Challenges to self-sufficiency
Chugach does use OEMs and/or independent contractors to repair some of the equipment at its plants as well as to refurbish certain gas turbine components. At Beluga, many of the repairs to gas turbine components are done on-site. For example, plant maintenance personnel use an on-site machine from IRD Balancing (Worthington, Ohio) to check for blade imbalances prior to installation of a new rotor on an ABB-11DM (Figure 5). The same technicians also repair gas turbine components such as combustion liners, transition pieces, turbine blades, vanes, and nozzles. Rotor and vane carrier rebuilds are completed on-site with refurbished or new parts, and Chugach maintenance crews have even coated compressors and restacked rotors at Beluga. Aiding the cause is a well-equipped on-site machine shop. Gas turbine disassembly and assembly during major overhauls is performed by plant maintenance personnel.
5. Balance it yourself. Plant maintenance personnel can balance gas turbine rotors on-site. Courtesy: Chugach Electric Association Inc.
After more than 25 years of service, the combined-cycle unit at Beluga—like many elsewhere (see box)—continues to present challenges to efficient O&M. Mechanical wear has caused HRSG tube leaks that have required weld repairs (Figure 6). Because cracking of the high-pressure-steam attemperator liner in each HRSG has been endemic, technicians recently installed inspection ports to facilitate borescope inspections (Figure 7). Highly stressed structural components—particularly in the high-temperature region near the gas turbines' outlets—require annual inspections and repairs. Since they were upgraded, both combined-cycle turbines now must be operated with their bypass stacks partly open to maintain the temperature of steam piping below its design value. This practice has increased stresses and cracking damage.
6. Seal it yourself. Weld repair of an HRSG tube leak. Courtesy: Tetra Engineering Inc.
7. Quenching steam. An attemperator spray port installed to facilitate annual inspections.Courtesy: Tetra Engineering Inc.
Water chemistry is maintained and monitored entirely by Beluga's on-site staff. The bronze impellers originally specified for boiler feedpumps have led to copper deposits in the steam system. The presence of copper suggests that the carbon steel components of the plant's steam systems have corroded. However, the contents of the steam turbine condenser have remained copper-free. Boiler and steam plant water chemistry is maintained by a conventional program based on the use of phosphate and amines.
At Beluga, corrosion damage required the low-pressure deaerator to be removed from service in 2004. A modification was made to provide low-pressure deaeration using the remaining high-pressure (HP) deaerator/storage tank. This was made possible only by a previous upgrade to the makeup water demineralizer system. The remaining HP deaerator components are subjected to thorough annual inspections that comply with NACE guidelines.
Flow-accelerated corrosion (FAC) wear is a common long-term problem at any combined-cycle power plant, and Beluga is no exception. Indeed, the plant's HRSGs include some carbon steel components that operate near the peak temperature for FAC wear damage (Figure 8). Difficulties with maintaining pH above recommended minimums, and years of operation with carbohydrazide (eliminated in the 1990s), have fostered long-term wear. More-recent operation has been at higher oxygen levels, and that has been beneficial, from an FAC perspective.
8. Just getting by. Structural repairs to an HRSG bypass damper. Courtesy: Tetra Engineering Inc.
Annual inspections of the HRSGs and related boiler piping components suggest changes in component wall thickness. In addition, a "guided-wave" inspection performed in 2004 provided a general screening of the thickness of main steam piping and helped in the assessment of the condition of other large-bore piping components. As a contingency, spare HRSG tubes were ordered and kept on-site should Beluga suffer a forced outage during the winter, when weather hampers or prevents cargo delivery.
24/7 O&M
Between two shifts, the Beluga plant has a staff of 50 craft and six management personnel. These two shifts rotate on a week-on, week-off schedule. The O&M crew comprises well-trained mechanics, machinists, instrumentation and control technicians, electricians, operators, and warehousemen. The crew and most supplies are brought to the plant by air.
Work orders are handled by the Cascade utility work management system from Digital Inspections (Corvallis, Ore.). A recent arrival at Beluga, the software suite also is used to input and record timesheet entries. Chugach orders, receives, and processes freight, and stocks and issues inventory using plant resources; these goods are stored in the plant warehouse.
Chugach also uses its own resources to operate and maintain camp facilities for housing employees and contractors on-site. The camp is staffed by a culinary crew, which serves well-prepared meals three times a day.
—The authors can be reached at pjackson@tetra-eng.com or 860-651-4622 and John_Kotrotsios@chugachelectric.com.
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