Noise limitation in a factory can and must be considered from two complementary angles: on the one hand (internally) safety at work and on the other hand (with regard to the outside) protection of the environment, i.e. the neighbourhood.
In terms of safety at work, it is a question of protecting personnel (e.g. those working in supervision or maintenance) from excessive exposure to noise, which could (irreversibly) damage their hearing or even (just as immediately) cause potentially serious accidents if warning signals are not correctly perceived (including those related to the movement of vehicles and machinery) due to an excessively noisy sound environment. In this area, European Directive 2003/10/EC, transposed into the national law to be considered, constitutes a common regulatory reference based on different indicators:
- limit exposure values for workers to noise: exposure level L EX,8h = 87 dB (A) and peak sound pressure pcrête = 200 Pa or 140 dB (C) ref. 20 μPa /li>
- higher exposure values for workers to noise triggering action: exposure level L EX,8h = 85 dB (A) and peak sound pressure pcrête = 140 Pa or 137 dB (C) ref. 20 μPa
- lower exposure values for workers to noise triggering action: exposure level L EX,8h = 80 dB (A) and peak sound pressure pcrête = 112 Pa or 135 dB (C) ref. 20 μPa
In terms of environmental protection, French regulations distinguish between the daytime period (7 a.m. to 10 p.m. except Sundays and public holidays) and the nighttime period (10 p.m. to 7 a.m. as well as Sundays and public holidays):
- in certain cases, a prefectural decree (when it is not ministerial) sets the noise levels not to be exceeded at the property boundary: this is particularly the case for Facilities Classified for the Protection of the Environment (FCPE) e.g. 70 dB(A) during the day and 60 dB(A) at night
- emergence i.e. the difference between the equivalent continuous A-weighted pressure levels of ambient noise (factory in operation) and residual noise (in the absence of noise generated by the factory, but measured over the period of operation of the factory) is limited in the Regulated Emergence Zones (REZ), which generally include, near the industrial establishment: homes, building zones and areas occupied by third parties e.g. Public Access Buildings (PAB), other industrial establishments. If the ambient noise level in the REZ (including factory noise) is between 35 dB(A) and 45 dB(A) then the admissible emergence is 6 dB(A) during the day and 4 dB(A) at night. If the ambient noise level in the REZ (including factory noise) is greater than 45 dB(A) then the admissible emergence is 5 dB(A) during the day and 3 dB(A) during the night.
- in the case of an Installation Classified for the Protection of the Environment (ICPE), the marked tone, the presence of which is linked to the difference between the sound pressure level in a given 1/3 octave frequency band and the adjacent frequency bands, is considered undesirable if it exceeds 10 dB in the frequency range 50 Hz-315 Hz or 5 dB in the frequency range 400 Hz-8000 Hz
Since no one is supposed to be ignorant of the law, it is therefore necessary to be concerned about the noise caused by hardware and equipment constituting a production or processing tool - whatever the sector of activity - and to keep it within acceptable limits.
This applies to noise sources located inside buildings, which must then have minimum characteristics in terms of sound transmission limitation (this is not always the case for dilapidated and not fully enclosing constructions in quarries, steelworks, industrial sites in the wood sector or the prefabrication of concrete products).
This also applies to noisy equipment installed outdoors (dust extraction networks, fans, air condensers, thermal or electric motors, pumps, compressors, grinders) as well as to the ends of air networks (air inlets and outlets, chimney outlets) and pressurized fluid networks (e.g. decompression vents).
The list of sound sources to be addressed in the context of limiting the noise of a plant varies according to the site considered, their acoustic power (which is an intrinsic characteristic) and, for specified locations (within the site, at the confines or beyond), the conditions of sound propagation (involving atmospheric absorption, and attenuation due to the ground or obstacles) as well as the objectives assigned in terms of sound pressure level.
In a gas-fired power plant with electricity production based on combustion turbines, the notoriously noisy equipment, subject to noise control arrangements involving ITS, are in particular:
- the turbines themselves
- the load compartments
- the plenums for an air intake duct and for an exhaust duct
- the gas and injection modules
- the generators
- (in the case of a combined cycle power plant) steam turbines
ITS participated in the noise limitation in a factory producing electricity in Eastern Europe, involving two combustion turbines of unit power 0.5 TeraWatt, with the noisy sub-assemblies mentioned above.
Study and manufacture of works for limiting noise in a factory
The study and manufacture of works to limit the noise of a factory are at the heart of ITS's activity:
- to limit the sound power level of noisy equipment:
- soundproofed hoods, enclosures and buildings (with soundproofed ventilation systems)
- air intake silencers, silencers for air discharge or for process fluids (chimneys, vents)
- to limit the sound pressure level in specified locations
- acoustic screens, noise barriers
- to limit the amplification of sound levels
- sound-absorbing linings
The definition of the most appropriate means for noise limitation in a factory must be carried out on a case-by-case basis, taking into account the context and objectives specific to each project. The experience acquired over time from similar projects is of course a key factor for the success of actions in this area:
- when it comes to soundproof buildings, easy to be dismantled to allow repair work on turbomachines, or maintenance work for turbines themselves, load compartments, gas and injection modules; specific ventilation systems are necessary for air renewal and temperature control
- when it comes to acoustic screens (noise barriers) around air intake ducts and exhaust ducts, and to complete the soundproofing of generators already equipped with noise control means
A complete multidisciplinary study is required, in the case of large-scale metal constructions:
- with regard to structural aspects: site conditions (rain, snow, wind, seismic activity) must be taken into account in order to design resistant and durable structures; the application of more or less specific calculation codes and local regulations is required
- with regard to predictive acoustic calculations using the SILDIS® software [1] e.g. Module 1 for predicting the acoustic and aeraulic (aerodynamic) performance of silencers; Module 2 for predicting the acoustic performance of flat partitions; Module 10 for predicting the noise emissions of buildings and other constructions
- for ventilation, without which the heating due to the heat emissions of turbomachines would be problematic for their proper operation
- for the electrical installation: involving fans, sensors
The manufacture of equipment for the limitation of noise in a factory involves a metal (steel) framework and:
- for fixed or easily removable parts, by elements: industrial soundproofing panels with an absorbing face (modular), without roof for the structures relating to generators
- for acces: (rotating) acoustic doorsets with, for translucent parts (with glazing guaranteeing residual safety in the event of accidental breakage): acoustic window sets
In addition, the ventilation air inlets and outlets (the latter: with fans, except for generators acoustic screens for which ventilation is natural) have been equipped with ventilation silencers (to ensure the necessary continuity of sound insulation).
The accomplishment of engineering tasks by an experienced human resource, the selection of first-class raw materials, the construction with proven assembly techniques meeting the highest quality standards for a high-performance and durable finished product, delivered on time: these are the key stages in the process of developing solutions for limiting the noise of a factory, perfectly mastered by ITS and its commercial partners.
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Sound enclosure for noise limitation in a factory |
Acoustical screen (noise barrier wall) for noise limitation in a factory |