thick-wall component assessment

Boiler headers and link piping failures can result in fatalities, injuries, and significant property damage. Damage mechanisms will vary for these components, ranging from long term overheating (creep) to thermal fatigue to FAC in HRSGs. Niantic Bay Engineering, LLC has assisted utilities in the development of Boiler Critical Component Assessment Programs to address the critical components in conventional boilers including superheater outlet headers, reheat outlet headers, and economizer inlet headers. Similar components are evaluated in HRSGs, but the scope is expanded to address evaporator sections where FAC damage may be present. These programs encompass materials ranging from carbon steels to advanced creep strength enhanced ferritic steels such as Grade 91.

Tubing Assessments

Boiler tube failures are the greatest source of boiler forced outage. Similar to other power plant components, there is no one damage mechanism responsible for these failures. Instead, the active damage mechanisms will vary depending on component, material, and local conditions.

Niantic Bay Engineering, LLC has developed effective condition assessment programs that include root cause analyses to identify the applicable damage mechanisms, nondestructive methods to detect the manifestation of that mechanism, and engineering analyses to determine component serviceability based on the examination findings. Then, as boiler tube-related damage mechanisms are often not resolvable in an economic manner, re-inspection intervals are calculated to manage the component serviceability with respect to that damage mechanism.

The images above show a section of waterwall tubing where circumferential cracking created a significant boiler serviceability issue related to numerous failure events. A root cause analysis was performed to identify the zones of the damage and likely causes for the cracking, and then a course of nondestructive examinations were performed to map out the extent of cracking including crack density and depth. From that, a mitigation plan was developed to replace those sections of tubing that were most susceptible to near-term failure.

Similar approaches have been used to address:

• Long-Term Overheating (Creep) Damage

• Dissimilar Metal Weld Failures

• Wastage Due to Localized Corrosion / Erosion

Analytical Tube Lifing

Niantic Bay Engineering has developed a comprehensive high temperature conventional boiler and HRSG harp tubing assessment approach, PiLOT (Probabilistic Lifing Of Tubing) that analytically evaluates the serviceability of complete sections. The method expands beyond traditional lifing with oxide measurements to address the complete span of tubing from inlet header to outlet header.

Boiler Tube Weld Acceptance criteria

To avoid boiler tube butt weld leaks after the major installation of boiler pressure parts, most construction projects use nondestructive examination techniques to evaluate the quality of the welds. That way, welding issues can be found while repair efforts are less onerous. Traditionally, radiography was used to evaluate the welds, but with the increase in computing power, other examination methods are now plausible. Linear phased array (LPA) ultrasonic examination methods are overtaking traditional radiographic (RT) methods to evaluate boiler tube butt welds. The reasons for this change include a lower cost per examined welds (more welds can be examined per hour) and the elimination of radiation-related safety requirements that allows welders to continue to work in the areas where examinations are performed. But there is a third reason that is more valuable – LPA is better suited to detect defects that will force the boiler offline.

Niantic Bay Engineering, LLC uses fracture mechanics analysis tools to develop appropriate allowable flaw sizes for boiler tube butt welds.  The approach is similar to that used to develop ASME Code Case 2235 (Use of Ultrasonic Examination in Lieu of Radiography) and as used in API 579 (Fitness-for-Service). The allowable flaw sizes are calculated based on client-specific conditions (i.e. design and operating conditions, geometries, materials) and concerns regarding risk. The resulting package is then tailored to support contractual weld examination projects, with appropriate guidance to the welders, NDE technicians, and QC professionals, with respect to expectations for an acceptable weld.

Boiler Tube laboratory Examinations

Laboratory testing of failed and service-aged tubes can provide invaluable information for managing boiler serviceability. The manpower and effort involved in removing a section of boiler tubing for metallurgical evaluation is not insignificant. As such, it is crucial that the laboratory testing performed on that sample provides as much information as possible. Thankfully, the laboratory costs to collect that information doesn’t have to be overly expensive, as long as the work performed in a methodical manner. We have partnered with Investigative Engineering Corp. to provide comprehensive and timely metallurgical services to evaluate boiler tubing.

Tube samples become valuable only after the analysis is complete, the findings documented, conclusions drawn, and effective recommendations identified. To that, we target tube sample turnaround to less than four (4) weeks from receipt.

The results of all these examinations are summarized in a letter report that contains all test data and representative images associated with all metallography. Distinct and meaningful conclusions regarding tube serviceability are provided, as well as clear recommendations for future examinations or mitigating actions as appropriate. Further, our engineering staff is available to discuss the findings with plant staff in more detail.

Re-Examination Timing

One crucial, but often overlooked part of a condition assessment program is the review of examination results and development of appropriate re-examination intervals. Intervals for each weld should weigh the likelihood of a future failure against the cost associated with the examination. A proper interval should detect the damage prior to a failure, but after some effective length of operating time where a change in the extent of damage is detectable.

The complexity of determining re-examination intervals varies depending on the system/component in question. Consideration should include:

• Active damage mechanisms associated with the weld;

• Examination technique(s) used during the outage; and

• Future opportunities for examination.