Plant:

• A 270 MW gas fired boiler, of once-through (Benson) type with 180,000h service.
• After 5 years base-load operation followed by 20 years of load following, the utility wishes to run in a two-shifting regime for 8 more years.

Objective:

A Phase 1 remaining life assessment of:

• all critical headers (22 types)
• some internal pipework (2 bends)
• some tubes (2 banks)

Basis of assessment:

• Current condition (calculated life consumed)
   - operating conditions to date
• Remaining life
   - proposed change from load following to two shifting
• Mechanisms
   - low temperature headers - fatigue only
   - high temperature components - creep and fatigue
   - tubes - creep, fireside corrosion, steamside corrosion
• Input data
   - actual and expected operating conditions
   - design materials data and dimensions
• Code based stress analysis (TRD)
   - all key locations

Operational events considered:

• Starts
   - Cold to 40 MW
   - Cold to full load
   - Warm to full load
   - Hot to full load

• Load changes
   - 40 – 270
   - 270 – 40 MW
   - 80 – 160 MW
   - 40 – 270 – 130 – 270 – 40 MW (full 24h cycle)

• Unit trip

Excursions and instabilities:

Known in advance:

• Economiser
   - cold-water pull-through from de-aerator on run-up
• Evaporator, level maintaining vessel, cyclones
   - feedwater choke on transition to once-through operation

Discovered during the assessment:

• Superheaters 3 and 4, Reheater 2
   - Attemporator instability
• Reheater 1
   - Bypass instability

Results - fatigue:

• Two components were calculated to be fatigue critical at the assessment time
• If instabilities have their maximum effect, then a further component was also calculated to be fatigue critical at the assessment time
• Three components were calculated to become fatigue critical during the next 8 years
• If instabilities have their maximum effect, then a further component was also calculated to become fatigue critical during the next 8 years

Results - creep and fatigue combined:

• Two components were calculated to be creep strain critical at the assessment time
• One of these was also critical with regard to calculated total damage at the assessment time and the calculated total damage was predicted to exceed 1 within the next 8 years
• The other was predicted to become critical with respect to calculated total damage during the next 8 years

Recommendations:

• Further data, necessary to refine the assessment for those components which were predicted to become critical within the next 8 years, were identified
• Future inspection plans were formulated
   - Scope and timing
   - Techniques
• Potential replacements were identified