Maintaining a 31-year-old power plant and fish lift is no easy feat. All older projects begin to show their wear-and-tear, whether it is rusted valves or leaking grates, and they need to be fixed. Some-times the fix is easy and can be handled in-house by a very skilled Charleston District employee, but sometimes the fix requires outside assistance. Recently, two separate underwater dive activities took place simultaneously at the St. Stephen Powerhouse to fix some issues.
One dive was in the plant intake canal where Tennessee Valley Authority divers isolated the cooling water intakes and replaced the unit cooling water supply piping, which will provide cool water to run generator operations for producing electricity. They also replaced the deteriorated intake grating, which prevents debris from entering the water supply to the generators.
The other dive was in the plant’s tailrace area where the Corps’ Nashville District dive team sealed off the units’ draft tube drains to the hydropower turbines. The turbines have to be free of water in order to conduct maintenance on them, but the valves and pumps to “unwater” the turbines are bro-ken. Sealing the drains allows the team to replace the broken parts since no water can come in and allow us to conduct unimpeded maintenance in the future.
While these were two separate projects, both projects required all three hydro units to be taken out of service, so to save time and money the District called on both Nashville District and TVA to work on the projects simultaneously.
Divers face several different challenges while working underwater, aside from trying to use a hammer and a wrench. They have to work wearing a constricting wet suit and a heavy helmet that is tethered to a boat, which can be difficult to move in, much less perform work in. Also, depending on the depth they are going to, divers are constrained by the amount of time they can only stay under water. A diver working at 50-feet can only stay under water for 92 minutes and once he surfaces, he has to stay out of the water for 11 hours and 52 minutes so that his nitrogen levels can return to normal. Typically several divers work on one project so they can rotate out to complete the project on-time and on-budget. Finally, a diver never knows what conditions the water may be until he is at his appropriate depth. The water at the powerhouse appears to be murky and muddy, however the divers were pleasantly surprised at how clear the water was, making their working conditions much easier.
Thanks to TVA and Nashville District for helping to keep the Charleston District’s power plant running efficiently.