APPLICATION: Individually designed to eliminate noise caused by atmospheric discharge of high-pressure gases and steam: Safety relief valves, Switch valves, Control valves, Anti-surge valves, steam ejector discharges, Turbine exhausts.

The vent/blow-off silencer is used to reduce noise levels created by gas or vapour discharging into the atmosphere. A silencer is installed in the discharge line or at the end of the discharge line, providing noise reduction while releasing gas or steam.

For maximum effectiveness, Noise and Pulsation Control Ltd. silencers simultaneously use two techniques of noise reduction: (1) absorptive,(2) reactive. Each silencer incorporates sound-absorbing material in addition to tuned reactive chambers.

The absorptive behaviour of the silencer provides broadband attenuation. Reactive behaviour is governed by a combination of changes in flow-section and forced phaseshifts, and provides high attenuation in the lower octave bands. Both techniques contribute to good noise reduction over the entire audible frequency range.

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Gases and noise enter the silencer through small drilled holes in a diffuser, giving a weighting-shift towards high frequencies. A subsequent expansion chamber gives reactive reduction of the low-frequency components of noise. Acoustic lining to the expansion chamber controls absorption of high-frequency energy. Gases and partially attenuated noise then pass to multi tubular acoustic modules, for a final reduction.



The diffuser is made from drilled pipe with hole diameters kept to a practical minimum and is then
finished either with a pipe cap or with a dished head. Full continuous penetration weldments are
used to join all of the seams. Adequate thickness is maintained to prevent disintegration under the
thrust of the flow. The diffuser can be designed to full pressure vessel code (without hydrostatic test)
if required.

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Acoustic Modules

Construction is in one, two or three bundles, determined primarily by the residual sound pressure level that is required. Separate bundles are connected by secondary expansion chambers to maintain optimum effectiveness. Diameters of flow tubes in the acoustic modules are designed to limit the fluid velocities, to control secondary noise-generation at the exits. Acoustic linings to the secondary expansion' chambers are protected, to prevent erosion into the flow stream. The degree of protection has to be varied according to flow velocity, density, and temperature.

Noise and Pulastion Control engineers select an acoustic lining-material which can withstand the anticipated temperature range, and can withstand fatigue of the sound-absorbing fibres under high stresses associated with intense noise fields, The material most commonly used over the temperature range -25°C to 600°C (-15°F to 1112 F) is long-strand glass fibre. Stainless steel wool is used for higher-temperature applications.


The overall diameter of the silencer is determined by the flow rate. Where as length of the silencer depends on the degree of attenuation required, itself governed by the size of expansion chamber, configuration of the reactive section, and lengths of the acoustic modules.

The external shell is constructed from seamless pipe/pipe caps in smaller sizes, and from continuously welded plate/dished head for larger-diameter silencers. Part of the shell and head assembly is lined to reduceinternal acoustic resonance's. 5 mm shell thickness is used to minimise noise-radiation from the silencer body.

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External supports on vertical or horizontal silencers

Silencers can be self-supporting from the nozzle depending on nozzle size, or can be supplied with support brackets/saddles for vertical and horizontal installations.

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Carbon Steel construction is used for standard duty. Construction can also be of stainless steel or other specified material.


Silencers can be supplied with a weather hood, flanged outlet or an open outlet.


All the silencers are fitted with drains to remove accumulated condensation or other liquids.


Noise and Pulsation Control Ltd.model VTS silencers are guaranteed to meet the customer's specified performance and integrity for a period of one and a half years after initial operation, or two and a half years after shipment.

Application data

For a specific recommendation on silencer, a customer should provide the following information:

  • Mass flow rate
  • Specific gravity and ratio of specific heats
  • Pressure upstream of reducing valve
  • Temperature upstream of valve
  • Reducing valve size, and flange type and rating
  • Line size, and flange type and rating
  • Required residual sound pressure level at a specified distance
  • Allowable pressure drop