V94.2 meeting highlights from Berlin (2019)

There were six V94.2 user presentations at the 2019 meeting, half based on experience in Europe, half in the Middle East. Speakers focused on the following: results of an extended HGP, follow-up on repairs to a compressor’s axial bearing after two years of operation, outage findings, damage to turbine blades attributed to domestic object damage, damage found in the solenoids of natural-gas vent valves, and finding the location of a generator ground fault.

   Here are thumbnail sketches of the assets upon which the presentations were based:

    • A two-decades-old, 486-MW, 2 × 1 combined cycle in CHP service having an overall thermal efficiency of 92%.

    • A 216-MW 1 × 1 combined cycle that began commercial operation 10 years ago.

    • A 480-MW 2 × 1 combined cycle (COD 1998) that went into service a few years earlier as two simple-cycle V94.2 (3) units.

    • A 10-yr-old, 380-MW, 2 × 1 gas-fired combined cycle with distillate back-up.

    • A two-decades-old, 450-MW, 2 × 1 combined cycle designed for resid/distillate and converted about 15 years ago to gas/distillate.

EHGPI. The speaker began with an overview of the unit’s operating history: about 29k EOH/400+ starts to the first major; second major at nearly 63k EOH and 500 starts; third major and lifetime extension at approximately 100k EOH/700 starts; and the extended HGP inspection, the subject of the presentation, at 42k EOH/230 starts following the third major.

About 10% of owner/operators were said to perform the Siemens HGPI, which has a workscope similar to a major, with rotor removal—but no rotor de-stack. Such an overhaul runs about 29 working days, each with two 10-hr shifts. The speaker recommended having some time in the schedule to rectify “as-founds.” Another recommendation: Plan for a six-day recommissioning of a dual-fuel machine that includes low part-load testing.

Here’s a list of what was done during this EHGPI:

    • All upper-half casings removed.

    • Rotor pulled along with compressor and turbine bearings. De-bladed rotor on roller rotor support stand but didn’t de-stack.

    • Compressor stator vanes and first six rows of rotating blades removed.

    • Full disassembly of the combustion system.

    • Exhaust casing removed.

    • Recoated compressor rotor blades and IGVs offsite (first time since COD). Stator vanes were new at LTE and recoating was not necessary.

    • Grit blasted all components removed from the rotor.

    • Overhauled all auxiliaries.

    • Installed upgraded fourth-stage divided seal rings for non Si3D vanes.

The as-founds included the following:

    • Damage to the trailing-edge tips of two Row 8 and one Row 9 compressor blades.

    • Fretting wear on tile support ring.

    • Excessive T hook wear.

    • Wear on mixing-chamber castellations.

    • Wear on some sharks’ teeth on the inner casing.

    • Wear and tear on exhaust-casing housing and on the exhaust expansion joint just before the diverter damper.

The as-founds were repaired by caulking, welding, etc, or parts were replaced—such as vanes, burner inserts, and divided seal-ring segments. Most of this work was captured in quality photographs and of significant value to attendees.

Of interest to attendees wanting to extract maximum value from their machines, compressor rotating blades here now have more than 140,000 EOH, while third-stage turbine blades and vanes have run north of 80k EOH, with about one-third of the run hours at partial load.

An outage was taken in 2019 for an engine that had accumulated 162k EOH since COD two decades ago with a “to do” list that included mods to reduce NOx emissions, combustion-chamber repairs, and implementation of a program to reduce low-frequency combustion dynamics.

The plant’s NOx reduction campaign began in fall 2017 in cooperation with Siemens. Key actions: Mods to pilot-gas control on a sister unit of the engine involved in the 2019 outage; plus, optimization of part-load pilot-gas flow. The trial project achieved the established goals and the changes were duplicated on the second engine.

Combustion-chamber repairs focused on the flame tube and mixing-casing castellation areas as well as on the HR3 burners. Burner rings were replaced and work was necessary on the bottom plates of the flame tube.

Improvements to the combustion monitoring system, a joint research project with the OEM, called for the addition of six dynamic pressure sensors and seven acceleration sensors on the combustion chambers (total for both). Low-frequency dynamics were thought to contribute to the wear of relevant parts. Metallurgical examination of the burner inset ring and an evaluation of dynamic pressure and acceleration pointed to “hammering” wear as the predominant damage mechanism.

The pilot-gas setting was adjusted to reduce pressure amplitudes while maintaining NOx emissions within prescribed limits.

Next step was to investigate the impact of ambient temperature on hammering using remote combustion monitoring tools. This was to have been done during winter 2019/2020 but no update on the project has yet been made available. Wear progress will be confirmed visually during the engine’s next minor inspection.

There was an unexpected finding to deal with as well during this outage: Loose and unattached burner-ring down holders. In round numbers, two-dozen down holders were loose on each combustion chamber, and a few others were either found out of position or tight but in the wrong position. Incorrect installation was the root cause; the work order provided was not followed.

At user meeting after user meeting there are examples of poor attitude, supervision, and training causing or contributing to issues that should not have occurred. Most senior plant personnel are aware of both this and the corrective actions necessary; however, schedule and budget constraints often dictated by others unfamiliar with plant operations militate against success.

During an engine inspection the leading edges of 26 first-stage turbine blades were found with varying degrees of distress; plus, the trailing edge of one was missing a 1-in. piece of material. Two blades in the second stage also were found damaged. In the fourth stage, dents were found on the leading edges of two blades, with a crack found emanating from one of the dents. There was no evidence of any loose material in the combustion chambers or exhaust diffuser to support the OEM’s belief that FOD was the cause.

A special inspection of the combustion chamber and mixing casing revealed a portion of the baffle plate was missing at the transition from the flame tube to the mixing chamber. That DOD caused the blade damage.

With the unit under an LTSA, Siemens pulled the affected combustion chamber and welded in a new baffle plate. On the other combustion chamber, the OEM trimmed four constellations in the transition area to prevent contact with the baffle plate during operation and avoid a repeat of the incident.

HGP inspection at 33k EOH: highlights. This case history begins two years ago at 16k EOH when the axial bearing for the compressor of this unit was found damaged. Bulletin PB3-13-5015-GT-EN-01 had been implemented in 2015. At 33k EOH bearing condition was determined “acceptable.” However, single pads that had suffered scratches were replaced as were pins and spring elements because of minor wear.

    • A crack was found in the inner casing in the region of the holder. No cause was identified. Local weld repair was the solution; no heat treatment was required.

    • Both transition rings were found with heavy wear. Replacement was preferred over weld repair because it was faster and less expensive. The mixing chamber outlet was modified as necessary to align with the new transition rings. Heat treatment was recommended and done.

    • Inspection of the F-ring upgrade implemented in 2015 found no issues.

    • Condition of the inner liner for the exhaust casing suggested installation of both a thicker end cover (9 mm instead of 5) and side compensators.

    • Upgrade of vanes and seal segments for the turbine’s third and fourth stages brought to light fretting wear not found previously.

Generator trip alarm was received on startup with turbine speed 260 rpm, rotor voltage at 270 V and current at 375 amps. Plant personnel reviewed the excitation drawing to identify test points. Next, all brush holders on the generator were removed and the alarm reset. The excitation side was tested and found acceptable.

An insulation resistance (IR) measurement on the generator rotor revealed less than 2k ohms. However, the resistance between one rotor terminal and ground was found low. Winding resistance for the rotor, measured after using external heaters to reduce humidity, was fine at 154.6 milliohms.

OEM Ansaldo Energia recommended repeating the IR measurement with a megger at 500V dc for 1 minute. Expecting the IR measurement would still be low, the manufacturer provided instructions for locating the portion of the winding affected by the ground and then cleaning it with acetone.

The ground gremlin was found in the area of the “B” slip ring and radial bolts. The speaker described the process of disassembling the slip-ring housing and the IR measurements taken to pinpoint the problem. Removal of the generator shields allowed a borescope inspection that identified the presence of lube oil on both shields and the rotor and evidence of foreign parts hitting the fan—as well as a piece of metal. An IR measurement after repairs confirmed the problem was solved.

Vent valves installed in the natural-gas supply line to the combustion chambers, an integral part of the generating unit’s safety system, are arranged to fail open. This means if the valves don’t operate properly, like the solenoids at this plant, the valves will open and the turbine will trip. In one year, the speaker said, four out of nine solenoids failed, negatively impacting plant reliability and availability.

The valves are located in different areas of the plant—some warmer than others. Personnel determined that heat was a primary factor in the failures. Changing out solenoid valves to the 240V dc coil type and cooling with air from the service-air system in locations of high heat seems to be a reliable solution. Operational observations continue.

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