Outage management is a hot topic at most-user group meetings because it’s not easy to remember everything you should do to ensure a successful gas-turbine overhaul, and how you should do it. With about half the owner/operators attending the 2012 Frame 6 Users Group meeting first-timers, one of the veterans offered his experience in outage planning as a segue to an open discussion on the subject.
Start planning 12 to 18 months ahead of the outage, the speaker said. This may sound like an extraordinarily long lead time, but some parts have normal delivery times of one year. If you have little history at the plant, he continued, gather up all the knowledge you can on the GTs: operations and mechanical history, parts lives, parts upgrades from the original installation. Establish your goals, your expectations after startup—including output (At what ambient temperature?), emissions, vibrations, oil leaks, etc. What guarantees are you looking for? What terms and conditions?
Background information and goals in-hand, it’s time to begin planning the outage. Some of the things that must be done to move forward include:
• Select an outage team leader and identify team members and their specific responsibilities.
• Decide what equipment will be addressed during the outage: gas turbine, generator, heat-recovery boiler, steam turbine (if installed), etc.
• Take a first cut at the scope of work: mechanical, instrumentation, electrical, inspections.
• Conduct an inventory of spare parts (mechanical, instrumentation, electrical), (1) verifying the condition of new/repaired parts in the warehouse, (2) identifying parts needed and ones that must be repaired, etc. Obtain quotes for needed parts, making sure they can be onsite before the outage starts. Assign a planner/scheduler to track all parts ordered and sent for repairs.
• Flesh out the workscopes outlined earlier, have team members review, incorporate changes, finalize. Develop bid packages incorporating the final workscopes.
• Decide which companies will be asked to bid, when the outage must start and when the plant is needed back in operation, what the work schedule will be (days only, two 10-hr shifts, work every day, etc).
• Schedule a day to meet with all bidders selected. Communicate expectations, review scope, conduct field-walk through, establish firm due dates for bids, stress the need for bidders to submit schedules for their work within the outage window and to provide labor rates for disciplines required.
• Select the successful bidders.
Borescope after you complete the outage work and before you restart the GT to check one more time that there are no loose parts or tools inside the machine. There’s a temptation to “tune out” during some discussions, perhaps this one on borescope activities, because they sound like the same thing you’ve heard a dozen times before. But there’s always something new brought to the floor and the challenge is to “remain engaged” to pick up on that material. Example: One of the users alerted the group to the fact that there are two sizes of borescope holes in Frame 6 model that he has—and the plug for one fits through the hole for the other. The user discovered this when the small plug fell through the larger hole into the machine and the upper casing half had to be removed to recover the “foreign object.” Plant best practice: Do not remove a borescope plug if one is already out.
Fire protection was another topic of discussion. Make sure package doors are closed and sealed, otherwise your protection will be compromised, a user told his colleagues. Another mentioned the need for operators to enter his plant’s package regularly to check gages. Doors remain open and are tied off as a safety precaution when operators enter, he added. New machines are designed to trip when you open the package door—another safety precaution. For units relying on CO2 as the extinguishing medium, package entry is prevented unless the CO2 system is temporarily disabled.
Jam-up and sticking of package louvers was reported by several attendees. Proper louver operation should be tested at least annually, one suggested. Liquid lubes can glob up and inhibit operation over time, he said. Experience with dry powder lubricants has been acceptable, reported several plant personnel. Lots of “little things” were tossed about, such as how to prevent wasps from building nests in CO2 dispersion nozzles.
Fall protection received some air time. Discussion focused on poor footing in the neighborhood of the compressor inlet plenum as the cause of several injuries. Floor is curved there on some machines because of the nature of the inlet scroll installed, and it can be difficult to maintain balance to exit the air inlet area after adjusting inlet guide vanes, for example. A couple of the long-term members of the group recalled that the purpose of the scroll—conceived by one of the founders of the Frame 6 Users and later adopted by the OEM as an upgrade—is to even out the flow stream entering the compressor and prevent failures of IGVs in the areas 45 deg from the vertical in the lower half of the unit. Not everyone agreed on the value of the scroll; a couple of attendees thought it inhibited maintenance access and felt they would be better off without it.
Arc flash discussion died out quickly. Not much mentioned other the proper attire and face protection. At one plant, all operators wear Level 2 fire-retardant clothing, so they are always protected. Safety precaution enforced at another plant: Only electricians are allowed to open breaker boxes.
Alternative fuels are pertinent to this group because many Frame 6 engines are installed in refineries and chemical plants. An attendee was investigating the use of a byproduct gas containing a high concentration of hydrogen. Colleagues offered some suggestions and contact info for follow-up discussions. Another user discussed the two byproduct fuel trains at his plant—one containing hydrogen, the other CO. Neither fuel is available all the time, he said, and transitioning between the two can be challenging. Nitrogen purge was cited as critical to safe transfer. One gleaned from the discussion that at least some systems for burning byproduct fuels to accommodate availability and specific concentrations of combustibles are unique and custom instruction in handling and combustion is necessary to assure a safe work environment. If burning alternative fuels in your gas turbine is a concern, consider burning them in the heat-recovery steam generator, assuming one is installed.
Control of exhaust-end temperature motivated discussion. Too-high temperatures pose operational problems and a safety hazard, an attendee said. Varnishing of bearing oil and tunnel fires were cited. One incident reported: A fire was caused by spark ignition attributed to an overloaded fan wire. The spark set on fire lube-oil-soaked insulation (caused by a leak). A major concern is instrumentation wiring in the exhaust end of the unit, which can have a thermal rating in the neighborhood of 300F—below the operating temperature in some overheated bearing tunnels. If sensor wiring is compromised, the fire protection system will not be activated and the fire will continue to burn.
The compressors open discussion covered the “waterfront.” One user said his plant replaced its galvanized filter house with one of stainless steel to prevent more galvanized surface from peeling off and entering the compressor. In addition, the old trash screen from which bits of stainless-steel wire had been liberated was replaced by a Nimonic 50 screen, thereby eliminating unnecessary dings on compressor blades. When replacing a filter house, another user offered, do not replace in-kind—redesign it. In many cases, you’ll find the original filter house is one with marginal air-flow entry area and by opening-up the design you can reduce the pressure drop through the unit. If you go this route, a suggestion was to consider HEPA filters and size accordingly. Users with experience reflected a positive attitude toward HEPA filters.
Effectiveness of fogging, location of fogging nozzles, importance of droplet size, and plate-out of zinc on compressor blades were among the usual discussion points—and ones of greatest value to first-timers. One user seemed concerned about the wear and tear on first-row compressor blades attributed to water droplets created by the fogging system installed at his plant. A colleague asked, “How much can you make by selling the excess power produced with foggers in service? He suggested conducting an economic analysis. His plant, he related, is on its fifth row of R1 blades. The cost of new blades in his situation is relatively low compared to the revenue gain.