The presence of progressive indications in critical components invariably leads to decisions about run, repair, or replacement. Likewise, because a growing number of plants on which the nation relies for its power are nearing or have already exceeded their design lives, the frequency with which these decisions confront plant owners is on the rise. The decision whether a component can continue to operate for some specified period or, instead, must be repaired or replaced can have tremendous economic consequences to plant owners. That being said, it is essential that every run/repair/replace decision be founded on sound engineering judgement, taking into consideration both the technical issues associated with the damage as well as the overall impact of a repair or replacement on plant operations.
The presence of surface cracks in a thick-walled component may not necessarily mean a steam leak is imminent and in certain circumstances, operating with cracks may be deemed acceptable. For example, a short-term repair that allows for a quick return of a unit to service can, in some cases, be justified technically with the understanding that a major repair or replacement will be required at some future specified time to maintain the highest level of plant safety. Assessments in such cases depend on accurate crack growth data and analytical procedures such as time of flight and phased array examinations. Given the uncertainties in operating conditions and the lack of crack growth data on the service-exposed material, many utilities opt for defect repair during planned outages.
Undoubtedly such options can only be exercised if suitable weld repair procedures are available and if the integrity of the surrounding base material is intact. A considerable amount of research has been conducted to improve weldment ductility by control of weld process variables, producing favorable microstructures or modifications to weld metal composition.
Service experience often indicates that the first observable creep damage occurs as cracking at weldments. However, examination of weldments is not usually sufficient to ensure overall structural integrity. In many cases, welds can be successfully repaired without reducing the overall component life. When problems have been identified, an alternative to replacement sometimes exist.
Furthermore, even when replacement is inevitable, remedial action to allow continued operation may be necessary. Some potential actions for common problems in boiler components includes:
- Control of temperature ramp rates to minimize thermal stresses
- Maintenance of supports to minimize system loading
- Proper control of welding processes and subsequent post-weld heat treatments
- Periodic inspections to monitor damage progression
Such a list cannot be considered as comprehensive, but in general, when problems with creep damage have been identified, significant improvements in service performance are invariably derived by reducing peak operating temperatures and/or the rate of temperature changes. However, this doesn’t come without consequences. The impact on efficiency resulting from such action are usually severe so that mechanical modification or alternative material selections are generally required.