AFRC-JFRC 2004 Joint International Combustion SymposiumOctober 10-13, 2004.

Flares - Noise Prediction and Thermo-AcousticEfficiency

Carl-Christian Hantschk and Edwin Schorer

Miiller-BBM GmbH,Robert-Koch-Strasse 11, D-82152 Planegg, GermanyPhone: +49.89.85602-269, Fax: +49.89.85602-111e-mail: CHantschk@MuellerBBM.de.ESchorer@MuellerBBM.de

Abstract

In many industries where combustible waste gases are obtained,flares are used to burn these gases in a controlled manner. Among otherenvironmental aspects the noise emissions associated with flaring arebecoming increasingly important in many countries as population den­sity goes up and residential and industrial areas move closer together.Installing noise control equipment on flares is almost impossible whilethey are in service, since flares are typically a safety related plant com­ponent that can only be turned off after the connected plant has beenshut down. Accordingly, predicting the noise emissions of flares is crucialas early in the design process as possible in order to plan appropriatenoise control measures in time and to avoid unnecessary costs. A con­cept to calculate flare noise is to use the "Thermo-Acoustic Efficiency"(TAE) which is the ratio between the acoustical power and the heat re­lease rate of the burning process. Values for TAE are usually determinedexperimentally in the test field and then used for similar flare designsand operating conditions. In the present work noise levels from fieldmeasurements on a number of different full-scale industrial flare typesare put together and the corresponding TAE values are compared. It isseen that the TAE concept has shortcomings resulting from oversimpli­fication of the noise generating mechanisms of flares. For improved flarenoise modeling the various noise sources contributing to the overall noiseemissions of flares must be taken into account. The present paper sum­marizes these sources and outlines the individual effects and parametershaving an influence on the acoustical characteristics of flares.

1 Introduction

The safe operation of hydrocarbon and petrochemical industry plants some­times requires the venting of considerable amounts of combustible gases either

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to control process conditions or to avoid excessive system pressures. In mostcases, environmental concerns forbid the venting of unburned gases directlyto the atmosphere since they may contain toxic or hazardous components andcould ignite somehow causing explosions or fires. Instead, it is common practiceto dispose of the gases by burning them in a flare.

Basically, a flare is a device by which combustible gases are reliably ignitedand burnt in a controlled manner at a defined location. This needs to be ac­complished in a safe way for all possible operating conditions and flow rates,protecting persons and environment from thermal radiation, hot gases, pollu­tant emissions and unnecessary annoyance.

Some of the safety and environmentally relevant effects associated with flaringare only pertinent in the vicinity of the flare (e.g. thermal radiation) while oth­ers can be important also at great distances but are not readily perceived (e.g.pollutants emissions). Other annoying effects, even at considerable distances,include visible light, smoke and noise.

2 Flare noise and noise control

This paper focuses on flare-related noise which is becoming increasingly impor­tant as population densities in many countries are on the rise causing industrialand residential or recreational areas to move closer together. Plant owners op­erating flares not only have to comply with environmental regulations butmay also need to take into account the public perception of flare noise and theassociated influence on the relationship between the facility and its neighbors.

There are a number of options on how to design a flare in order to meetspecific noise requirements. These range from primary noise control measuressuch as low noise burner design to secondary noise control measures such asmufflers and noise barriers. However, a decisive characteristic of most flaresis that they can under no circumstances be shut down without creating anintolerable safety hazard while the connected plant is still operational. As aconsequence, if a flare emits too much noise after being placed in operation,there will usually be no economically acceptable possibility, for several years,to modify its noise control concept.

Simply designing a flare according to the maximum achievable noise reductionby default is not reasonable since this can result in higher than necessary cap­ital and operating costs. Instead, the optimum noise concept must be workedout at the design stage of the flare. For this purpose it is essential to be ableto predict the level and characteristics of the flare's expected noise emissionswith sufficient accuracy.

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3 "Thermo-Acollstic Efficiency" flare noise pre­diction concept

3.1 Principle

Obviously one of the main noise sources of a burning flare is the flame itself.In the combustion process conversion of the reactants into the products takesplace in a highly turbulent reactive flow. The exothermic reaction involves asignificant rate of change of the volume of the reacting gases. This is due tothe change in molecular weight (resulting from the conversion process itself)and - to a far greater extent - due to the heat added to the flue gases whichcauses them to expand. Since this volumetric expansion partly translates intopressure fluctuations which are radiated as sound waves, it is reasonable toassume that the noise generated by the flame is in some way proportional tothe heat release rate of the combustion process.

This approach led to the socalled "Thermo-Acoustic Efficiency" concept for thecalculation of noise emissions from flames (e.g. [1]). It is based on the assump­tion that a certain portion of the energy Qcombust released in the combustionprocess will be transformed into acoustical energy Wacoust that is radiated fromthe flame into the surroundings as audible noise. The proportionality factoraccording to equation (1) is called "Thermo-Acoustic Efficiency", TAE. If TAEis known, the sound power emitted by a flame can readily be calculated.

Wacoust = TAE . Qcombust (1)

The TAE concept as outlined above is also employed to predict noise emissionsfrom flares. For reasons of simplicity, it is then often applied to the whole flare,including all associated equipment, although, strictly speaking, it is a conceptwhich exclusively refers to the flame alone. The TAE values needed for thecalculation are usually determined experimentally in the test field for certainflare configurations and then used to predict the expected noise emissions forsimilar designs and operating conditions - assuming that TAE will be more orless the same. As will be seen in the following discussion, this assumption isusually not valid.

3.2 TAE values for flares

In Figure 1 the sound power emitted by a number of industrial flares at differentloads is shown. The data is obtained from field measurements on several typesof flares under various operating conditions [2]. The noise intensity is given as

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the sound power level Lw according to equation (2) and is plotted versus theheat release rate of the flare Qcombust, for which a logarithmic scale is used.

L = 10 . 10 ( Wacoust )

W g 1. 10-12 Watts (2)

It is important to emphasize, that in this context" operating condition" doesnot only refer to the heat release rate of the flare alone but also to all otherparameters characterizing its operation, as, for example, flow rates of auxiliaryequipment (steam or air injection, etc.). Accordingly, flares shown in Figure 1can operate under different conditions at the same heat release rate.

..... TAB = 1.9E-8_._._.- TAB = 4.5E-5

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Figure 1: Sound power level Lw calculated from measured noise data, plot­ted versus heat release rate Qcombust for different types of industrial flares un­der various operating conditions [2]. HF = elevated (single-point) flare, BF= (enclosed) ground flare, RLU = smoke suppression by air, RDU = smokesuppression by steam, SS = equiped with advanced noise control.

In Figure 1 lines of constant TAE are straight lines. The corresponding linesfor the maximum and minimum TAE observed in the measurements - that is

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TAE = 1.9 . 10-8 and 4.5 . 10-5 - are shown as the dashed and dashed-dottedlines.

If a TAE value should exist that is more or less generally applicable to all typesof flares and operating conditions, the data points from the measurements inFigure 1 should all follow lines similar to the ones shown. Instead, it is obviousthat the TAE values in the measurements differ significantly not only fromflare to flare but even for different operating conditions of the same flare ­even at the same heat release rate. The TAE values correctly relating overallnoise emissions and heat release rate of the flare vary over a broad range andalmost seem to do so in a random fashion.

3.3 Shortcomings

A central problem of the approach described above certainly lies in applyingequation (1) to a whole flare system instead of the combustion process alone.Doing so means that TAE becomes a "lumped parameter" incorporating alleffects that have an impact on the acoustical behavior of the flare, i.e. noiseemissions from valves, injectors, smoke suppression devices etc. as well as anynoise control measures installed. Other approaches which are similar in thatrespect can be found in the literature [3].

However, even if equation (1) is applied to the flame alone, the TAE factorwill generally not be constant but will depend on the characteristics of thecombustion process. Besides being proportional to the total heat release rateof the flame (see section 3.1) the emitted noise will also strongly depend onthe local reaction rate and the overall power density of the flame, i.e. thevolumentric extent of the reaction zone. Generally, high local reaction rateswill lead to higher noise emissions. Similarly, burning the same rate of fuel ina small flame will usually generate more noise than if burnt in a large flame.

An example for these effects can sometimes be observed in industrial flaresthat use blowers to provide a good mixing between fuel and air for smokesuppression. As long as the fuel flow is well mixed with air, it will burn ina compact and clean flame. Burning the same rate of fuel with the blowersturned off will result in a very large, slowly burning (and often smoky) flame.The noise emitted from this flame is usually significantly lower - although theheat release rate is approximately the same in both cases l

.

Another problem with using equation (1) to predict combustion noise is that itdoes not take the frequency characteristics of the noise emissions into account.Frequency characteristics are very important for the design of adequate noisecontrol measures and will also determine the human perception of the noise

Ithe comparison refers to the actual noise emissions of the flame alone, i.e. the noisereduction described is not due to turning the blowers off in this example.

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emissions since the ear's sensitivity for noise is frequency-dependent. The lattereffect is usually taken into account by corrections applied to measured physicalnoise levels, as done for example by the socalled A-weighting. A-weighted levels(in dB(A)) represent the human impression of how loud a certain sound israther than the pure physical value (given in dB, and also referred to as "linear"or "non-weighted" level) and are, therefore, the levels relevant in regulationsfor noise protection.

Human hearing is most sensitive in the octave band frequency range from1000 Hz to 4000 Hz. In this range the average listener would rate the sound levelhe hears as being somewhat higher than the actual physical level a microphonewould measure. Accordingly, A-weighting in this range is done by adding upto 1.2 dB to the linear level. For the freqency range of 500 Hz and belowthe sensitivity of the human ear drops significantly with frequency and theperceived sound level is rated as lower than the linear one. Here A-weightingis done by making deductions from the linear level. For example, at the 32 Hzoctave band the A-weighted level is 39.4 dB lower than the linear one.

Due to the described characteristics of human hearing and their represen­tation in the A-weighting correction, the difference between the linear (i.e."measured") and A-weighted (i.e. "heard") overall2 noise level will depend onthe frequency spectrum of the noise in question. Generally, for low-frequencynoise the overall A-weighted level will be much lower than the correspondinglinear one but it can be somewhat higher for high-frequency noise.

This can also be observed in case of the flares shown in Figure 1, where theoverall linear sound power level Lw can be quite misleading in assessing thehuman perception of the flare noise: the difference between Lw and the corre­sponding overall A-weighted level LWA ranges between LWA = Lw + 1 dB andLwA = Lw - 20 dB - an effect not taken into account by the TAE concept.

In summary, it can be concluded that the TAE concept according to equation(1) and applied to flare systems will usually only allow a very rough estimateof the actual noise emissions and the associated effect of these emissions onpersons in the neighborhood of the flare. In some cases, the results can evenbe completely off the real situation.

A key issue in developing reliable noise prediction tools for flares that aremore generally applicable in a broad range of operating conditions lies in aproper treatment of the individual sources that contribute to the overall noiseemissions of a flare system.

2i.e. including all frequencies

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4 Advanced flare noise modeling

4.1 Flare noise sources

A theoretical model that is expected to predict the system behavior of a com­plex technical system in a reasonable time and at an affordable cost will alwayshave to be a compromise. It must go far enough into detail as to be applicablein the desired range and yield sufficiently accurate results but at the same timeit will have to generalize and simplify enough to keep efforts within a tolerablelimit.

Since the times when waste gases were vented through little more than apiece of pipe with a pilot flame burning at the exit, flares have indeed becomecomplex technical systems. Depending on the specific application they can bedivided into the following three major types [1]:

• single-point flares - generally characterized by a stack with the dischargepoint at the top end where the waste gases burn in a single large flamein the open. The height of the stack is mainly determined by safetyrequirements with regard to heat radiation.

• enclosed flares - typically with a number of burners firing into a kind ofstack, open at the top, with a large diameter. Combustion takes placeinside the stack and the flames are shielded from direct view.

• multi-point flares waste gases are split up over a large number of burn­ers. Usually, the flow rate for an individual burner is rather limited, alarge turndown is achieved by firing more or less burners. If arranged atgrade, a fence to shield off radiation is usually employed.

At first glance, it may seem that for all types it is the combustion processthat is the only relevant mechanism generating noise. However, while this maybe true when the flare is fired at maximum capacity, there are a number ofother noise sources present that can contribute significantly to the overall noiseemissions of the system depending on the respective operating condition andthe noise requirements to comply with.

The most important noise sources found in flare systems are:

• the combustion process

• noise from gas exiting the burner(s)

• pilot burners

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• smoke suppression equipment

noise from steam or air injection

combustion air blowers

• noise emitted from control valves and connected piping

How much noise is generated by these sources and how much of it is actuallyreceived at a specific location in the neighborhood depends on a variety ofparameters:

• flare geometry (e.g. stack height and diameter, number, design and ar­rangement of burner(s))

• steam or air injector layout (number of, arrangement, location, design,etc.)

• fuel, steam and air properties (flow rate, composition, temperature, pres­sure, exit velocities, etc.)

• fuel/air ratio and flame length

• (control) valve parameters (type, pressure drop, staging, connected pip­ing, etc.)

• air blower characteristics (e.g. type, shaft power, speed, flow rate, numberof impeller blades, pressure increase)

• spectrum, directivity, tonality of emitted noise

• geometry of eqipment acting as noise barriers (location and design ofradiation and wind fences, enclosures, etc.)

• noise control features (e.g. mufflers, absorptive linings, nOIse barriers,insulation)

• noise propagation conditions (location of point to be protected, groundproperties, vegetation, natural barriers, meteorology, etc.)

4.2 Modeling flare nOIse

Ultimately, it is the noise received at a specific point-of-interest (POl) in theneighborhood of a flare system - e.g. the nearest residential building or thelimits of the plant property - for which a specified limit exists that the noisecaused by the flare must not exceed. Predicting the flare-generated level at the

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POI can be split up into two steps - determination of the noise emitted by theflare system and a noise propagation calculation to find out what part of theemitted noise is received at the POL

In the following some general remarks and examples on how to perform thesesteps are summarized, based on the list of sources in the previous section. Thecompilation does not go into detail and is not at all expected to be completebut should only give some ideas about possible methods. Cited references areonly a small fraction of the existing literature on the respective topics. Focus ison general works and textbooks rather than publications on specific problemsor phenomena.

Clearly, the last of the two steps described in the first paragraph is the easierpart. Calculation of noise propagation in the open is well covered by vari­ous national and international standards and guidelines (e.g. [4-7]), the basicprinciples can be found in many textbooks (e.g. [8-10]). The same holds, inprinciple, for scattering and diffraction of noise waves around barriers that canlead to attenuation effects (e.g. [8,10,11]).

Guidelines for the treatment of noise control features like mufflers, absorptivelinings and insulation can also be found in the literature (e.g. [12-15]) andshould yield acceptable results if no uncommon and special designs are used.

In comparison, calculation of the source quantities - i.e. the sound power levelsof the emitted noise - can be much more difficult, depending on the equipmentin question. Since the effect of noise control measures and the noise propagationto the POI, as well as the perception by the human receiver depend on thefrequency spectrum of the noise emissions, the spectral characteristics of eachsource need to be assessed.

Where this cannot be accomplished with theoretical models, empirical ones,based on test data, may need to be developed. Naturally, in most cases, fol­lowing this approach will be a task to be performed by the manufacturer ofthe respective equipment.

The characteristics of jets generated by pressurized gases emanating throughnozzles or other discharge devices have been extensively researched. However,existing methods for the calculation of jet noise (e.g. [9,16,17]) can not alwaysbe applied with sufficient accuracy to the rather special situation of flares andflare equipment. Reasons are the extreme properties and non-ideal behaviourof the gases, multi-component flow, jet-structure interaction and others. Whereestimates obtained from general methods do not yield the desired predictionquality, measurements on representative designs of a flare manufacturer maybecome necessary. If a sufficient number of suitably chosen configurations istested, correlations between noise emissions and gas/flow characteristics canbe worked out.

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The same approach will be the most practical one with respect to the noiseemissions of the pilot burners. Computing those from theoretical considerationswill in general not be possible. However, since the number of different pilotburner designs of a given manufacturer typically is not too large, working outempirical models for pilot burner noise will usually be a practical alternative.

In some flare types air is forced into the discharging waste gas stream forsmoke suppression. Typically, the blowers used to feed the air to the burnerare common axial or radial fans as they are widely used in a multitude ofindustrial applications. It is characteristic for these blower types that theirnoise spectrum contains tonal components that are particularly annoying.

There are a number of references that deal with estimation of noise fromfans based on input parameters as fan type, design and operating conditions(e.g. [13,18-21]). For fans that are highly optimized with respect to low-noiseoperation, general noise prediction tools tend to overpredict the actual emis­sions and could lead to unnecessary expenses for noise control. In most casesthe fan manufacturer will be able to give detailed information on the noisecharacteristics of their products.

Control valves generate high-frequent noise when the compressed fluid up­stream of the valve suddenly expands after passing the flow restriction. Thenoise emissions are radiated into the upstream and downstream piping andemitted to the surroundings via the pipe walls if these are not acousticallyinsulated. Since the radiating surface of the piping is much larger than of thevalve itself it will always be the connected piping that is the main source ofvalve noise.

Typically, valve noise is proportional to the pressure drop accross the valvebut otherwise depends on a multitude of additional parameters. The mostimportant ones are valve type and design, number of relief stages, dimensions ofvalve and connected piping, flow medium, and process parameters. Predictionof valve noise is very complicated and usually makes extensive use of empiricalcorrelations. It will in most cases be necessary to rely on manufacturer dataor expert calculations.

Noise prediction is even more difficult for the combustion process itself. Thenoise emissions of the flame are mainly determined by the chemical reactionand the turbulence of the flow - both effects that even today are not completelyunderstood.

The possibility to numerically simulate noise radiation from combustion pro­cesses by Computational Fluid Dynamics (CFD) is intensly researched. How­ever, existing CFD approaches are still far from being applicable to the sit­uation encountered in full-scale industrial flaring. Simpler methods - like theTAE concept outlined in section 3 - usually suffer from serious shortcomingsand allow only a very rough estimate of the noise emissions to be expected.

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Due to the variety of applications and the large set of parameters affecting theflame's behavior, the development of reliable models for flare combustion will,at present, still have to rely on extensive work in the test field and require thejoint efforts of the flare designer and the noise specialist.

References

[1] C. E. Baukal Jr. and Robert Schwartz, editors. The John Zink CombustionHandbook. CRC Press LLC / John Zink Co., LLC, Boca Raton, 2001.

[2] Miiller-BBM GmbH. Noise emissions of different flare systems fieldmeasurements taken in various refineries and petrochemical plants. Pro­prietary data, 1990-2004.

[3] Verein Deutscher Ingenieure. VDI 3732 - Characteristic noise emissionvalues of technical sound sources - Flares. VDI guideline, 1999.

[4] American National Standards Institute. ANSI S1.26 - Method for thecalculation of the absorption of sound by the atmosphere. ANSI Americannational standard, 1978.

[5] International Organization for Standardization. ISO 9613-1 - Acoustics ­Attenuation of sound during propagation outdoors - Partl: Calculation ofthe absorption of sound by the atmosphere. ISO International standard,June 1993. 1. edition.

[6] International Organization for Standardization. ISO 9613-2 - AcousticsAttenuation of sound during propagation outdoors - Part2: General

method of calculation. ISO International standard, December 1996. 1.edition.

[7] Verein Deutscher Ingenieure. VDI 2714 - Sound propagation outdoors.VDI guideline, 1988. (In German).

[8] L. L. Beranek and 1. L. Ver, editors. Noise and vibration control engineer­ing - principles and applications. John WHey & Sons, New York, 1992.Wiley-Interscience Publication.

[9] M. Crocker. Encyclopedia of acoustics. John Wiley & Sons, New York,1997. Wiley-Interscience Publication.

[10] G. Miiller and M. Maser, editors. Taschenbuch der Technischen Akustik.Springer-Verlag, Berlin, 3. extended and revised edition, 2004. (In Ger­man).

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[11] Verein Deutscher Ingenieure. VDI 2720-1 - Outdoor noise control bymeans of screening. VDI guideline, 1996. (In German).

[12] M. L. Munjal. Acoustics of ducts and mufflers. John Wiley & Sons, NewYork, 1987. Wiley-Interscience Publication.

[13] L. L. Beranek, editor. Noise reduction. McGraw-Hill, New York, 1990.

[14] C. M. Harris, editor. Handbook of noise control. McGraw-Hill, New York,1957.

[15] D. Bies and C. Hansen. Engineering noise control theory and practice.Spon Press, Adelaide, 2. edition, 1996.

[16] W. K. Blake. Mechanics of flow-induced sound and vibration, volume 1.Academic Press, Orlando, 1986. Applied mathematics and mechanics.

[17] A. P. Dowling and J. E. Ffowcs Williams. Sound and sources of sound.Ellis Horwood Limited, Chichester, 1983.

[18] W. K. Blake. Mechanics of flow-induced sound and vibration, volume 2.Academic Press, Orlando, 1986. Applied mathematics and mechanics.

[19] M. E. Goldstein. Aeroacoustics. McGraw-Hill, New York, 1976.

[20] R. D. Madison. Fan engineering handbook. Buffalo Forge Company, Buf­falo, 5. edition, 1949.

[21] L. Bommes, J. Fricke, and K. Klaes, editors. Ventilatoren. Vulkan-Verlag,Essen, 1994. (In German).

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