How do you select the non-destructive examination (NDE) locations for your piping systems?
Prevent creep failures and high inspection costs by optimizing the NDE locations in high energy piping (HEP) systems at power plants
05 January 2016
High energy piping (HEP) system examinations are necessary to ensure the safety of personnel and reliable operation of power plants. Performing NDE at every location during each outage is not realistic as this can be costly, considering both the time and expenses (e.g. scaffolding, insulation removal and re-installation, surface preparation, implementation of NDE, and replacement power). Consequently, most plants have an HEP program that determines which NDE methods should be performed at each location and reexamination intervals. These programs usually include hot and cold walk-downs of the piping supports, stress analyses, life consumption or risk analyses, implementation of NDE, evaluation of NDE results (making the run/repair/replace decision), and implementation of necessary corrective actions.
Now, the question is, if utility companies are spending millions of dollars for HEP examinations across their fleet every year, why are failures still occurring after spending all that time, effort and money? Some failures occur due to factors that are hard to predict, such as manufacturing related defects and operational upset events. However, most of the failures are in fact predictable, but have not been prevented because of incomplete system evaluations.
There may be several advanced evaluations to be considered regarding the above tasks. For example, the program probably documents the condition of each pipe support during walk-downs. Does the program require simulation of the pipe movements between the cold and hot conditions and require an evaluation of current and expected (by design) piping displacements? The as-designed piping stress analysis may be in compliance with the ASME B31.1 Code, Chapter VII requirements. However, if the current piping system displacements are significantly different than expected (by design) and if there are malfunctioning supports, the actual piping stresses may not be in compliance with the Code.
Furthermore, the ASME Code stresses are not multiaxial stresses and are very optimistic for girth weld fitness-for–service evaluations. There have been many girth weld creep failures at locations that are in compliance with the Code. Is the program's life consumption analysis considered successful when it identifies a failed weld within the top 50 or top 5 ranked locations? Does the program implement cookie cutter recommendations for reexamination intervals of all NDE locations or are the intervals estimated quantitatively based on evaluations of current conditions? Each program might perform each of these steps differently. The key to successfully estimating the locations where piping is most susceptible to creep failures is to simulate the condition of the piping as close to the reality as possible, taking into account all the applicable factors and observed anomalies, not just the ones that are mandated by the Code.
How are you selecting the NDE locations for your HEP systems? Do you have a question about HEP systems? Please leave your comment below and one of our experts will get back to you.
Fatma Faham is a senior project manager with Intertek’s Asset Integrity Management Group and specializes in high energy piping systems. She has nine years of experience in the power industry and provides consulting on topics including life assessment of pressure vessels and piping, field inspections, plant operations and maintenance, and cost of cycling evaluations. Before joining Intertek, Fatma worked as a project engineer for a consultant firm and focused on metallurgical failure analysis, condition assessments and field inspections.