How a Vent Silencer Works
A pressurized fluid that expands generates noise—sometimes well above regulatory thresholds. This note explains the physical mechanisms at work and how the VS BOËT silencer controls them.
Industrial Relaxation Systems
In industry, many pieces of equipment are designed to reduce the pressure of a fluid or discharge a pressurized flow: safety valves, pressure-reducing valves, boiler blow-off valves, etc. These systems are ubiquitous in the energy, petrochemical, nuclear, and food processing sectors.
The common feature of all these devices is that the flow of fluid through a restricted cross-section (orifice, perforations, valve seat) causes expansion which, if not controlled, generates intense noise—a characteristic of jet noise.
The Vena Contracta — the source of the noise
When a pressurized fluid flows through a constricted section, the abrupt change in cross-sectional area and the significant pressure difference between the upstream and downstream sides cause high-velocity jets to form. This zone of maximum constriction is called the vena contracta.
The ratio of downstream to upstream pressures determines the flow regime within this zone:
- Subsonic — the fluid’s speed is less than the speed of sound.
- Sonic — the velocity is equal to the speed of sound. The flow is constricted: the velocity in the Vena contracta is at its maximum, and the pressure there is at its minimum.
- Supersonic — during a sharp drop in pressure, the velocity at the outlet of the expansion system can exceed the speed of sound.
The energy converted during expansion is largely dissipated as heat, with a small fraction converted into acoustic energy. It is this fraction that is the source of the perceived noise.
The 5 Noise Generation Modes
Noise generation can be divided into five regimes. The higher the upstream-to-downstream pressure ratio, the more intense the regime and the higher the noise level.
Starting at Mode 2, the pressure at the outlet of the Vena contracta creates a difference from the ambient pressure. Shock cells form to equalize the pressures, and the speed within these cells exceeds the speed of sound.
| Régime | Écoulement | Description |
|---|---|---|
| Régime 1 | Subsonique | Vena contracta bien formée. Forte récupération de pression en sortie (P2). Aucune cellule de choc ne se forme. |
| Régime 2 | Sonique | Des cellules de choc se forment en barrières mais n'interagissent pas encore entre elles. P2 conserve une récupération partielle, qui diminue à l'approche de la limite haute du régime. |
| Régime 3 | Supersonique | Aucune récupération de pression. Pas de vena contracta clairement définie — la chute de pression est continue. Les cellules de choc dites « quivering » interagissent fortement entre elles. |
| Régime 4 | Disque de Mach | Les cellules de choc fusionnent en un disque de Mach unique. Un saut de bruit est observé lors du passage du régime 3 au régime 4. |
| Régime 5 | Niveau maximal | Efficacité acoustique constante. Toute diminution supplémentaire de P2 n'augmente plus le niveau sonore — comportement propre à ce régime, absent des régimes 1 à 4. |
Nature of the expansion noise
The noise produced by the expansion of a fluid is the superposition of two distinct components, each with different physical origins and spectral signatures.
Mixing Noise (All RPMs)
Broadband noise, present regardless of the flow regime. It is generated by the turbulent structures in the jet: small structures generate high frequencies, while large structures generate low frequencies.
Impact Noise (Sonic Regimes Only)
Generated by the presence of shock cells or the Mach disc. It consists of a tonal component—the screech —and a broadband component. Its characteristic frequency can be calculated analytically.
Paramètres
Fréquence du screech
12 360 Hz
12.4 kHz
Zone spectrale
Hautes fréquences
Exemple — soupape vapeur industrielle
Une soupape de sécurité vapeur (c = 480 m/s, D = 8 mm) s'ouvre en régime sonique (M = 1,0). Avec S ≈ 0,30, le screech se situe autour de 18 000 Hz — au-dessus du seuil d'audibilité courant. C'est précisément l'objectif du détendeur perforé BOËT : réduire D pour repousser f hors de la plage audible.
The VS Silencer — Four Combined Mechanisms
The VS silencer consists of two complementary components: a pressure-reducing element (a perforated tube, possibly combined with a metal mesh) and a dissipative section. These two stages simultaneously activate four noise-reduction mechanisms.
Stepwise relaxation
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Building codes
Quality Certifications
French Code
CODAP
CODETI
U.S. Code
ASME Section VIII
ASME B31.1
ASME B31.3
European standards
EN 13445
; EN 13480
Nuclear sector
RCC-M (AFCEN)
