One of the strengths of the Structural Integrity Associates (SI) team lies in the diversity of the skills and capabilities in the organization. Sure, SI can perform inspection, analysis, design, metallurgy, failure investigations, risk assessments, and project management, but one of the real values of working with SI is when all of those aspects are brought together to solve an issue.
Recently, a client approached SI after finding a through-wall flaw in an autoclave at the head-to-shell weld as indicated by a visible dye liquid penetrant examination (Figure 1). The autoclave was one of eight similar vessels used for processing the client’s product. Three of the autoclaves are identical in construction to the flawed autoclave and operate with similar process conditions. Remote visual examination by the client indicated that all four autoclaves had similar observations at the inside of the head-to-shell weld, but only one was leaking. The remaining four autoclaves are smaller and are used infrequently. The initial call from the client was for SI to provide emergent support for inspection of the three autoclaves identical to the leaking one to meet production demands. SI responded quickly and examined all four autoclaves using a manual phased array ultra-sonic technique (PAUT) from the exterior of the vessel. The manual PAUT examination provided excellent coverage of the weld region and visualization of the through wall flaw (Figure 2).
The autoclaves are austenitic stainless steel, Type 304 and were constructed from a formed head, two shell sections, and a flanged opening. The sections were welded together using a single “V” weld preparation. The autoclaves are heated using electrical resistance strip heaters affixed to the outside diameter (OD) of the vessel. The autoclaves operate at a temperature of approximately 680°F (360°C) and a pressure of approximately 2690 psig (185 bar).
The PAUT technique was selected over conventional ultrasonic testing for the examination of the welds, because it interrogates the full volume of interest using multiple beam angles simultaneously. The objective of the examination was to quantify the current condition of the autoclave vessels. The autoclave with the through-wall flaw was examined, and the flaw extent was mapped (Figure 3). One of the three similar autoclave vessels contained a significant indication (about 50% through the wall) at the head to shell weld with a 360° extent. The other two autoclave vessels contained intermittent flaws, about 30% through the wall. Due to geometric constraints, only the head- to-shell and flange-to-shell welds could be examined using the manual PAUT technique from the autoclave vessel OD. Evaluation of the PAUT data indicated that significant intergranular stress corrosion cracking (IGSCC) to be present in the weld zones.
In parallel with examining the vessels, SI developed tooling to remove core samples from the vessel with the through-wall flaw (Figure 3). The core samples were submitted to the SI metallurgical services laboratory for assessment of the flaw and determination of cracking mechanism. An over-view of one of the core samples is provided [Figure 4]. The fracture path was adjacent to the weld, in the base metal heat affected zone (HAZ). Under higher magnification, it was observed that the fracture was intergranular and severely branched, to the point that several grains were released during the sample preparation process [Figure 5]. IGSCC of austenitic stainless steel has been reported across multiple industries representing a plethora of environmental conditions.
The environmental conditions (high temperature, high purity water) and cracking observed in boiling water reactor (BWR) environments are comparable to the conditions of the flawed autoclave. The removed core samples were metallurgically etched to reveal any sensitization of the HAZ. Substantial grain boundary sensitization was observed as given by the wide and dark appearance of the grain boundaries (Figure 6). The chemical composition of the base metal was determined as part of this investigation and met the expected composition for type 304SS. The carbon content of the base material was at a level sufficiently low that the materials could be considered to be near the upper boundary of what would be regarded as “L” grade materials. The “L” grade variant of type 304SS was introduced to reduce the impact of welding heat on the sensitization of the HAZ.
Additionally, a scanning electron microscope was used to evaluate the crack tip. It detected traces of chlorides and fluorides. Discussion with the client indicated that a potential source could be the service water being used in the vessel.
Once the extent of the flaws was established in the autoclaves without a through-wall flaw, a fitness for service evaluation was performed per API 579-1/ASME FFS-1 (API 579). API 579 provides industry established guidance in the assessment of pressurized components containing flaws for continued service and is recognized by the National Board Inspection Code.
A life assessment of the vessel was performed utilizing Part 9, fracture mechanics assessment, including SCC: involved cracking. The stresses in the vessel shell were evaluated using a finite element model (Figure 7). The life assessment of the vessel was performed initially using standard crack growth models from API 579 for “crack growth in a light water reactor environment.” It was quickly determined that the evaluation was inadequate as it showed that the life of the vessels should be extremely long. The current vessels were installed in the 1980s, thus operating for approximately 40 years. A model for IGSCC from NUREG 0313 Rev 2 was also evaluated based on the findings from the PAUT and metallurgical assessments. This model showed that the entire life of the vessel would be approximately 12 years.
Discussion with the client was needed to determine which of the crack growth models more closely matched the problem at hand. The initial reaction was that neither one could be correct. However, after some discussion, it was determined that the facility had changed from well water to city water around fifteen years before the failure.
A crack growth assessment based on the IGSCC involvement was then performed using initial flaw sizes, which were the actual size of the flaws detected in the vessel. This analysis determined both the expected rates of crack growth and the size of the flaw, which could result in catastrophic failure of the vessels (Figure 8).
Asset Management Planning
The final part of the story is working on getting the client back online with extended procurement schedules for new equipment. Planning for in-service inspection involves not only leveraging the proper rules and regulations, but also the information and data gathering for a specific unit to ensure that operation with known flaws can be done safely.
The plan was developed using guidance from not only the previous analysis performed but also with guidance from published sources such as the National Board Inspection Code, ASME PCC-3 on Risk-Based Inspection Planning, and Appendix B of ASME Code, Section VIII Division 3 on Requalification of Pressure vessels.
The plan was to determine the remaining life of the vessel from the API 579-1/ASME FFS-1 process discussed previously, including design margins on life, such as would be found in original vessel design codes such as ASME Code, Section VIII. The guidance documents then have the vessels inspected at an interval of 1/10 of the original design life to ensure that the crack growth rates are appropriate. This level of conservatism yielded a monthly inspection program for the vessels. The flaw growth was then tracked to ensure that the original predicted life was conservative (Figure 8).
The monthly inspections were able to be accomplished quickly and efficiently while minimizing downtime by leveraging the SI Internal Vessel Automated Scanner (SIIVAS) (Figure 9). This system allowed for the vessel to be examined quickly and easily in subsequent exams without the removal of external insulation or heaters, and without the need for confined spaced entry permits.
Another example of the value that working with a full-service company such as SI can bring to the table to quickly respond to clients’ issues, evaluate difficult failure mechanisms, and establish a long term management program to allow clients to get back online sooner and safely.