Silencers

THE ROAD TOWARDS SILENCE

1. ACOUSTICS AND SOUND

According to the dictionary, the term acoustic refers to the science of sound and in technical terms the phrase “acoustic techniques” are interpreted as one part of the science of acoustic systems. Attenuating noise from any type of machine or system that is used by society depends on this science. Sound is understood as changes in pressure within an elastic medium (the atmosphere, liquid, gas, etc.) imposed by something on its static pressure.
The human ear recives the results of these changes in pressure; in other words, changes caused by the movement of the fluid itself or by the movement of a solid, gaseous or liquid body within the atmospheric environment.
The air particles are aroused and start to oscillate, spreading in all directions, in what we know as “longitudinal waves”. The human ear is able to receive this changes from approximately 16Hz to 16,000Hz.
Types of oscillation (sound) are determinate according to the following table:

TABLA

The term “NOISE” is understood as any type of sound susceptible to causing discomfort or harm to human health.

2. DEFINITIONS OF A SOUND FIELD

Sound pressure P

This is the change of pressure caused by sound oscillations within the medium.
It is measured in N/m2.

Sound level L

This is the sound level that we measure, triggered by the aforementioned cause.
It is measured in dB.

Sound intensity J

This is the sound energy that penetrates per unit area from a vertical position to the direction of radiation per unit time.
It is measured in W/ m2

Acoustic power W and acoustic power level LW

Acoustic power generates the source as sound in the environment. It cannot be measured directly, but it is determined from sound pressure P and the surface A.
Acoustic power level Lw is measured in dB.

3. GENERAL IDEAS ON ACOUSTICS

3.1 At 1 dB change of sound level is perceptible to the human ear.
3.2 At 10 dB fall in sound level is perceived by the human ear as half of the initial noise.
3.3 At 20 dB drop in level is perceived as if the noise had dropped by a tenth (1/10).

4. MEASURING NOISE

Sound levels are, in general, measured and expressed in decibels (dB) in accordance with UNE standard 20464.

USO PREDOMINANTE / DOMINANT USE DIA (DE 8 A 22H) / DAY (FROM 8 TO 22H) NOCHE (DE 22 A 8H) / NIGHT (FROM 22 TO A 8H)
SANIDAD / SANITATION 45dB (A) 35dB (A)
RESIDENCIA UNIFAMILIAR / SINGLE FAMILY RESIDENCE (*) 50dB (A) 40 dB (A)
RESIDENCIAL / RESIDENTIAL 55dB (A) 45dB (A)
SERVICIOS / TERTIARY 65db (A) 55dB (A)
INDUSTRIAL / INDUSTRIAL 70 dB (A) 60dB (A)

 ACOUSTIC MEASUREMENTS

These measurements are made following the local standards in place and following criteria of good practice in the handling of this type of equipment avoiding screen effects, wind, distorsions, etc.
The following parameters will basically be taken into account:

  • Assessing the background or environmental noise and pointing the microphone towards the sound source.
  • Location of the sound-measuring device:A) on the ground: between 1.2 & 1.5m.

    B) walls of buildings: maximum 3.5 m.

    C) emission point: between 1 and 2 m.

Several measurements are made, with the maximum value of each measurement being considered. The arithmetic mean of the medians is then calculated.

5. NOISE GENERATION IN NATURAL GAS CONTROL STATIONS

We can distinguish two sources of noise within natural gas control and measuring stations.
5.1 The generated by the reduction of pressure (regulator).
5.2 That generated by the transmission of gas disturbance in the outlet area.
Both noises are to large extent generated by the speed of the gas. Normally this speed is much higher within the regulator, wich at this stage of reduction practically determines the sound level of the station.
It should be taken into account that this noise propagates by radiation through the flow of gas to the outlet of the regulator from its origin to a certain distance further on.

6. ORIGIN OF THE NOISE

Its origin may be explained by the very process of the cut in pressure.
The flow of gas ( open regulator) flows to the outlet pipe. This causes different masses of gas of different densities, or turbulences within the gaseous conduit, which likewise run from the outside to the inside, due to the friction between the gas and the walls of the tube.
This effect produces “pressure” oscillations from the inlet to the outlet. On occasions under critical conditions of pressure reduction, it produces Indeterminate changes in pressure. These are known as “percussion waves or strikes”.
This also produces vibrations on the regulator itself; although seconday with regard to the sound level, as they are low frequency waves they are no less important for the good functioning and duration of the internal parts of the regulator.

7. DETERMINING THE NOISE LEVEL

The factors that determine the noise level in the process of pressure reduction have been well-studied and well-defined.
These basically depend on the inlet and outlet pressure, flow volume, size of the equipment (valve stem, gas cross-section), size of the outlet pipe and type of gas.
It is clear that the speeds of gas play a major role.
All manufacturers have calculation programs to predetermine the sound level of any of their equipment under predetermined conditions.

8. NOISE ATTENUATION

These are several basic design norms to take into account that allow us to attenuate the noise produced by a jump in pressure (regulation) in a project; we will now describe them:
8.1 Increase the thickness of the regulator’s outlet pipe to the first accessory or valve. This measurement can be attenuated between 3 and 5 dB.
8.2 Although it is of secondary I mportance, limit the outlet speed to a maximum value of 20m/s. This helps stabilize against gaseous flow, therefore reducing the total sound level.
8.3 Prevent the speed of the fluid through the valve stem from being above 150 to 160 m/s. This also helps prevent further disturbance in the gaseous conduit and therefore stabilizes the flow.
8.4 Avoid fitting accessories immediately after the regulator. If this is not possible (for design reasons), use accessories with a considerable thickness to prevent resonance.
8.5 Use elements which prevent the propagation of the noise due to the reduction in pressure as we will see below:
8.5.1 Silencers in regulators or in pipes.
8.5.2 Flow straighteners adjoining the regulator
External oscillation attenuators

  • Absorvent coatings on the regulator
  • Absorvent coatings on outlet pipes

8.5.3 Cell silencers in ventilators with absorvent material
8.5.4 Speciall coatings (walls and roofs).
8.5.5 Coatings af covers and acces points with absorvent material.

TEYCO PRODUCTS

TEYCO “SRA” TYPE ACOUSTIC GRILLE SILENCERS

These acoustic grille silencers are made out of sheet metal with aerodynamic perforation that allows air or gases to pass through with minimum loss of charge, with the correct ventilation sections being maintained in each case.
The sheets are arranged so that the sound wave travelling through the air collides with them before going to the outside, weight efficacy that varies according to its configuration.

ACOUSTIC INSULATION MATERIAL.

This is a sandwich made up of fibra and an elastomeri sheet; although they have a similar density, there is a great difference in terms of elasticity, fragility and adherence between the heavy sheet and the fibrous layer. Due to its structure, this material has a high capacity for absorption to which should be added the high insulation capacity, due to the special composition of the high-density sheet which acts as a barrier against the noise.

EXPANSION SILENCERS SARES TYPE

Expansion mufflers are designed to be incorporated to the regulation outlet.
The assembling may imply an increase of the diameter size of the outlet pipe.
Gas flow is obliged to expand in a controlled way in this area by relaxing and linealizing this flow achieving a notable reduction of the sound level.
This kind of devices enables a reduction up to 15 Db according to gas speed.
Other commonly used complements also enable a reduction of sound level caused by gas expansion at the regulation outlet.
For guidance only, we can say that:

  • Assembling the muffler to the regulator: from 10 to 20 dB.
  • Using Sch80 pipe at the regulation outlet: 2dB.
  •  Acoustic insulation of cabinets or containers: 5dB.