Partial Rotor Rebuilds Save Time and Money
Creative partial rotor rebuild strategies are saving utilities time and money over conventional in-kind replacement. As current power generation units reach and exceed their expected design life, risk of severe structural damage leading to failure increases. But aggressive preventive maintenance regimens are producing cost savings and time efficient approaches for restoring availability.
“The reality is that in the absence of spare rotors, and low priority on preventative maintenance, OEM`s had to step up to the plate to offer creative alternatives,” said Greg Russell, ABB Power Generation Inc. (PGI) business development manager. “Partial rotor rebuild is just one of them.” Repair strategies include upgrading rotors that seemingly have reached the end of their useful life and redesigning critical components to make them stronger and more efficient than new.
Southern California Edison`s (SCE) Redondo generating station was experiencing problems with two HP/IP 1967 vintage 480 MW steam turbines. Control stage blade root and steeple fatigue cracking were attributed to the units` axial entry design and solid particle erosion (SPE) damage. To reduce stresses on the control stage blade roots, SCE was operating at an inefficient 63 percent initial arc admission. This configuration further overloaded the already vulnerable blade roots and rotor steeples. For fuel efficiency and even loading, the optimum mode of operation sought was 23 percent arc admission (see figure).
SCE evaluated several repair alternatives based on cost, reliability and outage schedule, and selected the joint-welded or integral blade ring design. This strategy is rooted in ABB`s traditional design for its own rotor control stages. It entails joint welding all blades on the 360-degree control stage ring wheel to form a durable, one-piece control stage ring. ABB performed a study to determine fatigue and creep stresses associated with operation at various valve point modes, demonstrating that with the integral blade ring design, stresses on the blades, shrouds and rotors would be within acceptable limits at all valve point configurations.
Next, the forging envelope was developed, defining critical steam path flow areas and critical dimensions of the assembled blade stages. Additional repair steps included custom production of one blade ring, treatment with SPE-resistant coating, and high-speed dynamic balance.
The work was completed during a seven week scheduled outage. SCE internal cost calculations indicate that using the welded impulse design and the associated operational flexibility results in annual savings of approximately $250,000.
Catastrophic creep damage to the control stage, down to the bore of a 325 MW IP/LP rotor at PECO`s Eddystone Generating Station, made it a weak candidate for weld repair (see photo). The untried alternative was to forge a new mid-section from a smaller forging. To save time and money, PECO chose this method, although it was more complex than weld build-up of the original part.
Reconstruction of the rotor`s mid-section and weld reattachment of the original shaft ends proceeded. All damaged material was removed along with any at-risk areas of the rotor that had operated at temperatures above 800 F.
With the smaller forging as the starting point, new dummy piston and wheel areas were weld built-up to size. ABB`s 12 percent chrome material was selected for its creep and high-temperature resistance. A low percentage chrome intermediate layer was also applied for added thermal resistance and to inhibit carbon migration.
The two salvaged ends underwent non-destructive testing, then were weld prepped. For reattachment, the three pieces were stacked vertically in the welding pit and centered with precision-machined welding joints. The rebuilt rotor center was completed within a four-month repair schedule.
Eddystone Generating Station