4AE3 U5 C2 Obj 4.pdf

Gas and Noise Emissions • Chapter 2 Objective 4 Describe the effects of noise pollution and methods of identifying, measuring, and controlling it. Noise Pollution Noise pollution can be defined as “Environmental noise that is annoying, distracting, or physically harmful.” The effects of industrial...

Gas and Noise Emissions • Chapter 2 Objective 4 Describe the effects of noise pollution and methods of identifying, measuring, and controlling it. Noise Pollution Noise pollution can be defined as “Environmental noise that is annoying, distracting, or physically harmful.” The effects of industrial noise on workers and the surrounding community have been the subject of much research. Chronic exposure to elevated noise levels has been shown to have adverse health effects. Living close to a high noise source area such as an airport can cause hypertension, stress, sleep problems, and psychological problems. These in turn can cause obesity, heart disease, diabetes, anxiety, depression, and headaches. The effects of noise on the communities near the source are mostly sleep interference and annoyance, depending on the frequency, intensity, and duration of the sound. For example, high frequency tones are perceived as louder than lower frequency tones at the same volume. Intermittent or impulse noises are often more annoying than a constant noise. Public concern over noise is increasing. This concern has led to stricter limits on noise production. Application for new, permanent facilities or for modifications to existing facilities requires a noise impact statement, if there is an expectation of continuous noise generation. Facilities such as pumping stations, gas processing plants, and gas pipeline compressors are examples of installations that may require noise impact statements. A noise impact statement should specify the design sound level at the nearest or most impacted permanently or seasonally occupied dwelling. Noise impact statements must consider possible noise impacts before a facility is constructed or in operation. Designers should discuss noise matters with residents during the design and construction phases of a facility. Otherwise, the cost to retrofit a facility already in operation may be significantly more than if noise mitigation was part of the original design. While residents near plants prefer no increases in sound levels, sometimes it is not possible to achieve. However, if proper sound control measures are incorporated into facility design, increases in sound levels can be kept to acceptable minimums. Sound levels are generally considered to be acceptable when overall quality of life or indoor sound levels for residents are not adversely affected. Even for facilities with no dwellings within 3 km, uncontrolled sound generation will not be allowed. Reasonable measures should be taken to reduce sound generation even though no sound level criteria are specified. Sources of Noise There are numerous sources of noise in a plant. These sources include: a) Machinery • Furnaces • Fans • Compressors b) Structural vibrations transferred from moving parts c) Construction activities Even leaving compressor room doors open can contribute to unwanted noise. 4th Class Edition 3 • Part A 2-25 Unit A-5 • Introduction to Plant Operations and the Environment Furnaces, fans, reciprocating compressors and coal pulverizers generally produce low frequency noise. Gas or steam passing through vents and valves produce high frequency noise. Noise impact from heavy truck traffic around operating facilities is not always specifically mentioned; however, its impact is not excluded from noise regulations. It is expected that every reasonable measure will be taken by industry to eliminate or reduce the impact of heavy traffic in any given area. Sound and Sound Measurement Sound travels as waves through the air, in the same way that waves travel through water. The waves are formed by vibrating bodies or air turbulence, which causes a variation in air pressure. This variation in pressure is actually a transfer of energy, passed through the air via molecular interaction. Because it is a form of energy, sound can be expressed as having specific power and intensity levels. Sound waves have varying frequencies and wavelengths. As well, sound waves can be reflected, deflected, and absorbed. Absorption of sound occurs when sound waves strike a non-rigid barrier. The energy of the wave moves the fibres on the barrier’s surface. Internal friction in the barrier opposes this movement and the energy of the air is converted to heat energy within the barrier. Reflection and deflection of waves occur when the sound strikes a rigid barrier. Very little of the energy of the wave is absorbed by the barrier; only the direction of the wave is changed. Variations in sound are caused by the different pressures and frequencies of the sound waves. A human ear is usually capable of hearing sounds which represent pressures from 0.00002 Pa to 200 Pa. Because of this large range, the concept of a decibel was created to convert sound pressure levels into a more meaningful scale. Levels of sound are measured by a sound level meter in Pascals, but are represented on the meter’s display in decibels. The decibel scale is not linear. Doubling a decibel reading does not double the sound intensity. The decibel scale is a logarithmic scale so that a decibel reading of 20 dB is 10 times more intense than 10 dB. Thirty dB is 10 times more intense than 20 dB, and so on. As the decibel reading increases by ten, sound intensity increases by a multiple of 10. Table 3 shows the relationship between the decibel scale and everyday sounds. Table 4 shows the kind of physical damage that can be caused by excessive exposure to sound pressure. Table 3 – Decibel Scale Decibels 2-26 Times more intense than 10 dB Examples Times louder than 10 dB 10 dB Barely audible (pin drop) 1 1 20 dB Whisper, rustling leaves 10 2 30 dB Quiet rural area 100 4 40 dB Quiet conversation 1 000 8 50 dB Louder conversation 10 000 16 60 dB Quiet traffic noise 100 000 32 70 dB Vacuum cleaner 1 000 000 64 80 dB Loud highway noise at close range 10 000 000 128 90 dB Lawn mower 100 000 000 256 1 000 000 000 512 10 000 000 000 1024 100 dB Jackhammer (pneumatic drill) at close range 110 dB Jet engine at about 100m. Human pain threshold. 4th Class Edition 3 • Part A Gas and Noise Emissions • Chapter 2 Table 4 – Auditory Damage Caused by Sound Pressure Decibels 85 dB Examples Hearing damage in about 8 hours 100 dB Hearing damage in about 15 minutes 120 dB Physical pain, hearing damage in a short period of time 160 dB Eardrum bursts instantly Noise Pollution Monitoring and Control Rules, regulations, and guidelines for noise pollution and noise remediation are generally managed by local jurisdictions (provincial, territorial, or municipal). Each jurisdiction is responsible for emissions within their jurisdiction. Currently, there is no consensus between jurisdictions on how to define and control noise. Most jurisdictions have an application process in place that requires all new facilities being built to conduct an environmental impact assessment which includes predicted noise levels. If the application shows the noise levels are reasonable, it can be approved. However, effort must be shown that noise emissions will be the lowest level achievable based on land control use and zoning. The noise generated from the facility is considered the point source; however, the sound field it will impact is complex. The amount and level of noise transmitted to the general public, and the distance it travels, are impacted by the topography, humidity, temperature, and wind. Each facility receives permits to build and operate. In that permit are specified levels of noise emissions that cannot be exceeded. Some legislation will look at noise emissions as an environmental release with potential consequences of fines or by allowing those who are impacted by the emissions to seek compensation for any loss the noise may have caused. Sound decibel levels do not distinguish between sound frequencies. Sound meters contain weighting networks which consider frequency and adjust the readout of the meter to correspond to how loud a person would perceive the sound to be. There are 4 weighting “scales” currently used: A, C, D, and Z. “A” is most representative of human hearing. When noise is expressed in decibels, there may be an extra letter after the dB, such as dBA. This letter, A, C, D, or Z indicates the weighting scale used to measure the noise. Sound meters are also rated by accuracy on a scale of 0 - 3. Type 0 meters are the most accurate. Type 3 are the least accurate. Figure 8 shows a typical hand-held sound level meter. Noise laws or regulations specify which type of sound meters and which weighting scale must be used to investigate noise levels, and to ensure regulatory compliance. 4th Class Edition 3 • Part A 2-27 Unit A-5 • Introduction to Plant Operations and the Environment Figure 7 – Sound Level Meter (Courtesy of B and K Precision Corp.) Efforts to control noise are aimed at lowering the sound intensity at a given location. The minimum noticeable noise reduction is approximately ten percent on the decibel scale which means a reduction in noise of 5 to 10-fold is necessary to be worthwhile and noticeable. There are three ways to reduce noise at a given location. 1. Reduce the sound at the source. 2. Modify the sound wave path. 3. Protect hearing with a barrier device, such as ear plugs or ear muffs. Noises are often controlled by a combination of these methods. Reducing the sound at the source can be achieved by: a) Installing vibration isolation mounts and damping equipment for machinery. b) Decreasing speed of operation. c) Decreasing blade tip velocities in fans. To be cost effective, most of these modifications must be included at the design phase. After the design stage, modification of the sound path to the receiver is often used. Enclosures are a form of sound path modification. They are designed using special materials which reflect sound back to the source, and in some cases, absorb it as well. All materials have some sound absorbing properties. The energy of a sound wave striking a material surface is either reflected, absorbed, or transmitted. The relative amounts will depend on the sound absorbing quality of the material. Soundproof materials absorb sound waves by trapping them in the material. As a result, very few sound waves are reflected back or transmitted through the materials. 2-26 4th Class Edition 3 • Part A Unit A-5 • Introduction to Plant Operations and the Environment The best materials for sound absorption are usually porous and easy to find. Lead, loose sand, brick, and almost all heat insulators (fiberglass, mineral wool, polyurethane, and cork) are good sound absorbers. Figure 8 illustrates how sound wave paths are changed by a reflective barrier and an absorbent barrier. Figure 8 – Sound Reflections and Absorbers Reflected Reflected Transmitted Transmitted Sound Sound (a) Reflector (b) Absorber Figure 9 shows two types of noise enclosures that absorb and reflect sound: mufflers and lined ducts. Inlet silencers on forced draft fans are also a form of enclosure. Figure 9 – Sound Path Modifiers Dissipative Muffler Sound Absorbing Material Perforated Tubes Lined Duct Parallel Baffles Reactive Muffler Sound-Absorbing Material Combination Design Rounded Edge Sound-Stream Absorbers Exhaust Mufflers 2-28 4th Class Edition 3 • Part A Ducts Staggered Baffles Gas and Noise Emissions • Chapter 2 Sometimes, it is necessary to enclose an entire piece of equipment. If such an enclosure is necessary, it must be completely free of leaks. A hole as small as 1/1000 of the total wall area of the enclosure would leak enough sound to make the enclosure non-effective. For high frequency noises, the best results are obtained using a double structure with a sound absorbing material between the two structures. Another form of sound path modification is by creating a buffer zone around the source. By doubling the receiver’s distance from the source, the sound pressure can be halved. 4th Class Edition 3 • Part A 2-29 Unit A-5 • Introduction to Plant Operations and the Environment Chapter Summary Activities carried out at facilities where Power Engineers work can generate emissions that can pollute the environment, and cause annoyance and stress to the general public. Emissions caused by combustion of hydrocarbon fuels contribute to the greenhouse effect and acid rain. There are processes in place to reduce the generation of these emissions at the source or to strip toxic gases before they reach the atmosphere. More research and development will improve these systems. However, facilities that burn coal as fuel and generate significant amounts of SOX and NOX are being pressured to shut down or convert to a cleaner burning fuel such as natural gas. This can be a very costly conversion and not realistic for some plants. Reducing the production of pollutants in a process is a better option than trying to remediate the problem afterwards. Through careful, diligent, and mindful operation of a plant, Power Engineers can do their part to reduce the effect of gaseous and noise pollution on the environment. 2-30 4th Class Edition 3 • Part A