Like many other types of rotating machines, generator sets powered by internal combustion engines emit noise and vibrations. Whether these generator sets operate continuously as in the case of mains power sources or only operate occasionally, in many cases operating noise must still be reduced to meet local and state regulations. or federal. In North America, the maximum allowable general noise level ranges from 45 dB(A) to 72 dB(A), depending on the locality or region. In fact, recently, some states and communities have begun to introduce noise restrictions at property boundaries using octave band frequencies to reduce low-frequency noise. spread to neighboring areas in the community. Noise levels from untreated generator sets can reach 100 dB(A) or higher, so the location of the generator set as well as noise mitigation are both very important.
Limit the noise of Diesel generators
Like many other types of rotating machines, generator sets powered by internal combustion engines emit noise and vibrations. Whether these generator sets operate continuously as in the case of mains power sources or only operate occasionally, in many cases operating noise must still be reduced to meet local and state regulations. or federal. In North America, the maximum allowable general noise level ranges from 45 dB(A) to 72 dB(A), depending on the locality or region. In fact, recently, some states and communities have begun to introduce noise restrictions at property boundaries using octave band frequencies to reduce low-frequency noise. spread to neighboring areas in the community. Noise levels from untreated generator sets can reach 100 dB(A) or higher, so the location of the generator set as well as noise mitigation are both very important.
In general, there are two types of noise regulations that affect individuals or residents: State or local noise regulations and federal safety regulations issued by the Department of Occupational Safety and Health. Health (Occupational Safety and Health Administration - OSHA) issued. The first type of regulation applies to noise that may travel outside the property's boundaries and cause inconvenience to residents, but is usually not loud enough to pose a safety hazard. The second type of regulation applies to noise exposure in the workplace to protect workers' health. OSHA regulations generally only apply to workers who may be exposed to generating set noise greater than 80 dB(A) for significant periods of time.
Workers can limit exposure by wearing appropriate ear protection when working near an operating generating set. Europe and Japan, as well as many other countries, have also set standards for noise control in the workplace and in the environment in general.
DIESEL GENERATOR
Definition of noise
Sound is what the human ear hears; noise is simply unwanted sound. Sound is created by vibrating objects and transmitted to the listener's ears in the form of pressure waves in the air or other medium. Technically, sound is pressure fluctuations in the area adjacent to the ear. When the volume is loud enough to cause discomfort or distress, it means that changes in air pressure near the ear have reached too large an amplitude. The human ear has a fairly wide dynamic range, so people use the decibel (dB) scale to represent sound levels. The dB scale is a logarithmic scale because the ratio between the softest sound the human ear can detect and the loudest sound the human ear can withstand without damage is about 1 part per million, which is 1/106. If we use a base 10 logarithmic scale, we can express the entire range of sound intensity levels that humans can hear with a more convenient number, from 0 dB (threshold of normal hearing) to 140 dB (threshold of earache). . There are two types of dB scale: A and L.
The dB(L) scale is linear, all audible frequencies are considered to have the same value. However, the human ear does not perceive all sound frequencies as equally loud. The human ear is particularly sensitive to frequencies in the range of 1,000 to 4,000 Hz, and is less sensitive to sounds of lower or higher frequencies.
- Therefore, people use an “A-weighting filter”, which is essentially a way to approximate the sound loudness, to calibrate the sound pressure level to more accurately reflect what the human ear perceives. receive. From frequency weighting came the dB(A) scale, adopted by OSHA in 1972 to describe sound levels in official regulatory documents. Figure 1 shows typical noise levels associated with different surroundings and sound sources.
Noise source of generator set
Generator set noise is generated by six main sources (see Figure 2):
- Engine noise. Mainly due to mechanical and combustion forces, typically in the range from 100 dB(A) to 121 dB(A), measured at 1 m distance, depending on engine size.
- Cooling fan noise. Because air moves at high speed through the engine and radiator. Noise range from 100 dB(A) to 105 dB(A) at 1 m.
- Alternator noise. Caused by cooling air and carbon brush friction. The noise range is approximately 80 dB(A) to 90 dB(A) at 1 m.
- Noise due to induction phenomenon. Due to current fluctuations in the alternator windings, mechanical noise ranges from 80 dB(A) to 90 dB(A) at a distance of 1 m.
- Exhaust air from the engine. Without an engine exhaust muffler, noise ranges from 120 dB(A) to 130 dB(A) or higher, but is usually reduced to a minimum of 15 dB(A) when fitted. standard silencer.
- Noise due to mechanical structure. Due to mechanical vibrations of various structural details and parts, radiation occurs in the form of sound.
Noise measurement
Before determining where noise mitigation is needed, you need to take accurate measurements of the existing ambient noise and the noise to which the generator set contributes. Accurate and meaningful data on generating set acoustic noise levels should be measured in an “open environment”. An open field differs from a “reflected field” in that it is a sound field where the influence of sound reflected from surrounding or obstructing objects is negligible. Noise measurements should be made using a minimum of a sound level meter and an octave band filter to enable more detailed analysis by the acoustics consultant.
When measuring sound levels at a distance of 7 m, the microphones are placed in a circular array around the generator, measuring positions 45 degrees apart. The measuring rig is placed 7 m away from an imaginary rectangular box that fits the generator set, usually determined by the dimensions of the space occupied by the base or frame.
When measuring sound intensity levels for European applications, a rectangular microphone array as defined in ISO 3744 is typically used.
You can refer to the acoustic performance data for Cummins Power Generation Inc. generator sets. on the company's design software CD (named "Power Suite"). You can also refer to the website www.cumminspower.com.
Initial measurements are usually made over eight-octave bands, from 63 Hz to 8,000 Hz, although the maximum sound power typically lies in the range from 1,000 Hz to 4,000 Hz, which is the most sensitive sound range for with human ears.
Although measurements are made over the entire frequency spectrum, the logarithmic sum of all frequencies is the most important reading. However, when the overall sound level exceeds the allowable level for the project, frequency band data is used to determine what design changes are necessary to lower the overall sound level to comply with the project's requirements. requirements.
Get the sum of all noise sources
The total noise level from a generating set is the sum of all individual sources, regardless of frequency. However, because the dB(A) scale is a logarithmic scale, dB(A) measurements cannot be added or subtracted in the usual arithmetic way. For example, if one noise source produces 90 dB(A) and a second noise source also produces 90 dB(A), the total noise produced is 93 dB(A) rather than 180 dB(A). . Increasing the noise by 3 dB(A) means doubling the sound intensity, but it is almost imperceptible to the human ear.
Table 1 illustrates how to add decibels based on the numerical difference between two noise levels. As in the example above, there is no difference between source 1 and source 2, their combined dB(A) measurement only increases by 3 dB(A) - from 90 dB(A) to 93 dB(A). . If source 1 is 100 dB(A) and source 2 is 95 dB(A), their combined measurement will be 101 dB(A).
Noise laws and regulations
In North America, state and local codes regulate maximum noise levels allowed at property boundaries. Table 2 lists some representative regulations on outdoor noise levels. To comply with these noise regulations, it is necessary to have an understanding of the existing ambient noise levels at the property boundary when the generating set is not in operation and what the final total noise level will be when the generating set is installed. Generator running at full load.
In Europe, regulations on generator noise based on law 2000/14/EC/Phase II have been applied since 2006. For generators with primary engine power below 400 kW, the noise level allowable is calculated according to the following formula: 95 + log Pel = dB(A) where Pel is the nominal power of the generator primary motor.
Generators with a rated power of 400 kW or more must be labeled with an LWA figure (European measurement of “sound power level”) calculated from the manufacturer's implementation test results. For the European market, most generators from 11 kVA to 550 kVA are housed in standard enclosures so that the unit meets most regulations. In general, standard enclosures reduce radiated noise to a minimum of 10 dB(A).
Posted by caothangtech, at 7:45 PM, August 1, 2013