Frame 5: Timely borescope inspection prevents turbine damage
It wouldn’t be difficult to convince yourself that a robust MS5001P gas turbine (GT) averaging two fired starts per month and 40 fired hours per year over its 30-year lifetime doesn’t require much—if any— attention by the maintenance staff. However, that would be an inappropriate decision.
Gabe Fleck, an electrical engineer with Associated Electric Cooperative Inc (AECI), Springfield, Mo, who is responsible for long-term maintenance of the utility’s peaking GTs, offers the following case history illustrating just how important it is to inspect your generating assets regularly—no matter how often or how long they run.
Fleck, perhaps best known in the industry for his work as chairman of the 501D5/D5A Users, shared this recent experience with the editors of the COMBINED CYCLE Journal over lunch at his group’s 2006 annual meeting in Memphis. AECI and Fleck are proactive in sharing information with other owner/operators and there’s no better forum for accomplishing that than at a user’s group meeting.
AECI has two oil-fired peakers at its unmanned Unionville GT Power Plant in rural Missouri, where pipeline gas is not available (Fig 1). They were installed in spring 1976 for the primary purpose of maintaining voltage stability in an area served by one transmission line. In fact, it has been more than five years since these units were started with the express purpose of supplying kilowatthours to the system.
The site is checked frequently by personnel from the company’s coal-fired Thomas Hill Energy Center— located about 100 miles away— which has operational responsibility for the GTs. Until recently, one service provided by Hill technicians was conducting monthly reliability tests of the units. Simple arithmetic reveals that about half of the starts were for those hour-long reliability tests.
Inspection prep, results
Borescoping of the units typically is done on a three-year cycle, most recently in September 2005. Fleck developed the specification for the last inspection, won in competitive bidding by Floridabased Advanced Turbine Support Inc (www.borescopeinspections. com). Several borescope inspection ports are provided on the Frame 5—two on each side of the engine, one on each of the 10 combustion cans. Utility personnel prepped the engines for the inspection—including removal of the 10 fuel nozzles to facilitate borescope access. At the same time, AECI removed oil check valves and sent both the valves and nozzles offsite for cleaning and inspection.
ATS’s Rick Ginder spent about a day with each machine to ensure a thorough visual inspection from the inlet guide vanes (IGVs) to the exhaust plenum. Borescoping of Unit 1 gave no reason for immediate concern, but inspection of Unit 2 did. The most important of Ginder’s findings was that spot welds retaining the inner seals provided between adjacent first-stage nozzle segments had cracked in two instances (Figs 2, 3).
This allowed those seals to migrate downstream into the space between the first-stage nozzles (called vanes on machines made by vendors other than GE) and buckets (rotating blades). Rubbing was observed on platforms of first-stage buckets, but there was no indication of significant damage (Fig 4). Note that when a weld breaks, the seal does not just come loose and travel downstream; the resistance fit restricts its movement to small increments over a period of time. The seals are about 7 in. long and slightly less than a quarter of an inch wide; the gap between nozzle and bucket is 0.345 in.
Vendor selection. Fleck stressed the importance of proper due diligence in the selection of a borescope inspection service. Another vendor might have missed the traveling seals, he said, but Ginder knew exactly what to look for on these particular machines.
The editors tracked down Ginder on the road and he said there’s nothing like experience as the teacher on where to look for what. “Each machine has its idiosyncrasies,” he added, “and in the case of the Frame 5, we’ve done many, many inspections.”
“We started out monitoring a firststage nozzle shim for a Florida-based utility,” Ginder recalled. “For two years we watched the shim move back and forth between the trailing edge of the first-stage nozzles and the leading edge of the first-stage buckets. Initially, the customer and we thought that there wasn’t enough room for the shim to come out—that it would just continue to move back and forth in the seal slot.
“However, we learned over time that shim contact with the rotating buckets was actually pulling the shim out from between the nozzle segments. Upon confirming this, the unit was removed from service and repaired.
“Based on this experience, ATS implemented an inspection program for all Frame 5s that requires the checking of each shim to see if it is moving and/or if the tack weld holding it in place is cracked. To facilitate this inspection, we designed and manufactured some special guide tubes for our borescopes. We also inspect closely the first-stage buckets for signs of rubbing.”
Ginder added that since finding the first migrating seal, ATS inspectors have identified loose seals on at least five more units. More might have been found if all customers had been diligent about removing all 10 fuel nozzles. Some owner/operators choose to pull only two to four nozzles despite knowing this type of damage might be overlooked.
Revised maintenance plan
The “traveling seal” problem prompted AECI to schedule an emergency hot-gas-path (HGP) inspection for repairs. Fleck prepared the specifications for both field service activities and component repairs. Wood Group’s Houston-based field services unit was the low bidder for onsite work that began in November 2005. Major items in its workscope were these:
- Remove combustion and turbine components—including liners, cans, transition pieces, and turbine stationary parts.
- Inspect all bearings.
- Inspect load and accessory gearboxes.
- Inspect inlet and exhaust ducts.
- Inspect and clean inlet manifold, IGVs, and row 1 (R1) compressor blades.
Wood Group identified major damage to the compressor and exhaust bearings, which the utility believes was caused by an inadvertent tripping of the lube-oil pump for about 10 seconds earlier in the year (Fig 5). Service personnel also noted that both rows of turbine blades on this two-stage machine were fused to their respective disks (no blade rock here), and that they could not budge the lower half of the second-stage nozzle assembly.
These observations sent Fleck back to the drawing board. He again expanded the scope of the job, this time to include (1) removal of the GT rotor, (2) disassembly and inspection of the accessory and load gears (because of the wiped bearings), and (3) inspection of the generator bearing.
With the rotor out of the machine, Wood Group was able to extract the second-stage nozzle blocks with the assistance of PB Blaster, a penetrating catalyst that breaks loose the surface tension of frozen parts. Wear and tear on nozzles was relatively minor.
One of Wood Group’s challenges at Unionville was the fabrication onsite of several specialty tools that could not be located. As you can imagine, many people were involved with the plant over its 30-yr lifetime because there was no permanent O&M staff; no one could recall where these tools were stored for safe keeping.
The rotor lifting tool in Fig 6 made by Wood Group craftsman is an example. Fleck said Project Manager Tom Lamprecht and his mechanics were very resourceful. Unionville is in rural Missouri and there are relatively few places within 50 miles of the plant where you could purchase materials required by the job. The local junkyard helped considerably under these circumstances.
The rotor and related components were packed and shipped to Houston for refurbishment— rotor to GE Preco, nozzles and other HGP par t s t o ACT (Advanced Combustion Technology Inc )—and the onsite Wood Group crew demobilized. GE Preco handled rotor and turbine- blade cleaning and inspection.
One of the first tasks facing GE Preco when the turbine rotor arrived at its shop was removal of the buckets from the disks. The company first tried turning the rotor through a heated solution formulated to penetrate the rust between the bucket roots and wheel dovetails. Didn’t work. Next, it hired a specialty contractor that used induction heating to expand the disks and dry ice to shrink bucket roots (Fig 7). “Buckets came out as if there was grease on them,” related Fleck.
Why so much rust? Fleck explained that the Unionville peakers operate in a high-humidity environment and that about half of their lifetime starts had been for reliability testing. Company procedures called for a onehour test, mindful of the price of fuel , emissions concerns, cost of personnel to conduct the tests, etc.
However, one hour was not sufficient to heat-soak turbine components and the resulting moisture took its toll over the years. New procedures call for running three consecutive hours quarterly. Operating the engine longer also allows more thorough performance testing, acquisition of vibration data, etc.
Next, GE Preco grit-blasted the rotor and buckets. The minor tip rubs on first-stage blades and the few indications found on the rotor disks were blended out. On reassembly, machinists found that the fit between blade roots and disk dovetails was within the OEM specification and no work was required. However, the locking device had excessive play: In effect, the turbine disk had “shrunk” because of material loss. A 0.030 in. coating of metal spray on the disk enabled the locking device to fit properly; no machining was necessary.
ACT’s workscope included the manufacture of new seals for the nozzle rings; material was upgraded to Hastelloy X (Fig 8). Operations Manager/President Joe Cosart said ACT completely refurbished first- and second-stage nozzle assemblies and all combustion hardware on a fastturnaround basis. The weld-bead barriers installed by ACT to prevent seal movement are more robust and perhaps better positioned than the ones that failed, noted Cosart. The company has done this work successfully many times previously, he added.
ACT also did some machining on the compressor case to facilitate access to the forward bearing in the future. Fleck mentioned that the rabbet fit between the upstream compressor case and the bellmouth requires compressor case removal to access the bearing. While the rabbet fit probably enabled proper fit-up of the GT during installation, it is an annoyance when trying to access the bearing for maintenance. AECI had ACT machine off the rabbet on the upper half of the compressor case so now only the bellmouth had to be lifted to access the bearing on the cold end of the machine.
With work complete at GE Preco and ACT, and new bearings poured, components were returned to Unionville and Wood Group remobilized its field force the third week of January, completing work about one month later. Sub-zero wind chill made easy work difficult. Among the tasks accomplished:
- Replacement of inlet-duct hardware (bolting) that had rusted out and required replacement.
- Refurbishment of IGVs to eliminate excessive play. Work was done consistent with recommendations in a TIL (Technical Information Letter) issued by the OEM on the subject.
- Cleaning of lube-oil reservoirs; oil flush.
- Replacement of compressor bleed valves.
- Realign and re-shim the entire package using laser technology.
Restarts after a major overhaul generally have some challenging aspects. At Unionville, it took seven attempts to synchronize Unit 2 on February 18. Four failures to start were caused by servo issues, two because of operator oversight. Problems corrected, the machine ran for several hours the next day and recorded enviable vibration data.
End notes. While no nozzle shim movement was observed during the inspection of Unit 1, it is logical for this to happen given that it is the same model as Unit 2 and operates in the same environment. AECI’s plan is to borescope every spring and if migration is observed the unit would be placed in emergency standby status and planning/ budgeting initiated to make the repairs the following spring.
Experienced operations personnel also must be wondering about the condition of the fuel oil at Unionville given an average 40 hours of operation on an annual basis. Fleck says they maintain oil in good condition by use of chemicals to prevent microbiological growth. Plus they periodically drain the tank and refill with fresh oil. Oil drained is trucked to Thomas Hill for use as startup fuel.
AECI took advantage of the outage to abate the asbestos gaskets that still remained. Recall that this was the first inspection of the machine. ccj oh