Personnel at the 380-MW Safe Harbor plant discovered a gap between a wicket gate stem and bearing surface in one unit. To temporarily repair the unit, personnel inserted pieces of steel pipe in the gap and welded them in place. The repair functioned successfully during high spring flows. Safe Harbor Water Power Corporation is now working to determine whether to rehabilitate the unit or install a new runner.
Discovering the problem
Unit 5 at Safe Harbor – on the Susquehanna River in southern Pennsylvania – has been operating since 1934. In 2006, personnel at Safe Harbor conducted a routine annual inspection of this unit. During this inspection, they discovered a sizeable gap between one of the wicket gate stems and the brass bushing. On a unit where the normal clearance is 0.005 inch, the gap was more than 0.5 inch!
The arrangement on this unit consists of a wicket gate stem made of mild steel with a stainless steel sleeve shrunk fit to the stem. This sleeve was designed to rotate with the stem inside the bushing. A more detailed inspection of this wicket gate stem revealed that the stainless steel sleeve had come loose from the stem and was rotating freely. Because of this finding, plant personnel determined that the wicket gate stem had worn away significantly, causing the gap. This problem was not seen during the annual inspection in 2005, so it is unknown how rapidly the wear was occurring.
Plant personnel discovered a gap (at left) between a wicket gate stem and brass bushing on Unit 5 at the 380-MW Safe Harbor plant. Repairing the unit by inserting pieces of steel pipe into the gap and welding them in place (at right) allows the unit to operate successfully and gives Safe Harbor Water Power Corporation time to plan for rehabilitation of the unit.
This gap was a serious concern for several reasons. First, if the wicket gate stem continued to wear at a high rate, it would eventually break and could go through the unit, causing significant damage. Second, taking this unit apart would cost about $1 million in labor and lost generation. Third, there was neither a plan in place nor the money budgeted to totally rehabilitate the unit. And fourth, another unit at the plant was being rehabilitated, so there was not sufficient laydown area to take Unit 5 apart.
Based on these considerations, plant personnel decided first to reduce operating time for the unit by making it the last unit on and the first unit off. This was a temporary solution to give personnel time to determine the best way to repair the unit. A repair was needed to allow the unit to continue operating and to give Safe Harbor Water Power time to plan a more extensive outage that would involve either rehabilitating Unit 5 or installing a new runner. The goal was to develop a repair that would allow the unit to operate for at least another two years.
Developing a solution
In 2007, engineering and maintenance personnel at Safe Harbor met to discuss possible solutions to the problem with Unit 5. Personnel knew the repair would involve placing some sort of material in the gap. The best solution would be to choose a material that could be welded and that would hold up to significant wear and tear. Personnel considered and dismissed car body filler because it cannot be welded and would not hold up under these operating conditions.
The solution chosen was to obtain a pipe with a diameter similar to the size of the wicket gate stem and with sufficient wall thickness to become the interface between the worn stem and the stainless steel sleeve. Personnel chose steel pipe because it can be welded and is strong enough to last for the required two years.
Plant personnel used a jack to move the existing gate stem until it was roughly centered in the gap. They then measured the gap again and obtained a piece of steel pipe 8 inches long (the length of the stem). Personnel machined the inside diameter of the pipe to be slightly smaller than the gap. The next step was to cut the pipe longitudinally into 12 pieces, each about 2 inches wide.
Personnel then inserted these pieces of pipe into the gap and drove them in place with a hammer. There was not room to get all the pieces into the gap, so the gap was not completely filled. Personnel then welded the pieces of pipe to both the wicket gate stem and the stainless steel sleeve. Because personnel only had access to the top of the stem, all welding had to take place in this location. Thus, this was not a perfect repair. However, it only needed to last about two years.
After the repair was complete, in the fall of 2007, personnel rotated the unit several times while visually checking the repair. The pipe kept the wicket gate stem centered so that it was able to rotate without binding.
Once the unit was repaired, plant personnel agreed to put the unit back into normal operation. They planned to take the unit out of service in early spring after the high runoff flows had decreased and determine if the repair had held up.
Results to date
Unit 5 was operated normally during the 2008 high spring flows. In early May, personnel took the unit off line to inspect the repair. Everything held together well and the unit continues to operate.
Safe Harbor Water Power is now working to determine whether to rehabilitate Unit 5 or to install a new runner. This plant was built in the 1930s and contains 14 units. All of the units have some sort of problem. Rehabilitation work has been completed on one unit, and another is currently undergoing rehabilitation. The success of this repair to Unit 5 has allowed the company to prioritize repairs required for several units.
– By Richard A. Johnson, P.E., manager of engineering, Safe Harbor Water Power Corporation