1.    Background

This combustion file gives a qualitative outline of the main sources of noise arising from flames and ancillary equipment used in combustion systems. Quantitative guidance on noise sources specific to combustion is given in a (growing) portfolio of separate combustion files. These may be accessed through links on each combustion file or by returning to the ‘Search Combustion Files’ facility and entering the keyword ‘noise’.

The topic of noise uses specialist terminology which is explained the first time a term occurs within each CF by means of flying windows linked to the Handbook glossary. Terminology requiring a more in depth exposition is described in separate combustion files. Where they exist, such files may be found using the handbook search facility, entering the specialist term as the keyword.

2.    What are the main sources of noise arising from the flame itself?

There are two principal sources of noise associated directly with flames and the combustion process:

–          Combustion roar

–          Combustion driven oscillations

Combustion roar

Combustion roar is the continuous broad band noise that arises in all turbulent flames. It reflects the way in which turbulent mixing controls the combustion process. As a rule of thumb, about one ten millionth (10^-7) of the thermal power released in a flame is radiated as [GLOSS]sound power[/GLOSS]. Generally speaking, the fraction of the thermal power of a flame appearing as sound power increases as flow velocities, and therefore combustion intensities and turbulence, increase. Quantitative guidance on broad band [GLOSS]Sound Power Level[/GLOSS] arising from flames is given in CF6. The same Combustion File also gives guidance on the way the sound power is distributed across the [GLOSS]octave bands[/GLOSS] normally used for spectral assessment of noise.

In the case of industrial flames, combustion roar may be the cause of high workplace noise, leading to a poor (acoustically) working environment, or in extreme cases to a risk of hearing damage. There is also a possibility of noise breakout from factories and flues to cause local (work place or neighbourhood) annoyance. Combustion roar is usually less of a problem when flames are confined within a well-sealed combustion chamber.

Combustion driven oscillations

Combustion driven oscillations arise when positive coupling occurs between the flame and the acoustics of the combustion system, ie when the flame acts as an amplifier of disturbances (acoustic or fluidic) at some natural frequency of the combustion system. Although this phenomenon is relatively uncommon, when it occurs, it can give rise to extremely high noise levels within a relatively narrow frequency range. Hence the descriptions of this phenomenon as:

–          Combustion oscillations

–          Combustion resonance

–          Pulsations

–          Combustion hum

Combustion oscillations normally occur if there is some similarity between a characteristic frequency of the flame (eg the mean residence time in the flame, or an eddy shedding frequency in the fuel or air supply to the flame) and a natural [GLOSS]Helmholtz resonator[/GLOSS] or [GLOSS]organ pipe[/GLOSS] frequency in the combustion chamber, flue ways, or air and fuel supply systems (often in combination).

The consequences of combustion oscillations can be intolerably high tonal noise in the vicinity of the burner, leading in extreme cases to mechanical failure of system components. The noise can occur at high (whistles), medium, (hum), low (resonance) or very low (infrasonic) frequencies. The high frequencies can cause unacceptable disturbance in the work place. Medium and low frequencies are the most likely cause of mechanical failure. Low frequency and infrasonic oscillations can travel long distances in air with little attenuation apart from inverse square law spreading. They can be the cause of neighbourhood complaints over large areas.

It is often the case that the noise from combustion oscillations is difficult to attenuate. Where such oscillations arise, a solution must be found which breaks the link between the combustion process and the system acoustics, and hence eliminates the oscillations at source. Active noise control (‘anti-noise’) may be an option where the frequencies are too low for effective attenuation by passive silencers.

3. Other noise sources

Noise is also associated with ancillary equipment found on combustion systems. Examples are:

–          Jet noise, from high pressure (usually gaseous) fuel or air jets

–          Jet pumps and boosters

–          Valves and regulators

–          Fans and air movers

–          Motors and mechanical pumps

–          Solid fuel grinders and  mills etc

Jet noise

Jet noise occurs on all gaseous jets, but being proportional to about the eighth power of jet velocity, it becomes obtrusive at high gas and air pressures (typically when supply pressures are measured in ‘bar’). Noise from large high-pressure jets can dominate combustion roar as the principal noise source in a combustion system. It has a characteristic ‘hiss’ associated with a broad band source peaking at about 10kHz (as opposed to combustion roar which peaks at 1 kHz or lower).

Jet noise can be an issue on plant using high pressure gaseous fuels or high-pressure atomising air or steam (eg cement and lime kilns, bulk glass melters).  It can also affect certain types of air or steam blast atomised oil burners used on boilers and other plant. 

In extreme cases, the jet may give rise to a pronounced whistle, often associated with a burr or sharp edge near the jet exit.

Jet noise may be reduced by using a lower pressure system, or by attention to the design of the nozzle to avoid sharp edges, burrs and to reduce turbulence. Because of its relatively high frequency, the impact of jet noise may be reduced using passive acoustic absorption.

Jet pumps and boosters

These devices, also called injectors, ejectors or eductors (depending on their application) depend on a high-pressure fluid jet to act as the driver to force air, gas or combustion products into or out of a burner or combustion system. Generally speaking, the comments made above about jet noise apply to jet pumps. They can, however, be even more obtrusive than high-pressure fuel jets because of their location outside the combustion chamber.

Noise from jet pumps is generally amenable to attenuation by the use of passive acoustic shrouds. The source noise levels from jet pumps may be reduced by 10 [GLOSS]decibel[/GLOSS] or more through the use of low turbulence jets and mixing systems, such as [GLOSS]Coanda effect[/GLOSS] devices.

Valves and regulators

Many valves and pressure regulators operate by restricting the flow of a fluid to reduce pressure by creating a high velocity jet within the valve port. Thus, all the comments made above about jet noise apply to these devices. Generally, the noise propagates downstream of the device (particularly if the pressure drop across the valve is super critical). The noise may be radiated from large areas of downstream pipework, which may make noise control by acoustic lagging uneconomic. In-line passive silencers are available to reduce transmission of the noise through pipelines. For clean fluids, quiet valves and regulators using multiple small passages to shift the frequencies upwards, or sometimes friction based pressure drops to reduce jet noise at source, are available.

Fans and air movers

Fans are often the most obtrusive source of combustion system noise, particularly if the flame is well enclosed or the fan is located remotely in a quiet or external location. There are well-established rules for calculating the sound power emitted by the various common fan types, as well as its octave band distribution.

There is a risk of tonal noise emissions from fans at the blade passage frequency. Where this causes problems, solutions should be sought to avoid its creation by attention to the geometry of the inlet or exit ports from the fan. When fan speed is constant, this type of noise may also be suppressed by the use of tuned passive silencers.

Other noise sources

Other sources of noise associated with combustion systems include electric motors, pumps, mills, etc. These sources are not specific to combustion systems. They are not considered further here.

4. Which noise sources are of most concern?

There are no universal guidelines on which of the above noise sources cause the most concern.

–          Where combustion oscillations occur, they will often dominate all other noise sources, and must be suppressed at source.

–          When the flames are in the open, or in a furnace requiring access for loading or unloading, combustion roar can dominate. High velocity burners can be particularly intrusive.

–          When the flame is enclosed, for example in a shell or water tube boiler, often the ancillaries dominate,  particularly combustion air fans.

–          Jet noise can be the main cause of problems on systems using high pressure fuel, air or steam jets, such as flares, cement and lime kilns, and bulk glass melters and certain oil and gas industry process heaters.