In these times of growing need fornew energy sources, geothermal hasshown great promise. Geothermal is

a green, relatively nonpolluting energysource that can provide power on ascale large enough to make a significantcontribution to our needs. One of thechallenges of geothermal developmentis noise emission. This occurs after awell encounters steam and before aplant is constructed. It also arises fromthe necessity of shutting down a power plant for periodicmaintenance. While the power plant is down, the steam andnoise is vented to the atmosphere.

Geothermal wellsGeothermal steam is accessed by drilling a well in areas

of natural volcanic activity where the Earth’s magma is closeenough to the surface to encounter ground water. These areasare scattered around the world in such places as Hawaii,Iceland, Italy, New Zealand, and the Geysers area of SonomaCounty, California. In Sonoma, wells are drilled to a depth ofup to two miles. Wells in northern California yield so-calleddry steam, a relatively clean gas, which, after the removal ofentrained rocks and debris, can be used to power steam tur-bines directly. Once enough wells have been drilled, thesteam is collected and piped to a plant.

A geothermal well has been compared to inserting astraw into a closed teapot. The water deep within the Earth’scrust is heated by hot rock and the resulting super hot steamis forced out of the pipe (straw). When the pipe is open at thetop the steam vents directly to the atmosphere. At the nar-rowest opening in the pipe, usually near the wellhead, theflow reaches a velocity of Mach one. Once the steam passesthis point, if the pipe expands, its velocity can even increasefor a short distance. If vented directly to the atmosphere itcan produce sound pressure levels as high as 130 dBA at fiftyfeet. 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 ImpactReports in the Geysers area of California, we studied noisegenerating activities associated with geothermal develop-ment. Some noise is caused by equipment used for welldrilling. Most comes from the high pressure steam emanat-ing from an active well. During several trips to the Geysersarea, I met with many of the field engineers. These menwere a rugged group, having gained their experience on oildrilling rigs. Their reaction to a young engineer with verylittle oil field experience was polite tolerance. They wouldexplain the drilling process and what they had tried, and I

would explain how the noise was gen-erated.

Their approach to a suggestion wasto build whatever it was, based on a ver-bal description or a crude sketch, to seeif it would work. They usually foundtheir own solutions to drilling problems.

Wells are drilled through a numberof valves, as shown in Fig. 1. The middlevalve has a flow diverter, which routesthe 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 encountereduntil there is sufficient pressure and flow to sustain a pro-ducing well. The drilling engineers explained to me the stepsthat had been tried to quiet the wells. A muffler has to be ableto tolerate rocks and debris. The mean-time-between-failuresof several conventional mufflers that had been tried werequite short. During drilling, the steam is directed into a

GEOTHERMAL NOISE CONTROLMarshall Long

Marshall Long Acoustics13636 Riverside Drive

Sherman Oaks, California 91423

“The mean-time-between-

failures of several

conventional mufflers

that had been tried was quite

short.”

Fig. 1. Steam well at the Geysers geothermal field near Calistoga. (Courtesyhttp://www.flickr.com/photos/kqedquest/1544212039/).

Geothermal Noise Control 23

cyclonic separator sometimes called a blooie muffler, asshown in Fig. 2. This is a large, ten-foot diameter, verticalcylinder with the inlet off axis to induce cyclonic motion.Rocks are flung to the outside and collected in pockets wherethey can later be removed. Water is often introduced to coolthe steam and reduce its volume, which lowers the flow noise.The steam exits the silencer vertically through a smaller,three-foot diameter pipe. Under strong flow conditions(170,000 lbs/hour) blooie silencers still generate loud noiselevels (108 dBA at 50 feet without water injection and 103dBA with water injection) and require large quantities ofwater, typically 150 gallons/minute. Thus a blooie muffler ismore effective as a rock separator than as a noise silencer.

The realities of well drilling and handling the steamintroduce a number of interesting engineering problems forwhich clever solutions have been developed by the drillingcrew. A good example of the in-field ingenuity was the solu-tion to the valve replacement problem. Like oil wells, geot-hermal steam wells are drilled using long sections of pipescrewed together into a drill string. The drilled hole is casedwith a steel pipe about twelve inches in diameter surroundedwith concrete that has been fitted with one or more large gatevalves at the top. The valves are operated by hand with a

wheel about the size of a car’s steeringwheel, and are used to cap the well ordivert the flow when steam is encoun-tered. Due to the rocks and debris in thesteam flow there is wear on the valvesand they have to be periodicallyreplaced. The practice is to use two ormore valves, mounted on top of oneanother. After the drilling is finished thetop valve can close off the well whennecessary, while the bottom valve isused only when the top valve needs tobe replaced. However, periodically, thebottom valve must also be changed.

The technique for bottom valveremoval was to have a worker approachthe valve, while the well was venting, andremove two bolts in the valve flange, oneon either side of the pipe. He would thenthread a large diameter steel cable upthrough the bolt holes and secure a cableclamp near the end of the cable. Tensionwas then taken up on the two cablesusing a D8 Caterpillar tractor. The volun-teer would then gingerly remove theremaining flange bolts and retire to a safedistance while tension was being main-tained on the valve via the tractor. Thetractor would then drive toward the wellto release the tension on the cables andallow the valve to dance around until itfell to the ground. To install the new valvethe process was repeated in reverse, usinga bucket loader to hold the new valve.

Rock mufflersIn free jets, noise is produced by the high velocity fluid

mixing with the quiescent atmosphere. This is similar to air-craft exhaust noise, where the sound power follows the eighthpower of the velocity. The cause of the noise is the turbulencecreated by the mixing, which occurs five to eight pipe diame-ters downstream of the pipe opening. Thus the key element incontrolling the noise is to reduce the flow velocity.

Clearly, conventional mufflers would not work becausethe exit pipe diameter is almost the same as the inlet pipediameter, so the flow would not slow down. The exit area hadto be much larger than the entrance area. A silencer wouldalso have to tolerate high velocity rocks and debris. I suggest-ed blowing the steam into the bottom of a large bed of rocksin a pit dug into the ground or placed in a large steel or con-crete enclosure. The open top of the pit would be the exitarea, which could be made quite large. The pipe below therocks extends across the enclosure to allow debris to impactagainst a steel plate at its end, which could be removed forcleaning. A perforated pipe could be used to distribute thesteam evenly over the bottom of the pit. This was the firstrock muffler.

I subsequently learned that a trial muffler was built on

Fig. 2. Drilling a steam well at the Geysers (Courtesy Calpine Corporation).

24 Acoustics Today, October 2009

the back of a flat bed truck. In the first prototype, the wellpipe was introduced horizontally into the base of a large steelbox. The truck was backed up to a well and the steam pipewas welded to the entrance pipe of the box. The pipe at thatpoint had a diameter of about 30 inches. Unfortunately no

one had determined the depth of rock required toslow the flow down and keep the rocks in the box.The first trial turned out to be more of a rocklauncher than a rock muffler since quite a few rockswere blown out of the top of the box. Later, addi-tional depth was added until the weight of therocks counterbalanced the pressure of the flow.

Eventually rock mufflers were installed at eachplant so the entire stream could be vented during shutdown. I measured 72 dBA while standing next to oneduring a full plant vent test, almost a 60 dB reductionfrom raw well venting. This has become the industrystandard methodology for controlling geothermalpower plant noise throughout the world (Fig. 3). Later,Dick Stern and I worked under a Department ofEnergy program to quantify geothermal noise duringdrilling and production.1AT

References1 M. Long and R. Stern, “Evaluation of Noise Associated with

Geothermal Development Activities,” Department of Energy,Department of Environment, under Contract DEAC-01 79-EV06200 (1982).

Fig. 3. Rock mufflers in El Salvador (Courtesy LaGeo).

Marshall Long received a BSEdegree from Princeton Universityin 1965, attended the Universityof Grenoble in France and theUniversity of Madrid in Spain in1966. He received M.S. and Ph.D.degrees in engineering fromUCLA in 1971. While still a grad-uate student, he founded his ownacoustical consulting firm, now inits 38th year. Marshall Long

Acoustics specializes in architectural acoustics, audiovisualdesign, noise and vibration control, and other technical areasrelated to acoustics. He enjoys sailing, judo, soccer, reading,and writing, and is living with his family in Sherman Oaks,California. He is a Fellow of the Acoustical Society ofAmerica.

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Geothermal Noise Control 25