Which noise reduction of control valves ?

When it comes to limiting noise emission due to discharge, noise reduction of control valves can be obtained by means of a suitable silencer, installed in the end of the piping line.

Such silencers are generally composed of a diffuser (upstream) and of a dissipative stage (downstream).

The diffuser is a perforated element, at which a change occurs (that is desired to be downwards) of the turbulence noise and shock noise, the presence of small diameter perforations causing a peak in noise generation for  high frequency. In addition, the presence of the diffuser causes a total loss of pressure on which attention must be paid (*).

The dissipative stage, in turn, consists of a lining (preferably: with high sound absorption), often used as filling of splitters, sometimes concentric (else: transverse), allowing for noise attenuation in a frequency range more or less extended namely depending on the acoustic characteristics of the porous medium and of its possible surfacing, on the geometry of the dissipative stage and on the nature and passage speed of the fluid.

In addition, the presence of the dissipative floor causes a total loss of pressure (usually less compared to that of the diffuser) on which attention must be paid (*) and generates self noise for which it is important to make sure that it is compatible with the noise reduction target to be considered as part of a project for which the implementation of a silencer is envisaged.

* Especially vis-à-vis the operating conditions of the valve on the downstream pressure which affects (upward)

Link to learn more about control valves noise calculation

What are the main parameters influencing control valve noise ?

Control valves (a fortiori when operating under conditions of high pressure drop) can contribute significantly to industrial & process plant noise, namely due to aerodynamic noise generation depending on valve data & process data, main parameters being as follows:

  • Valve inlet absolute pressure
  • Valve outlet absolute pressure
  • Liquid pressure recovery factor of a valve with or without attached fittings
  • Pressure differential ratio factor of a control valve with or without attached fittings at choked flow
  • Valve style modifier
  • (Required) Flow coefficient
  • Acoustic power ratio or Valve correction factor for acoustical efficiency
  • Molecular mass of flowing fluid
  • Inlet absolute temperature
  • Density of fluid at inlet
  • Specific heat ratio
  • Mass flow rate
  • Strouhal number
  • Outlet absolute temperature
  • Density of fluid at outlet
  • Speed of sound at downstream conditions
  • Valve outlet diameter
  • Internal downstream pipe diameter
  • Contraction coefficient for valve outlet or expander inlet

When valve style modifier is not an available explicit input data, additional parameters must be accounted

  • Area of a single flow passage 
  • Wetted perimeter of a single flow passage
  • Number of independent and identical flow passages in valve trim

When flow indicators are not available explicit input data, additional parameters must be accounted

  • Liquid pressure recovery factor of a valve without attached fittings
  • Upstream inside pipe diameter

Links to learn more about control valves noise calculation

What are the main fluids of which flow rate can me changed by a control valve ?

A control valve can change the flow rate of many fluids used in various industrial processes:

  • Acetylene
  • Air
  • Ammonia
  • Argon
  • Benzene
  • Isobutane
  • n-Butane
  • Isobutylene
  • Carbon dioxide
  • Carbon monoxide
  • Chlorine
  • Ethane
  • Ethylene
  • Fluorine
  • Freon 11 (trichloromonofluoromethane)
  • Freon 12 (dichlorodifluoromethane)
  • Freon 13 (chlorotrifluoromethane)
  • Freon 22 (chlorodifluoromethane)
  • Helium
  • n-Heptane
  • Hydrogen
  • Hydrogen chloride
  • Hydrogen fluoride
  • Methane
  • Methyl chloride
  • Natural gas
  • Neon
  • Nitric oxide
  • Nitrogen
  • Octane
  • Oxygen
  • Pentane
  • Propane
  • Propylene
  • Saturated steam
  • Sulphur dioxide
  • Superheated steam

What is the definition and what are the main noisy control valve types ?

A control valve is a power operated device which changes the fluid flow rate in a process control system. It consists of a valve (i.e. an assembly forming a pressure retaining envelope containing internal means) connected to an actuator that is capable of changing the position of a closure member in the valve in response to a signal from the controlling system.

Main control valve types are as follows:

  • Globe, single port
    • 3 V-port plug
    • 4 V-port plug
    • 6 V-port plug
    • Contoured plug (linear and equal percentage)
    • 60 equal diameter hole drilled cage
    • 120 equal diameter hole drilled cage
    • Characterized cage, 4-port
  • Globe, double port
    • Ported plug
    • Contoured plug
  • Globe, angle
    • Contoured plug (linear and equal percentage)
    • Characterized cage, 4-port
    • Venturi
  • Globe, small flow trim
    • V-notch
    • Flat seat (short travel)
  • Rotary
    • Eccentric spherical plug
    • Eccentric conical plug
  • Butterfly (centered shaft)
    • Swing-through (70°)
    • Swing-through (60°)
    • Fluted vane (70°)
  • Butterfly (eccentric shaft)
    • Offset seat (70°)
  • Ball
    • Full bore (70°)
    • Segmented ball

Which acoustic and aeraulic performance for silencers ?

The acoustic and aeraulic performance of silencers is a double problem with respect to which technological compromises (sometimes: sophisticated) must often be found in the perspective of the definition of soundproofing equipment allowing the normal operation of a network of fluid, especially when high speeds are involved.

Acoustic performance of silencers

Acoustic performance of silencers can be expressed in terms of difference (with and without silencer) of overall A weighted sound pressure levels or of sound pressure levels per octave bands at specified locations (such as maximum value at 1 m from the outlet plane of the silencer, average value of an enveloping surface) - also known as insertion difference of sound pressure level - or in terms of difference of overall A-weighted sound power levels or of sound power levels per octave bands of the outlet of the silencer (or of the mouth, or of the silenced noise source) - also known as insertion loss of the silencer -.

For orders of magnitude (and with respect to a noise spectrum like "pink noise"), a level difference of up to 10 dBA can usually be obtained without special requirements, while a level difference from 10 to 20 dBA requires a standard silencer without significant by-pass, while a level difference of 20 to 30 dBA requires a standard silencer with transverse partitionning devices of the absorbing filling and a resilient mounting, and while that a level difference of 30 to 50 dB often involves high performance silencers carefully designed and installed (a higher level difference shall involve a special achievement or 2 silencers installed in series with sufficient spacing).

Acoustic performance of dissipative silencers (at room temperature: ventilation, air conditioning, various industrial processes and for engines, gas turbines or at high temperature: stacks, exhausts - for what concerns the downstream stage of the soundproofing device when it comes to thermal engines - ...) is very frequency dependent. Dissipative silencers (or: dissipation silencers) are devices allowing an attenuation of sounds on a wide frequency range ; however, their efficiency is good neither at low frequency nor at high frequency.

Its is primarily related on the one hand to the behavior of the absorbing filling (e.g. its flow resitance for service conditions which can - in particular for the temperature - differ greatly from the laboratory conditions - quasi-atmospheric - under which the measurements were carried out) given the spacing of the airways and the length of the silencer (characterized by the propagation loss) and also, in many cases (for silencers other than those with a sound-absorbing lining on the - sometimes partial - periphery of the duct) to the geometry of the splitters (characterized by the reflection loss) and also to bypass phenomena: acoustic energy transmission through the casing of the silencer as well as, if appropriate (for silencers other than with simple dissipation) through splitters themselves, and finally to noise regenerated in relation to the speed of passage of the fluid.

 

160 silencer propagation loss sound absorbing filling flow resistivity

Acoustic performance of a dissipative silencer - variation of the propagation loss i.e. of the longitudinal attenuation (dB/m) as a function of the airflow resistance of the lining (results of evaluations with SILDIS® software)
 

The acoustic performance of reactive silencers (for compressors, for thermal engines exhausts) is linked to the geometric singularities of the internal parts (often: these are chambers - single, double or triple - connected by perforated or non-perforated tubes with possible changes in the direction of the gas flow; each dimension has its importance) which condition the reflections of the acoustic waves - basing the efficiency -, and also to the self noise (i.e. linked to the flow, therefore in relation to the passage speed of the fluid).

 

210 acoustic performance reactive silencer transmission loss of a triple expansion chamber

Acoustic performance of a reactive silencer - transmission loss of a triple expansion chamber (dB) at low and medium frequency (result of a simulation with SILDIS® software)
 

Aeraulic performance of silencers

Whether it concerns dissipative silencers or reactive silencers, the aeraulic performance is, as for other components of aerodynamic circuits and piping assemblies for pressurized fluids, mainly linked to sudden section changes (widening, narrowing ) and obstacles opposing the gas flow; the total pressure loss is, as always, increasing with the density of the fluid and with the square of its velocity. The thermodynamic conditions of use of the silencer have therefore, all other things being equal, their importance.

As orders of magnitude, the total pressure loss allowed for noise reduction devices of air condensers or cooling towers (even when the fans are of very large diameters e.g. above 10 m and induce colossal flow rates, especially when there are 10 units in the same facility set) are of the order magnitude of one millimeter of water gauge (i.e. 10 Pascals) when the pressure drop allowed to an exhaust silencer for very large engines (the power is counted in MW) or for high-capacity combustion turbines (the power is counted in tens or hundreds of MW and the mass flow in hundreds of kg/s) is generally around 10 mbar (i.e. 100 mm H20 or 1000 Pa).

In the specific case of dissipative silencers, an additional pressure drop (often: low, but should not always be neglected in the case of silencers without splitters i.e. with only a peripheral sound absorbing lining) is to be considered due to the friction of the fluid against the rough surfaces which constitute the sound-absorbing lining (of variable importance depending on the nature of the surface layer, the hydraulic diameter and the length of the section considered for such a linear loss).

In the specific case of reactive silencers, if necessary, an additional pressure drop is to be considered in the case where the gas flow is forced to pass through a perforated sheet (sometimes: this is the thickness of a tube e.g. for exhaust mufflers of cars, trucks, or even small machines - such a noise attenuation stage is also found in depressurization mufflers, for the expander).

longitudinal section - axial speed mapping for the noise attenuator in fig. 1
Aeraulic performance of a silencer consisting of a single splitter with profiled ends - Computational Fluid Dynamics - axial velocity mapping (longitudinal section of a rectangular duct, ITS simuation result)

Determination of the acoustic and aeraulic performance of silencers

As often in the field of acoustics, the determination of the performance of silencers can be carried out by means of calculations or measurements.

The prediction of the acoustic performance of dissipative or reactive silencers can be performed by ITS with the simulation software SILDIS®:

  • with Modules 1 and 1A, for dissipative silencers
  • with Module 1B, for reactive silencers

SILDIS®

end faq

For (absorption or reflection) silencer's geometries which are not those pre-programmed in the SILDIS® software, ITS human resources can carry out calculations using other means of simulation [1] [2] [3], to determine the acoustic and aeraulic performance as well.

In all contexts, the human resources of ITS, qualified in building physics, with extensive knowledge in acoustics, aeraulics-aerodynamics (measurements, calculations, design and project management) is able to determine, as acoustician specializing in this field, the performance of silencers for reliable and optimized sizing of such noise reduction equipment, for common projects (e.g. in the building sector) or for high-tech projects (e.g. in industry) , regardless of the level of expertise required.

Measurements relating to silencers can be made (for ambiant temperature, and when they are dissipative) according to NF EN SO 7235 Acoustics - Laboratory measurement procedures for ducted silencers and air-terminal units - Insertion loss, flow noise and total pressure loss; calculations with SILDIS® software or with other simualtion tools that ITS has and results of performance measurements are then perfectly comparable.

The acoustic performance of silencers, whatever they are and whatever the conditions of service, can also be evaluated on site, according to standard NF EN ISO 11820 Acoustics - Measurements on silencers in situ.

[1] FEM : Finite Element Method
[2] BEM : Boundary Element Method
[3] CFD : Computational Fluid Dynamics (Mechanics)

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