7FUG 2020: Strut cracks dictate exhaust-frame replacement

A major inspection in March 2020 on one of the gas turbines for a 2 × 1 7FA.03-powered combined-cycle power block at Gila River Power Station revealed severe rubs in the compressor and turbine sections upon unit disassembly. The machine, which did not run at all for 18 months beginning in 2017, operated baseload after returning to service. It had accumulated 53,000 operating hours (1760 starts) since commissioning in the early 2000s. Since the unit’s prior outage, a 2014 HGP, the gas turbine had run a nominal 20,000 hours (300+ starts).

Severe rubs were in evidence on one side of the turbine’s first-stage shroud blocks and on the compressor stator airfoils in Rows 13, 14, and 15 on that same side. Plant personnel found rotor clearances tight on the right in almost all rows of the turbine and compressor.

Schaffer Precision Alignment was contracted to check unit alignment (laser) and found the exhaust frame had shifted 0.071 in. to the right. Advanced Turbine Support, onsite to perform a borescope inspection, was asked to inspect the exhaust struts for any obvious signs of damage. Visual indications were found on two struts with confirmation by eddy current on one. A through-wall crack also was confirmed.

The search for a replacement exhaust frame ensued. The owner opted to purchase one in an as-is/where-is condition to minimize schedule impact. The replacement frame was removed from an unfired unit and shipped to the site. Upon arrival, it was inspected with the following observations:

    • Large gouge in the stainless-steel liner.

    • Support legs had a water jacket around them.

    • Bearing housing was a three-piece design.

Plant personnel believed that the gouge probably occurred when the weld was ground out during removal of the exhaust frame. The gouge was filled in with weld material and the frame installed at Gila River. Other actions taken were these:

    • Weld repairs were made on the exhaust manifold.

    • New oil seals were installed on the old bearing housing.

    • The existing support legs were reused to avoid having to deal with the water jacket on the new legs. All units in the fleet were believed by staff to be working well without the water jacket.

    • Stainless-steel exhaust seals were replaced with Inconel 718.

Schaffer Precision returned to check alignment with the replacement exhaust frame. It was located slightly to the left and high. The condition of bearing No. 2 and its position were checked and the experts agreed that it could be reinstalled and satisfactory alignment could be achieved by adjusting the bearing’s position. That’s what was done.

End notes:

    • The unit was restored to service after a 45-day outage, nine days less than originally scheduled.

    • The old exhaust frame is awaiting root-cause-analysis investigation to determine why the struts cracked.

    • Balance was not an issue after the work was finished.

    • Damaged airfoils in the compressor were simply swapped out because replacements were available and the job could be completed quickly with new stator blades.

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