In an Air Separation Unit (ASU), gas decompression (relief) vents often allow mixtures of air and Nitrogen (N2), or pure Oxygen (O2), to be released into the atmosphere, which makes it necessary to reduce the noise caused, both for the protection of site personnel and for the neighborhood, because the sound power level before soundproofing can exceed 150 dB(A), which is considerable.
If the considered site is in Europe, Directive 2003/10/EC of the European Parliament and of the Council, of February 6, 2003, concerning the minimum safety and health requirements relating to the exposure of workers to risks due to physical agents (noise) should be respected: for the level of daily exposure to noise L EX, 8h: 80 dB(A) and 85 dB(A) are the thresholds, respectively lower and upper, triggering action; 87 dB(A) is the authorized limit value for the peak sound pressure ρpeak: 112 Pa i.e. 135 dB(C) and 140 Pa i.e. 137 dB(C) are the thresholds, respectively lower and upper, triggering the action; 200 Pa i.e. 140 dB(C) is the authorized limit value.
In addition, French regulations require a reduction in industrial noise in factory environments, distinguishing 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 certain cases, a prefectural decree (when it is not ministerial) sets the noise levels not to be exceeded at the property line: this is particularly the case for Facilities Classified for Environmental Protection (FCEP) e.g. 70 dB(A) during the day and 60 dB(A) during the night
- emergence i.e. the difference between the equivalent A-weighted continuous pressure levels of ambient noise (industrial site in operation) and residual noise (in the absence of noise generated by the industrial site, but measured over the period of operation of the industrial site) is limited to Regulated Emergence Zones (REZ), which generally include, near the industrial site: homes, building zones and areas occupied by third parties e.g. Public Acces buildings (PAB), other industrial establishments. If the ambient noise level in the REZ (including industrial site) 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 industrial 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 Environmental Protection (FCEP), the marked tone, the presence of which is linked to the difference between the sound pressure level in a 1/3 octave frequency band data 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
These noise limits are not exactly applicable in every country in the world, but they nevertheless always provide a solid basis for achieving a choice soundscape in noisy industrial sites.
ITS participated in the noise reduction of gas decompression (relief) vents of an Air Separation Unit (USA), in a petrochemical industrial site located in Central Asia:
- a decompression vent for a mixture of air and Nitrogen (N2)
- a decompression vent for pure oxygen (O2)
Design and construction of equipment for noise reduction of gas decompression (relief) vents
The design and construction of equipment for noise reduction of gas decompression (relief) vents (sets of baffles with sound-absorbing lining inserted in large-section concrete ducts: of the order of 15 m2 for one , 5 m2 for the other) involved:
- a structural study: to ensure the necessary stability of the different subassemblies constituting the soundproofing device
- acoustical calculations: to determine the insertion loss and the self noise of the sound-absorbing baffle arrangements constituting the silencers with a view to obtaining a sound pressure level not exceeding 85 dB(A) at 1 meter, which is ambitious in the circumstances
- aeraulic calculations: to evaluate the total pressure loss linked in particular to the open area ratio of the silencer, its length and - of course - the density and speed of the considered fluid, which must not exceed a certain limit for the proper functioning of the process
In such a context, Module 1 of the SILDIS® software (cf. https://www.its-acoustique.fr/en/sildis/acoustic-aerodynamic-performance-silencer-prediction-software ) was a valuable tool for predicting the acoustic and aeraulic (aerodynamic) performance of silencers for the noise reduction of gas decompression (relief) vents (this software can also be used for the sizing of devices to limit the sound impact of ventilation networks, air intakes or exhausts for engines or turbomachineries); further information relating to silencers marketed by ITS - operating principle, applications, input data useful for sizing, performance, can be found elsewhere on this site cf. https://www.its-acoustique.fr/en/supplement-acoustics/f/noise-reduction-by-the-means-of-a-silencer -.
The sound-absorbing baffles were made of mineral fiber filling, covered with a fabric - respecting the constraints linked to the presence of pure oxygen (O2) for one of the silencers - and protected by a perforated metal sheet. The selection of first choice materials (for which the meticulous consideration of the impact of all their intrinsic and dimensional characteristics being a basic know-how for the sizing of such equipment: each detail has its importance), the study of an optimized combination of different parameters for the best compromise between performance (acoustics and aeraulics), size (imposed) and cost, and careful manufacturing within the allotted time contributed to the success of this project to reduce the noise of gas decompression (relief) vents allowing, once again, to illustrate the capabilities of ITS and its commercial partners in the field of high-tech industrial acoustics.
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