Cavitation damage to four of the six units in the 810-MW Lower Granite powerhouse was so severe, exposed concrete was visible in some areas below the discharge ring. Repair work involved machining the discharge rings and repairing the blade cavitation damage.
By Mallory Davis and Kenneth D. Hostler
In four of the six turbine-generator units in the powerhouse of the 810-MW Lower Granite Lock and Dam project, the discharge ring suffered such severe cavitation that exposed concrete was visible in some areas below the ring. The runner blades suffered severe cavitation as well, with significant damage to the blade tips. The U.S. Army Corps of Engineers, which owns and operates the project, needed a method to fix this problem and bring the units back to their original condition.
Machining work was performed on the discharge rings and turbine runners, with satisfactory results. The remaining two runners in the powerhouse are scheduled to undergo similar work in the future.
Background on the project
Construction of the Lower Granite facility, on the Snake River in Washington state, began in 1965. The main structure – which consists of the dam, navigation lock, powerhouse, fish ladder and associated facilities – and three generators were completed in 1975, with the additional three generators finished in 1979. The concrete gravity dam with an earthfill right abutment embankment is about 3,200 feet long with an effective height of 100 feet. The lake created by the dam extends upstream about 40 miles.
The project provides hydroelectric generation, navigation, recreation and incidental irrigation. The Corps says power generated during fiscal year 2011 was 31,700 GWh.
The facility has been undergoing a great deal of work over the past few years. In July, work was performed to replace the intake gantry crane on the top deck of the dam, including assembling the new 130-ton-capacity crane, then disassembling and removing the existing 100-ton crane. And in June the Corps awarded a contract to supply two central non-overflow vertical turbine pumps and two navigation lock vertical turbine pumpes for the project. The Corps also is planning repairs to a trash shear boom that directs floating trash and debris away from the powerhouse turbine intakes.
|New stainless steel blade strips were welded to the edges of the runner blades during the final step in the cavitation repair to four of the six turbines in the 810-MW Lower Granite powerhouse.|
Scope of the work
With regard to the machining work discussed in this article, the Walla Walla District issued a request for bids in March 2011 for the in-place repair of four of the six turbine-generator units in the Lower Granite powerhouse.
Requirements of the work included:
– Machining the 312-inch-diameter discharge ring for each unit;
– Installing anchors in the discharge ring to control distortion;
– Overlaying the discharge ring with stainless steel welding wire;
– Filling in the area between the discharge ring and the original concrete with epoxy;
– Machining the overlay to fit the original equipment manufacturer tolerances; and
– Repairing the cavitation on the six runner blades.
Corps policy is to award a contract to the company whose total evaluated price combined with technical experience provides the best value to the government. These factors include: experience, past performance and the best management approach.
There was considerable interest in this contract. After all proposals where submitted and evaluated, Hydro Consulting & Maintenance Services Inc. (HCMS) of Rotonda West, Fla., was awarded the contract, which is valued at $4.62 million.
Investigating the problem
Once onsite, HCMS personnel evaluated the condition of the four units to be repaired at Lower Granite Dam. After this review, personnel determined that not only were the discharge rings cavitated to the exposed concrete, the turbine runner blade tips also were in need of repair.
When HCMS personnel air arced the cavitation damage on the runner blade edges, the blade tip pieces that were originally installed started cracking and coming apart from the blades. An initial review of the OEM drawings indicated a potential area of concern. Apparently, a 0.5-inch stainless steel strip was welded, using a partial penetration weld, on the low- and high-pressure sides of the blades when the turbine runners were manufactured. This area was then machined to the proper diameter during the initial manufacture of these units.
Further investigation of the OEM drawings indicated how these stainless steel strips were installed. At that point, Corps personnel decided these strips needed to be replaced to ensure each turbine unit maintained the original hydraulic profile.
After a meeting between the Corps and HCMS, it was decided the blade tips needed to be addressed before any weld repair was made because of the amount of stress put on these areas. HCMS personnel found that the cavitation damage to the runner blade tips could not be repaired without removing the stainless steel strips.
It is important to note that there was added difficulty in repairing the runner blade tips with the turbine runner in place because the clearance between the discharge ring and runner blade itself is less than 0.25 inch on these units. So, a decision was made to remove the existing strips and replace them with new pre-made stainless steel strips.
Performing the work
Once a procedure was prepared and approved by the Corps, HCMS personnel removed the stainless steel strips from the blades by air arcing the existing welds. Personnel then ground the runner area to receive the new stainless steel strips. Templates were made to create new stainless steel strips. The templates were traced from the blade tip and then transferred to a computer-generated drawing.
HCMS next had the stainless steel strips machined and sent to the site for installation. The strips were tacked into place and held there by mechanical devices to ensure little movement during the welding process. The welding itself was very labor-intensive. However, HCMS personnel were able to complete all the repairs and stay within the tight tolerances as per the original drawings in less than 10 days.
Once the stainless steel blade strips were welded to the each of the runner blades, the discharge ring was machined and the anchors were installed. To complete the process, HCMS personnel “clad welded” more than 20,000 pounds of weld material on the discharge ring, allowing for a final clearance of 0.188 inch between the runner blade and discharge ring.
Since this repair was complete, the four units have operated satisfactorily.
HCMS personnel believe these same repairs will be needed on the remaining two units at Lower Granite as well. Current plans indicate HCMS will start work on the next unit in October 2012 and the last of the six units in early 2013. HCMS personnel also plan to use the same procedures as described in this article to bring the tolerances back to the OEM specifications for these two remaining units.
Mallory Davis is manager of machining and cavitation repair and Ken Hostler is executive vice president of Hydro Consulting & Maintenance Services.