CASE STUDIES

Oglethorpe Power Corp. Trims Turbine Upgrade Schedule with the FARO Laser Tracker

Oglethorpe Power Corp. Trims Turbine Upgrade Schedule with the FARO Laser Tracker

Oglethorpe Power Corp. Trims Turbine Upgrade Schedule with the FARO Laser Tracker

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Rocky Mountain Hydro Electric Plant is a pumped storage facility located in the Appalachian mountains of Northwest Georgia, approximately 62 miles from Chattanooga, Tennessee. The facility is co-owned by Oglethorpe Power Corporation (75%) and Georgia Power (25%), with all operations and maintenance controlled by Oglethorpe. Oglethorpe is the nation’s largest power supply cooperative which provides electricity to 4.1 million Georgia citizens.

The Rocky Mountain pumped storage facility features an upper reservoir on a plateau at the top of Rock Mountain and a lower reservoir that wraps partially around the base of the mountains. Two auxiliary reservoirs located adjacent to the lower reservoir function as storage pools for make-up water during dry weather conditions. The generating facility houses three reversible pump/turbine motor/generator units that are connected to reversible Francis pump turbines rated at 353,000 horsepower at a flow of 17,200 cfs which operate at 225 rpm. Based on projected load forecast for the next 5-10 years, Oglethorpe Power determined the most cost effective way to increase system capacity was to upgrade the three units at Rocky Mountain. As a consequence of this turbine upgrade, the three-unit operating range would increase from the existing range of 320-486 MW to a new range of 390-1,050 MW.

During the planning and scheduling process of the Unit 3 turbine upgrade project at Rocky Mountain, Oglethorpe was tasked with finding ways to trim the schedule. A Senior Engineer had been looking at a system that was gaining notoriety in the metrology industry for attaining measurements with phenomenal precision using laser technology. This instrument — a laser tracker — can be used to perform critical measurements for “as found” and “as left” conditions pertaining to disassembly and reassembly of their pumped storage turbine and generator. During the demonstration of the FARO Laser Tracker, supervisors and engineers were amazed with the equipment and the possible uses.

Measurement of Wicket Gate Bore Alignment

Measurement of wicket gate bore alignment is typically a very time-consuming process. The commonly used methods typically require a significant amount of setup time at each wicket gate. In this project, engineers would need to check the alignment of the three bores. The traditional method for obtaining this measurement involved hanging piano wire through the center of the bores. A large plumb bob submerged in oil for damping movement would hang on the end of the wire. The bores would then be checked relative to the wire using an electric micrometer. If it was determined that line boring would be required, the entire setup would have to be removed for boring and then replaced afterwards for a quality control check. This process requires quite a few man hours to perform. Since they needed to measure a total of twenty gates with three bores each, management and engineering personnel at Rocky Mountain realized a great potential for time-savings if these measurements could be expedited. They planned to shorten this time by taking advantage of their newly acquired tool — the FARO Laser Tracker.

For this application, technicians needed to take measurements straight down from the top bore. This is not physically possible in the standard or vertical orientation of the Laser Tracker — these measurements would require mounting the Laser Tracker horizontally. Oglethorpe designed and constructed a custom adapter made out of aircraft aluminum, and worked with FARO technicians to modify and download different software to slow down the speed of the gyros. Eventually they were successful in attaining the correct balance of speed and inertia to use the Laser Tracker in the horizontal position. This saved numerous man hours during the initial line bore checks. After the line boring was complete, they again measured the bores to ensure they maintained the specified tolerances.

Reverse Engineering

Thrust Bearing Oil Cooler Tube Sheet

At Rocky Mountain, the turbine/generator thrust bearing lube oil cooling is provided by four tube-type heat exchangers, per unit, mounted in the thrust bearing housing. The heat exchangers consist of a bronze tube sheet, several stainless steel tube support sheets, and various loops of finned copper tubing. The copper tubing had developed leaks on several occasions. This failure allowed mixing of the cooling water and lube oil. Repair consisted of draining the thrust oil, entering the thrust bearing to determine which tube was leaking, and plugging the failed tube. As more tubes were plugged and cooling capacity decreased, the management staff at Rocky Mountain determined a spare cooler would be of great value. This would allow leaking coolers to be replaced while defective coolers were sent out for re-tubing. With the upgrade of Unit 3, all coolers would be removed and re-tubed. During this time it was determined that one of the coolers would be used to create engineering drawings for the manufacture of additional coolers.

Technicians were tasked with exploring the possibility of using the FARO Laser Tracker in the duplication of the thrust bearing cooler tube sheet and tube support sheets. Creation of a drawing for these parts using standard methods would be challenging due to the difficulty of precisely locating the 671 tube holes, o-ring grooves, and various raised surfaces of the sheets.

The task began by breaking the parts down into a collection of simple geometric shapes. This is the way objects are measured using the Laser Tracker. A number of points are created by measurements taken at locations along the object. The Laser Tracker software then uses the XYZ coordinates of these points to create a best-fit feature (circle, plane, etc). The parts were then broken down into a series of lines, planes, circles, and points for measurement. With the parts lying on a stable surface, each of these features were measured and saved.

Inlet Valve Servomotor Base Plate

Due to the favorable results of the first project, Oglethorpe technicians have had several other reverse engineering opportunities. One involved the base plate of their Unit 3 inlet valve servomotor. A larger servomotor was being installed due to the increased forces expected from higher water flow after the upgrade of this unit. The servomotor was being manufactured in Germany for delivery near the end of the project. The project manager asked for several dimensions that determine the location of the current base plate mounting holes relative to the centerline of the servomotor. His intention was to keep this relationship the same on the new base plate. This would allow the new servomotor base to bolt in with no modification. The current base plate was measured with the Laser Tracker while still in place by removing one mounting bolt, measuring the hole location, then replacing the bolt and moving to the next hole. A 3-D model was created and dimension drawings emailed to the manufacturer in Germany for location of the mounting holes. When the new servomotor arrived it fit perfectly, greatly simplifying installation.

Summary

Richard Weekley, Craft Supervisor for Oglethorpe sums it up best: “We continue to improve our processes in each of the applications with the FARO Laser Tracker. The time to complete tasks has and continues to decrease as we learn new methods and tricks. We have faced some challenges due to involvement with a relatively new and not yet widely used technology for this industry, but persistence does pay in the form of precise measurements and the satisfaction of learning and working with a very interesting technology.”

Tim Watson, Level II Predictive Maintenance Specialist at Oglethorpe adds, “We have found the FARO Laser Tracker to be a very powerful tool. It has many applications as long as you have an imagination and think outside the box.”

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