Voith Siemens refurbishes Bath County (VA) pumped storage station

Jan. 21, 2004 -- You know how it is when a simple chore turns into a monumental undertaking. You set out to sharpen the knives and wind up remodeling the kitchen.

That's not quite how it happened to Voith Siemens Hydro Power Generation of York, Pennsylvania, when they set out to refurbish the Bath County (Virginia) Pumped Storage Station. They knew it would be a huge job. Still, the scope of work is cause for daily amazement among those involved.

"No part of this job is small," said Voith Siemens president E. Mark Garner, P.E. "No part is mundane. Even the simplest task requires our greatest skill."

The concept of a pumped storage station isn't hard to understand. Demand for electricity is highest during the day and lowest during the night. Most power generating stations don't respond quickly to variations in demand, so they end up producing more power than is needed at night and operate at or near capacity during the day.

A pumped storage station helps flatten out these curves so that output more closely matches demand. It has three primary components: a lower water source, an upper water source, and a hydroelectric generating station situated between the two, near the bottom. The lower water source can be a stream. The upper source is almost always an impoundment or reservoir. At Bath County, both sources are impoundments.

During the night when there's excess electrical capacity from other power stations, electricity is supplied to the turbines in the generating station, turning them into giant pumps. They pull water from the lower source and pump it to the upper source. During the day when the other stations need some help in meeting demand, the water is released from the upper source. It flows back through the turbines to generate electricity. Overall efficiency of this process is good. It runs about 79 percent at Bath County; for every four kilowatts of output, five kilowatts of input is required.

When pumped storage stations were first conceived, nuclear power was growing. You may recall claims that nuclear-generated electricity would be "too cheap to meter." The efficiency of pumped storage units, although good, was almost inconsequential. The important thing was that they were available to take advantage of fluctuations in demand for power and flatten out the delivery curve.

Now, of course, power isn't cheap. Growth has brought a booming demand, and meeting that demand takes money. Nowadays, efficiency is anything but inconsequential.

At Bath County, the issue is magnified by the fact that the plant runs as an intermediate power station. In the early days of pumped storage, plants would run infrequently, usually only on the hottest and coldest days or when another station was taken off of the grid for maintenance or by equipment failure. Bath County's six turbines run in both modes - pump and generate - virtually every day.

"We run a lot more than we did when the plant opened 20 years ago, and more now even than we did six years ago," said station director Michael Wood. "Since the plant started, demand has changed and the generation mix has changed. Facilities such as ours are very dependent on the spread between on- and off-peak power."

Wood works for Dominion, which operates the plant. Ownership is split, with Dominion having 60 percent and Allegheny Energy Supply having 40 percent.

Construction of Bath County Pumped Storage Station began in March 1977. The plant began commercial operations in December 1985. By the late 1990s, it was due for service. Because the original contractor, Allis-Chalmers, was purchased by Voith Siemens, the company was uniquely qualified to do the work.

Part of the project was to replace wear items, such as bearings and seals. Part of it was to install new components that would improve the efficiency of the plant.

"The design tools available to engineers have improved dramatically since Bath County was built," said Voith project manager Bob Steele. "We're also able to more fully test new designs than we were 20 years ago. The net effect is that we will raise the power of the station by eight percent."

New designs are useless if manufacturing processes can't hold the tolerances they require.
Tolerances on bearings are a mere plus-or-minus five-ten thousandths of an inch. In the heart of the turbine, the 195,000-pound runner, tolerances as tight as plus-or-minus five thousandths of an inch must be maintained. By comparison, a sheet of paper is four thousandths of an inch thick. This five thousandths of an inch tolerance must be held on a part that measures 140 inches - nearly 12 feet - in diameter.

Manufacturing these huge, precision pieces is one challenge. Getting them to the site in the mountains of Virginia and then placing them within the plant is quite another.

Components are hauled on a 19-axle trailer, where both the front and rear axles were steerable. There is a semi tractor at the front pulling and another at the rear pushing. The trailer has hydraulic cylinders allowing it to be lifted nearly four feet to clear guardrails and other obstacles. A crane with a rated capacity of 760 tons plucks the parts from the trailer once they reach the power station.

"The hardware is just part of the solution," said Garner. "Logistics is another. Many of the structures used 20 years ago during construction have been removed or destroyed or are simply no longer available. Some states require us to travel at night. Some require that we travel during the day. We get just so far then we have to park and wait for either daylight or darkness. Then there's always road construction going on somewhere. It took us six months just to plot our course and arrange the necessary permits.

"It's about 270 miles from our plant to the Bath County site, and it takes us four days to cover it. The last 40 miles up the mountain takes one full day. To service all six turbines, we'll have to make 30 of these trips over five years."

The size of the power plant components is in scale with the rest of the infrastructure. The tunnels connecting the upper and lower reservoirs are 3,780 feet long, 28.5 feet in diameter, with concrete walls 18 to 24 inches thick. The dam on the lower reservoir is 135 feet high and 2,400 feet long. It holds back 9.1 billion gallons of water in an area covering 555 acres. The upper reservoir covers 265 acres and contains 11.6 billion gallons of water. Its dam is 460 feet high and 2,200 feet long.

Pressure on the 114-inch diameter valves at the bottom of the tunnels is roughly 80,000 pounds per square foot, the equivalent of supporting a fully loaded semi tractor-trailer rig on a floor tile. Flow during power generation is 2.4 million gallons per minute. Output is 510,000 kilowatts. Flow during pumping to fill the upper reservoir is 1.8 million gallons per minute. Pumping power is 480 MW. (All figures are maximum values for each of the six turbine pumps.)

Rotational speed of the pump turbines is 257.1 rpm in either pump or generation mode. The system can be switched from one mode to the other in a matter of minutes, and can go from a dead stop to full load in six minutes. Total rotating weight of each unit is nearly 1,000 tons.
The power house in which the pump turbines and other equipment is located is 500 feet long, 150 feet wide, and 200 feet high.

As if all this mechanical servicing weren't enough, the plant's operating system is also being upgraded. "We're doing a controls upgrade at the same time as the turbine rehab," said Wood.
"We're changing from analog to digital systems."

Remember that kitchen remodeling project? Well the reason the dishwasher, the microwave, the garbage disposal, the trash compactor, the refrigerator and the lights all work is because power companies are upgrading their plants to meet your needs. Compared to what they're doing, hanging cabinets is a cinch.

Sponsored by FLSmidth

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