Hydro Life Extension Overhauls Historic Dams

Pam Fleming reaches for her two-way radio and pushes the talk button.

“Attention plant personnel,” she calls out to the team working at Tennessee Valley Authority’s Fort Loudoun Dam near Knoxville, Tennessee. “Please be aware that the thrust bridge lift is imminent. Please clear the area.”

Fleming repeats the message twice.

The day’s extensive inspections and briefings have already been completed.

After years of preparation, it’s go time.

Time to hoist an 80-year-old, 140-ton piece of machinery from deep inside the dam out into the light of day.

When you’re dismantling a hydroelectric generating unit for repairs, there are a lot of lifts. Some parts that need lifting are relatively small, just 500 pounds or so. But a few, like the thrust bridge assembly, are impressively huge.

This is classified as a high hazard lift. Special protocols are required.

A 50-foot gantry crane will raise the massive component barely an inch off its foundation. Then it will hold the load steady for five minutes to stabilize it.

From there, the lift will proceed almost imperceptibly slowly.

There’s very little margin for error – only about a foot of clearance to pull the enormous structure out the bore hole in the top of the dam.

Seen from inside Fort Loudoun Dam, the Unit 2 thrust bridge assembly inches its way toward the light of day.

‘Crane’s Gonna Creak’

Machinist foreman Richard Cook and construction manager Heath Jones have just wrapped up their safety briefing.

“Pay attention to what you’re doing,” Cook told the crew of TVA machinists. “Be here now.”

Jones reiterated that the team needed to stay alert and monitor the lift every inch of the way.

“The crane’s gonna creak,” he said.

That’s normal due to the weight being lifted.

But Jones’ orders were clear: Listen, he told the crew.

“Anytime you hear something that doesn’t sound right?"

Speak up.

Machinist foreman Richard Cook watches the thrust bridge being lowered onto a platform.

Another 40 Years

Thanks to all the vigilance and preparation, the lift goes off without a hitch.

For Fleming, it’s just another day at the office.

And just one step among thousands she supervises as a construction manager for TVA’s Hydro Life Extension program, which overhauls TVA’s iconic dams to provide renewable power for decades into the future.

“It's a bit like rebuilding a car from the ground up,” David Rowland, who oversees the program, said.

A very large, very complicated car.

The units get new turbines, new generators and new electrical power trains. Other components, such as bearings and shafts, are remanufactured at TVA’s Power Service Shops in Muscle Shoals, Alabama.

“The equipment is nearing the end of its operational life, so it has to be rebuilt,” Rowland said. “We either replace or remanufacture every single component. Our goal is to make these units last for another 40 years of reliable operation.”

The gantry crane at Fort Loudoun Dam displays the weight in tons of the thrust bridge assembly plus the lift beam.

Money Well Spent

TVA’s hydroelectric fleet includes 113 generating units. Most are housed inside 29 conventional dams spread around the Valley region. Four are at Raccoon Mountain Pumped Storage Plant near Chattanooga, Tennessee.

TVA optimizes the dams’ performance – like that of all its electricity-generating assets – with rigorous maintenance.

Life extension projects are scheduled about every 40 years. To rotate through TVA’s entire hydro fleet, the life extension program aims to overhaul roughly three units per year.

When a unit comes due, it has to be taken offline. Hundreds of thousands of gallons of water must be drained from the tunnels.

“You could actually climb into the water passageways if you wanted,” Rowland said.

The overhaul outages last about a year. And before that can even happen, there’s a lengthy phase of planning and ordering parts. All told, the overhaul process for each unit stretches about five years.

Each project costs $40 million to $50 million.

For TVA, the investment is well worth it.

“These are forever assets for TVA,” Rowland said. “As long as the dams are there, it will always be economically profitable to capture that water, pass it through a turbine and make good, clean, green energy.”

Behind David Rowland, head of the Hydro Life Extension program, this man door is used to seal off water passageways at Cherokee Dam.

Detail-Oriented

To rebuild a generating unit, you must first take it apart.

It’s a mindbogglingly complex operation.

The unit must be methodically disassembled so it can be put back together exactly as it was.

Fleming’s role includes tracking a whole lot of moving parts.

“Every part is matchmarked – any piece, if there’s more than one,” she said.

For instance, the rotor – the major moving part of the generator – is a 25-foot-diameter structure that looks a bit like a carnival ride. The outside rim is covered with oblong panels called rotor poles, which generate voltage and current.

There are 68 of them.

Each must be numbered – first with paint, and then by etching the metal.

That way, after months of work to rebuild the rotor, the panels can be reassembled in the right order.

At Fort Loudoun Dam, numbers are painted and etched into rotor poles awaiting remanufacturing.

‘A Thing of Beauty’

About 100 miles upstream at Cherokee Dam on the Holston River, construction manager Frank Barber’s team recently completed a life extension outage on Unit 4.

Barber has an analogy for the job he and Fleming do: It’s like conducting an orchestra, or a band.

Only rather than trumpets, clarinets and percussion, this ensemble features machinists, electricians, carpenters, pipefitters, welders, crane operators and special remediation teams.

“It’s a real symphony to get it all to work together, and it takes all the people here to make it happen,” Barber said. “When all this gets assembled? To me, it's a thing of beauty.”

Barber used to play the tuba. He aspired to be a high school band director. Although life took him down a different path with a career at TVA, he’s never lost his connection with music.

“When you have a group of people playing in unison, it makes life very good,” Barber said. “You get everybody together on the same page and then we end up playing in harmony.”

Construction manager Frank Barber, right, with Rowland, says his job is like conducting a band or orchestra. 

‘Jazz Ride’

Despite all the planning that goes into the dam overhauls, the unexpected often crops up.

This requires some improvisation, or as the music-loving Barber calls it, “Going on a jazz ride.”

A case in point involved the governor cabinet at Cherokee Unit 4.

The original plan was to replace some terminal strips, the plastic connecting strips for the cables and wiring to all the instruments and components that run the unit.

“You’ve got hundreds of these, and each one has its own place,” Barber said. “Each one has to be checked. If there's one out of place, then something's not going to work, or you might not get the unit on at all. It's very tedious and demanding work.”

Once field work was under way, Barber’s team spotted an opportunity to make additional improvements.

“We ended up rewiring the governor cabinet,” he said.

The work to upgrade wiring in 80-year-old governor cases at Cherokee Dam is tedious and demanding, Barber says.

The Power of Air Bubbles

Fleming’s team at Fort Loudoun recently went on a jazz ride, too.

It’s normal to see wear and tear in the throat ring, the band of metal encircling the walls where the water swirls around the blades of the turbine. Known as cavitation damage, it’s essentially pockmarks from decades of air bubbles in the water bumping against the steel.

Ordinarily the repair crew machines the throat ring to buff out any damage, then adds a layer of stainless steel to ward off future corrosion.

But with Fort Loudoun Unit 1, the cavitation damage was worse than usual. The metal was cracked and resistant to repair.

Welders were able to repair the cracks, but it took a tremendous amount of work.

Fleming, a bit of a TVA history buff, took it in stride.

“This unit was built for the people of the United States back in the 1940s,” she said. “What was going on during that time? They were busy building planes, ships, tanks, everything. That had to have priority because we were in war. That’s why we think maybe the metal wasn't as good. It was just the supply that we had.”

The Hydro Life Extension program replaced this 80-year-old turbine with this new one last year at Fort Loudoun Dam Unit 1. 

Greater Efficiency

There’s nothing cookie-cutter about hydro life extension projects, Rowland said. The scope of work must be customized to each unit.

“These are not easy projects. They’re very difficult,” Rowland said. “We always find new challenges with each project in the field.

“You would think, ‘Well, gosh, here you've got this dam. And this dam. And they're almost the same size. But each one is very unique.”

The beauty of the life extension program is that its dedicated staffers amass more and more expertise with each new project, which makes the process as efficient and streamlined as possible, Rowland said.

The overhauls also give TVA the chance to upgrade each unit on a case-by-case basis.

For instance, some units get new aerating turbines that can help oxygenate water downstream. Others are able to return to their full capacity after having operated at a reduced level while awaiting repairs.

And all units improve their efficiency by about 3% to 4%, Rowland said.

“We're able to produce more energy with the same amount of water,” he said.

In this diagram of a hydro generating unit, No. 1 marks the rotor, No. 3 indicates the thrust bridge and No. 5 designates the turbine.

Precise and Robust

Hydro Life Extension team members share a sense of awe about these majestic dams, built so long ago and still running strong.

“These units were designed with a slide rule. A lot of people don’t even know what one is,” Fleming said. “There were no computers.”

The precision was – and remains – remarkable.

Even without modern tools like laser trackers, the units were built with clearances of just a few thousandths of an inch.

“You can’t even fit a credit card in there,” project manager Jake Bussey said.

“The cool part about hydro is these units have been around a long time and they're very robust,” Bussey said. “They just continue to keep spinning. They’re workhorses.”

Photo Gallery

This heavy man door at Cherokee Dam ordinarily seals off the scroll case, which guides water flowing into the turbine.

With the thrust bridge assembly removed for an overhaul, there’s an open hole at Fort Loudoun Unit 2.

Fort Loudoun Dam, built between 1940 and 1943, is the uppermost of nine TVA dams on the Tennessee River.

PHOTO AT TOP OF PAGE: From inside Fort Loudoun Dam, construction manager Pam Fleming shows the hole from which the Unit 2 thrust bridge will be lifted.