Daniel P. Duffy 2015-04-20 15:44:14
Aside from always being useful and often necessary, gensets, turbines, and engines have one other thing in common: They can be very noisy. A listener standing adjacent to such a power source can experience discomfort, or even hearing loss, if exposed to their high noise levels. To counteract and minimize the noise generated by these machines, they can come equipped with silencers for their engines, and be housed in sound attenuating enclosures. This may seem like a secondary consideration compared to their direct function of generating electrical power, but the need to reduce their noise levels is just as important to their proper functioning. Acoustics 101 So what exactly is sound? The dictionary defines it as: “Vibrations that travel through the air, or another medium, and can be heard when they reach a person’s or animal’s ear.” Or more scientifically, sound is “mechanical radiant energy that is transmitted by longitudinal pressure waves in a material medium (such as air) and is the objective cause of hearing.” So, sound takes the physical form of pressure waves moving in a medium (air, water, or even vibration through a solid object). Noise on the other hand, as perceived by the human ear, is unwanted sound. It depends on the circumstances and the listener. Noise is “a sound, especially one that is loud or unpleasant, or that causes disturbance.” This is why regulations exist to minimize noise and eliminate noise pollution. So, to answer the age old question: if a tree falls in a forest and no one is around to hear it, it still makes a sound. It just doesn’t make a noise due to the lack of a listener. (The more metaphysical aspects of this question are best left to Zen philosophers.) There are various means of measuring sound. The energy of a sound wave is measured using “A-weighted” sound levels. The A-weighted sound level closely matches the perception of loudness by the human ear. This can vary from sea level pressure to mountain pressure, depending on where one is. It is graphically represented by the amplitude (height) of the wave. Decibels (dBA) are measured on a logarithmic scale, which means that a small change in the number of decibels results in a huge change in the amount of noise and the potential damage to a person’s hearing. Therefore, 115 dBA (the sound level of a large generator) is equivalent to 10,000,000 micro-Pascals, but 50 dBA (the sound level in a typical urban dwelling) is about 50 dBA, or only 8,000 micro-Pascals. Further examples of sound levels are provided in Table 1. Since the decibel scale is logarithmic, as a simple rule of thumb, the addition of 3 dBA means a measured doubling of the acoustical energy. However, an addition of 10 dBA represents a perceived doubling of the sound pressure. The pitch of a sound wave is measured by frequency, which is defined as cycles per second. Since the speed of sound (1,126 feet per second in air at sea level—designated as Mach 1) is constant, frequency equates to the inverse of the sound’s wavelength. For example, a sound wave with a wavelength of 2 feet would have a frequency of 563 feet (velocity/wavelength equals frequency; or 1,126 feet per second/2 feet = 563 per second). Frequency is measured in Hertz (Hz), with 1 Hz being equal to 1 cycle per second. The whole frequency range of human hearing has been divided up into octave bands and 1/3 octave bands. A frequency is said to have an octave width when the its upper band frequency is twice its lower band frequency. High frequency is defined as 2,000 Hz and higher. This frequency is considered to be annoying, if not painful, and can cause permanent hearing damage. This frequency has very short wavelengths (0.56 feet). Low frequency is rated at 31 Hz (perceived as a low rumble, almost a vibration), to 250 Hz. Sound waves in this frequency have very long wavelengths (36 feet to 4.5 feet). Table 2 provides a comparison between Hertz and wavelength. Sound patterns attenuate over distance due to divergence. This usually takes the form of spherical (or hemispherical, if a solid wall or floor is involved) radiation as the sound spreads out over a wide area. This divergence pattern can be made more complicated by environmental factors and the presence of buildings or natural obstructions. As a simple guide, sound measurements are usually made as Near Field (either 1.5 times the largest dimension of the equipment generating the noise, or about one wavelength from the source) and Far Field (over 300 feet from the source). Far Field is also the region where sound waves can be considered to be planar. This is not the same thing as a Free Field where there are no reflective surfaces that could complicate the sound wave patterns. Outside of the Free Field is the Acoustic Reverberant Field where complicated sound wave patterns are formed by bouncing off of reflective surfaces. Natural sound attenuation (sound reduction that is not caused by reflective surfaces or attenuation devices like silencers) over distance is measured by the result of the Inverse Square law. Like hemispherical light radiation, the pressure of a sound wave decreases with the square of its distance from its source. So, doubling the distance results in a four times reduction in sound pressure. This is equivalent to a 6-dBA reduction in sound energy. Goals and Methods of Acoustical Attenuation The primary acoustical goal of sound attenuation is the avoidance of a noise disturbance in residential areas and the minimization of potential hearing loss in work areas. Local noise ordinances define a noise disturbance in different ways, depending on location, hours of the day, zoning, permitting, and such. Unless authorized by a site specific permit, a regulatory noise disturbance typically means any sound level across a property line that: exceeds 60 dBA in residential areas, exceeds 55 dBA in a noise sensitive zone, exceeds 80 dBA in a commercial/light industrial district, or generally disturbs the peace and quiet of neighboring residential inhabitants from 50 feet away. Certain work place environments require systems that provide personnel protection. OSHA hearing protection standards are based on a worker’s time weighted average over an eight-hour work day (or a time weighted average of eight hours per day). The permissible exposure limit is 90 dBA for all workers. For every 5-dBA increase in noise levels, the amount of time that a worker can be exposed to this noise is cut in half. However, the National Institute for Occupational Safety and Health (NIOSH) has recommended that all worker exposures to noise should be controlled below a level equivalent to 85 dBA for eight hours, to minimize the potential for hearing loss due to occupational noise. NIOSH also recommended a 3-dBA exchange rate for sound exposure equivalency. Therefore, OSHA has ruled that all workers exposed to more than 85 dBA must be enrolled in a hearing conservation program. In addition to measuring noise levels and providing hearing protection, employers must reduce exposure to noise and achieve noise attenuation via engineering controls such as enclosing and isolating noise sources. Attenuation is a term used to describe the amount of sound reduction a silencer or enclosure will provide. The ability of each type of surface to achieve sound attenuation is measured by its attenuation coefficient. Specifically, it is a measure of the energy loss of sound propagation in media (air or water) and is also measured in decibels. There is no such thing as a perfect media for the propagation of sound waves; air and water have inherent viscosity that changes somewhat with elevation and temperature. This viscosity results in a natural attenuation of sound energy over distance. Enclosures and similar structures provide mechanical attenuation. Mechanical attenuation can be achieved over short distances by means of absorption and annulling. Acoustic absorption occurs when a material absorbs sound energy instead of reflecting it back off of its surface. This absorbed sound energy gets transformed into waste heat and vibration transmitted into the absorbing material. As there are no perfect sound reflectors or absorbers, all surfaces reflect back some of the impacting sound and absorb the remainder. The amount of sound energy absorbed is a function of both the incident angle (the angle at which the sound wave impacts the surface), the structure of the material’s surface and the physical nature of the material. Smooth surfaces will tend to reflect more while porous and pitted surfaces tend to trap sound waves. Physically speaking, soft, flexible materials absorb sound waves, while hard, dense materials reflect size and shape of the sound-absorbing structure. Both its large-scale configuration, and the micro structure of its absorbing pores, influence the sound wave if its size and shape interact with the sound’s wavelength. How well a structure absorbs sounds depends on it overall surface area and the absorption coefficient of the materials. Whether fixed or mobile, gensets, engines, and turbines tend to come with their own enclosures. Most are supplied by companies that specialize in the engineering, design, manufacture, and installation of enclosures that provide model specific (if not customized) sound attenuation. These capabilities of these enclosures range from simple weather protection to extensive sound reduction. Utilizing an innovative post and panel design, Girtz Industries supplies the Z-GUARD product line of stationary sound attenuated enclosures. Their post and panel design is structurally stronger than standard formed panel enclosures. This increased structural strength makes their enclosure applicable for exposed conditions, extreme weather, and seismic zones. The complete design includes unique elements such as a completely removable, fully welded roof to prevent water intrusion. Modular in concept, the design allows both the creation of large, side-by-side structures and stacked structures to minimize the site footprint. These enclosures are tailor-made for generators, pumps, chillers, compressors, switchgear, and boilers. Girtz has a wide variety of projects under its belt. One of its largest was a recent service to an end customer, a large hospital in Memphis, TN. This project included four double-stacked boiler modules, three chiller modules, three genset modules, and two switchgear/control modules. The project included the entire utility campus. MTU Onsite Energy produces customized power systems designed to work together as integrated systems. These system components include sound enclosures and weather housings, silencers, controls, tanks, and monitoring systems. The enclosures are designed for a wide variety of engine generator applications. Built with modular construction, they protect the engine from extreme weather, while protecting the public from extreme noise. The company’s basic Weather Proof Enclosure is an inexpensive means of protecting gensets form the elements. When matched with an MTU Energy unit of 125 kW, its sound rating is no more than 78 dBA with the use of intake and exhaust scoops. The Ultra Quiet Enclosure model is similar in design to the Weather Proof enclosure, but with more sound attenuation features (foam, large air chambers, fixed air intake louvers). This allows for an additional 4-dBA reduction in noise, compared to the Weather Proof model. Usually this design is combined with MTU Energy models rated 250 kW and above. The Crystal Quiet Enclosure is specifically designed to meet both severe noise and wind requirements. Able to stand against 125-mph wind loads and 45-degree rain. Its noise reduction capabilities can reduce sound to 69 dBA, about the same as an exposed residential central air conditioner. Universal Acoustic & Emission Technologies (Universal AET) provides air inlet and exhaust systems, ducting, diffusers, and enclosures. Working with Ojibway (Ojibway Enclosures is a manufacturer of sound-attenuating enclosures, tanks, metal products, and genset packaging services), Universal offers custom-designed enclosures for a variety of applications. From a simple weather enclosure (no insulation, no plenums) multiple levels of sound protection are provided. Level 1 enclosures provide 2-inch wall insulation and discharge plenums. Level 2 includes 8-inch foil faced insulation in walls, intake, and discharge plenums. Level 3 provides 8-inch insulation in walls, insulated intake, and discharge plenums with baffles. The highest level, Level 4, comes with 12-inch insulation in the walls, insulated intake, and discharge plenums with double baffles. These enclosures provide up to 40 dBA of sound attenuation. Since 1959, Universal AET has been a designer and manufacturer of noise control systems for engine and generator set applications. The company’s silencers have been supplied to OEMs, packagers, and distributors as acoustic and emissions solutions for gas or diesel engine applications in the power generation industry. Universal offers engine silencers for models as small as 10-inch inlet size (equivalent to a 1-meg kW engine). Each configuration can be customized with a variety of inlet and outlet options, and value-adding features such as weld-on support brackets, explosion relief valves, drains, and more. Available accessories include: flex connectors, insulation blankets, rain caps, elbows, and gaskets. Silencers are point specific noise attenuators that are affixed to the original source of the noise. Not be confused with the device attached to a barrel of a gun (which is actually called a “suppressor”), an engine silencer is designed to fit the exhaust manifold and reduce the noise created by engines and generators while in operation. On automobiles, these silencers are referred to as mufflers. Certain companies specialize in the design, manufacture and installation of silencers for different configurations and levels of noise reduction. SEMCO LLC (a FlaktWoods company) offers a full line of sound attenuating products designed to eliminate a wide variety of noise problems (HVAC systems, clean rooms, extended/supply/mixing plenums, engine test cells, etc.). These include rectangular & round silencers, media free silencers, custom silencers, and acoustic enclosure panels. The company’s Acoustic/Thermal Panel System Enclosures include 2-, 4-, 6-, and 8-inch thickness, in a wide range of materials and performances. These structures have multiple applications (standard built-up air handling units, clean rooms, jet engine test cells, extended plenums, material handling enclosures, supply/return chase plenums, mixing plenums, etc.). They also supply larger scale pre-engineered acoustical barrier assemblies for airport and highway noise, manufacturing equipment, and enclosures. At a smaller scale, SEMCO LLC provides specialty silencers, such as elbow-shaped silencers for applications with limited space constraints. The company has also developed media free silencers for sensitive applications (such as hospitals, pharmaceuticals, electronics, and clean rooms) that provide broadband attenuation without the use of fiberfill media. Custom-made silencers, engineered and tested for individual applications, are available for HVAC systems. Harco Manufacturing produces a wide variety of exhaust silencers. At the high end are the company’s Extreme Grade. (EXT silencers, capable of sound attenuation between 45 and 52 dB. The EXT was designed for engines operating in environments where the surrounding noise level is low—in hospitals, for example.) Super Critical Grade and Critical Grade silencers (the SCSC and CSCS series) both are designed to force exhaust to flow through the silencer in a vortex pattern with attenuation being accomplished by a combination of absorption and annulling of noise waves. This allows for the reduction of the silencer diameter and attenuation levels between 18 and 32 dBA, suitable for light industrial facilities and schools. In addition to performance grade, Harco silencers are categorized by their physical profiles. Their Low Profile Super Critical Grade Exhaust Silencer (SFH) has insulated internal surfaces (top, sides, and bottom) which reduce both radiant heat and external noise. Both low in profile and operating with low back pressure, this silencer is known as a “hockey puck” style. Providing the same 18–32 dBA attenuation, this low-profile model is suitable for engine generator exhaust systems in attenuated enclosures, ISO containers for emergency power generators, and military exhaust systems. Critical Grade Exhaust Silencers (CFH) are useful for generator exhaust systems in buildings with low overhead such as parking facilities. For non-industrial uses, Harco has a series of low-profile residential grade (RFH) exhaust silencers. Not all applications can be satisfied with the company’s standard products. Between 40% and 50% of Harco’s products are custom designed. Customizing could include making the package smaller, mounting it on legs, or some other form of customizing. Residential Grade silencers for low levels of attenuation (18–23 dBA) are also available from Harco. These smaller silencers are recommended for use in emergency gensets, mobile equipment, military equipment, and diesel engine air intakes. Industrial grade exhaust silencers (M) allow attenuation in the range of 15–22 dBA for construction, logging, oil fields, irrigation pumps, and other industrial applications where background noise is relatively high. MIRATECH is a comprehensive manufacturer of industrial silencers for power generation, oil and gas, construction, mining, mobile industrial equipment and rail transport. Most industrial gas and diesel engines emit exhaust noise at about 110 dBA. MIRATECH’s silences can reduce this noise level to 60 dBA—equivalent to the noise level of background conversation. These silencers include the EM and COWL product brands. In addition to their standardized lines of silencers, MIRATECH engineering staff can design and manufacture silencers for specialized needs. Whether standardized or customized, these silencers meet both ISO 9001 Quality Management and ISO 14001 Environmental Management standards. Additionally, MIRATECH Acoustic Silencer Technology (AST) is built into the housings on the VX, RX, ZX, and Ground Access product lines. Their Ground Access units are combination catalyst/silencers designed for large gas compression and power generation engines. This design allows the housing to stand vertically, eliminating the need for access structures (stands, ladders, and platforms). In addition to improving aesthetics, this design reduces the installation footprint and improves safety. Acoustic annulling is the active negation of sound due to noise cancellation. This is a method for reducing unwanted sound by the addition of a second sound specifically designed to cancel the first. The cancellation wave has the same amplitude but an inverted phasing of the noise to be annulled. As a result, the peak of one wave matches the valleys or the other, and vice versa, resulting in a greatly reduced amplitude for the combined sound wave. BE Daniel P. Duffy, P.E., writes about topics related to energy and the environment. SIDEBAR More to Consider In addition to sound reduction, emission controls are an integral part of power production units. In addition to the immediate discomfort created by uncontrolled exhaust emissions, these emissions are the source of long-term damage to human health and the environment. Companies that provide these controls to gensets and other power sources use a variety of methods such as catalytic reduction. Miratech is involved in controlling sound and emissions. One location that has successfully incorporated Miratech technology is the OneOK ICE station located near Calumet, Oklahoma. The site utilized a total of eight CAT 3608 power packages onsite supplied by Exterran. OneOK, convinced of the superiority of Miratech technology originally considered MIRATECH’s Z-Flow housing and catalyst. However based on OneOK’s needs, MIRATECH suggested a ground accessible option. The use of these housings and catalysts results in a 75% reduction in CH2O, a 95% reduction in CO, and a 50% reduction in NMNEHC emissions. Though the overall cost was roughly equivalent (due to the ground access modifications) the site gained in user friendliness and improved safety that comes from being able to service the catalysts at ground level. The client has since used this design on a duplicate site, Northern rows, built in the past year, and plans to use it on their new Cottonwoods faculty currently under construction. In this more specialized area of emissions controls, Johnson Matthey Emission Control Technologies provides controls for three major industries: auto-catalyst (for cars and light trucks), heavy duty diesel engines (buses and trucks and construction equipment), and stationary emissions control systems (coal fired power plants, gas turbines, diesel engines, locomotive, marine and waste-to-energy industries). Their catalytic systems are based on Continuously Regenerating Trap (CRT) and Selective Catalytic Reduction (SCR) technologies. Catalyst can be supplied on either ceramic substrates for rich burn engines, or on metal substrates for lean burn engines. This technology allows Johnson Matthey to meet stringent California Air Resources Board (CARB) and US EPA standards for reducing NOx, CO, and HC emissions. The INDECK Power Equipment Company is a supplier of gensets including large 125–2,200-kW trailer-mounted commercial generators and industrial gensets for FEMA, municipalities, building owners, industrial plants, food refrigeration warehouses and senior housing facilities. These units include sound attenuated trailers and produce low emissions capable of meeting California code requirements. Their I-SCR System (Indeck Selective Catalytic Reduction System) is one of a series of Indeck product offerings that reduce NOx emissions (9 ppm or less). Mobile I-SCR systems are also available. It uses a selective catalytic reduction process that converts combustion flue gas nitrogen oxide (NOx) emissions into harmless nitrogen (N2) and water (H2O) without forming secondary pollutants. This is accomplished by the controlled injection of anhydrous ammonia (NH3) into the boiler exhaust gas and passing the gas ammonia mixture through a catalyst. Utilizing the latest flow modeling along with fluid, thermal and structural computational analyses, their engineering design ensures safe operations while meeting emissions standards. There is more to consider in the choice of system to generate power than the not so simple creation of electricity. The human element is always of vital importance. These generators, engines and turbines would be well nigh unusable without ancillary fixtures that limit noise pollution and air pollution.
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