Page 24 - Winter 2009

GEOTHERMAL NOISE CONTROL
Marshall Long
Marshall Long Acoustics 13636 Riverside Drive Sherman Oaks, California 91423
“The mean-time-between- failures of several conventional mufflers that had been tried was quite short.”
In these times of growing need for would explain how the noise was gen-
new energy sources, geothermal has
shown great promise. Geothermal is
a green, relatively nonpolluting energy
source that can provide power on a
scale large enough to make a significant
contribution to our needs. One of the
challenges of geothermal development
is noise emission. This occurs after a
well encounters steam and before a
plant is constructed. It also arises from
the necessity of shutting down a power plant for periodic maintenance. While the power plant is down, the steam and noise is vented to the atmosphere.
Geothermal wells
Geothermal steam is accessed by drilling a well in areas of natural volcanic activity where the Earth’s magma is close enough to the surface to encounter ground water. These areas are scattered around the world in such places as Hawaii, Iceland, Italy, New Zealand, and the Geysers area of Sonoma County, California. In Sonoma, wells are drilled to a depth of up to two miles. Wells in northern California yield so-called dry steam, a relatively clean gas, which, after the removal of entrained rocks and debris, can be used to power steam tur- bines directly. Once enough wells have been drilled, the steam is collected and piped to a plant.
A geothermal well has been compared to inserting a straw into a closed teapot. The water deep within the Earth’s crust is heated by hot rock and the resulting super hot steam is forced out of the pipe (straw). When the pipe is open at the top the steam vents directly to the atmosphere. At the nar- rowest opening in the pipe, usually near the wellhead, the flow reaches a velocity of Mach one. Once the steam passes this point, if the pipe expands, its velocity can even increase for a short distance. If vented directly to the atmosphere it can produce sound pressure levels as high as 130 dBA at fifty feet. Noise from well venting has an impact on the neighbor- hood even in a sparcely inhabited region but it can be atten- uated in a straightforward manner.
In the 1970s, while working on Environmental Impact Reports in the Geysers area of California, we studied noise generating activities associated with geothermal develop- ment. Some noise is caused by equipment used for well drilling. Most comes from the high pressure steam emanat- ing from an active well. During several trips to the Geysers area, I met with many of the field engineers. These men were a rugged group, having gained their experience on oil drilling rigs. Their reaction to a young engineer with very little oil field experience was polite tolerance. They would explain the drilling process and what they had tried, and I
erated.
Their approach to a suggestion was
to build whatever it was, based on a ver- bal description or a crude sketch, to see if it would work. They usually found their own solutions to drilling problems.
Wells are drilled through a number of valves, as shown in Fig. 1. The middle valve has a flow diverter, which routes the steam out horizontally away from
the well. The upper valve is fitted with a rotating coupling, which allows the drill string to turn while sealing off the flow. Drilling continues after the initial steam flow is encountered until there is sufficient pressure and flow to sustain a pro- ducing well. The drilling engineers explained to me the steps that had been tried to quiet the wells. A muffler has to be able to tolerate rocks and debris. The mean-time-between-failures of several conventional mufflers that had been tried were quite short. During drilling, the steam is directed into a
Fig. 1. Steam well at the Geysers geothermal field near Calistoga. (Courtesy http://www.flickr.com/photos/kqedquest/1544212039/).
Geothermal Noise Control 23