Calpine Corp.’s South Point Energy Center in Mohave County, Ariz. (Figure 1) is a 520-MW natural gas-fired, combined-cycle facility with two Siemens Westinghouse 501FD gas turbines and one steam turbine-generator with a BB33 high-pressure (HP) section, a BB65 intermediate-pressure section (IP), and a 65CC intermediate/low-pressure (LP) section. The plant entered commercial service in June 2001.
A recent crash of South Point was a plant engineer’s worst nightmare. One of the final-row blades in the steam turbine’s LP section failed, causing extensive damage to the turbine and generator. The huge vibrations produced by the liberated blade then shook loose some of the lube oil drain and supply lines, and the released lube oil ignited when it came into contact with turbine casings, at around 1,000F. The failure also caused condenser tubes to puncture, dousing the casings with water as well. The vibration caused by the quenching increased the friction between all mating components of the turbine and generator, welding them together. All turbine and generator bearings and the generator’s rotating and stationary components appeared to be extensively damaged (Figure 2).
In the merchant power business, time is money. Repairing the steam turbine and getting South Point back on-line became a top priority. So Calpine brought in a Houston-based team of six field-service specialists from Wood Group Field Services Inc. (WGFS) and put them under the direction of its Turbine Maintenance Group with instructions to work around the clock to repair the turbine.
A close inspection of the turbine confirmed that the damage was extensive. The WGFS engineers and technicians began by removing the generator rotor and the rotors of the HP, IP, and LP turbines. Next, they removed all internal components of both the turbine and the generator and sent them out for repair by third parties. Rather than wait for the parts to be returned, the WGFS personnel reinstalled diaphragms; repaired crush pads, centerline keys, and ledge keys; and replaced all turbine and generator bearings that had been destroyed beyond repair. In addition to this work, WGFS did some on-site repair and machining to restore selected parts to spec, eliminating the need to send them off-site for repair and helping to minimize costs and downtime.
Coordinating traffic in the large number of repaired and replacement parts was a team effort. The parts repaired on-site included seals, the turbine’s throttle, intercept, and reheat stop valves, and the unit’s turning gear. The turbine casings were inspected for warpage and subsequently correctly set following the installation of new sole plates. All internal alignments were done at night, because high daytime temperatures can cause components to expand, causing misleading readings. Final adjustments were made the following day. At the end of the outage, and following the completion of all field work, WGFS personnel performed a successful final balance of the steam turbine.
Taking advantage of the forced outage, the South Point staff and WGFS completed other maintenance work on both of the plant’s gas turbines as well as many of its auxiliary systems—including the cooling tower’s fans, which needed to be aligned.