Expert paper DESIGNING MAIN FAN NOISE PROTECTION SYSTEM IN MINE JARANDO BALJEVAC

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1 UNDERGROUND MINING ENGINEERING 15 (2006) UDK 62 FACULTY OF MINING AND GEOLOGY, BELGRADE YU ISSN ABSTRACT Expert paper DESIGNING MAIN FAN NOISE PROTECTION SYSTEM IN MINE JARANDO BALJEVAC Petrović Zoran 1, Radičević Branko 1, Bjelić Mišo 1 In article are given the principles of environmental noise protection system design. The noise is caused by main fan that ventilate mineshaft Jarando in Baljevac. System of absorbers is defined by permitted noise level in environment and frequency analysis of noise. We also give recommendation for further reduction of environmental noise around the noise source. Key words: Noise protection, fan, and noise absorbers INTRODUCTION Absorption materials and special structures are often used in modern civil engineering, architecture and industry. Their application is especially important in reducing noise in environmental and working areas. The aim of building noise protection system for reducing noise in environment is to protect environment and people from noise caused by main fan of Hard Coal Mine "Jarando" JP PEU Resavica, Baljevac. Assessment and measurement of noise pressure level are made in accordance with regulations S.G.RS 54/92 and JUS U.J6.090 and JUS U.J EXISTING LEVELS NOISE ANALYSIS 2.1. NOISE SOURCES The noise source is main fan for ventilation of mineshaft "Jarando". Characteristics of main fan are: Type: AV1000; Air flow: Q=20m 3 /s; Total dynamic head H=1250Pa; Electro motor power P m = 45kW; RPM=1460 rpm/min. The fan works continuously during 24 hours MICROPHONE POSITIONS The fan represents the biggest source of noise in its surroundings. The nearest housing building, which is endangered by noise, is 1 Faculty of Mechanical Engineering Kraljevo, Kraljevo ( petrovic.z@maskv.edu.yu; radicevic.b@maskv.edu.yu: bjelic.m@maskv.edu.yu

2 134 Petrovic Z.; Radicevic B.; Bjelic M.; located about 35 meters far from the absorption room, where the main fan is settled. The equivalent continuous sound level in residence is measured in rooms and outside. Microphone positions were selected at places most exposed to noise source. These are sleeping room on the first floor and two positions in the yard. (See Fig. 1.) m 35 m 40 m 2 4 Legend: 1. The administration building 2. Building with fan equipment 3. Main fan 4. Diffuser position on absorption room 5. Barrier wall 6. Supporting wall 7. The closest housing building 8. Microphone position No.1 (MP1) 9. Microphone position No.2 (MP2) 10. Microphone position No.3 (MP3) Figure 1. The sketch of noise source and microphone positions 2.3. RESULTS OF NOISE MEASUREMENTS Measurement results are shown in Table 1. It can be seen that the measured noise levels exceed permitted levels of noise both in rooms and in yard. Therefore, main fan for ventilation of mineshaft Jarando at Baljevac is source of noise pollution in its surroundings. The analysis of existing situation in domain of noise is based on measurements of equivalent continuous sound level on three microphones positions during the day and night. The equivalent continuous sound level is average level of noise and has the same energy content and consequently the same hearing damage potential as the varying sound level: 1 T T LA() t /10 L =10log eq 10 dt 0 ( ) db A (1) where is: L A (t)- weighted varying sound pressure level, T -measurement time interval.

3 Designing main fan noise protection system Microphone Position ( ) eq( ) eq( ) L ( sp ) =10log 10 L u /10-10 L p /10 ( ) eq db A (2) The specific sound pressure of source is given by equation 2. Environmental Level of noise L eq(p) Equivalent sound pressure level L eq(u) Specific sound pressure of source L eq(sp) Table 1. Existing noise levels Permitted Level of noise day night Comment MP Exceeds MP Exceeds MP Permitted MP Exceeds 3. DESIGN OF NOISE PROTECTION SYSTEM The designed system for environmental noise elimination includes activities on revitalization of building and passive structure for protection like barriers and absorbers. The next activities are realized: 1. Construction of acoustics barrier between building, where the main fan is settled and the nearest houses. Acoustic barriers are commonly used for the reduction of noise in outdoor applications. In general, barriers are more effective in reducing high-frequency noise than for low-frequency noise. 2. The closing of all holes on the building, especially between walls and roof construction. 3. The exchange of all damaged cover elements on roof construction. 4. The repair and exchange of damaged elements of roof construction in absorption room with new elements. 5. The building of double layer (panel) ceiling in absorption room 6. The porous acoustic absorbing material in the air space behind the panel is used for design of acoustic absorber in the ceiling and on the walls too ACOUSTIC ABSORBERS Before the selection of the most efficient absorbing material, we made frequency analysis in 1/3 octave bands at appropriate microphone position, near fan absorption room. On the basis of obtained results of analysis the most appropriate sound absorber is selected. Practically, in absorption room, the highest level of sound pressure is measured at frequencies between 50 and 500 Hz (see Fig. 5.) for which membrane absorber is selected. Generally speaking, in practice there are three types of acoustic absorbers: a)porous, b)membrane and c)resonator (See Fig. 2.).

4 136 Petrovic Z.; Radicevic B.; Bjelic M.; P P a α =. (3) u The absorption coefficient is the most important factor for effective absorbers and it is defined as ratio between absorbed P a and total P u power on boundary surfaces. Figure 2. Methodology for selection of type of acoustic absorbers in correlation with frequency spectra and absorption coefficient 3.2. MEMBRANE ABSORBERS Membrane absorbers consist of one or more layers of metal, glass, plastics or plywood with an air space behind the panel. The absorption coefficient is larger for the thinner panel and when in the air space behind panel porous acoustics absorbing material is placed. The absorption for the membrane absorbers is related to the transition loss in the panel. Microphone position (MP1) is the most critical related to noise and its position is selected as represented for defining of noise protection system. In 1/3 octave band spectra, measured on microphone position (MP1) it is visible that the highest levels of sound power are: at central frequency 31.5 Hz equivalent level of noise is 61.1dB(A), and at central frequency 500 Hz equivalent level of noise is 61.4dB(A). Considering frequency spectra of noise, the greatest effect of sound absorption may be achieved by membrane or resonant absorbers. The strongest power losses arising at resonant frequency: f r = M d (4) where M represents surface mass of membrane, and d represents air space thickness.

5 Designing main fan noise protection system Absorption coefficient may be increased by placement of acoustic absorption material within air space (usually it is not needed to fill whole volume of air space). membrane air space absorption material Fig. 3 Membrane absorbers with and without absorption materials 3.3. PRACTICAL REALIZATION OF MEMBRANE ABSORBER Considering the fact that the highest noise level arises at frequency of 500 Hz, a membrane type of absorber is selected. As absorption material is selected Azmafon A with thickness of 20 mm in combination with Azma AD plates, both manufactured by Azma Kragujevac. Non absorption surface Fig 4. Sketch of designed membrane absorber with absorption-frequency characteristic of embedded material 4. RESULTS OF MEASUREMENTS OF NOISE LEVEL AND FREQUENCY ANALYSIS OF NOISE AFTER REALIZATION OF NOISE PROTECTION MEASURES Frequency spectra of noise show great reduction of noise level at all frequencies except at lowest frequencies range from 31,5 Hz to 250 Hz. Microphone Position Table 2. Noise levels after realization of noise protection measures Equivalent Permitted Environmental Specific sound sound Level of Level of noise pressure of pressure level noise Comment L eq(p) source L L eq(sp) eq(u) day night MM Permitted MM Permitted

6 138 Petrovic Z.; Radicevic B.; Bjelic M.; Level of noise [db] Frequency [Hz] Fig. 5 Frequency spectra at measurement point MP1 before and after construction of sound insulation After realization of noise protection measures results of noise measurements, results of measurement of noise of main fan for ventilation of mineshaft Jarando at Baljevac do not exceed values set by defined standards and regulations. Further reduction of noise levels in lowfrequency range is possi- ble to be achieved by resonant absorber for resonant frequency of 31,5 Hz. CONCLUSION After realization of noise protection measures, results of measurements of noise level cause by main fan for ventilation of mineshaft Jarando at Baljevac shows that there is no more noise pollution of surrounding environment. Further reduction of noise in low-frequency range is attainable by application of resonant absorber. BIBLIOGRAPHY [1] Petrović, Z., Radičević, B., Bjelić, M., [2006]: Zaštita od buke glavnog ventilatora za provetravanje jame Jarando JP PEU Resavica, Ibarski rudnici kamenog uglja Baljevac, Mašinski fakultet Kraljevo; [2] Praščević, M., Cvetković, D., [2005]:Buka u životnoj sredini, Fakultet zaštite na radu u Nišu, Niš; [3] Veličković, D., [1990]: Buka i vibracije 2, Fakultet zaštite na radu u Nišu, Niš; [4] Standardi: JUS U.J6.090 i JUS U.J6.205; [5] Pravilnik o dozvoljenom nivou buke u životnoj sredini, Sl. Glasnik RS, br. 54/92; [6] Translation into English: Authors