Reducing the noise of technical facities quite often motivates the recourse to a company specializing in soundproofing such as ITS, to prevent or treat nuisances linked to hardware located:
- outdoor e.g. in relation to energy production, refrigeration, waste disposal or in relation to more or less specific production or processing activities; the sound propagation outdoor is then at stake and can be reduced either "at the source" e.g. by means of enclosures (canopies) and silencers, or by means of screens (noise barrier walls)
- inside buildings (in technical rooms: gensets, pumps, Air Handling Units - AHU - and other Heating Ventilation Air Conditioning - HVAC - equipment also often with outdoor units); the transmission of sounds (e.g. to other spaces in the building in question - home, office space, health or educational establishment, hotel, gym or leisure hall) may require the use not only of soundproofing hardware of the kind mentioned above but also of sound-absorbing materials to reduce the reverberation of enclosed spaces, and of anti-vibration devices
Each project has its specificities (noise sources, location, position of locations to be protected from noise, acoustic objectives, constraints for selection or development of noise attnuation devices) which is an opportunity for the acoustician to rise challenges of various kinds and of varying importance, which mean that his daily life is not routine.
ITS participated in the noise reduction of a big technical facility on the borders of Eastern Europe and Asia.
On the one hand, it was a question of complying with the minimum safety and health requirements relating to the exposure of workers to risks due to physical agents. In this area, European Directive 2003/10/EC (which can be considered a reference document beyond the geographical scope of the Member States) sets the noise level limits for the preservation of hearing:
- 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
On the other hand, it was necessary to protect the neighbourhood from any noise disturbance, with regard to which French regulations, if applied, 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 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 (technical facility in operation) and residual noise (in the absence of noise generated by the technical facility, but measured over the period of operation of the technical facility) is limited in the Regulated Emergence Zones (REZ), which generally include, near the technical facility: homes, building zones and areas occupied by third parties e.g. Public Access Buildings (PAB), industrial establishments. If the ambient noise level in the REZ (including the noise of the technical facility) 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 technical facility 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 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
As this was a combined cycle gas-fired power station, the main sources of noise were as follows (the steam turbine was not included in the scope to be considered for the planned phase of works):
- the combustion turbine itself
- plenums for an air intake duct and for an exhaust duct
- a generator
When it comes to producing electricity (with a power of around 900 MW), noise emissions are very significant - the sound power level of some of the sub-assemblies listed above reaches 145 dB(A) which is considerable - and it is necessary to use very specific noise reduction equipment for such technical facilities, in order not to exceed the noise levels permitted at various specified locations:
- in nearby acoustic field e.g. at 1 m from enclosed noisy hardware
- at the edge of an industrial property
- at local residents' premises
Design and manufacture of equipment for noise reduction of a technical facility
The design and manufacture of equipment for reducing the noise of a technical facility such as those marketed by ITS not only in the context of energy production, but in many other industrial contexts, involves, in the case of metal constructions, a (steel) frame 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 avoid noise transmision trough openings).
As regards the turbine itself, the noise reduction equipment takes the form of a soundproof building (of large dimensions) with four walls and a roof (forming a closed and covered unit) with soundproof air inlet and outlet; for the other sub-assemblies of the technical installation, these are acoustic screens (noise barrier walls), complementing the limitation of sound propagation of already soundproofed equipment.
Of course, studies are crucial for the success of such projects:
- structural calculations: to ensure the stability of the equipment, taking into account site constraints e.g. temperature (possibly particularly low in winter, in the context of the project covered by this article) and the need to make possible in complete safety interventions related to the maintenance and replacement of some enclosed equipment, which involves the dismantling of certain elements - sometimes of large surface area - whose temporary absence must not weaken the whole too much (for what simulations with predictive acoustics software [1] are necessary)
- acoustic calculations: for the selection of construction elements and the assessment of the noise impact of the technical facility at the various locations of interest
- aeraulic calculations: for the dimensioning of the ventilation system intended to evacuate the heat output dissipated by the turbomachines and related noisy hardware inside the enclosed constructions
- electrical study: in relation to the installation of fans, sensors
Manufacturing is not a formality when it comes to noise reduction equipment for technical facities that must be delivered without delay (the consequences would be disastrous), and with impeccable quality (both in terms of the choice of raw materials, components of all kinds, and in terms of assembly and corrosion protection techniques).
Experience then counts for the provision of equipment that complies with all technical specifications, and generally meets all expectations: ITS and its business partners are far from being devoid of it, having faced such situations for decades, on the occasion of similar projects, for the most demanding customers.
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Equipement for noise reduction of a technical facilty |
Equipement for noise reduction of a technical facilty |