damaged component, they will need to answer: Is it fit to remain in operation as is without sacrificing plant safety or reliability? Or, are repairs absolutely necessary before it can return to service? Or, is it beyond the point of repair and requires replacement?”
Recognized codes or standards used today by power generation engineers to answer these questions are inadequate because the information they provide is general in nature. Methods typically used to assess a power plant component’s fitness for service do not consider the components, materials, and damage mechanisms unique to the power generation industry. Indeed, common fitness-for-service standards are used by several industries. As a result, the technicians and engineers who apply the methods need to make assumptions, which could excessively increase the uncertainty of fitness-for-service assessments.
Much of the available mechanical data used in fitness-for-service assessments comes from databases populated with information about new materials. This can result in additional uncertainty because an assessment may need to consider a power plant component that has been in service for hundreds of thousands of hours.
“There hasn’t been a systematic study on power industry fitness-for-service methods in more than 20 years,” said Siefert. “Plant operators need well-engineered methods to conduct targeted studies on component and damage scenarios specific to the power industry. For example, a useful method might tell you that if you have a certain pipe weld made of material X exhibiting damage mechanism Y, you need to collect Z data to execute a well-informed calculation to assess the weld’s integrity.”
A Comprehensive Fitness-for-Service Framework
EPRI has launched a four-year project to develop a technically rigorous fitness-for-service framework to assess the plant components at greatest risk of in-service damage as well as those that require the most technically challenging repairs and replacements. The objectives are to provide a well-engineered basis for safe, cost-effective plant operations and to inform critical decisions regarding inspection, monitoring, and maintenance. Plant owners, engineering organizations, nondestructive evaluation companies, and other specialists and service providers involved in fitness-for-service assessments are invited to participate in this collaborative research.
As a first step, EPRI and the project participants are prioritizing an extensive list of scenarios defined by specific components, damage mechanisms, and materials. “Based on the most common questions I hear from plant operators, I expect that the priorities will include high-temperature headers, high-energy piping systems, steam and gas turbine casings, and large-bore valve bodies,” said Siefert. “These components are very expensive to replace, and many plants already take a ‘repair and run’ approach with them.”
Methods to assess components will be designed to keep analyses simple when possible. “You don’t want the process to be more complicated than it needs to be,” said Siefert. “Depending on the complexity of the problem, the fitness-for-service framework could lead the user to make a simple calculation or conduct a more detailed analysis. If the component is deemed unsafe using the simple route, then the framework might direct the user to a more complex set of data and calculations to confirm or revise that conclusion. EPRI is investigating how modern computer software can facilitate complex analyses.”
To provide flexibility, the fitness-for-service methods will offer users a set of options on how to address damage. For example, as an alternative to a costly repair for a component, a method may lead a user to conclude that a less expensive welding technique is sufficient to achieve fitness for service—or that repairs can be avoided altogether if the plant’s operational mode is adjusted.
The best path forward will be different for each scenario. “An evaluation of one component may find that a crack is likely to lead to an inconsequential leak—and is therefore potentially fit to operate,” said Siefert. “Meanwhile, an assessment of another component may determine that a crack may result in a catastrophic rupture and a costly plant outage in an unacceptable timeframe. Replacement may be the best option in this case.”
Installing sensors on components to track degradation is an important part of effective fitness-for-service evaluations. In order to capture useful component data, such as temperature or strain, the project will provide specific guidance on where to place sensors, when to collect the data, and how to use the data to assess component integrity. For instance, welds have multiple constituents, each with unique properties—information that can be critical to informing where and how to monitor a component.
Project participants are encouraged to provide EPRI with service-damaged components and case studies on component damage, its causes, and its progression over time. The components and case studies can be used to validate the fitness-for-service methods developed in the project or to develop relevant material property data. Evaluating components made by many different manufacturers makes EPRI uniquely qualified to provide industry-wide fitness-for-service guidance.
“EPRI invites the participants to share their experiences and perspectives on the challenges of applying fitness-for-service methods,” said Siefert.
To reduce the uncertainty of the inputs for fitness-for-service assessments, EPRI plans to develop specifications and best practice guidelines as well as recommend additional training. EPRI and the participants will regularly review these and other outcomes of the project.
To facilitate application of the methods by plant operators and fitness-for-service providers, EPRI plans to transfer key findings to the relevant code and standard organizations, where they will be subjected to peer review and potentially codified. EPRI technical staff have already discussed the project with the National Board Inspection Code (NBIC) and plans to involve this organization in the project’s review process.
Key EPRI Technical Experts:
John Siefert and Jonathan Parker
For more information, contact techexpert@eprijournal.com.
Additional Resources:
Artwork by Craig Diskowski/Edge Design
