During the recent overhaul of its 663-MW Shasta plant, the U.S. Department of the Interior's Bureau of Reclamation needed a method to quickly and efficiently replace the stationary seal rings. Personnel developed a device that would allow the seal rings to be in the turbine pit with the headcover and boring bar installed. The seal rings then were installed and machined while the contractor worked on the stator rewind overhead. This device was used on the final three of the five units overhauled at this plant and reduced total outage time by several hundred man-hours.
Developing the device
Reclamation owns 83 hydroelectric plants. For the most part, Reclamation uses its own staff to assemble and disassemble the 131 turbine-generator units in its plants. The use of Reclamation staff has resulted in some remarkable innovations. In particular, one such innovation at the Shasta plant, on the Upper Sacramento River in California, significantly reduced the outage time during a recent unit overhaul.
The five units at Shasta feature vertical shaft generators. Original plant maintenance practices relied almost exclusively on the plant main gantry crane to remove the generator and turbine components during disassembly and reassembly. Traditionally, these units were disassembled from the top down, and parts were staged throughout the plant.
In 2007, Reclamation staff finished the complete replacement of the second Shasta turbine assembly. Alstom, the contractor Reclamation selected for the turbine replacement, determined that the turbine assembly efficiency could be improved by replacing the turbine, wicket gates, and operating linkages. The contract was awarded for all five main units at Shasta. For two of the five units at Shasta, this overhaul also included a full stator rewind. Normal practice was to completely tear down the unit, from the pilot exciter down to the draft tube. Then the lower guide bearing bracket was reinstalled, and a work platform was erected above it for the generator rewind. Once the rewind was completed, the work platform and lower guide bearing bracket would be removed.
For those units that had already been rewound by Alstom, the practice would be to remove the headcover, invert it, then install the upper seal ring on the power plant deck. The lower seal ring would be installed separately in the discharge ring. The headcover and boring bar would be installed and the two rings turned to the final dimension.
The Shasta turbine replacement involved installation of new stationary seal rings. This was needed because the original seal rings, designed in a segmented style, had a history of failures and seizures. Seal rings, also called wear rings, reduce the water that either bypasses the turbine blades or applies force to the top of the turbine, thus increasing the thrust bearing load. Alstom redesigned the seal rings for the Shasta units into a continuous style. This new design would require that the seal rings be installed with an interference fit to the headcover and discharge ring. Figure 1 shows the design and location of typical seal rings, much like the new Shasta seal rings.
Once plant personnel removed the work platform for the rewind, the new seal rings normally would have to be lowered into the turbine pit and installed. Once installed, the seal rings would need to be machined, which requires the installation of a boring bar that would rely on the headcover for its upper support.
In preparation for the turbine replacement for Unit 4, performed in 2002, a plant mechanic named John Boughton had the idea that the seal rings could be in the turbine pit with the headcover and boring bar installed. In this situation, the two seal rings could be installed and machined in one set without having to either turn the headcover upside down or remove the headcover. The idea turned out to be very beneficial when Alstom personnel worked on the stator for Units 1 and 2. Boughton sketched out his idea. Owing to his other duties, Boughton was not able to fully develop the design.
Another plant mechanic, John Martin, moved ahead with the concept and designed the device. The device needed to be designed so that it could be assembled under the headcover and would support the seal rings in preparation for installation. The device had to fit around the boring bar, which was centered in order to machine the seal ring to the final dimension. The device resembled a twin-arm bearing bracket, with two long elements roughly equal to the diameter of the smallest seal ring. The device featured six bolted segments. Two of these segments separated the long elements, and four more (perpendicular to the two long elements) extended the line of the two separating elements to form a symmetrical cross. All the elements were made of 2-by-6-inch channel and featured bolted joints.
This design contained a 3-inch gap for dry ice to shrink the diameter of the seal ring. With this design, the seal rings were supported by six bolts that would be threaded through a gusset welded to the end of a 13-inch-long channel that was 2 inches wide by 4 inches deep. This arrangement also would support an internal micrometer to measure the change in diameter of the seal ring.
During fabrication of the device in 2002, Martin altered the design, assisted by John Hays, to replace the 13-inch-long channel with a flat plate and threaded block. This plate would be used to support the seal ring. A bolt through the block was reduced in size from the original design and now used only to center the ring. Figure 2 shows the final design of the device inside a cross-section of a discharge ring and the lower seal ring.
The device Martin designed could accommodate two different upper and lower seal ring diameters and ensured the seal rings could be centered and measured for the proper fit.
One particular benefit is that the smaller-diameter seal ring could be installed with little modification of the device. The design relies on the symmetry of the bracket to rotate the support plate end for end, move the plate to the opposite arm, and reverse the jack bolt. With that change, the other-diameter seal ring could be installed. Unfortunately, owing the different radius, another set of dry ice trays had to be fabricated.
The entire assembly could be removed through the scroll case access hatches once both seal rings were installed.
Using the new device
This device was used on all Shasta units, including the final two unit rewinds and turbine replacements. Reclamation personnel estimate that the device reduced the outage time by several hundred man-hours compared with installing the new seal rings after the rewind was completed. This equates to about $40,000 in savings due to reduced labor costs alone.
This device can be adapted to any unit that is undergoing seal ring replacement during a rewind. The device is particularly suited for power plants with larger turbine diameters and similarly-designed units.
One final note: The device was improved on several times during its evolution. John Martin took the lead in the evolution of the device, but the ideas and suggestions provided by others that resulted in further improvements were not recorded. The entire Shasta maintenance and engineering staff undoubtedly contributed and should be recognized. Finally, credit should be given to the management personnel in the Shasta office, who continue to encourage this sort of drive and innovation.
– By Martin A. Bauer, manager, Compliance Audit and Incident Evaluation Program, Reclamation, and former chief of operations and maintenance for Reclamation's Eastern Colorado Area Office
The authors thank Dave Poore, retired Chief, Mechanical and General Maintenance Branch, who provided the original insight into the use of the device, and Joe Ascoli, current Chief, Mechanical and General Maintenance Branch, who provided a detailed explanation and photos of the use of the device.