7B major inspection case study reveals hidden value in old iron

Marty Magby was at the 7EA Users Group’s annual conference in St. Augustine the week of October 23, as the editors expected; he’s been a meaningful contributor to discussion sessions at this forum for years. However, he came to the 2017 meeting wearing Allied Power Group (APG) gear after attending the last 11 years in shirts with an owner/operator’s logo. Nothing should surprise anyone in the electric power industry these days given the ongoing structural changes.

Magby’s forte is large rotating machinery. His gas-turbine inspection and overhaul experience is a good fit for APG given its “bundled solutions” initiative and expansion plans. The recently appointed director of the company’s field services group, Magby is responsible for planning and managing work for both union and non-union customers and for growing the firm’s service offerings.

The opportunity to speak with Magby for about an hour in a quiet spot was rewarding: He shared his experiences on a Frame 7B major inspection completed shortly before joining Allied. The project scope and notes below provide a checklist others overhauling 7B-EA engines may find helpful.

But before digging into the details, he urged his colleagues to begin their projects with a business case to justify the proposed work scope. Of course most users in plant management know this, he said, but they sometimes jump-start outage planning—their comfort zone—based on previous experience. In his view, this may not produce the desired outcome.

Magby’s recommendation: Take a clean sheet of paper and write down how the asset scheduled for an overhaul makes money for the owner and how it is dispatched. Sounds elementary, but the exercise can be revealing. Next, check the unit’s permits to be sure it is offered to the market in the most favorable way for the owner. That’s obvious, most might say. But given today’s rapidly changing market dynamics, it might not be so. “Opportunities” can change virtually overnight.

Case in point: The dual-fuel 7B Magby was lining up for a major inspection was in its fifth decade of service and approaching 2500 starts and 15,000 hours of operation. Firing temperature for this “standard” engine was about 1850F. The unit was capable of 55 MW on a favorable summer day.

Interestingly, about three years ago the owner had decided to retire this machine, and two sister units, in the first couple of years of the coming decade while continuing to operate a conventional steam plant for the next two decades. “This was the plan when I went on vacation,” Magby said, “but by the time I returned, the old plan had been scrapped and I was told to do what was necessary to keep the 7Bs running for 20 more years; the steam unit would be retired instead.”

Clearly, the gas turbines were better suited than a steam unit to accommodate the start/stop operation required to fill in around rapidly increasing renewables generation in the area. The 7Bs could go from cold iron to rated load in 10 minutes. They also required less O&M manpower. Management had determined upgrades performed during the major would enable a site staff reduction from 19 to three with no personnel onsite for some hours.

A review of the “grandfathered” air permit revealed the true value of the old iron. The gas turbines only have an annual limit on NOx emissions. Thus they can be offered into the market at any at virtually any output—even as low as 1 MW, if that makes financial sense—and be available to fast-ramp to meet market demand. Magby’s challenge in planning the outage was to be sure no modification would trigger a New Source Review.

Orders in hand, it’s important for the project manager to do his or her homework before proceeding. Magby recommends reviewing operating records and previous outage reports and then conducting in-depth discussions with the following groups:

      • Plant personnel, to identify issues requiring correction.

      • Users with relevant experience in engine overhauls, to take advantage of industry best practices and lessons learned.

      • Vendors, to conduct a preliminary evaluation of the alternatives considering experience, cost, lead times, special installation requirements, etc.

While conducting the foregoing discussions, he said, it’s important to keep in mind the critical project drivers—in this case, 20 more years of reliable service, reduced staffing, and periods of unstaffed operation.

Project scope

      • Remove the engine rotor. De-stack, clean, and coat the compressor section to protect against corrosion; correct blade migration. De-stack turbine rotor, clean, and coat fir trees to eliminate bucket rock; remove bore fan as suggested by the OEM in one of its Technical Information Letters.

      • Replace exhaust cylinder and diffuser.

      • Conduct wheel-space thermocouple mod to allow t/c access without lifting the casing.

      • Conduct bearing-metal thermocouple mod to monitor bearing temperature.

      • Bently mod to upgrade vibration monitoring capability.

      • Replace turbine and load-compartment enclosures; expand turbine compartment enclosure.

      • Upgrade flame scanner and eliminate water cooling.

      • Rewind the air-cooled generator with existing copper; correct shorted turn.

      • Replace magnetic retaining rings on the generator rotor with ones of 18-18 stainless steel.

      • I&C and valve refurbishment (conducted by plant staff).

      • Re-wedge the generator stator.

      • Switch from hydraulically actuated gas valve to one with electric actuation.

      • Replace original lube-oil pump with two motors with separate ac and dc pumps.

      • Modify SSS clutch for today’s service.

      • Upgrade purge bleed valves.

      • Recharge accumulator.

      • Replace radiator and clean the coolant system.

      • Inspect and clean lube-oil cooler. Install a booster cooler to address high-oil-temp issue.

      • Upgrade mist eliminator.

      • Replace starting motor.

      • Upgrade voltage regulator.

      • Modify the generator protective relay to satisfy new NERC rules.

      • Rewire unit to eliminate cannon plugs and degraded wiring. Circuits were rewired externally to eliminate to the degree possible all connections inside the enclosure.

      • Install an isolation switch between the generator and generator step-up transformer.

      • Replace switches (on/off) with transmitters in control circuits to eliminate failures with the former.

      • Paint visible exterior surfaces with an environmentally acceptable coating system.

      • Documentation upgrade. Big effort to update drawings and instruction books and convert them for electronic access.

      • Controls. The gas turbine’s Bailey Infi 90 system was retained. Staff did not want to replace it; had plenty of spares and the internal talent to maintain it.

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