One of the most troublesome and perhaps the most common noise source in industrial environments is the gas jet. This basic noise mechanism is often referred to as aerodynamic noise and some examples include blowoff nozzles, steam valves, pneumatic control discharge vents, gas or oil burners, etc.
To emphasise the word troublesome, the sound pressure level 3 ft from a typical 1/4″ diameter shop air blowoff nozzle is often in the range of 105 to 107 dBA. As has been the practice throughout this text, we shall first consider briefly the basic physical parameters which determine the level and character of the noise mechanisms and then proceed to the measures for noise control.
Gas Jets:
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The simplest example of a gas jet is the high-velocity airflow emanating from a reservoir through a nozzle. The gas accelerates from near zero velocity in the reservoir to the peak velocity in the core at the exit of the nozzle. The peak velocity of a gas jet is a strong function of pressure difference between the reservoir pressure pr and the external ambient pressure pa.
In short, as the pressure ratio pr/pa between reservoir and ambient discharge is increased, the velocity of the gas at the discharge nozzle increases. However, when a pressure ratio of approximately 1.9 is reached, the flow velocity through the nozzle becomes sonic, i.e., reaches the speed of sound and further increase in reservoir pressure does not significantly increase the flow velocity. When this critical pressure ratio of 1.9 is reached, the nozzle is said to be choked.
The actual generation of the noise from gas jets result from the creation of fluctuating pressures due to turbulence and shearing stresses as the high-velocity gas interacts with the ambient gas. Complex radiating sources, called eddies, are then formed, with the high-frequency noise being generated near the nozzle in the mixing shearing region and the lower-frequency noise being generated downstream in the region of large-scale turbulence. With only turbulent flow present, the pressure fluctuations are random functions of space and time and only statistical methods can be used to describe the character of the noise. Therefore, the spectral character of gas jet noise is generally broadband.
Shown in Fig. 23.2 is an octave band spectral analysis of an air jet from a ¼ inch diameter copper tube (nozzle) crimped to blow parts from the die area of a power press. Since the reservoir or shop air pressure is usually in the range of 45 to 90 psi, the gas nozzle can be considered choked and the peak velocity in the core of the jet near Mach 1. Subjectively, the character of the noise is a broadband hiss with peak levels near 101 dB in the octave band whose centre is 2000 Hz.
The magnitude and spectral character of the noise from jets cannot be accurately predicted. This is due to the complex nature of the jet itself and uncertainties associated with such factors as turbulence, nozzle configuration, temperature, etc. However, from the monumental work of ‘lighthill’ and others and reams of empirical data generated dominantly by the aerospace industry, first-order estimates of the acoustical power spectral character can be obtained.
Noise Control Measures for Gas Jets:
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For large high temperature, high-velocity and highly turbulent gas jets such as found on jet engines or gas turbines, sophisticated engineering noise control measures are required. These machines are extremely sensitive to upstream and downstream pressure losses and velocity gradients.
Hence, a careful system type design approach is necessary to meet the rigorous aerodynamic constraints. In short, the aerodynamic constraints for these test facilities or installations are often the pacing items and somewhat beyond the scope of this text.
However, the basic approach to controlling noise is generally the use of absorptive silencers. For the simple high velocity air jet, common in industrial environment, such as those used to eject parts, for cooling purposes, as air curtains, for pneumatic control vents, to power air tools, etc., rather straightforward noise reduction measures can be applied.
The basic guidelines to noise control, in somewhat rank order of effectiveness, are as follows:
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1. Reduce the required air velocity by moving the nozzle closer to the part or scrap being ejected, thus preserving thrust.
2. Add additional nozzles, thus reducing the required velocity and again preserving thrust.
3. Install quieter diffuser and air shroud nozzles.
4. Interrupt the airflow in sequence with ejection or blow off timing.
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With respect to measures 1 and 2 the noise reduction follows from reducing the jet velocity. Increasing the nozzle exit area and moving the nozzle closer to the ejection site will also usually allow considerable velocity reduction. Similarly, the addition of two or more nozzles will also allow air jet velocity reduction and corresponding noise level reduction.
Experimental results have shown that halving the distance between parts and nozzles or adding another nozzle will allow air velocity to be lowered 30 per cent (preserving thrust), with an overall noise level reduction of 8 to 10 dB being typical.
In summary, the noise reduction of air shroud silencer nozzles is typically 10 to 20 dB in the critical high-frequency range of 2000 to 8000 Hz for small high-velocity air jets.
Impingement Noise:
Another source of noise which is associated with gas jets is impingement noise. Impingement noise gets its name from the sharp increase in noise level one notes as a gas jet is brought close to and impinges upon a solid surface or object. Shown in Fig. 23.3 for comparison, are the octave band levels from a ⅛ inch diameter air jet in free air and then impinging upon a hard metal surface. Note the noise level increase, especially in the higher-frequency range of the spectrum.
Studies of this phenomenon have shown that when the gas jet impinges upon a surface, additional unsteady forces are produced. These pressure fluctuations take the form of aerodynamic dipoles, which can be described as a pair of point sources of equal magnitude separated by a small distance and oscillating with an angular phase difference of π rad or 180°. In short, they are out of phase; that is, when one of the point sources is positive, the other is negative.
Now it is beyond the scope of this text to proceed with further analysis except to say that these dipole sources are the basic mathematical models used to describe the noise and radiation of many common noise sources including propellor noise, loudspeakers, many musical instruments, valves air duct diffusers, grills, etc. Further, there is usually strong directivity patterns present when the noise originates for dipole source.
Returning to impingement noise, the amplitude and spectral character of this noise mechanism like jet noise, is very difficult to estimate. However, from both analytical and experimental studies, the radiated sound power for impinging subsonic jets depends to the first order on the fifth or sixth power of the flow velocity. Hence, again as for the free jet, even small reductions in flow velocity yield dynamic reductions in impingement noise.
From experience, as a jet flows over a sharp edge or discontinuity, additional noise is typically produced. In some cases, whistle like edge tones will also result. The periodic components, as well as impingement noise, can be minimised by reducing the flow turbulence created as the jet flows over a cavity or obstruction. Illustrated in Fig. 23.4 are some basic, almost intuitive methods for reducing impingement noise.
Gaseous Flow Noise in Pipes or Ducts:
Generally, the velocity of gas or steam in pipes or ducts is in the very low subsonic range. However, in some industries where valves or vents are used to regulate high gas pressure or flows, extremely high noise levels are common. Examples of these sources of noise are often found in the chemical, petrochemical and electric power generation industries. Measured noise levels downstream of reducing valves in large steam lines often reach the 130-140 dB range.
In most cases, the pressure drop across a control or regulator valve is sufficient to choke the flow at the discharge. As such the flow of the gas jet is sonic or near sonic velocity with the corresponding generation of high-intensity aerodynamic noise. The resultant high-intensity noise is then transmitted through the pipe walls to adjacent surrounding and even worse, conducted downstream with little or no attenuation.
Prediction of the magnitude of aerodynamic noise in pipes is very difficult because the source mechanism is complex and there are other uncertainties, such as the transmission loss of the pipes or ducts. In addition, extensive service conditions such as pressure, pressure ratios and temperature are required.
It should be noted that the principal area of noise generation is in the turbulent mixing region immediately downstream of the valve. Now if the pipe downstream is light walled (schedule 40 or less) overall noise levels from a choked steam valve will be in the range of 110 to 120 dBA at 1 m from the pipe. For heavy walled pipe (schedule 80 or heavier), the levels are typically 6 to 10 dB lower.
The spectral character of the noise resembles high-velocity gas jets, i.e., broadband with peak levels in the range of 2000 to 8000 Hz. Shown in Fig. 23.5 is an octave band spectral analysis of the noise from a control valve measured at 1 m downstream of the valve and 1 m from the pipe (schedule 40).
In summary, where high-pressure steam and gas are regulated or discharged through valves, noise levels in excess of 100 dB can be expected. Since the valves have relatively thick metal housings, the piping system itself downstream of the valve is usually the primary source of externally radiated noise. As such the propagation of noise generally follows the characteristic of a line source with cylindrical divergence, i.e., 3 dB per doubling the distance.
Noise Control Measures for Noise in Pipes:
There are three basic approaches to reducing the noise radiated from these control valve areas:
1. Changing the dynamics of flow.
2. Including an in-line silencer to absorb acoustic energy.
3. Increasing the transmission loss of the pipe walls.
Of these three approaches, changing the dynamics of the flow is noise reduction at the source and hence is preferred. Changing the dynamics refers actually to reducing the flow velocity through multiple stages of pressure reduction or diffusion of the primary jet. Shown in Fig. 23.6 is an illustration of multiple stages of pressure reduction utilising expansion plates.
Here the gas flow velocity is sequentially lowered in each expansion chamber. In addition, the plates act as a diffuser, reducing turbulent mixing. With a threeplate configuration such as shown in the figure, noise level reductions up to 20 dB have been reported. However, each chamber introduces back pressure, which is often undesirable and can be a limiting factor.
Another but similar approach is the use of diffusers to reduce flow noise. Note that the single orifice element in the valve body is replaced by a slotted cage which diffuses the flow into smaller interacting jets. Noise reduction in the range of 10 to 18 dB has been reported with this approach alone.
In combination with a downstream in-line diffuser, as illustrated, an overall noise level reduction of up to 30 dB can be expected. In either approach, expansion plates or diffusers, careful attention must be given to proper sizing to assure proper valve operation. Guidelines for selections are generally simple and provided by the manufactures.
The inclusion of an in-line silencer to reduce aerodynamic noise in pipes and the most serious deign constraint is pressure loss limitations. Note that at the inlet a diffuser is included and that thereafter the construction is similar to a flow through tubular absorptive silencer. The absorbing medium in these silencers behind the perforated sheet metal is either fibrous glass or metal wool.
Typically, the flow area of the diffuser is 2 to 3 times the valve discharge area and in this way the flow velocity is sharply reduced in the silencer. The acoustical performance of these in line silencers follows closely the performance of tubular silencers. However, because of unusually high flow velocities, care must be taken to account for performance reduction in accordance with the manufacturer’s specifications.
Installation guidelines are simple and can be summarised as follows:
1. The in-line silencer should be installed no closer than 3 or 4 pipe diameters from the valve discharge.
2. Pipe treatment between the valve and silencer must be considered, since the silencer will only reduce noise downstream of the silencer itself.
There are two basic approaches to increasing the transmission loss of pipe walls:
1. The pipe wall thickness can be increased.
2. The pipe can be wrapped or lagged with acoustical materials.
With respect to pipe wall thickness, no simple generality exists, for estimating the transmission loss of pipe walls. However, for carbon steel pipe, most commonly specified for high-pressure, high velocity flow installations, schedule 40 is considered standard up to pipe sizes of 12″ inside diameter (ID).
For larger pipe sizes, standard wall thickness is constant at about 3/8″. Now because the thickness and hence the surface density of the pipe wall change with pipe size, the transmission loss of the standard pipe cannot be simply given.
However, from empirical data, a table of attenuation increase, such as Table 23.1, can be very useful. From this table the increase in noise reduction due to pipe wall thickness increases (schedule) is given for a representative range of pipe sizes. An example will illustrate the use of the Table 23.1.
Pipe Wrapping or Lagging:
For especially critical noise-sensitive areas, treatment of the noise path may also be required and for pipes this is accomplished by wrapping or lagging the pipes. One very effective wrapping scheme. Here a 1 to 3″ layer of acoustical quality absorbing material such as fibrous glass, mineral wools or polyurethane foam is wrapped next to the pipe wall.
This absorbing layer is then wrapped with sheet lead, dense vinyl or sheet metal (1 lb/fit2 minimum). Shown in Fig. 23.8 is the typical noise reduction achieved with this wrapping scheme. For this example, the absorbing material was 1″ thick acoustical quality polyurethane foam and the other layer was dense vinyl (1.5 lb/ft2). It should be emphasised here that the outer dense layer plays the key role in achieving a high level of noise reduction.
Additional noise reduction can obviously be obtained by using a more dense outer layer or adding more composite layers. For ease of installation, various manufactures, of thermal pipe insulation have developed and made available commercially preformed lengths of absorbing materials or lagging sized to conventional or standard pipe diameters.
Additional guidelines for wrapping include the following:
1. Avoid any mechanical coupling between the pipe wall and the outer layer of treatment.
2. Seal all edges and joints.
3. Use special materials such as fibrous glass or mineral wool for high-temperature applications.
4. Means to avoid accumulation of condensation should be included for cold piping or icing will reduce the performance of the treatment.
5. Duct tape or metal packing bands are best for securing the treatment.
One disadvantage of wrapping or lagging alone as the only measure for noise reduction is that long lengths of pipe will probably require treatment. Therefore, in summary, it is strongly recommended that source noise reduction be given the first priority, that is, the utilisation of multiple pressure reduction stages or diffusers in conjunction with in line silencers. If additional noise reduction is required, wrapping or lagging the pipes will usually bring levels within the most stringent criteria.
Furnace and Burner Noise:
The noise from furnaces, burners and similar combustion equipment has its origin in complex interactions associated with high-velocity flow, turbulent mixing and combustion. The spectral character and noise levels vary dynamically with furnace or burner configurations and the fueling method.
Examples of industrial applications where natural or forced draft furnaces are used include refineries, chemical plants, boilers, smelting furnaces and heat-treating furnaces. The peak noise levels are generally low frequency in spectral character, usually below 1000 Hz and subjectively the noise is often referred to as a roar. Shown in Fig. 23.9 is an actual octave band spectral analysis of a large natural draft furnace measured 2 m from the burner.
Several methods for predicting the magnitude and spectral character of burner noise have been developed and the flow velocity in the flame seems to be the most important parameter. Other parameters of lesser importance include mass flow rate, volumetric expansion, ratio of air to fuel mixture, etc.
More important, most studies regarding burner combustion noise have yielded little encouragement for significant noise reduction at the source. In short, combustion burner noise reduction is usually followed by a sharp decrease in burner efficiency which is usually unacceptable to the operators. On a positive note, however, higher combustion noise levels are often encountered when poor flame control exists. These conditions, easily recognised by the operators, can be remedied by properly adjusting the burners.
Thus with little hope of source reduction, the acoustical engineer must look for some method of containing the noise. J. G. Seebold, has shown that acoustically treated plenums have been both effective and practical. Examples of these plenum configurations are shown in Fig. 23.10. Here the interior walls are constructed of highly absorbing materials, yielding a labyrinth of parallel baffles and lined bends. The plenums are basically sound traps and typical noise reductions achieved with these plenums are shown in Fig. 23.11.
With respect to design guidelines, walk-in-sized plenums can be constructed of typical acoustical panels with the following modifications:
1. The solid external sheet metal should be 12 gauge minimum.
2. The acoustical absorbing material must be 4 inch thick in order to have effective absorption at lower frequencies.
3. Since oil mist is often present, a 1 or 2 mil layer (maximum) of plastic film enclosing the acoustical absorbing material is recommended to prevent wicking or absorption of the oil, thus avoiding a possible fire hazard.
It should be emphasised that the plenums illustrated here are by no means exhaustive. The only limitations to the number of bends, baffles, switchbacks, etc., are those associated with pressure losses. Here again, each bend or switchback represents a constriction to the flow of air to the secondary registers. An adequate volume flow of air with minimal entry distortion is essential for good burner efficiency.
Mixer (Spud) Noise:
In premix-type furnaces, another source of noise is usually encountered where the primary air is mixed with the fuel gas. Here the air is inspirited into the mixing unit or spud by the high velocity fuel gas jet. The source of the noise is of course the high velocity gas and since the pressure ratio is usually well above critical, a choked condition is prevalent.
As such the noise levels are extremely intense and the spectral character of the noise is broadband with peak levels in the 2000 to 8000 Hz range, often above 120 dB. Because of the delicate aerodynamic balance required for proper mixing, significant noise level reduction is rarely available at the source. However, commercially available silencers can be obtained which sharply lower spud noise.
These silencers are basically absorptive types relying for the most part on lined bends for the attenuation. Because of size and flow constraints, silencers of this configuration yield noise level reductions of only 10 to 20 dB in the critical range of 2000 to 8000 Hz. As such, resultant overall levels may well exceed 100 dBA within a 10-ft radius of the spud.
Another approach which is popular, especially in new installations, is to enclose the spuds in a separate equipment room. The spud room is a walk in type, generally of masonry construction, extending the entire length of the furnace complex. The mixing units require little monitoring of maintenance and hence lend themselves to enclosure isolation. One note of caution – A generous air supply is required to the spud room and this can usually be achieved through a duct penetration with absorptive parallel baffle silencer to treat the noise.