Reduce outage time, cut cost by repairing Frame 5 compressor with the rotor in-situ

Coverage of the Frame 5 rightly belongs in CTOTF’s™ GE Legacy Roundtable chaired by NRG Energy Inc’s Ed Wong, PE. But that forum is held only at the user group’s fall meetings and the hook-fit repair experience TVA’s Nathan Holland, vice chair of the GE F-class Roundtable, had to share was timely. Plus, most attendees at the GE E- & EA-class Roundtable, where Holland presented, likely have one or more Frame 5s in their portfolios.

As most Frame 5 owner/operators are aware, the thin ligament at the 10th-stage extraction slot in the compressor is a weak spot in the machine. It forms the hook that holds one side of the ninth-stage compressor stator vanes in place. Fig 1 shows the area of interest as well as the damage caused when the casing cracks, allowing one or more vanes to work free and go downstream. Fig 2 shows the damage south of the ninth stage in greater detail.

TVA Frame 5 Fig 1

Rodger Anderson, manager of GT technology for DRS Technologies Inc, told the editors he has seen this type of failure in many Frame 5s over the years. He said it is the result of a gray cast iron casing weakened by (1) corrosion, (2) the uplift loading on the ligament area created by airflow through the compressor, and (3) the numerous start/stop cycles associated with peaking service. Keep in mind that gray cast iron has poor tensile and fatigue properties.

TVA Frame 5 Figs 2, 3

The casing cracks before failure, Anderson continued, and the vane loading will propagate the crack. Thus regular checks of casing condition in the extraction cavity during borescope inspections can warn of impending failure (Fig 3) and enable corrective action before serious damage occurs.

Industry experience with cracking of the ninth-stage hook fit is that it can be expected on peaking units with more than about 1000 starts/5000 service hours over a nominal 30-yr period. TVA’s 16 dual-fuel peaking Frame 5Ns (a/k/a Nancys) at one site were beyond that, approaching 40 years of service; in round numbers they averaged 2000 starts/6000 operating hours. Plus, two engines in this group had suffered forced outages because of in-service hook-fit failures.

The utility’s original repair technique required performing a major inspection with the unit rotor removed to access the compressor casings for hook-fit repairs—a costly approach. With outages planned for the remaining 5Ns at this site by the end of 2019, an alternative solution was sought to reduce outage cost and duration, and still provide a quality hook-fit repair.

Industry sources suggested a less intrusive method, which eliminated the need to remove the rotor to make hook-fit repairs. These are the steps:

      • Remove the upper half of the forward and aft compressor casings.

      • Roll out the lower half of the aft compressor casing (Fig 4).

      • Machine the ninth-stage hook fit offsite.

      • Bolt the patch ring in place axially.

      • Reinstall the lower-half aft casing.

      • Reinstall the upper half of the forward and aft compressor casings.

TVA Frame 5 Fig 4

Six patch rings already have been installed using the rotor-in technique (Method 2) at about one-third the cost and one-third the duration of the original method. Even in cases where the rotor must be removed, the speaker recommended pulling the lower half of the aft compressor casing to machine the hook fit. This allows bolting together both casing halves and machining a 360-deg patch ring, thereby reducing the risk of radial clearance issues.

Lessons learned included the following:

      • Weld and grind flush axial patch-ring bolting.

      • Where multiple units are involved, project synergies can reduce cost and schedule.

      • Secure compressor stator vanes into the casing before rolling out the lower half.

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