Planning and executing your 100,000-hr overhaul

By Marty Magby, Turbine Generator Maintenance Inc

The 100,000-hr overhaul of your gas turbine/generator (GT) typically occurs during the second of what the OEMs refer to as a “major inspection.” That’s appropriate, because the 100,000-hr inspection is indeed a major event.

The list of tests, measurements, and maintenance tasks that should be performed at the 100,000-hr interval, according to your OEM manual, is probably mind-numbing. Complicating matters is the fact that, after running for this extended period of time, many things on your unit will have changed—including the unit’s very shape and location. Axial growth, creep strain, piping-hangar deformation, and other inevitable deviations from baseline will have occurred. Hardware upgrades implemented over the years to improve capacity, efficiency, or reliability may have further altered the unit since a good baseline was documented during commissioning.

Faced with these challenges, and constrained by today’s lean staffs and even leaner O&M budgets, plant managers often have difficulty defining a detailed work scope for the 100,000-hr overhaul, and incorporating that work scope into an overall plant outage plan. That’s why it’s important to start early. Treat the overhaul-planning process as you would any other large, ongoing plant project. Conduct planning meetings, set action items with specific deadlines, and hold assigned team members accountable during frequent progress updates. Don’t forget to identify all of the labor resources you’ll need—both in-house and outsourced— and bring those resources into the planning process as soon as they’re identified.

For long outages like the 100,000- hr overhaul, outsourced contractors are an essential part of the team. Be sure to treat them so, by contractually obligating their involvement in the outage’s success. Clearly worded bonus and penalty clauses can help here. Also, you should equip the contractors with the same scheduling, cost-control, and communications tools that your in-house folks have. That will be particularly critical with multiple contractors onsite, if you hope to keep all jobs moving with only minimal conflicts.

Pre-outage inspection

The first tangible step in your 100,000-hr overhaul should be a preoutage inspection. It should have two major goals: Allow for development of a comprehensive work scope, and establish a baseline on your unit for reference.

This inspection could be performed by plant personnel, but it’s probably best to bring in a specialist. Besides his technical expertise on the GT, a specialist can contribute “fresh eyes” that are less likely to overlook subtle, lingering issues that may be affecting the unit. That’s no “knock” against plant personnel. It’s just human nature that as we work around the same environment day after day, problems begin to seem “normal” (Fig 1). Types of information that should be collected during the pre-outage inspection are:

  • Visual condition of each piece of equipment. A thorough visual inspection should be performed, looking for leaks, loose or broken fasteners, wear, dirt, corrosion, and any other abnormal condition that is causing—or if left untreated can lead to—problems (Figs 2, 3, 4). Digital photos are a great way to document your visual findings.
  • Vibration data. These readings should be collected on all rotating machinery, using both installed instrumentation and portable, hand-held test equipment. If you do not have an in-house vibration specialist, this is a critical task that should be handed over to a specialist.
  • Hot- and cold-position reference points of the unit. This task typically will require an outside contractor. Record the elevation on all of the equipment, up and down the train, during both hot and cold conditions (Fig 5).
  • Axial growth caused by thermal loading of the unit.
  • Bearing temperatures while operating at load. High bearing temperatures are indicative of potential problems, including these: improper loading, contaminated oil, partially wiped bearings, insufficient oil supply, hot air leakage into bearing area, and improper insulation. Elevated bearing temperatures should be taken seriously and efforts should be made during the inspection to identify the root cause.
  • Bearing vacuum readings.
  • Operating pressures throughout the unit’s fluid streams—air, combustion gas, fuel, and lube oil.
  • Acoustic readings. An acoustical map of the unit should be made. The sounds your machine makes can be an indicator of a potential problem. This information cannot be used without an understanding of what your unit typically sounds like. Gathering and saving acoustic data provides an additional and valuable diagnostic tool.
  • Valve positions throughout the operating load range. Positions should be documented based on both the control-system indicated position, and the actual position observed at the valve.
  • Check inlet position and operation of inlet guide vanes. IGVs have a significant impact on overall unit performance and should be given commensurate attention. If possible, stroke IGVs from the full-open position to fully closed while someone watches the actual movement of the vanes. Pay particular attention to any vanes that don’t operate smoothly or ones that rub on the inner or outer bellmouth. Vane angles and clearances should be recorded. All IGV components should be inspected for any unusual wear or loose components (Figs 6, 7).

Additional work scope

The pre-outage inspection is the foundation of your detailed work scope. Here are some other tasks, not typically identified by that inspection, which should be added to your 100,000-hr overhaul work scope:

  • Starting-motor cleaning and electrical testing. Also consider having the starting motor dipped and baked, depending on operating environment, operating history, and the results of its electrical tests.
  • Motor and motor-coupling maintenance. There are many smaller motors on the GT that drive fans, blowers, or pumps. These motors should be cleaned and inspected, and possibly even dipped and baked. Motor couplings should be inspected closely, and replaced as needed.
  • Torque converter service. Torque converters often are neglected because they tend to be durable, low-maintenance equipment. Problem is, if your torque converter fails there is likely to be a long lead-time for parts or a replacement. Suggestion: Call the toque-converter OEM during the development of your work scope, to discuss the history of your particular unit and obtain service recommendations (Fig 8).
  • Valve and valve-operator refurbishment. All the valves and valve operators for your GT should be refurbished at 100,000 hours (Fig 9).
  • Gear-box inspection. Gear boxes routinely are inspected, but only through access ports. At the 100,000-hr interval, a compete gear-box inspection should be considered (Fig 10). The bearings should be inspected and repaired as needed; the gear mesh should be recorded; and the gears and shafts should be inspected using non-destructive evaluation techniques— such as dye penetrant and magnetic particle inspection.
  • Instrument calibration. The old saw “garbage in, garbage out” truly applies to the relatively lowcost instruments that monitor and control your high-cost gas turbine. A good inspection, test, and calibration of all of the unit’s instruments will deliver a more reliable machine.
  • High-velocity oil flush. Many plants do not perform oil flushes after their major inspections. The ones that do, often perform merely a “maintenance oil flush.” This typically comprises installing mesh screens in the supply lines of the oil system, running the system for two to 12 hours, and inspecting the screens. If there is no major debris or accumulation of metal flakes in the screens, the operators pull them and return the oil system to service. At the 100,000- hr interval, a more thorough, high-velocity oil flush should be conducted to scrub away the contaminants that typically cannot be removed with a “maintenance flush.” Such debris, over the subsequent 100,000 hours of operation, could break loose and cause bearing overheating or complete failure.
  • Shell leaks. Virtually all GT shells leak, creating both a loss in unit efficiency and a safety risk to your employees. The 100,000-hr overhaul is a great opportunity to address those unavoidable leaks. There are many ways to do so. The key is to identify the leaks prior to the plant outage, evaluate the possible repairs, and insert the best option into the work scope. Your field-service contractor should be able to offer several methods to address your shell leaks (Fig 11).
  • Generator testing. If the stator on your electric generator has not been subjected to an electromagnetic core-imperfection detector (El CID) test, you need to add that to the work scope. An El CID test verifies the integrity of the interlamination insulation. Degradation of this insulation will cause a hot spot on your stator core, and potentially a major breakdown of the generator. The stator windings also should be checked—such as with a Hi-Pot test.

Execution and close-out

Once the planning is completed and the work scope is fully defined, it’s time to conduct the outage. Executing a 100,000-hr overhaul can take as long as eight weeks, depending on the type of GT (frame or aero), the GT model, and, of course, the work scope defined for your project. There’s an entire discipline—some may say even an “art form”—to properly managing an outage and recommissioning the plant after a major overhaul. But that’s the subject for another article.

However, it is important to emphasize one more often neglected aspect of the 100,000-hr overhaul: formal closeout of the project. Once the unit is placed back into service, all of the information from the pre-outage inspection and the final outage report should be combined into a book that documents the new unit baseline. The contents of this book should include all new parts, products, and vendor information. Also, it should contain all of the detailed data sheets from the pre-outage and outage inspection reports, as well as a photographic record of your unit in the various stages of pre-outage condition, disassembly for repair, and re-assembly before restart.

The goal is to consolidate as much information into one place as possible, so that in the future questions about unit condition can be found easily in one location. ccj oh