ITS will participate in the soundproofing of boosters / expanders, major componets of Air Separation Units. Although such hardware is not very large (the surface area of its footprint is around 10 m2, its height typically being around 4 m), its noise emissions are nevertheless far from negligible when taking inventory of the noise sources of such an installation, since the sound power level is around (for each one) 125 dB(A), which is considerable; this is all the more true when 6 such pieces of equipment have to be taken into account - as in the context of the project to which this publication relates - because the correction term to be added to the unit sound power level is 10 * log10 (6) = 7.8 dB(A).
As is often the case in the context of industrial projects, the reduction of noise from pressure multipliers/reducers meets the dual requirement of maintaining acceptable working conditions for personnel likely to be nearby (employees, external contractors) and preventing noise pollution in the environment:
- regarding working conditions, European Directive 2003/10/EC concerning the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (noise) is a reference document, fixing:
- 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
- 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
- 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
- regarding environment, French regulations, if applied, distinguish between the daytime period (7:00 a.m. to 10:00 p.m. except Sundays and public holidays) and the nighttime period (10:00 p.m. to 7:00 a.m. as well as Sundays and public holidays):
- in some 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 (considered noisy site in operation) and residual noise (in the absence of the considered noisy site, but measured over the period of operation of the considered noisy site) is limited in the Regulated Emergence Zones (REZ), which generally include, near the power plant: 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 the considered noisy site 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 considered noisy site) 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 a Facility Classified for the Protection of the Environment (FCPE), 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
In such cases (as for more common air compressors), it will be necessary to enclose the boosters/expanders in a metal structure with a frame and with:
- for fixed or easily removable parts, by elements: industrial soundproofing panels with an absorbing face (modular)
- 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 are equipped with ventilation silencers; it is a matter of the continuity of the acoustic insulation, without which the degradation of the soundproofing performance at the level of the openings - if they remained gaping - would not be compatible with an objective of reducing noise emissions the slighest bit restrictive, especially when a sound pressure level at 1 m below 80 dB(A) is sought as in the context of the project which is the subject of this publication.
Reconciling the requirements related to the evacuation of the heat power dissipated by the enclosed equipment - which will require the installation of a fan that is noisy by nature - and the challenges of soundproofing will be an important aspect of such a project, the task being often complex due to the space constraints for the installation of noise attenuators.
Another key point for its success will be the proper integration of the enclosures into an environment consisting of multi-level metal platforms (as is often found in the industrial chemical sector). In this case, it will not have five sides (4 walls and 1 roof) as in the case of an installation on a concrete slab - which would be sufficient to prevent the propagation of noise - but six, because - in the absence of such a concrete slab - it will be necessary to prevent downward sound transmission by means of custom-made sound insulation panels which will have to be fixed to metal structural elements provided separately - which will form a mesh creating numerous discontinuities taking into account the presence of pipes and ducts of all kinds, which will require the consideration of just as many interfaces.
Careful design work, based on precise and comprehensive three-dimensional modeling - no screw or nut will not be defined and its placement meticulously considered - will be necessary. This will involve taking into account all the details contributing to the development of an airtight soundproofing system (without which it would be ineffective in terms of acoustic insulation), and ensuring sufficiently simple and rapid installation. It will also encompass the proper consideration of the dismantling (and reassembly) procedures for the equipment used to soundproof the boosters/expanders. This will allow for safe maintenance and repair work for the personnel assigned to these tasks, who will have to handle sub-assemblies whose location must be adapted to the specific needs of enclosed hardware, and whose geometry and weight must be adapted to the site conditions, which are particularly constraining in this instance.
These aspects of the study (which will also include numerous calculations - in acoustics, in mechanical resistance of metal structures, in ventilation aerodynamics, in electrical engineering for fans and associated sensors) will be crucial to the project's success.
The rigor, ingenuity, and experience - with prestigious references in all sectors - of the human resources assigned by ITS and its business partners to such tasks, using the best Computer-Aided Design tools - which contribute to their reputation, as does the robustness of the constructions, adherence to deadlines, and, more generally, fulfillment of all commitments made - were key factors in the awarding of this contract by one of the most demanding Clients. It will be a point of honour to ensure they are not disappointed!
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