ITS was involved in lowering the noise level of a noisy industrial facility in Southeast Asia, in connection with the production and processing of non-ferrous metals by means of processes requiring the use of large amounts of energy, supplied by two combustion turbines (unit power output 80 MW).
For such a noisy industrial facility, lowering the sound level is a necessity, because the sound power levels emitted by several sub-assemblies of such turbomachines (e.g. air intake, casing, exhaust of burnt gases) are considerable, namely: greater than 135 dB (A) ref. 1 pW per unit - an increase of 3 decibels is to be considered in the case of 2 decorrelated noise sources -, and therefore likely to have a strong impact on the sound environment :
- near machines: it is not possible for personnel (being employees or being commissioned by an external company for supervision and maintenance) to be able to work or even circulate (even for short periods) in the absence of effective soundproofing, because exposure to noise levels such as those resulting from such sound power levels (even if they are not always above the pain threshold, especially when wearing hearing protection) would be such as to cause sometimes irreversible impairment of hearing faculties [1]
- at longer distance :
- in locations being inside the perimeter of the noisy industrial facility (outdoors: for remote workstations [1] or indoors i.e. in ordinary buildings e.g. in offices [2]), it is in most cases not possible to carry out an activity under normal working conditions without a quality soundproofing device
- in locations being beyond the limits of the noisy industrial facility (with regard to the neighborhood), on the property line [3] and even several hundred meters away [4], the desirable calm can not exist (e.g. in gardens, on balconies or even inside buildings and homes) without noise reduction equipment
Thus, lowering the sound level of a noisy industrial facility like this requires the use of different noise reduction techniques :
- silencers, for the air intake and for the exhaust of burnt gases, which must limit the propagation of noise (their insertion loss accounts for their performance, expressed in decibels - the symbol is dB -, variable with the frequency, having to take into account their self noise, related to the flow) while not opposing too much the passage of the fluid (the total pressure loss accounts for their performance, expressed in pascals - the symbol is Pa - or by means of any other pressure unit such as the millimeter of water gauge - the symbol mmWG -, the millibar - the symbol is mbar -); they generally consist of a very thick metal casing and, in the case of dissipative silencers, of sound-absorbing elements - most often in the form of splitters (baffles), in the form of boxes, i.e. with a peripheral framework - of variable geometry and nature depending on the case :
- air intake silencers for gas turbines are often rectangular in cross section - and so the splitters are parallelepiped in shape (the filling being mineral wool panels, protected by a covering and by a perforated sheet) -; such constructive provisions are used, moreover, for many industrial silencers, in connection with various manufacturing or transformation processes of noisy industrial facilities
- the exhaust silencers for gas turbines are often of rectangular section - and then the splitters are of parallelepipedic shape - or cylindrical - and then the splitters are of parallelepipedal shape or are concentric, being sometimes integrated into a chimney - (the filling being , depending on the case, panels or mattress in mineral wool or in ceramic fiber wool for use at high temperature, protected at least by a fabric and by a perforated sheet); such construction principles are often used for other noisy industrial facilities such as those involving combustion gases e.g. heat engines, furnaces, boilers
- enclosures, i.e. constructions enveloping the various parts of the machines, so as to limit sound transmission (their sound reduction index reflects their performance, expressed in decibels - the symbol is dB -, variable with frequency); depending on the size of the machines, such enclosures can be - given their dimensions - buildings, sometimes of great height, which must have a sufficient tightness (to air, to noise, and to bad weather) while allowing appropriate evacuation of the calorific power dissipated by the turbomachines. The renewal of air in these buildings is not solely based on the consideration of their volume and of an hourly rate: it is necessary to carry out a precise dimensioning taking into account the thermal loads (sometimes: very important) and the admissible temperature rise (sometimes: low, because it is not only a matter of creating conditions favorable to the resistance over time of hardware, but also of making possible the presence - sometimes prolonged e.g. in the event of inspection or repair works - of personnel inside such enclosures): sophisticated ventilation systems, involving high speed fans, often redundant to overcome any failure, and specific noise reduction equipment are required. Such soundproofing devices are often made by means of a steel frame and acoustic insulation panels:
- the vertical (walls) and horizontal (roof) partitions constituting such assemblies are often made of rock wool, protected by a glass veil and by a perforated sheet); such constructive arrangements are used, moreover, for many industrial soundproofing carterizations and hoods, in connection with various machines and production lines
As part of the project to lower the sound level of a noisy industrial facilty referred to in this post, ITS did not intervene either for the intake or for the exhaust (while this is the case in the context of other projects for which the human resources of ITS is able to carry out tailor-made sizing taking into account the technical requirements of the project, first and foremost the acoustic performance and the aerodynamic issue, which influences the productivity of the facility).
ITS participated in the construction of soundproof buildings constituting machine halls for turbines, according to the general principle mentioned above.
As always in such a case :
- the studies took into account the site data (snow, wind, seismic risks), the interfaces of buildings with other sub-assemblies (e.g. alternator) as well as the specificities related to access for personnel (doors) and for equipment (trap doors, removable wall elements), also covering related equipment (electrical devices, overhead crane)
- the workshop manufacturing was carried out with a view to maximum durability of the construction (choice of top-of-the-range raw materials and components, work and checks carried out by a qualified workforce, with a love for a job well done )
The (long-standing) cooperation with a specialist builder, having perfect knowledge of the ins and outs of such a project of lowering the sound level of a noisy industrial facility has been a key success factor.
[1] according to European directive 2003/10/EC, for the daily noise exposure level L EX, 8h: 80 dB(A) and 85 dB(A) are the thresholds, respectively lower and upper, triggering the action; 87 dB(A) is the authorized limit value
[2] regulatory and normative requirements vary from country to country; in France, the standard NF S 31-080 Acoustics - Offices and associated spaces - Levels and criteria of acoustic performance by type of space (2006) constitutes a reference document
[3] in France, a limit is set by a prefectural decree e.g. 60 dB (A)
[4] in France, emergence - i.e. the difference between the A-weighted equivalent continuous pressure levels of ambient noise (noisy industrial facility in operation) and residual noise (in the absence of noise generated by the noisy industrial facility, but measured over its period of operation) - is legally limited - depending on the case to 5 or 6 dB (A) during the daytime and to 3 or 4 dB (A) during the night period in regulated emergence zones (REZ)