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:
- Carbon dioxide
- Carbon monoxide
- Freon 11 (trichloromonofluoromethane)
- Freon 12 (dichlorodifluoromethane)
- Freon 13 (chlorotrifluoromethane)
- Freon 22 (chlorodifluoromethane)
- Hydrogen chloride
- Hydrogen fluoride
- Methyl chloride
- Natural gas
- Nitric oxide
- 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
- Globe, small flow trim
- Flat seat (short travel)
- Eccentric spherical plug
- Eccentric conical plug
- Butterfly (centered shaft)
- Swing-through (70°)
- Swing-through (60°)
- Fluted vane (70°)
- Butterfly (eccentric shaft)
- Offset seat (70°)
- Full bore (70°)
- Segmented ball
Which performance in terms of acoustics for silencers ?
Acoustic performance of silencers (at room temperature: ventilation, air conditioning, various industrial processes or at high temperature: various industrial processes, stacks, exhausts ...) is very frequency dependent and is primarily related on the one hand to the performance of the absorbing filling given the spacing of the airways and the length of the silencer (characterized by the propagation loss) and also where appropriate (for silencers other than with simple dissipation) 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.
This performance 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 -.
Dissipative silencers (or: dissipation silencers) are devices allowing an attenuation of sounds on a wide frequency range.
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 40 dB 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).
The prediction of acoustic performance of dissipative silencers can be performed with the simulation software SILDIS®®.
Measurements on silencers can be made (as applicable) 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 or according to standard NF EN ISO 11820 Acoustics - Measurements on silencers in situ