French regulations regarding the limitation of environmental noise stipulate the following, with regard to a Facility Classified for Environmental Protection (FCEP):
- if the ambient noise level in the Regulated Emergence Zones - REZ - (including installation 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 Regulated Emergence Zones - REZ - (including installation noise) is greater than 45 dB(A) then the admissible emergence is 5 dB(A) during daytime periods and 3 dB(A) during periods nocturnal
Emergence i.e. the difference between the equivalent A-weighted continuous pressure levels of ambient noise (facility in operation) and residual noise (in the absence of noise generated by the facility, but measured over the period of operation of the facility) is limited in Regulated Emergence Zones (REZ), which generally include, near the facility: homes, building areas and areas occupied by third parties e.g. Public Acces Buildings (PAB), other industrial establishments.
Furthermore, a 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 adjacent frequency bands, is considered undesirable if it exceeds 10 dB in the frequency interval 50 Hz-315 Hz or 5 dB in the frequency interval 400 Hz-8000 Hz.
This makes it necessary, in many contexts, the implementation of specific constructive measures in the fight against noise, which constitutes the heart of the activity of ITS, which has participated in the lowering of sound levels at an industrial site in Western Europe, as part of the installation of two gas turbines - with a total power of around 170 MW - dedicated to the production of electricity.
The aim was to ensure that the limit values of admissible emergence were not exceeded, to preserve the vicinity of this facility located in the heart of a conurbation (with numerous office buildings), from the impact of these imposing thermal machines, as well as gas modules and associated alternators - by nature: all very noisy -, by means of ventilated sound enclosures.
Means for lowering sound levels at an industrial site
The means for lowering the sound levels of an industrial site can be classified into different categories:
- silencers, which make it possible to reduce the sound power level (generally expressed in dB ref. 1 pW) of gaseous vents (discharges to the atmosphere of fluids under pressure, air intakes and discharge of aeraulic networks, lines of exhaust and chimneys)
- anti-noise walls, which provide a screening effect in more or less specific locations, with a reduction in the sound pressure level (generally expressed in dB ref. 20 μPa) depending (apart from the frequency) on the distances to the the screen of the noise sources and the observation point, the dimensions of the screen, and the possible presence of reflective surfaces at the start or finish of the sound waves path; at elevated points of reception of sounds emitted by noise sources (such as balconies or even rooms in high-rise buildings), the effectiveness of such noise attenuation devices is less good
- ventilated enclosures (hoods, soundproofed buildings), making it possible to reduce noise "at the source" i.e. by diminushing the sound power level (generally expressed in dB ref. 1 pW) of noisy equipment; this means of soundproofing has the advantage (which is not secondary) of protecting personnel (e.g. working on supervision, maintenance) of the energy production site (i.e. avoiding situations of auditory trauma , accidents when warning signals are not heard); European directive 2003/10/EC stipulates noise exposure limit values for this purpose: limit exposure values for workers to noise:
- exposure level L EX, 8h = 87 dB (A) and peak sound pressure ppeak = 200 Pa or 140 dB(C) ref. 20 μPa
- higher exposure values of workers to noise triggering the action: exposure level L EX, 8h = 85 dB (A) and peak sound pressure ppeak = 140 Pa or 137 dB(C) ref. 20 μPa
- lower exposure values of workers to the noise triggering the action: exposure level L EX, 8h = 80 dB (A) and a peak sound pressure ppeak = 112 Pa or 135 dB(C) ref. 20 μPa
As part of the project that is the subject of this article, it is this last solution which was marketed by ITS, with a scope including:
- structural studies: sizing of sub-assemblies of very large metal constructions, which must withstand bad weather, seismic risks and must allow the dismantling of some parts of the works (with large areas) during maintenance or repair work of enclosed hardware
- sound insulation calculations: when one foresee, at a short distance from the partitions of structures dedicated to soundproofing, sound levels as low as 85 dB(A) at 1 meter, or even lower in some cases - while some of the noisy equipment has an overall weighted sound power level greater than 135 dB(A), which is considerable - the sound levels at long distances (several hundred meters) - at the property boundary or beyond - do not pose a problem for the environment
- aeraulic studies: regarding the renewal of air and the evacuation of the (very significant) heat power dissipated inside the sound enclosures
- electrical studies: for components such as fans, sensors
As part of the current project of lowering the sound levels of the concerned ndustrial site, was foreseen a metal frame (steel) and:
- for fixed or easily removable parts, by elements: industrial soundproofing panels with an absorbing face (modular)
- for acces for employees and some hardware: (rotating) acoustic doorsets with, for translucent parts (with glazing guaranteeing residual safety in the event of accidental breakage): acoustic window sets
In addition, the apertures provided for the ventilation of constructions intended for noise limitation have been equipped with ventilation silencers:
- for the noise reduction of the combustion turbines and gas modules (in a closed space), fans (also: noisy by nature, the silencers having also served to reduce their noise impact) to force the evacuation of hot air
- for the noise reduction of the generator (to completement the effectiveness of a standard enclosure: by means of a noise-proof wall, which is only distinguished from what has been said about ventilated acoustic enclosures by the absence of a roof), a principle of ventilation by natural convection was used (with air intakes in the lower part of the structure, and an evacuation by the top)
This works constituted an additional opportunity for ITS and its commercial partners to demonstrate their mastery of all aspects of an operation to lower the sound levels of an industrial site with rigorous multidisciplinary design, harmonious project management and quality construction, meeting the highest level of requirements, delivered within the announced deadlines.
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Structures for lowering the sound levels of an industrial site in a power plant similar to those covered by this article |
In addition to sound enclosures and noise-barrier walls, ITS markets, for lowering sound levels in industrial sites (visible in the photo opposite but not delivered as part of the project subject of this article) soundproofed air intake systems for combustion turbines / gas turbines, exhaust silencers for combustion turbines / gas turbines, and also Vent silencers for pressurized gas | |