Rigorous inspections
Starting in 2005, detailed inspections of the HP deaerator vessel and storage tank were performed, compliant with the standard practice specified by NACE in RP0590-96 (the latest version is now NACE SP0590-2007) “Standard Practice for Prevention, Detection, and Correction of Deaerator Cracking.” These included visual inspections, pit depth surveys, ultrasonic testing of remaining shell wall thickness, and wet fluorescent magnetic particle testing of pressure vessel welds. An inspection opening was cut in the tray shroud (baffle) to provide access to the LP steam piping nozzle and weld connection. Additional cracks and preexisting damage were repaired in both 2005 and 2006.
During a scheduled outage in April 2007, a relatively large area on one end of the HP deaerator vessel closest to the LP steam nozzle was identified as having noticeable wall loss since prior inspections (Figure 2). The damage mechanism was identified as flow accelerated corrosion, which is common in deaerator systems and is caused by high local velocities and temperatures that maximize the corrosion rate.
2. Engraved invitation. This photo of FAC wear damage to the HP deaerator vessel shell was taken in April 2007. Courtesy: Chugach Electric Association Inc.
FAC damage has also been confirmed as the primary mechanism responsible for recurring damage to HP boiler feed pump impellers (Figure 3) and for wall thinning of HP economizer tubes in the HRSG, whose progression has been trended for several years.
3. Total loss. FAC wear damaged each of the plant’s three HP boiler feed pump impellers. Courtesy: Chugach Electric Association Inc.
Plant management decided to schedule several interim inspections of the HP deaerator to better characterize the FAC damage and its rate of progression. The first special inspection was in August 2007; a second was in December 2007. These special inspections confirmed a relatively rapid rate of progression and a need to plan for component replacement (Figure 4).
4. Death sentence. This was the HP deaerator vessel remaining life projection made in December 2007. Source: Chugach Electric Association Inc.
As an interim measure, a deflector plate was installed to attempt to reduce the flow rate near the most degraded part of the vessel shell. Subsequent inspections indicated that this measure was in part responsible for reducing the rate of local shell wall thinning.
Fitness for continued service and estimates of remaining lifetime were demonstrated by the procedures prescribed in Parts 5 and 6 of ASME FFS-1 (formerly referred to as API Standard 579 “Fitness-for-Service”) for pitting damage and local wall thinning. The procedure for local wall thinning was validated by the Welding Research Council, as documented in its Bulletin No. 505 released in 2005. These standards and procedures provide a “more sophisticated assessment of metallurgical conditions and analyses of local stresses and strains can more precisely indicate whether operating equipment is fit for its intended service or whether particular fabrication defects or in-service deterioration threaten its integrity. Such analyses offer a sound basis for decisions to continue to run as is or to alter, repair, monitor, retire, or replace the equipment.”
Calculations performed using these fitness-for-service standards indicated that the HP deaerator vessel was fit for continued service until August 2008, provided that the rate of continued thinning did not exceed an amount assumed in the assessment. Three successive inspections confirmed that this was indeed the case. By late March 2008, FAC damage had consumed about one-third of the initial wall thickness, clearly representing a significant degradation since the initial detection in April 2007 (Figure 5).
5. Surface of the moon. This March 2008 photo shows FAC wear damage to the HP deaerator vessel shell in the same location as Figure 2. Courtesy: Chugach Electric Association Inc.
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