Enel works to reduce noise at Fallon plant

The Stillwater Geothermal plant in Churchill County has been noisier than expected since Enel North America brought it online in April of 2009, and Enel spent recent months implementing a noise-reduction plan whose cost ran upwards of $500,000.

"Stillwater in particular was loud because it operates at a higher pressure ratio," says Bill Price, vice president of geothermal construction and engineering. "It was louder than expected."

Engineers expected a high level of noise from gases expanding in the plant's turbines, so they built in noise-reduction elements such as insulated walls. But noise traveled through the project's piping and was transmitted outdoors, much to the unhappiness of the plant's neighbors and Enel.

"It is not like it is in the middle of nowhere in the desert. It is right next to where people live," Price says. "It was very important for us to make it as quiet as possible."

Enel's special-use permit with Churchill County stipulates that noise from the plant cannot exceed 70 decibels at the fence line of Stillwater or nearby Salt Wells geothermal plant. Noise from the Stillwater Geothermal plant reached 100 decibels before completion of the noise-reduction work. (A noise of 100 decibels is about the same as a gas lawnmower heard from three feet away.)

Enel hired Tom Norris of Consultants in Engineering Acoustics in San Francisco as a noise consultant, and noise from the plant now is in the low 80-decibel range at the source and appears to meet the standard at the property line after a series of specially made silencers were installed on each of Stillwater's four turbines.

Price describes the silencers 20 feet long by 42 inches in diameter as similar to the muffler on a car. Engineers had to find a way to reduce noise without impacting the operating efficiency of the turbine. CEA's Norris designed a series of "resonator strips" inside the pipes that cancel noise as superheated gas travels across them.

Enel fabricated a quarter-scale model that was tested and refined in a wind tunnel before installing a full-sized prototype model in May. Production of the silencers required more than 35,000 individual welds, or 140,000 welds total.

Colorado TBC of Fallon performed the welding. Butch Clark, project manager of Colorado TBC, says the biggest challenge during fabrication was time and cramped quarters. Welders worked inside the 42-inch piping for several weeks.

Clark kept the same three welders on each of the four silencers, and a team of six to seven men installed them once finished.

Brad Goetsch, Churchill County manager, says noise complaints have ceased.

"Enel initiated a substantial and costly process of identifying noise sources and custom design of turbine/silencers to replace and modify expensive components," Goetsch says. "The outcome is a decrease in noise to well below all permit requirements. Residents in the area have reported a marked decrease in noise. I stood in the middle of the plant and was able to carry on a conversation."