What is the process for performing an Inspection Scope Optimization?
Inspection Scope Optimization (ISO) for Flow Accelerated Corrosion (FAC) can be performed several different ways. Components are selected from several areas of the FAC program for inspection and different program areas require different approaches. Let’s look at the methodology used for each program area.
Component re-inspections are added to the scope based on remaining service life calculations from a previous inspection. Re-inspections typically make up the largest portion of a scope, and many of these are scheduled prematurely due to the overly conservative nature of wear calculation methods that were available to program owners when these evaluations were performed. In recent years, more advanced wear calculation methods have been developed for the purpose of removing the extra levels of conservatism without increasing risk to the plant.
During the scope optimization process for re-inspections, the remaining service life calculation from the last inspection is reviewed and, if necessary, recalculated using modern wear calculation methods to ensure the component has the most accurate remaining service life possible. Reinspection evaluations are the source of the majority of inspection deferrals that result from a scope optimization. Component reinspection reviews are performed for both single-outage and historical scope optimizations.
New CHECWORKS™ SFA Inspections
CHECWORKS™ SFA is software that predicts wear rates in FAC-susceptible components. FAC Program owners use the results to select inspections on previously-uninspected components based on the predicted wear rate and time to critical component thickness. The scope optimization process reviews the results of the CHECWORKS™ SFA Wear Rate Analysis to verify the necessity of new inspections selected from the model and to make suggestions of other locations that may have been overlooked. CHECWORKS™ SFA inspection reviews are only performed during single-outage scope optimizations due to the frequent recalibration of the model.
New Susceptible Non-Modeled (SNM) Inspections
Roughly 60 percent of all FAC-susceptible piping operates under conditions that cannot be modeled using CHECWORKS™ SFA. Because conditions in these lines can be unpredictable, this group of susceptible piping presents a greater risk of failure.
The optimization of SNM inspection selection includes comparing the results of past inspections on high wear lines to determine if further inspections on that line are warranted as well as determining the risk level of uninspected lines to determine if an immediate inspection is necessary. New SNM inspection reviews are only performed during single-outage scope optimizations due to the industry norm of only planning these inspections one cycle ahead of the outage.
New Inspections from Operating Experience
It is important to account for plant and industry operating experience when constructing an FAC inspection scope. During the scope optimization, recent operating experience is compared with components selected for the scope to determine if the new inspections are adequate and necessary. New OE inspection reviews are only performed during single-outage scope optimizations due to the industry norm of only planning these inspections one cycle ahead of the outage.
Engineering judgment inspections are often inspired by notification of off-normal conditions in the plant or are simply inspections performed by the FAC program for other programs with a less robust inspection schedule during the outage. As a result, these inspections usually experience little or no change during the scope optimization process. New engineering judgment inspection reviews are only performed during single-outage scope optimizations due to the industry norm of only planning these inspections one cycle ahead of the outage.
Combination with Critical Thickness Recalculations
GSE TrueNorth teams are qualified in the calculation of minimum allowable (critical) component thicknesses for several utilities. Recalculation will often result in a lowering of the critical thickness for a component, extending the life of the component even if the previously calculated wear rate cannot be changed. We estimate that the reinspection deferral rate combining new critical thicknesses and optimized wear calculation methods is between 60- 90 percent. Previous critical thickness calculations in your program may not be optimized because of:
- Overly conservative screening criteria: Historically, many sites utilized screening criteria using a standard percentage of the component nominal thickness (e.g. 87.5% or 70%) instead of a formal critical thickness calculation. Often times, this value is never refined prior to the scheduling of the reinspection, resulting in a premature reinspection and a waste of up to $10,000 for each early inspection. It is also possible that these screening criteria were non-conservative. Several plants in the industry have at least one system that is considered a “low-margin” system. This means the nominal/un-degraded thickness of the pipe is very close to the critical thickness, sometimes even within the manufacturing tolerance, meaning that the slightest wear or manufacturing variance could put the plant in violation of code standards.
- Outdated material allowable stress values: Newer revisions of the ASME B31.1 piping standard have increased the allowable stresses of several common materials used for piping in power plants. While safety-related piping is tied to the code by which the plant is licensed, allowable thickness calculations on non-safety related piping components can be performed without a formal code reconciliation. While a formal code reconciliation has less of an impact on the FAC program due to small amount of safety-related piping, the impact to other plant programs is significant.
If you are considering outsourcing your Inspection Scope Optimization, talk with the experts at GSE TrueNorth or visit https://www.gses.com/balance-of-plant-bop/.