Report on Mill Noise levels and noise level attenuation for consideration at the proposed Coega and Empangeni clinker grinding sites.

Transcription

1 Client: OSHO Ventures 9 October 2012 Report on Mill Noise levels and noise level attenuation for consideration at the proposed Coega and Empangeni clinker grinding sites. Compiled by: A.F. van der Merwe

2 Table of Contents 1. Executive Summary Introduction Noise and Noise Calculations Noise Attenuation in Free Space Noise and Noise Attenuation in Cement Grinding Facilities References... 8 Cement Mill Noise Level and Attenuation Report Page 1

3 1. Executive Summary The study was conducted to establish if the noise radiated by two different cement grinding technologies, i.e. a conventional ball mill and a vertical (roller) mill, would be sufficiently attenuated at the boundaries of the proposed sites to adhere to regulations. It was found that for conventional ball mills and with no other obstacle, either natural or man-made in the vicinity, and with optimum atmospheric conditions allowing maximum sound travelling capabilities, a noise level below 80 db is found at 100 meters from the source, a noise level below 70 db is found at 250 meters from the source and noise levels below 60 db can be experienced at distances of 750 meters and beyond from the source. For vertical mills, due to the lower noise level at the source, lower noise levels are generally experienced on the site at the different distances from the source. Here, for example, a noise level below 80 db is found at 10 meters from the source, a noise level below 70 db at 26 meters and beyond from the source and noise levels of 60 db and below are experienced at 100 meters and beyond from the source, again calculated with no obstacles in the path of the sound and at optimum atmospheric conditions for maximum sound travelling. However, even with much of the noise being attenuated without noise barriers (obstacles) or enclosures at relative far distances from the source, care should be taken as to the location of administrative buildings, workshops and laboratories to protect staff members from continuous exposure to high noise levels. It is also imperative that staff members working in the vicinity of the source of noise should be protected through proper ear protection (PPE) and the use thereof should be enforced on employees and visitors alike. Cement Mill Noise Level and Attenuation Report Page 2

4 2. Introduction This study was conducted to evaluate noise levels of both conventional ball mills and vertical roller mills for cement and/or slag grinding and to determine levels of attenuation of such noise in a space free from any obstacles and at optimum atmospheric conditions for sound travelling, as a function of distance from the source. Conventional ball mills and vertical roller mills are currently being investigated as potential grinding technologies for two cement producing facilities at the South African coastal region and it is required that the facilities adhere to environmental as well as other pollution regulations, including noise as potential pollutant. 3. Noise and Noise Calculations Noise can be defined as an unwanted sound and is generally classified as an undesired disturbance. Due to regulations, noise above a certain sound pressure level (SPL) can be classified as a pollutant. A noise can either be of only a certain frequency range, or can cover all or most of the audible sound frequency spectrum. Additionally can noise be intermittent of nature, it can be erratic or of a continuous (steady state) nature. In general can the noise radiated from a cement mill (as point source) be classified as continuous with frequencies consisting of a large range of audible frequencies. Sound pressure levels (SPL), or more commonly referred to as noise levels, are measured in decibel (db), which is dimensionless and is defined as the logarithmic ratio of the sound level with respect to a reference sound level (or a zero level). This reference level is usually a sound pressure of µbar, and is referred to as 0 db, or the level (sound pressure) of the weakest audible sound. The following equation is used to determine SPL: =20 log With: P = the root mean square pressure of the sound (µbar) The following table gives the relationship between the SPL and the sound pressure (P): SPL (db) Pressure Pressure (Pa) 74 1µbar lb (f) /ft lb (f) /in atm Cement Mill Noise Level and Attenuation Report Page 3

5 4. Noise Attenuation in Free Space Air has the ability to attenuate noise, meaning that noise levels will reduce with an increase in the distance from the source. Simply stated, and from mathematical considerations, noise levels can diminish in a free field due to only the air in the atmosphere. The following equation, also known as the inverse-square law, describes typically the attenuation that can be expected in a free field, meaning that noise produced at a source diminishes spherically in all directions: = 20 log With: L p = SPL at a specific distance from the source L x = SPL at the source (1 meter distance) r = the specific distance from the source where the SPL = L p r x = 1 meter Using the above equation, the SPL can be calculated at any distance from the source of the noise based on the SPL at the source. This holds for ideal conditions where: (a) the relative humidity is less than 50%, (b) ambient temperature is above 16 C and (c) the wind velocity is below 8 km/hr. However, due to alternative atmospheric conditions and the presence of natural and/or man-made obstacles, the measured levels are generally lower than those predicted by the model (inversesquare law). Also to be mentioned is that higher frequency noise is attenuated more by air than lower frequency noise. Noise attenuation prediction for noise in the Hz range will be accurate to about 1 db in 300 meters from the source, while noise attenuation predictions in the Hz range will be accurate to 6 db in 300 meters, meaning that high frequency noise will be more attenuated by air than low frequency noise. The attenuation of higher frequency noise will in effect reduce the overall SPL with distance, however, with the wide frequency range found with conventional ball milling, this effect might be negligible in calculating the overall SPL. 5. Noise and Noise Attenuation in Cement Grinding Facilities Various sources reported on measured noise levels of conventional ball mills (as very potent generator of cement plant noise). Other sources were consulted for noise levels of vertical (roller) mills where it was found that the noise was mainly originating from the motor and gearbox. Cement Mill Noise Level and Attenuation Report Page 4

6 The table that follows give a good indication of noises that can be expected at around meter (15 feet) from the source: Source of noise SPL (db) Cement mill (finish grinding mill) 105 Raw grinding mill 103 Rotary compressor 97 Bucket elevator (head) 88 Vertical (Roller) mill Taking these values into account, the inverse-square law was used and the attenuation in a free field at desired atmospheric conditions was calculated to yield the noise levels at different distances from the source. The first calculation was done for a conventional ball mill: BALL MILL Noise level Measured at: Distance from source Noise level at distance (db) (meters) (m) (db) Cement Mill Noise Level and Attenuation Report Page 5

7 This data yields the following relationship between SPL and distance from the source (ball mill): 140 Noise level (db) vs distance (m) From this data it is seen that, for conventional ball mills and with no obstacle, either natural or manmade in the vicinity, and with optimum atmospheric conditions for maximum sound travelling, a SPL below 80 db is found at 100 meters from the source, a SPL below 70 db is found at 250 meters from the source and SPL s below 60 db at distances of 750 meters from the source. Consequently, for vertical mills, taking the worst-case noise level of 85 db at meters (15 ft), the following calculated attenuation data were obtained for a free field with optimum atmospheric conditions and no obstacles in the way of the travelling sound: VERTICAL MILL Noise level Measured at: Distance from source Noise level at distance (db) (meters) (m) (db) Cement Mill Noise Level and Attenuation Report Page 6

8 From this data, the following graph was obtained: 120 Noise level (db) vs distance (m) It is clear that, due to the lower SPL of vertical mills at the source, lower noise levels are in general experienced on site at the different distances from the source. Here, for example, a SPL below 80 db is found at 10 meters from the source, a SPL below 70 db at 26 meters and beyond from the source and a SPL of 60 db and below at 100 meters and beyond from the source. As the distance to the site boundaries could be sufficient to reduce the SPL to acceptable limits, it is still important to keep in mind that the SPL in the vicinity of these sources are still potentially injurious to hearing. Care should be taken to adhere to the following guidelines: Staff working in the close proximity of the source of noise should be equipped with the required PPE and the use thereof should be enforced. Administration buildings, laboratories, workshops etc. should be planned and positioned to protect staff working there on a full time basis. The following table outlines noise exposure limits set out by the United States of America (Federal Safety and Health Act, April 28, 1971), indicating the maximum duration per day an employee should be exposed to certain SPL s: SPL (db) Duration per day (hours) or less Cement Mill Noise Level and Attenuation Report Page 7

9 6. References The following sources were consulted during this study: 1. Johnson, David A., A Study in Ball-Mill Noise Control, IEEE Transactions on Industry Applications, Vol. IA-9, No. 6, November/December 1973, pp Jorgensen, Soren W., Cement Grinding a comparison between vertical roller mill and ball mill, Cement International, 2/2005, FL Smidth Cement Mill Noise Level and Attenuation Report Page 8