Noise attenuation can be a complex engineering challenge. The marketplace is responding to stricter regulatory and market-oriented specifications.
Noise attenuation at onsite power generation facilities is a complex subject. Several experts in this area of environmental protection indicate that only recently have power facility designers and contractors begun to use available tools and site-specific engineering to attenuate noise with the necessary precision to meet local requirements with certainty. The prevailing sentiment, based on these experts’ experience, is that older practices have been used as a one-size-fits-all approach even in the face of ever-stricter noise attenuation requirements and increasingly tighter site constraints—and the practices often do not produce the desired result if and when enforcement activity occurs. (Elden Ray, an acoustic/noise-control expert with Universal Silencer, Stoughton, WI, provides several facts about industrial noise that serve as a starting point for understanding the issue.)
The term “sound level” is understood to mean an A-weighted sound level, i.e., that which is generally used to assess community response to noise. So, the term dB(A) refers to the number of A-weighted decibels emitted by a facility. No US federal standards or laws with regard to limiting noise from power generation facilities and industrial sites exist, except in regard to energy transportation facilities—i.e., gas compressor stations. For these, the Federal Energy Regulatory Commission (FERC) mandates sound levels not to exceed a day-night level (DNL) of 55 decibels (dB) at the nearest residential receptor; this is equivalent to producing 48.6 dB on a 24-hour basis. Noise regulations vary greatly across states, regions, and communities. In some states or regions, the regulatory or permitting board imposes some noise limit at the property line or into the nearby community after study and having hearings.
A power plant has numerous main sources of noise. The two types of plants most frequently planned today are those using combustion turbines or diesel engines as prime drivers for turning generators; often, waste heat is used to generate steam for auxiliary services. Each prime driver has three principal sources of noise: inlet, casing, and exhaust. Common sources of noise in power generation facilities include:
- Combustion turbine (CT) inlets, which emit significant blade passing tones and harmonics
- Heat recovery steam generators, when connected to CTs
- Diesel engines, which have firing rates that produce strong, low-frequency tones (multiple units can create a beating phenomenon when two tones come in and out of phase with each other)
- Superchargers on diesels, which create tones similar to that of a CT inlet
- Steam generators, regulators, bypass and control valves, and piping
- Generators spinning at 50 or 60 Hertz, which may produce unpleasant, low-frequency noise for the community
- Air-cooled condensers
- Cooling towers, which produce fan and gearbox noises that can travel large distances
- Fuel forwarding or pumping stations that are remote from the main facility
- Fuel gas pressure regulating, metering, and valve stations
- Main step-up transformers, which produce significant tones at harmonics of the line frequency
- Condensate pumps, condenser units, and associated piping
- Openings in enclosures and barrier walls for piping and electrical penetrations that are not sealed
- Piping and pipe hangers that are not acoustically isolated from structures
- Blow-off and venting processes
- Remote water-pumping stations
The amount of required noise mitigation depends on the distance to the nearest property line or noise-sensitive receptor. Noise-sensitive receptors are generally residential areas, schools, hospitals, and parks, but also of concern are any nearby industrial facilities operating precision machinery that could be affected by low-frequency sound energy (infrasound).
Predicting or modeling noise from equipment requires an understanding of the sound field, i.e., how the sound will propagate from the equipment or the sources of noise. The near-field region is probably the most difficult to understand. It is the region where noise propagation is neither well developed nor can be accurately measured, because of the spatial variation of sound levels. Complex structures and equipment arrangements also add to the variability.
The far field starts where the sound field is more stable and propagation is fairly uniform. This location depends on frequency (wavelength) and is usually two to four major source dimensions away from the noise source. The free field describes where the sound level theoretically decreases by about 6 dB for every doubling of distance. For a “line source” such as a long pipe or duct, the theoretical lateral decrease is 3 dB per doubling of distance.
The reverberant field occurs where freely propagating sound waves are reflected back from a wall, a ceiling, or other surfaces, causing variations in sound levels. The sound field becomes complicated when the near and reverberant fields overlap, i.e., there is no free field. This is frequently the case inside industrial facilities and buildings.
The principal method for predicting sound levels is based on the following physical parameters: the power of the sound source (LW), the path (A) over which the sound travels to a receptor resulting in a sound pressure level (Lp) in decibels, which is heard and measured by a sound level meter at that compliance location. The mathematical calculation is Lp = LW – A. The total noise at a receptor location is the cumulative addition of all the noise sources as adjusted by “A” to account for the unique sound path geometry between each source of noise and each receptor location. This equation is applied to each of the nine octave bands as each frequency has unique values for both the source and path of sound. To fully model a complex site, each source of noise is modeled on a master three-dimensional coordinate grid and geometrically defined as to its shape, size, and sound power level.
Noise attenuation involves affecting one of three elements: the source of noise, path of noise, and received noise. Controlling the path of noise by the use of acoustic enclosures, barrier walls, duct silencers, and other similar noise-control treatments is the most widely used and, generally, the more economical approach. Reducing the noise at the source of noise can be expensive, because most equipment manufacturers assemble their product using commodity parts that are economically produced for the industrial market and a complete redesign would be necessary.
Frequently, more silencing is needed than the bare minimum in order to account for noise from other equipment or sources. A balance of plant or total noise analysis should be performed to account for all possible sources of noise.
James Longacre, operations and engineering vice president for Generator Service Corp., Bristol, WI—a wholly owned subsidiary of the Edward D. Newell Company that provides design consulting and operation of power generation and electrical systems—points out that increased scrutiny of industrial noise emissions are part of an overall regulatory environmental focus. “It’s emissions across the board,” he says. “You have sound emissions, air emissions, water emissions, heat emissions—all of those aspects are starting to fall under the environmental profile. What little bit of land left over that is appropriate for different things is typically going to butt right up against encroachment areas of residential to industrial.”
Another reason for increased enforcement of noise ordinances is the demand for increased output from onsite power generation facilities. “The reliability requirements of what customers want today compared with what they required 20 years ago are two very, very different things,” says Longacre. “We didn’t have Tier 1, 2, 3, and 4 compliances; we didn’t have n+1, n+2, and n+3 requirements. Even for standby applications, you’re talking about extremely high reliability, and, generally speaking, you’re seeing the entire industry transition to more of a technical approach than you ever saw before in the past. Other drivers are things like quality, the products themselves, and how they’re manufactured, as well as building codes and regulations.
“Natural disasters here in the United States—things like earthquakes and hurricanes—are mandating much higher-quality equipment,” he continues. “Typically, noise suppression was a mechanical function, and many, many HVAC people went out and bought inexpensive systems and incorporated them into their systems using baffles and dampers, et cetera; that isn’t happening anymore.”
Noise-attenuation solutions that address demands for tighter compliance and technical mitigation approaches include enclosures, silencers, and computational fluid dynamics (CFD) services that model noise emissions from a facility.
Containing the Entire Source
Effective use of an enclosure, which is typically constructed of steel or aluminum, to surround an engine or genset is subject to more variables that can affect its noise-attenuation performance, argues Mike Witkowski, vice president of sales for Pritchard Brown, in Baltimore, MD, a designer and manufacturer of enclosures, primarily aluminum.
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Photo: RMF Engineering Inc.
A recent installation at the Kennedy Krieger Institute in Baltimore proved challenging. The only space available was a narrow alleyway with an opening only 50 feet from an adjacent commercial zone, and the city's ordinance calls for a 61 dB (A) level at a property line to a commercial zone. |
Obviously, the party responsible for noise attenuation should know the local noise ordinances. Another important question to ask is who and how close the neighbors are, as well as the temperature range at the location. A location near the ocean that is subject to extremes of snow or wind will emit different noise levels from one in a desert environment.
Additionally, the manufacturer should know the layout of surrounding buildings and the general topography. A large nearby structure, a grass-covered berm around the site, or an asphalt parking lot leading to the neighbor’s yard can greatly influence the propagation of sound and hence the design.
It is also critical to determine the source mechanical and fan noise under full load—information that is routinely available from equipment manufacturers. Dimensions, noise, and airflow requirements can vary greatly from manufacturer to manufacturer for a given electrical kilowatt rating, so sizing the enclosure based on worst-case specifications is often a good idea if more than one genset supplier is being considered. After these inputs are determined, the noise attenuator can determine if noise attenuation is necessary and, if so, how much. This determination often dictates the enclosure size, air-handling choices, and even the construction materials.
Witkowski cautions that the noise attenuator should remember that the decibel, the fundamental unit of sound pressure measurement, is a logarithmic ratio. This means that as the required amount of attenuation increases, the relative size, weight, air-handling complexity, and cost tend to increase exponentially. With this in mind, he recommends ascertaining the true site noise requirements from the start.
For example, most communities have ordinances regarding maximum permissible sound levels at the property line, but it is sometimes unclear how a standby genset, that runs one hour a week for “exercise” or during the occasional utility power outage, is defined as a noise source.
Conversely, prime power and cogeneration applications have a greater noise impact on the environment because of their extended periods of operation. A phone call to the appropriate authorities can often provide insight and affect the budget accordingly.
When no specific level is to be met at a given distance, it is common to specify only the amount of attenuation required by the enclosure itself, Witkowski points out. Many manufacturers, therefore, standardize on certain levels of attenuation at some predetermined distance, e.g., 10 dB(A) reduction at 10 feet. These numbers usually represent an average of the reduction measured at various points around the enclosure, he stresses. He recommends verifying that the design is such that there are no points at which the actual attenuation is more than 3–5 dB(A) less than the promised average.
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Photo: RMF Engineering Inc.
Overcoming the challenge required construction of a basic aluminum enclosure within the small genset footprint and a secondary enclosure on top that contains air-handling devices and noise-attenuating baffles. The air intake at the top, also referred to as a "scoop," formed an awning. |
Two recent projects illustrate some of the difficulties presented by site-specific examples, and how enclosures and equipment need custom engineering in order to compensate. In one, William R. Thornton, PhD, P.E., a nationally recognized expert in the field of acoustics, consulted on a 1.8-MW diesel genset installation on an urban site for a major telecommunications company in the Northeast. A local ordinance limited the noise emission to 45 dB(A), which necessitated a noise attenuation of more than 50 dB(A) for the genset, which was 50 feet long, 15 feet wide and 40 feet high.
“They [the owners] were confronted with the fact that they had neighbors surrounding them, particularly some very well-heeled ones living in apartment buildings that were very, very critical about what they would see and hear,” says Thornton. “The contractor asked me to write a specification, which I did, and the spec was so airtight, that when Pritchard Brown got it along with a few other bidders, they were being pushed to the state of the art.”
The owner approached Thornton and asked him to help get the genset noise level to within the specification. “They said, ‘OK, Bill, you wrote the specification-now you can let them pay you to teach them how to meet the specification,’” he says. “So, it was kind of a crazy situation. I got out there, and one of the first things Pritchard Brown found out was that the silencers they had been sold for the air inlets and discharge were choking the machine so badly that the machine couldn’t perform reliably. So we tried to do some fairly clever and innovative things and pushed the envelope to find several ways of working within the envelope of the existing enclosure.”
Thornton says he faced a common dilemma: maintaining a necessary level of air flow without allowing the noise emission to exceed the local ordinance. “The science of acoustics says keep the passages as small as possible and the smaller you make them, the more attenuation you’ll get,” he notes. “But the science of air flow says that the smaller you make the passages, the more pressure drop you’ll get, and you’re going to choke the air flow.”
Thornton and Pritchard Brown involved the Quietflo Noise Control Division of Flaregas Corp., of Nanuet, NY, which specializes in fluid-borne noise control associated with blowers, compressors, engines, fans, gas turbines, and similar equipment. “One of the problems was that the passages weren’t really large enough,” he recalls.
Part of the solution, he says, was to build a transition piece between a passage and silencer out of noise-absorptive fiberglass. The cooling fan inside the enclosure presented another challenge.
“One of the other things we were faced with was that the fan itself was putting out blade passing tone,” says Thornton. “I showed Pritchard Brown how to modify the fan scroll to actually diminish and virtually eliminate the blade passing tone. We had to do some very basic work to smooth out the transition.
“The air intake became a problem, too,” he continues. “The engine was producing noise levels on the order of 110 db(A), which is extraordinarily loud. We had this passage that originally had been built to accept these silencers that did not work. We had to design a silencer that would actually attenuate the noise in the limited passage length and cross-sectional area. What we ended up doing was going in and literally turning the enclosure; what we were trying to do was absorb as much of the noise inside as we could.
“Normally, you call that a reverberant field where you’re in a hard echoing room like a racquetball court where the sound ricochets around like ping pong balls,” adds Thornton. “What we tried to do was actually modify the interior of the enclosure to the extent to where the acoustical ping pong balls hit a surface, they get absorbed, and they’re not made available for additional ricochet. We added a very thick treatment of very heavy-density fiberglass board. That works in two ways. We added a thick treatment to actually increase the transmission loss and the noise-reduction properties of the wall. The other thing it does, at the same time, is provide an enormous amount of low-frequency absorption for the interior of the engine enclosure, which means that the acoustical ping-pong balls that are bouncing around get gobbled up by the fiberglass, and they’re no longer being made available to the reverberant field, which means that now we don’t have the energy escaping out that opening.”
The second Pritchard Brown project was enclosing an installation of a 600-KW diesel genset by RMF Engineering Inc., Baltimore, at the Kennedy Krieger Institute in Baltimore last September. The genset, located in a small alcove next to a parking garage, was installed mainly to provide backup power to elevators for evacuation—a major concern as many patients in the children’s outpatient medical facility are wheelchair-bound, making stairwell egress unfeasible for many.
The institute is located about 250 feet directly south of a residential area, but only 50 feet from a commercial zone to the south. The Baltimore City Health Code Title 9, Noise Regulation, mandates that through Article 9-206 the maximum permissible sound level would be 61 dB(A) at any point on the property line of a commercial zone. If a residential zone exists, the maximum sound level at any point on the boundary between the residential and commercial zone could not exceed 58 dB(A). RMF decided to adhere to the 61 dB(A) requirement after determining that the distance from the residential area to the north was sufficient to minimize the impact of noise on it.
Still, the building design dictated that the only place to locate the genset was in a 19-foot-long by 10-foot-wide section of an alleyway.
“All of the other sides of the building are either right on the street, on the sidewalk, or there’s a therapy garden on the north side of the building, then there’s a big entrance loop on the north side beside the garden,” says Wesson Miller, P.E., LEED AP, an associate with RMF. “There was really no other place to put the generator.”
Witkowski reports that, when he explained how large the enclosure would have to be in order to attenuate noise to the specified level, RMF realized there was a problem. The solution was building a basic enclosure within the small genset footprint, then building a secondary enclosure on top that contains air-handling devices and noise-attenuating baffles.
Targeting System Components
Silencers, which surround individual system components such as air-handling equipment, use a more targeted approach to noise abatement. This equipment could be described as a larger version of a car muffler or an oversized air duct. They may incorporate interior channels of perforated metal that use noise-absorptive material such as fiberglass. In addition to the principle of absorption, these units use the principle of sound-wave reflectivity, i.e., causing long sound waves to bounce around within the body of the enclosure, breaking up the sound waves into shorter ones. Shapes vary according to the design of the system, into which they are installed, and larger sizes provide greater sound attenuation.
As demonstrated in the telecommunications facility example, though, silencers can restrict air flow, creating a trade-off between output and noise emission. More powerful air-handling equipment may need to be installed and more space may need to be allowed to fit silencers; a retrofit such as in the telecommunications facility can present challenges.
Universal Silencer designs and manufactures noise-control equipment for gas-fired turbines and diesel engines. The company reports that it has worked on applications from 100 kW to several hundred megawatts and has custom-designed systems installed on all major turbine brands.
Another silencer manufacturer, GTE Industries, offers 200, 300, 400, and 500 Series standard silencers, catalytic silencers, and diesel particulate filter (DPF) silencers—the latter two designed to reduce air emissions as well. The standard silencers differ by design and noise attenuation level.
Units in the 200 Series are cylindrical and provide attenuation of 10–17 dB(A) to 45–52 dB(A). The 300 Series is described as residential grade (10–17 dB(A) attenuation) and also shaped cylindrically but shorter than the 200 Series. The 400 Series has a disk or “pancake” design, a low profile, and provides attenuation of 27–31 dB(A) to 34–40 dB(A).
Additionally, the low-profile silencer the 500 Series is shaped much like a car muffler and provides attenuation of 31–38 dB(A) to 45–52 dB(A). The manufacturer notes that most of its projects call for a standard silencer but require about 20–30% “engineered-to-order” configurations.
Notable engineered-to-order projects in which GTE Industries has been involved include the Union Park Combined Sewer Overflow Detention/Treatment Facility in Boston, MA, which had gas turbine engines retrofitted with catalytic silencers to meet the city’s noise-attenuation specification; and the Seattle-Tacoma International Airport, in Washington, where the company customized a 500 Series silencer to fit a configuration in which the exhaust run was at a 30-degree angle to the engine.
Sophisticated Modeling
A method of simulating noise emission for the purpose of engineering a power generation system to meet noise specifications is the use of CFD. Noise-emission simulation can provide cost savings as it facilitates optimal designs prior to the construction of any physical structures. Savings are also possible when CFD analysis is used following system installation, when compared with making physical alterations and retesting the system.
Longacre is a proponent of CFD analysis for noise emission and attenuation modeling and has used it for several common noise challenges.
“Number one is probably misapplied sound attenuation, trying to achieve a given level of reduction and, at the same time, losing flow, because the products you selected were of inferior quality and/or design,” he says. “Amazingly, CFD analysis mimics the environment. Wind, heat, temperature, ambient temperatures, air flow, the position of the building, the air flow to and through the equipment, the air demand requirements, the precision pressures, and/or barometric drops, positive pressure building versus negative pressure building—they’re all interrelated and impact sound attenuation.”
Longacre says that simulation tools such as CFD analysis will soon become the industry standard. “I think our customers are getting smarter, and they’re starting to realize that pre-evaluation is the better process, because it doesn’t blow your budget,” he states. “You’re getting much more power density than ever in a much smaller facility, and they’re only going to get bigger and bigger as far as the heating, engine, and cooling tower requirements.
“I would generally say that, as we move into more advanced technology and more requirements associated with these facilities, that pre-examination at the 20% to 30% design level is going to become an essential element,” he adds. “Right now, it’s considered a ‘nice-to-have,’ not a ‘have-to-have.’ The discerning client that wants to do it right the first time will utilize the CFD process.”
Financial reality is ultimately the factor pushing the industry in the direction of using tools such as CFD analysis; a facility shutdown due to non-compliance can be catastrophic, indicates Longacre.
“There’s another factor: operational reliability,” he says. “More and more, customers today are insuring their production, meaning that their systems are designed such as to guarantee that they’re going to produce X, Y, and Z, and their income is actually insured. You could be talking about downtime being measured in millions of dollars a minute. Examples would be pharmaceutical companies, IT companies, data centers—there’s not a major player in the Internet world that doesn’t insure its productivity.”
Market forces will eventually lead to consolidation in noise-attenuation equipment and services, predicts Longacre. “The owner of the future is going to let things like CFD modeling and harmonics modeling drive his design, because, if he meets those functions, he is then able to meet the entire project delivery,” he argues. “Owners are going to get smarter and smarter, and facilities management teams are going to get better and better. Ten years from now, CFD analysis will be used on a regular basis for modifications for all structural designs on all critical facilities; it’ll be standard, part of the package.
“It’s also going to eliminate in the end the marginal manufacturers,” continues Longacre. “It’ll force a quality level on the industry that has never been seen before. It’s going to get down to a few high-quality manufacturers. The smart [enclosure] company is the one that will comply with that and make his products better and lighter, and still meet the demands of what’s required out of those products. What before was a low-level-demand product is now an extremely high-demand product.”
A company that specializes in simulation services is Engineered Aeroacoustics Inc. (EAI), which combines the science of acoustics and aerodynamics into “aeroacoustics.” Its use of two- and three- dimensional modeling technologies provides quantitative, visual, and animated predictions of system performance such as inflow pressure distribution, flow patterns dynamic and static stress points, laminar or turbulent flows, flow dilution, and flow velocities. In addition to modeling, the company recommends corrective system design alterations.