Inspecting Welds for Creep Damage
Inspection Solutions No. 67, March 2000
Success for ESI
Inspection News from AEA Technology Energy
AEA Technology Engineering Services, Inc. has recently completed inspection and project management at a series of electricity generating stations for a major US utility. The inspection services included ultrasonic testing using TOFD (time-of-flight diffraction). The aim of the inspection was to identify and size service-included defects, but the main focus of interest was checking the longitudinal seam welds, tor creep damage.
The condition of these systems is a matter of great concern in the US. For thirty years from the 1940s onwards, many fossil-fuel power generating facilities used longitudinal seam welded piping in their construction. The pipes are used to carry steam at high temperatures and pressures, and the use of the longitudinal seam welds enabled manufacturers to produce pipes more cheaply from rolled steel plate.
However, the failure of these welds can be catastrophic: in recent years there have been both fatalities and damage to plant components. The power generation utilities are understandably concerned about the safety and liability issues as well as loss of income.
The primary cause of most failure is creep, a common damage mechanism precipitated by stress during service. It often occurs at welds, which concentrate localised stresses. Creep develops through several stages to stage 5, macro-cracking: once cracks are established, failure of the weld is dynamic.
Previous non-destructive inspection techniques were ineffective at distinguishing creep damage early enough to be useful, but TOFD has changed all this. Now hundreds of feet of welding can be inspected within days, surveying the entire system rapidly and accurately. AEA Technology Engineering Services have refined and specialised the technique so that it can identify and assess stage 3 and even stage 2 creep damage, enabling the utility to determine fitness-for-duty, and make repair or replacement decisions in plenty of time.
Before this particular inspection exercise, AEA Technology ESI was required to demonstrate the procedure at EPRI (Electric Power Research Institute) in Charlotte, N Carolina. All the required indications were detected and correctly sized, and highly accurate flaw characterisation to determine type and location was achieved
The actual inspection was conducted during scheduled unit maintenance outages, and the plant operators were reassured to be using such a highly recommended system for their inspection. One of the units was found to have sustained significant creep damage, so for immediate safety and to avoid a similar situation arising in the future, the piping outside the boiler was replaced with new seamless piping. Other units had also sustained stage 2 and creep damage and the managers have elected to initiate a more aggressive inspection schedule so as to monitor the longitudinal seam welds for additional initiation sites, and growth or change of the existing damage areas.
The utility managers are delighted with the outcome all the actions they have taken will either prevent similar problems occurring or enable them to detect the first signs of damage much earlier. In all cases this improves the levels of safety and confidence ill the plant, enables them to manage the site more effectively with a more extensive and accurate knowledge base, and provides them with cost-effective solutions to what is potentially an expensive and damaging problem.
AEA Technology plc
Sonomatic Products and Services
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