Inspection of Otahuhu B’s triple-pressure HRSG revealed safety issues outside the casing; reliability issues inside. The Siemens single-shaft combined cycle, powered by a V94.3A gas turbine, had a complex history, with several consultants and engineering contractors onsite since its commissioning in 2000.
Steering Committee Member Mark Utley of Contact Energy Ltd (CEL), owner/operator of the Auckland (New Zealand) facility explained that the as-supplied HRSG was of paramount concern. CEL assumed responsibility for the boiler soon after installation to address non-compliant welds and long-term risks. Operations were acceptable until 2008; further boiler defects and lingering construction faults surfaced at that time.
The first major tube failure was in 2008, specifically creep damage where a T91 superheater tube was connected to a P22 header. CEL replaced all at-risk headers with P91 (photo). That same year, the owner called for EPRI chemistry benchmarking and launched a significant steam-cycle chemistry upgrade. Gas-turbine firing temperature was increased to recover lost capacity and HRSG safety valves were resized.
Before this outage, the plant had half of Structural Integrity’s 19 recommended cycle-chemistry instruments. This was brought to 100% by 2009. Benchmarking improved from “average” to “above average.”
But for the next four operating years, disturbing HRSG events and findings continued. These included faulty welds and cracks in lugs supporting headers, among others. A long layup for repairs caused fin and tube fouling and backpressure limitations on the gas turbine attributed to gas-side corrosion. There was corrosion under insulation as well. The plant entered permanent shutdown in 2015.
Specific HRSG defects were shown to meeting participants—including the lack of penetration on tube-to-header welds, tube-to-header misalignments, and general poor weld quality. Acceptance criteria also were clearly defined.
The primary Otahuhu message: Challenging risks can be managed over time with an active and flexible approach. A wait-and-see attitude will not work. This put emphasis on Steering Committee Member John Blake’s “Don’t just set and forget” caution earlier in the day.
