GENERATION

Temporary Structures Expedite Repairs During Steam-Turbine Outage

At one such plant, TurboCare "a Siemens company and leading aftermarket supplier of turbomachinery support and replacement "was called on to remachine and reblade the low-pressure steam-turbine r'tors of two 750-megawatt, natural gas-fired units. The goal was to increase operational reliability while recovering 22-megawatts of lost capacity from each unit. Restoring output and revenue would reduce the need to build expensive new plants.

TurboCare would perform a "ongshank"repair on the two units, removing damaged material and remachining new blade attachments at a radius lower on the turbine wheel.

The "ongshank"would lower the attachment point of the r'tating blade, with the airfoil in its original radial position along the steampath.

The project was logistically challenging due to the very limited outage window. It was also the first time TurboCare had worked on two r'tors simultaneously, with multiple wheels on each r'tor. Moreover, with each of the 30 to 40-fo't long r'tors weighing over a hundred tons, special consideration had to be taken to house them from inclement weather, since they would be removed from their turbine casings to complete the repair, and the outdoor laydown area next to the turbine deck offered no pr'tection from the elements. Proper housing and workspace was critical to attempt and complete the project.

"We required large, weatherproof, temporary structures with quickly removable roof sections, as heavy rains weren't unseasonable and could delay the project,"said Charles M. Kaslow, Operations Manager for TurboCare's Perris, California facility. "of logistics was the primary challenge, time and space were our biggest enemies. The utility had a very limited outage window and very limited space. To recover power output, the unit had to go offline with the plant losing revenue and the electrical system losing capacity. The state's independent system operators (IS's) approved the outage for only a given number of days, so as not to cause power disruption to consumers."ÒFrom turning gear to turning gear, the first unit's outage could last just 30 days, which was extremely tight,"continues Kaslow. "The second unit's outage was longer but still extremely aggressive since we were just one repair on the critical path. There were a number of other contractors, so it was important that we complete our work on time so we wouldn't delay anyone else and adversely affect our customer."On the second outage, space was at a premium since a number of contractors had to share the limited amount of workspace. For both outages, buildings with 100 percent usable interior space were required, since the rotors, lathes, and tooling would run almost the full length of each building. A further difficulty: the buildings would need adjustable foundations to compensate for a sloped ground surface, engineered for water drainage. Finally, the right buildings had to meet California structural codes for both seismic and wind loads, as the site was subject to both high winds and potential seismic activity.

TurboCare looked at a number of temporary structures, but they were unsatisfactory dimensionally and didn't have removable roofs. They required a crane operator to lower an entire structure over each rotor, lathe, and associated tooling, which was risky and impractical. Through industry sources, TurboCare turned to Kelly Klosure of Fremont, Nebraska, a leader in pre-engineered relocatable panel structures for over 30 years.

The manufacturer sent a representative to facilitate erection of its pre-engineered metal buildings, each of which assembled "out of the box"in under a week with a crew of four, using a crane, scissor lift, forklift, and standard hand tools. Weathertight and built to California codes, erection of the 30x19x51"and 30x19x60"buildings was sped up by the structure, sheeting, and insulation pre-installed in modular panels, along with doors, ventilation, and skylights.

An adjustable baseplate foundation allowed fast, level installation on the utility's sloped laydown surface using the forklift and wooden or metal shims.

"Since speed was critical, Kelly modified the roofs so that 39"and 48"sections could be removed as a single piece,"says Kaslow. "What allowed us to remove a roof section, lower the turbine rotors inside, and replace the roof section in under two hours - protecting the rotors and our personnel from rain and the elements. With no column interference, 100 percent of interior space was usable to us, which was essential when working on the oversized rotors."The modular pre-fabricated buildings allow mixing and matching of panels to create multiple smaller buildings, or a single larger building if needed. The wall and roof panels may be unbolted, removed, or reconfigured to change length, width, and height dimensions at any time for facility flexibility to accommodate future temporary, or even permanent structural needs. The pre-engineered buildings are designed to last at least 30 years, and changing out a relatively small number of panels, in fact, would enable the structures to handle snow loads. The panel frames themselves offer sturdy, interior attachment holes for lights, piping, or mechanical-electrical installations.

With the pre-engineered buildings, TurboCare finished "ongshanking"ahead of schedule and on budget, boosting the electrical output of the two steam-turbine units by 44-megawatts.

"Kelly Klosure was the only product that met all our project requirements so we didn't have to compromise our planning and logistics,"Kaslow sums up. "and their pr'tective storage racks simplify material handling, including shipping the structures to 'ther locations and storage when necessary. It's a well-engineered product, a great value."Del Williams is a technical writer based in Torrance, California. ET