It goes without saying that safety is an important goal of any company. Although there is an inherent risk in operating high temperature and high-pressure components, it is important to reduce this risk to minimal levels. When flaws or other damage are detected, the decisions on how to deal with such imperfections have enormous economic implications. If flaws are discovered during normal operation, a fitness-for-service assessment can determine whether or not it is safe to operate the equipment until the next planned outage. If the outcome of the FFS assessment is favorable in such a case, then the owner can avoid a costly unplanned shutdown.
Fitness-for-service assessment involves several engineering disciplines that are used to determine if the equipment is fit to continue operation for some desired future period. The equipment may contain flaws, have sustained damage, or have aged so that it cannot be evaluated by use of the original construction codes. API 579-1/ASME FFS-1 is a comprehensive consensus industry recommended practice for this assessment that can be used to analyze, assess, and monitor equipment for continued operation. The main types of equipment covered by this standard are pressure vessels, piping, and tanks.
Thielsch Engineering recently performed fitness-for-service assessments on a high temperature, high-pressure header located at a power facility in the Southeast. As a basis for the inspection, Thielsch utilized recent inspection data and metallurgical evaluations of the component. The inspections revealed numerous indications of internal surface cracking. The cracking was observed in the ligaments between the bore holes on the inside surface of the headers. Typical internal damage conditions are depicted in Figures 1 and 2 below.
Due to the extent of damage noted in the evaluations reviewed, it was determined that a Level 3 Assessment be required. A Level 3 is an advanced assessment requiring detailed data, computer analysis and considerable technical knowledge and expertise in FFS assessment procedures. Figure 2.1 below shows an overview of the FFS Analysis utilized by Thielsch Engineering during a Level 3 Assessment.
This particular Level 3 assessment of the high-temperature headers included the identification of damage mechanisms, remaining life evaluations and methodologies for in-service monitoring for the known thermal fatigue type defects acting on the headers. Additionally, Thielsch performed solid model finite element analysis and fracture mechanics to determine critical flaw size, crack growth rates and leak before burst outcome. The results show the highest stresses on the inner surface at the bore holes, consistent with the observed ligament cracking described above.
Based on the detailed stress input from the FEA (as illustrated in Figures 3 and 4 below), the fracture mechanics analysis indicates that the crack would grow through the wall thickness before reaching the critical size for fast fracture (leak before burst).
Because the cracking conditions were not such that would result in a catastrophic failure of the header, Thielsch provided recommendations that a formal inspection program be maintained to closely monitor the propagation of the ligament cracks.
Of course, during an outage, whether planned or not, it is advantageous to avoid or postpone repairs is safely possible. Provided the FFS assessment indicates that the equipment can be safely operated until the next planned shutdown, unscheduled repairs can be avoided, saving the facility both time and money. Additionally, the unnecessary early retirement of components can be particularly costly, as long lead times for delivery of replacement components can result in extensive delays in production. Fitness-for-service assessments provide a rational basis for determining whether or not a damaged component can continue to operate until a replacement can be delivered.
To learn more about Thielsch Engineering’s Fitness For Service Assessment capabilities please contact Peter Kennefick at Pkennefick@thielsch.com.