Excelsior ever upward with capacity
A project similar to the upgrade of Labadie Plant was recently completed at KeySpan Corp.'s four-unit, 1,520-MW Northport Power Station on Long Island. Northport has 1960s- and 1970s-vintage steam turbines from General Electric. In the fall of 2004, it was the 375-MW Unit 3's turn for rejuvenation. The main reliability problem of Unit 3's turbine was internal seal leakage that was reducing the unit's peak output and increasing its heat rate.
KeySpan typically tests its units regularly and overhauls them about every seven years. Northport Unit 3 hadn't gotten a facelift since 1997. Before its scheduled turbine overhaul in 2004, Unit 3 hadn't been performing up to snuff. For example, tests indicated that the unit was load-limited by its 15-MW main boiler feed pump throughout 2003 and early 2004. Main steam temperature also was limited—by increasing hot reheat temperatures and by the steady falloff in first-stage pressure since the unit's last overhaul. Due to these issues—and many others—Unit 3's corrected load had dropped 20 to 30 MW below expectations. All signs pointed to increased HP/IP leakage of an unacceptable level.
When the 2004 overhaul began, operators noted that the two horizontal joint studs on the right side of the HP inner shell were completely relaxed when the shell was removed. Both washers were split and removed without removing the nuts. A contact check of the inner shell half joint in the HP to IP mid-span area revealed a significant leakage path. GE confirmed and estimated the leakage at over 18,000 lb/hr, penalizing capacity by 470 kW and heat rate by 7.9 Btu/kWh.
Pre-overhaul estimates promised recovery of 4 MW and a heat rate reduction of 257 Btu/kWh by improving turbine stage efficiency and reducing HP to IP leakage from 11% to 4% of steam flow. The first step was to send the turbine shell out to have the mating joint machined down by 0.031 inch to bring the flatness into design specs.
The steam seal optimization package KeySpan chose came from TurboCare Inc. (Chicopee, Mass.). In it were standard labyrinth packaging rings, retractable packing rings, brush seals, and conventional blade and brush-tip seals—all sized for Unit 3's turbine. Seal rubbing is not uncommon in steam turbines, and it is particularly common with single-casing HP/IP designs that use a packing box to seal the flow between the HP and IP sections.
A relatively long bearing span often initiates a rub in the mid-span seal area. Conventional packing can open after the first few turbine starts, leaving blade-tip seals with as much as 0.025 to 0.040 inch of clearance for years after the first few restarts after an overhaul. The retractable packing included in the TurboCare package maintains a 0.015-in. clearance by using springs to force the packing ring segments away from the shaft during start-up. For the shaft seals between stages, brush seals were added to the body of the retractable packing, reducing clearance to nearly zero (Figure 8).
8. Sealed up tight. Installing a new brush seal (left) and brush-tip seal (right) significantly reduced steam seal leakage within the turbine of Northport Unit 3. Courtesy: TurboCare Inc.
Steam seal design is as much art as science. On Unit 3's turbine, the brush seals were analyzed over a wide range of operating conditions to determine the effect of wear on them. Good turbine design practice requires the HP rotor to withstand twice the expected destabilizing force from seals as the worst-case stability condition. Because the original seals did not meet this condition, the new seals incorporated three anti-swirl design features in the HP mid-span seal (N2 rings) and in stages 2 and 3 of the HP section. The new design now can withstand 2.3 times the worst-case destabilizing force. Overall, it was estimated that improved sealing alone was responsible for a 2.3-MW boost in capacity.
Northport Unit 3 was returned to service in March 2005 with its heat rate lower by 465 Btu/kWh (net)—almost twice the predicted 257 Btu/kWh gain. Similarly, capacity was up 14.1 MW (gross)—more than triple the 4-MW gain predicted before the overhaul. Bottom line for KeySpan: This was a very successful overhaul, indeed.
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