Cross-section of VANEC vent silencer

VANEC Natural Gas Blowdown Silencer


Silencer Sizing/Selection 

The following information is required to optimize a vent silencer selection;
  1. Venting gas
  2. Molecular weight or specific gravity
  3. Flow rate (lbs. / hr, ACFM, SCFM)
  4. Upstream pressure & temperature
  5. Valve manufacture, size, model no.
  6. Unsilenced noise level (octave band analysis)
  7. Allowable pressure drop
  8. Noise reduction criteria (Overall level & distance)
  9. Installation particulars (horizontal, vertical)
10. Pipe sizes
11. Materials of Construction
12. Weight limits
13. Loading requirements (wind, seismic, nozzle.)

Due to the many variables and the complexity of vent silencer sizing, selection, and design, computer optimization methods are employed. The size is generally dictated by velocity, required noise reduction, and allowable pressure drop.

More About Vent Silencers
 A vent silencer is a device that is intended to reduce the noise levels created when high-pressure gas or steam is expanded to the atmosphere.  This noise is produced by the rapid turbulence of the mixing gas jet and energy release of the compressed gas. Most of the noise occurs at the expansion across the valve and is emitted at the stack opening. The silencer is installed at the exit to muffle this noise.

There are two noise reduction principles used in a vent silencer:
1) Absorption of the high frequency audible noise into a sound-absorbing material. 
2) A reactive section to attenuate the low frequencies and provide broad band noise reduction.
Vent Silencer Calculations

Pressurized Volume - Blowdown Time Calculation: 

Model Pressure Drop Factor
412 10
521 13
552 20
553 30
561 11.75
562 12.25
711 Presser Reduction/Diffuser 10

These velocity limits are based on fully expanded flow, i.e. ACFM at the operating temperature and downstream pressure.

                  Pressure Drop Factors 

Intermittent 15000 to 20000 FPM
Continuous 10000 to 15000 FPM
Max. Allowable 20000 FPM
Critical Less than 10000 FPM

VANEC vent silencers are available in carbon steels, galvanized steels, stainless steels, or with numerous protective-coating systems. Diffusers are highly engineered, designed and fabricated to ASME Section VIII requirements if specified, and may be multi-stage.

                                                                                                                       Recommended Operating Velocity Limits 
                                                                                                                                        Blowdown Duration

Vent & Blowdown Silencers

4 VANEC blowdown silencers installed at

natural gas compressor station

Sketch of silencer internals

V= Pressurized Volume (ft3)
A= Effective Area in Square Inches (Valve or Orifice Throat Area x Flow Coefficient). Use 0.85 Flow Coefficient unless known.
SG= Specific Gravity of Gas
Z= Compressibility Factor (at initial Pressure)
Ta= Initial Temperature (˚R)
Pa= Initial Pressure (psia)
Patm= Atmospheric Pressure (psia)
Natural Logarithm or Loge

Product Information
VANEC vent and blowdown silencers are designed for use in suppressing the noise associated with venting high-pressure gas or steam to atmospheric pressure.  VANECoffers models capable of supressing gas discharge noise to a community located as close as 400 ft from the vent or blowdown.  We also offer models that reduce noise to an acceptable level at 800 ft or 1600 ft from the source.  Available models: 412, 521, 552, 553, 561, 562, 711

Our vent and blowdown silencer models typically have a three element design, each providing a critical function:
1) Diffuser
•    Redirects the flow 90° and expands flow into the plenum chamber
•    Absorbs thrust forces
•    Shifts frequencies into the audible range
•    Disrupts the jet mixing zone
•    Provides a pressure staged expansion to atmosphere
2) Plenum Section
•    Expands the gas and provides a reduced flow velocity
•    Provides a reactive element to attenuate the low frequencies
•    Redirect the flow through the upper splitter panel sections
•    Where conditions dictate the plenum and head may be acoustically
•    lagged to improve the (TL) transmission loss and reduce radiated
•    noises
3) Splitter Panel Sections
•    Primary high frequency noise absorption section
•    Panels consist of perforated metal surfaces through out the
•    perimeter with acoustic packing inside each section. The splitter
•    panel sections diameter thickness, open area spacing and lengths
•    are optimized for maximum (IL) insertion loss.