Gas Dispersion - Draeger · 2016-11-04 · dispersion model. Near-source dispersion models must therefore rely more on the thermodynamic conditions and physical properties of the

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Gas Dispersion COMPETENCE IN GAS DETECTION

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Learning from historyRecognising that accidents do happen,Governments worldwide publish Directivesand enforce these Directives throughlegislation to mitigate against accidents. In Europe the Seveso II Directive(98/82/EC) is an example of one suchDirective. In the USA, the Risk and Management Program (RMP) is anotherexample of how Governments are trying toprotect the public and environment.

Government Directives require that companies, who hold excess amounts ofdangerous substances, perform a HazardAssessment. The main demands of mostHazard Assessments are to detail Prevention, Preparedness and EmergencyResponse Plans.

Gas detection systems play a major role inthe prevention or mitigation of a major accident or incident. Prompt detection of atoxic or flammable release allows people tobe informed early enough to make their

escape, or where the release is explosive,to automatically switch off ignition sources.Other countermeasures may include theactivation of local and remote alarms, isolation of process fluids by automaticallyclosing valves or initiating additional emergency response services. Gas detection systems do not prevent theinitial release. Gas detection systems onlyrespond to a release event.

In order to determine the magnitude of anyEmergency Response Plan it is necessaryto simulate a large scale gas release. Thissimulation will determine the boundaries orend point where a release of a toxic substance will no longer be harmful to peopleor where explosive gas/air mixtures will nolonger ignite. Knowledge of gas dispersionis fundamental to conduct a simulation.

Analysing accidentsThe gathering of data from all accidents isvery important. Data gathered from accidentsenables investigators to give reasons of why

accidents occur and the consequences ofsuch accidents. There are three main reasons for why accidents occur.

Major accidents do not just happen, theyare a combination of minor events, including small gas releases, which collectively result in a major accident or incident. The impact of large scale accidentscan be reduced or even avoided by earlydetection or a gas or substance release,with appropriate action being initiated. Awell designed gas detection system, wherethe position of the gas detectors are optimised will act as a first line of defenseagainst the dangerous release of toxic orexplosive substances. A prompt detectionsystem is a valuable asset.

The figure on the bottom details the mostcommon substances involved with industrial leakages. It shows that there area handful of substances which are most difficult to handle safely.

Risk Management Program

02 | RISK MANAGEMENT PROGRAM GAS DISPERSION

Wherever toxic or flammable chemicals are being manufactured, processed, stored or shipped, there willalways be a chance of an accident or a substance release. Even small releases of substances can causeharm to people, damage the environment or even destroy property. History informs us that accidents dohappen and when they happen they can be catastrophic.


RISK MANAGEMENT PROGRAM GAS DISPERSION | 03

The role of gas detection systemsA gas detection system is not simply ahandful of gas detectors spread across anindustrial plant. The choice of detectiontechnology, quantity of detectors and routineservice and maintenance of the entire gasdetection system are all important. However,the real challenge is to identify the possible migration path of any gas releasebased on a variety of factors wind direction,ambient temperatures, terrain,

process pressures etc, which establishesthe correct location of gas detectors. It isnot practical to saturate an industrial plantwith gas detectors, nor is it sensible toinstall only a single gas detector in a largearea. There must be a compromise between cost and risk reduction.There are two main types of gas detectors;a point gas detector which monitors theimmediate vicinity of the gas detector, or anopen path gas detector which monitors a

much larger area between two points. Eachtype of gas detector has its own strengthsand weaknesses.

It is also important to choose the correctmeasuring technology as some gas detectors may be poisoned by other chemicals in use, or be susceptible to highhumidity or give invalid readings due tocross sensitivities. In all applications it isimportant to avoid spurious gas alarms.

35 % Wear and tear of material and equipment

Reason for accidents

30 % Human error 30 % Process out of control 5 % Others

50 % Release of a substance

Accident consequences

30 % Explosion 20 % Fire

AmmoniaChlorineHydrogen chlorideHydrogenPropane/ButaneNatural gasHydrogen fluorideMethanolFluorineHydrogen sulfideSulfur dioxide

Chemicals involved in accidents; decending order. (MARS database 1984-2004)

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04 | ACCIDENTAL RELEASE GAS DISPERSION

Invisible gas hazardUnfortunately, many gas releases are invisible to the human eye. This means thatwe have little experience about how gasreleases behave and where the released gaseventual disperses. We can base our judgement on the behavior of gas releaseson smoke clouds from fires which we havegood experience of. However, this technique has its limitations because fires generate their own thermal energy which isdifferent to the motive force behind a gasrelease. What we do understand is that gasreleases have a high concentration withintheir initial central core and with turbulenceat their fringes a lower gas concentration canbe measured. Distance dilutes a gas releaseuntil the release reaches a boundary or endpoint where harmful effects are unlikely.

Software simulationIn recent years gas dispersion software topredict the movement and dispersion characteristics of gas releases over largeareas has become available. These soft-ware packages are not accurate at characterising gas releases within a fewmetres of the release source because thereis simply insufficient information availableclose to the source to generate an accurate

dispersion model. Near-source dispersionmodels must therefore rely more on thethermodynamic conditions and physicalproperties of the gas components releasedthan on meteorology or site-specific conditions. Simply using gas density as theonly parameter may lead to dangerous misinterpretation of the predicted gasdispersion.

Stages of matterTo fully understand the characteristics ofany gas cloud we must first understandmatter. The three stages of matter are– Gas– Liquid– Solid

If the temperature of a substance is greaterthan its boiling point, then the substance isin its gas phase. The volume of a gas willchange as temperature and pressure changes. An increase in temperature willexpand any gas increasing the volume and decreasing the density. The result of this isthat the warm gas cloud will rise. Gaseswhich are stored and compressed in a vessel have a higher density. At higher densities or pressure many gases changestate and take up a liquid state.

Many solids and liquids below their boilingpoint have a tendency to evaporate to agaseous state which is called vapour. Thereis a limit to the amount of vapour which canbe in the gaseous state. This limitation is afunction of temperature and pressure. Thevapour concentration can never exceed thesubstance’s specified value called the saturation vapour pressure. With changingambient conditions, saturated vapours cancondense.

Condensation can either form small droplets suspended in the air or form aliquid layer on cold surfaces. Small dropletsin the air move freely and are called aerosols. When aerosols become tooheavy or large they fall to the ground likerain. Aerosols are not a gas but the liquidphase of a substance.

Although pure water is not a gas at roomtemperature, water in the vapour phasemixes readily with dry air, until the partialpressure of the water vapour reaches thesaturation water vapour pressure at the current temperature. Such moist air is lighter than dry air at the same temperature,because the molecular mass of water islower than the average molecular mass of

Accidental release of toxic and flammable matter

Industrial plants are not designed to leak toxic or flammable gases. However, each piece of equipmentused in a process line does have a potential to leak, especially if service routines are not maintained. Ineach industrial plant there are thousands of devices and pieces of equipment used, such as e.g. gauges,valves, pumps, compressors, storage vessels, etc. Each joint or connection point is a potential source ofrelease. The goal is to detect an accidental release before it develops into a major hazard.


ACCIDENTAL RELEASE GAS DISPERSION | 05

dry air. The maximum concentration ofwater vapour (saturated vapour pressure)in air at 20 °C / 68 °F is 23000 ppm. At 10 °C / 50 °F it is only 12000 ppm and atfreezing point only 6000 ppm. It almost doubles every 10 °C / 18 °F temperatureincrease. When cooling down the surpluswill condense as clouds, fog or rain.

Property of gaseous matterThe density of a substance is the ratio between its mass and the volume it occupies. All substances in a pure formhave a different but specific density. Theweight of a substance is related to the substance’s density. If the densities of allindividual substances are compared withair then we find that there are substancesheavier than air, lighter than air or neutral

with air. Substances which are lighter thanair show buoyant characteristics and generally float upwards; such as hydrogenand helium which are used in balloons.Gases which are heavier than air are affected by gravity, thus flow towards andalong ground level.

If the density of a gas is neutral with air,then this gas will move with air and be pushed and pulled by air currents. Neutralgases have no self motive force.

When comparing relative densities it isimportant to remember that the informationgiven in reference tables supports puresubstances which are at the same tem -perature as the surrounding air e.g. 20 °C /68 °F. If a gas is heated then its densitydecreases and the motive force dictatesthat the gas cloud will rise; this is why hotair balloons float. Whereas, if a gas is cooled its density increases resulting in thegas cloud falling naturally towards theground; the motive force being the pull ofgravity. A rule of thumb is that the densityof a substance can change by 3 % for every10 °C / 18 °F change in temperature.

At room temperature 100 vol.% Methanehas a relative density of 0.55 compared withair. This means, it is a light gas and will rise.For transportation and storage Methane isliquefied at – 162 °C / – 260 °F (LNG). IfMethane leaks a very cold gas cloud is formed, which, due to the high density ofcold Methane, is heavier than air. At – 112 °C / – 170 °F Methane density will beequal to air and when warmed further, thelight gas properties will dominate.

As a gas cloud disperses two events occur;the concentration of the gas cloud decreases and with this change the gascloud density will approach that of air. Therefore, as a gas cloud disperses itsbehaviour changes and finally becomesneutral with air. A diluted gas will neverseparate again from air to produce higherconcentrations.

Buoyant gasesNeutral gasesDense gasesVapoursReference

DENSITY

Substance Formula Rel density Boiling temperature

Hydrogen H2 0,07 gas – 253 °C – 423 °F

Helium He 0,14 gas – 269 °C – 452 °F

Methane CH4 0,55 gas – 162 °C – 260 °F

Ammonia NH3 0,59 gas – 33 °C – 27 °F

Hydrogen fluoride HF 0,69 gas 19 °C 66 °F

Acetylene C2H2 0,90 gas – 84 °C – 119 °F

Hydrogen cyanide HCN 0,93 vapour 26 °C 79 °F

Carbon monoxide CO 0,97 gas – 192 °C – 314 °F

Nitrogen N2 0,97 gas – 196 °C – 321 °F

Ethylene C2H4 0,97 gas – 104 °C – 155 °F

Air at 20 °C / 68 °F – 1,00 gas

Formaldehyde HCHO 1,04 gas – 21 °C – 6 °F

Nitrogen monoxide NO 1,04 gas – 152 °C – 242 °F

Ethane C2H6 1,04 gas – 89 °C – 128 °F

Air at 0 °C / 32 °F – 1,07 gas

Methanol CH3OH 1,10 vapour 65 °C 149 °F

Oxygen O2 1,10 gas – 183 °C – 297 °F

Phosphine PH3 1,17 gas – 88 °C – 126 °F

Hydrogen sulfide H2S 1,18 gas – 60 °C – 76 °F

Hydrogen chloride HCl 1,26 gas – 85 °C – 121 °F

Fluorine F2 1,31 gas – 188 °C – 306 °F

Propylene C3H6 1,45 gas – 48 °C – 54 °F

Ethylene oxide C2H4O 1,52 gas 11 °C 52 °F

Carbon dioxide CO2 1,52 gas – 79 °C – 110 °F

Propane C3H8 1,52 gas – 42 °C – 44 °F

Nitrogen dioxide NO2 1,59 vapour 21 °C 70 °F

Methylchloride CH3Cl 1,74 gas – 24 °C – 11 °F

Acrylonitrile CH2CHCN 1,83 vapour 77 °C 171 °F

Acrolein (Acrylaldehyde) C2H3CHO 1,94 vapour 57 °C 135 °F

n-Butane C4H10 2,01 gas – 1 °C 30 °F

Sulfur dioxide SO2 2,21 gas – 10 °C 14 °F

Chlorine Cl2 2,45 gas – 34 °C – 29 °F

Benzene C6H6 2,70 vapour 80 °C 176 °F

Hydrogen bromide HBr 2,79 gas – 67 °C – 89 °F

Phosgene COCl2 3,41 gas 8 °C 46 °F

Bromine Br2 5,52 vapour 58 °C 136 °F

List of substances sorted byrelative density compared to air.


06 | ATMOSPHERIC DISPERSION GAS DISPERSION

Depending upon the temperature of theleaking gas additional thermal effect maybe experienced. A hot gas will tend towarm the surrounding air and create thermal currents which rise. Alternatively,cold gases will generate downward thermals.

Buoyant gasIf a gas release has a steady state then itimmediately generates a cloud or a plume.If this cloud or plume is more than 2 % lessdense than air then the release is said tobe buoyant and will want to rise naturally.Hot gases react in an identical manner. Therelative motion of a rising gas cloud generates turbulence at its fringes, resulting in rapid mixing of the gas and air.This mixing expands the rising gas cloudslaterally. As the gas clouds rises, dilutesand expands laterally its density decreases,resulting in the gas cloud becoming neutral with respect to air. Once the gascloud looses its buoyancy, then naturalambient conditions become dominant andthe gas cloud can move anywhere.

To detect such a release, the recom -mended location for a gas detector isabove and close to any potential release

point, keeping the predominant wind direction in mind. Additional baffles or collecting cones can be used to direct thebuoyant gas towards the gas detector. Dueto the characteristics of a buoyant gasrelease it is often found that gas measurements are unstable due to the frequently changing gas concentrationswithin the gas cloud.

Typical examples of buoyant gases areMethane, Ammonia and Hydrogen. Statistically, only 20 % of all gas releasesare of a buoyant nature. Like all gasreleases the final stage of the gas cloud iswhen it becomes neutral with air.

Neutrally buoyant gasA neutrally buoyant gas has almost thesame density as air. Typical gases with neutral buoyancy are Ethylene, Ethane,Carbon monoxide and Ethanol. Neutrallybuoyant gases do not have any intrinsicmovement of either up or down. Gasclouds are driven by wind or artificial airstreams. Neutrally buoyant gas clouds mixextremely quickly with the surroundingatmosphere due to turbulence and vortexes. When toxic gases mix with airdown to their work-place limits in the ppm

range, they have reached density equilibrium with air and will behave neutrally buoyant.

To detect such a release the recommendedlocation for a gas detector is level with anypotential release point, keeping the predominant wind direction in mind.

Dense gasDense gases and vapours are much heavier than air and collectively form thelargest group of all dispersing substances.This group of dispersing substances includes heavier than air gases, vapoursfrom evaporating liquids and cold gasclouds. The motive force for the migrationof dense gases is gravity, therefore thedispersion normally follows the gradient ofthe terrain. A dense gas cloud will fall likea waterfall, flow along the surface like waterand can travel long distances before naturaldilution occurs or turbulence disperses thecloud altogether. Long dispersion distancescreate greater areas of danger. Dense gasclouds are not easily distorted by wind,however structures, walls and dikes canalter or control the flow of the moving gascloud. Dense gas clouds are extremely

Atmospheric dispersion

When a gas leaks from a process there is a boundary between when the gas is influenced by its processcharacteristics or thermodynamics (i.e. pressure, temperature, etc.) and the point where it becomesinfluenced by the ambient conditions (i.e. wind speed, terrain, temperature, etc.) It is extremely complicatedto model a gas release due to the number of variables acting upon the released gas. It is not accurate tobase a gas dispersion model on gas densities alone. Even on a calm day, the average wind velocity is 3 m/sec. which is enough to displace gases even though the wind can not be felt.

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ATMOSPHERIC DISPERSION GAS DISPERSION | 07

dangerous as the can disappear by entering basements, tunnels, wells etc,which makes countermeasures very difficult. However, as their migration pathsare very predictable the location of gasdetectors is relatively simple and straightforward. All gas detectors should be mounted close to the ground and in thepresumed pathway of the gas cloud. If thegas cloud is made up from cold gaseswhich are normally buoyant at room temperature then the gas cloud will act alittle differently. Initially the temperature ofthe gas cloud will make it behave as adense gas cloud. As the gas cloud heatsup, its characteristics will change from‘dense’ to ‘buoyant’. Dense, cold gasclouds are sometimes easy to see as theycondense water vapour from the surroundingatmosphere to produce visible fog.

AerosolsAn aerosol is not a gas, but a liquid madefrom small droplets which are suspendedin air. The droplets are formed fromvapours or gases under certain thermo -dynamic conditions or by flash evaporationof pressurised liquids. The scattering oflight within an aerosol cloud frequentlymakes the cloud visible to the naked eye.

The dispersion of an aerosol may vary between the behaviour of a dense gas ora neutrally buoyant gas. However, as theaerosol droplets absorb temperature fromthe surrounding environment they will evaporate and generate a gas/vapourcloud.

Substances with a hygroscopic propertycan form aerosols by absorbing moisturefrom the surrounding atmosphere. Substances like HCl, HF and SO3 are typicalexamples. The gas SO3 which has a verystrong hygroscopic property will absorbmoisture from the air immediately uponrelease. The reaction between SO3 andwater produces droplets of sulfuric acid.As the weight of the sulfuric acid aerosolincreases droplets will fall to the ground toproduce pools of acidic liquid. For thedetection of acid gases it is recommended to position the gas detectorlow to the ground.

VapourLiquids naturally have a vapour pressurewhich is a function of the temperature. The process of evaporation requires energywhich normally comes from the liquid andthe surrounding environment. As a liquid

vaporises the temperature of the liquiddecreases resulting in a cooling effect,which in turn slows down the evaporationprocess. The temperature of the vapour islower than the original liquid, which causes the vapour to act like a dense gas.The concentration of the liberated vapouris not easy to predict as it is a function ofthe evaporation rate, temperature of theliquid and the surrounding air flow. Gasdetectors should be located close to theground in the presumed pathway of thevapour cloud.

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08 | SOURCE CHARACTERISTICS GAS DISPERSION

The characteristics of any substance leakdepends upon the following main processconditions

– Gas at different temperatures andpressures

– Gas liquefied under pressure– Gas liquefied by refrigeration– Liquids or solvents

Most leakages occur slowly. Process linescorrode, pumps seals age and pressurevalves weep slowly. Where leakages of thissize are continuous it may be possible todetect reasonable concentrations withlocalised gas detectors as the gas disperseswith air movement. When leak sourcescontinue without repair, there may be an

instant when the process leak becomes anaccidental discharge.

Gaseous releaseAn over pressure gas release will producea gas cloud (concentration at the point ofrelease equals the process concentration)at the release point e.g. pump seal. Thedensity of the gas and the prevailing windwill move the gas cloud in a predeterminedmanner. Light gases or hot gases having adensity lower than air will tend to float,whereas heavy gases or cold gases havinga density higher than air will tend to sink.Irrespective of whether the gas cloud isbuoyant or dense the uniformity of the gascloud is not constant and changes drama-tically with distance and time.

As the gas disperses from its point ofrelease a gas plume is formed. The concentration within the plume changesconstantly: it may rise and fall dependingupon the movement of the plume. A gasplume can be compared with the charac-teristics of visible smoke from a chimney.

Plumes of gas moving across a gas detectorwill generate random gas measurements.Sometimes the gas detector will measurehigh concentrations, and as the plumemeanders, the gas detector may measurelow concentrations – random gas valuesare typical. This type of gas dispersionmodel is called the Gaussian dispersion.Eventually, the gas plume will completelydisperse and its contents become part ofthe ambient atmosphere.

Medium pressure gas release reacts diffe-rently to an over pressure gas release. Athermodynamic effect called adiabaticexpansion comes into play. As the highpressure gas escapes, it expands andcools down. The resulting cold gas cloudtherefore acts likes a dense gas.

High pressure gas releases will initiallyproduce jets of released gas. The shapeand distance of the jet varies, however theformat of the jet is reasonably stable,having a high concentration along its axis,with concentrations decreasing towardsthe edges. The shape of the jet release is

Source characteristics

To fully understand gas dispersion it is insufficient just to consider the characteristics of how gasclouds disperse. It is necessary to understand that different phases of dispersion occur from different sources.

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High pressure gas jet displaying gasconcentration by different colours.

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SOURCE CHARACTERISTICS GAS DISPERSION | 09

tapered outwards with distance. Due to thehigh gas velocities experienced the released gas quickly mixes with air due toturbulence. Quick dilution creates a natu-rally buoyant gas cloud. Due to the con-centrated direction of gas jets the positio-ning of gas detectors is quite difficult,because jets can form in any direction. Baffles located around possible leakpoints help to destroy a jet release, thus amore turbulent release occurs which easilydisperses across gas detectors.

Liquefied under pressureThe storage of gases in their liquefiedphase is very common in industry. Theliquefied phase of a gas reduces the totalspace required for storage and makestransportation of the gas easier. The volumetaken by the liquid phase of a gas is reducedby a factor of 100 to 500 depending uponthe gas. There are two ways to liquefy gases;the first way is to increase the pressure and the second way is to reducethe temperature.

When a pressurised liquid escapes, thereare two phases associated with the leak.First, a jet of liquid will be released whichinstantly evaporates. This evaporation is

called ‘flash’. Second the evaporatingliquid pulls energy from itself and the surrounding atmosphere and in turn coolsdown the leaking fluid. The cooling of thefluid prevents total evaporation therefore anaerosol is produced. If the leak is largeenough then cold pools of fluid can accumulate on the ground which will evaporate to produce a gas release. Thecold aerosol cloud will act like a dense gas.A pressurised liquid release can often beseen by the naked eye as the cooling effectof evaporation will condense ambienthumidity to produce a vapour cloud.

Liquefied by refrigerationWhen a gas is refrigerated below its boiling point it will become a liquid. If arefrigerated gas leaks, a cold pool of liquidis formed at ground level. As the cold liquidpulls energy from the surrounding atmosphere the liquid will boil naturally.This is a self controlling process. Limitedheat transfer will only allow a certain rateof gas to be generated. The temperature ofthe liquid and the evolving gases are low,therefore both the liquid and gases will actlike a dense gas. Gravity and wind flow willdictate the direction of liquid and gas flow.Like dense gases, the use of dikes or bund

walls can be used efficiently to direct orhold the flow of all leakages. The mosteffective place to mount a gas detectorwould be inside of the dike or bund wall.

Liquid spillageThe spillage of liquids will always form apool on the ground unless the surface isabsorbent. The vapour pressure and evaporation rate of the liquid will form avapour cloud at the liquid’s surface. Themaximum concentration of the vapourcloud is determined by the vapour pressureof the specific liquid and temperature. Athigher temperatures, higher gasconcen-trations will be experienced.

The rate of evaporation of any substanceis fixed, therefore the concentration build-up is a function of time. If the gas isdispersed by air currents, then only lowconcentrations will be experienced.Vapours liberated by non-boiling liquidswill act like a dense gas e.g. flows alongthe ground where walls and dikes caneffectively control their direction.

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Horizontal jet of pressure liquefied Ammonia. Visible aerosol 0.7 sec after valve rupture.


10 | GASEOUS RELEASE INSIDE BUILDINGS GAS DISPERSION

The indoor differenceFor indoor applications, the prevailing windfor an outdoor application is replaced byinternal air currents. Air currents within abuilding can be forced e.g. heating andventilation systems, by convection from hotprocess equipment or by thermal effectson the walls and roof from external sunlight. There are many other factorswhich can affect internal air currents e.g.the movement of people or product.

In an open plant, a gas cloud can spread in all directions and has no boundary. Wit-hin a building, a gas cloud can only consume a fixed and finite volume, thus theconcentration of the leaked gas will increase. Given sufficient time, even thesmallest of leaks can exceed the LEL(explosive) or TLV (toxic) levels within theentire room volume.

An additional hazard, in buildings, is thereplacement of ambient Oxygen by a leaking gaseous substance. Gases whichdisplace Oxygen are not limited to gaseswhich are either explosive or toxic, butinclude nonflammable and non toxic gasessuch as Nitrogen, Helium, Argon or Sulfurhexafluoride (SF6).At a concentration of 10 vol.% Argon in the air the Oxygen concentration drops by2 vol.% from 20.9 vol.% to 18.9 vol.%.

Where there is a risk of Oxygen depletion,additional gas detectors must be installed.

The location of gas detectors inside a building is not a simple and straight forward task. It may be sensible to locatea gas detector adjacent to a potential source of release. This would give the earliest warning. However, it must be

remembered that leaks occur in all directions. This would mean many sensorsacross a three dimensional grid. Installinga grid format of sensors in a room may bemore practical and economic, however thistype of installation stretches the alarmresponse time as the gas must travel to thesensor.

The correct location of a gas detector canonly be determined if the air movement wit-hin the building/room is analysed. The firststep is to identify all of the possible sources which generate air movement e.g.ventilation systems, hot surfaces, movementof people and product, thermal radiationetc. The second step is to simulate the airmovement by using smoke tubes, takingcare to monitor dead spaces like corners,roof voids and sumps.

Gaseous release inside buildings

The behaviour of a gas release inside a building is completely different to the known behaviour ofthe same release occurring outdoors. Not only do you have the containment of the gas, resulting inan increase in the potential concentration, but there is an additional hazard of oxygen displacement/ deficiency. To position a Gas Detector correctly, it is imperative that the characteristic of the room´sair flow is known. This can be achieved by using smoke tubes.

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TECHNICAL DATA

Air exchange Dilution Recommended Room class Driving force rate Flow efficiency Gas dispersion placement

Air tight Density, convection, 0.1 /h < 20 cm/sec Low Dense gas descending Ground

motion diffuse Light gas buoyant Ceiling

Thermal sources Machine, heater, > 0.2 /h > 20 cm/sec Good Ascending, convection Ceiling above heat exchanger, thermal source

lumination Circulating streamline In the stream

Active ventilation Ventilation, Calculated > 1 m /sec High Following streamline In the stream

air condition, Ductopenings


, Air tight roomsAn air tight room means that there is verylittle air exchange between the volume ofthe room and the outside. This is typical forlarge warehouses which store chemicals,gases and other goods. Within such aroom, air movement does exist, however it’sso small that it has no influence or motiveforce to move or dilute a gas cloud. Theexchange of air with the outside can be aslow as 0.1/h. This means that 10 % of theair volume within the room will be exchan-ged with the outside air through naturalgaps in the room. This exchange is notenough to dilute any gas release, thereforeinsufficient to guard against a gas hazard.With very little air movement within a room,a gas release will behave as a buoyant,neutral or dense gas depending on its density.

Examples of gas releases in an air tightrooma) An evaporating pool of solvent will

create a dense cloud at floor levelb) Carbon dioxide will fall to the ground

and accumulate at the lowest pointc) Methane will rise to the ceilingd) Ammonia will disperse and rise to

the ceiling

Gas mixtures have different dispersioncharacteristics with respect to their particular contents.

12 | GASEOUS RELEASE INSIDE BUILDINGS GAS DISPERSION

Gaseous release inside buildings

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Different scenarios which are typical for gas leaks within a building

Thermal source devel-oping convection cur-rents.

Room with ventilationsystem.

Air tight room; lightgas „blue”, dense gas“yellow”.


GASEOUS RELEASE INSIDE BUILDINGS GAS DISPERSION | 13

Hydrogen does not always rise just becau-se it is lighter than air. In a galvanic process Oxygen and Hydrogen can begenerated at the same time and at thesame location. Oxygen is heavier than air(relative density 1.1) and Hydrogen is lighterthan air (relative density 0.07). An undisturbed mixture of Oxygen and Hydrogen, up to a concentration of 10vol.% Hydrogen remains denser than airand will fall to the ground. The LEL ofHydrogen is 4 vol.%, therefore at floor levelthere exists an explosive level of Hydrogen.

The location of a gas detector in an air tightroom is totally dependent on the relativedensity of the leaking substance, includingthe combined influences of gases in mixtures.

Thermal sourcesThere are many heat dissipating sourceswithin a room, especially machinery rooms.The energy from the heat source heats upthe surrounding air which rises to the ceiling. This warm air will spread sidewaystowards the walls. As the air cools it beginsto fall down towards floor level, thus creating a circular motion. This circular aircurrent is so strong that it carries gases

irrespective of their density. The turbulentcharacteristics of the circular air currentsencourage any leaking gas to mix with theair resulting in dilution of the concentration.The concentration will increase with time.

Gas detectors should be installed at ceiling height above the heat source, orwhere the air current can be defined.

Active ventilationFor safety reasons it is common to findrooms where forced ventilation is installed.This is to ensure that there is a constant airexchange to guard against any build up oftoxic or explosive gases. The air movementwithin the room is calculated (rate of change)and the profile of air currents is known, the-refore the location of the gas detectors ismade easier. There are two options for thelocation of gas detectors

a) The gas detector can be located in theknown air stream within the room, or

b) The gas detector can be located in theventilation system (outlet)

Gas detectors with a lower measuringrange will be needed as the constant airexchange will dilute any gas leak. Gasdetectors may be required to monitor for

gas emissions. In the case of a gas emission the ventilation system may berequired to shutdown to contain the gas.

Air flowWithin any controlled environment e.g. insideof a building, it is recommended that an airflow check is performed on a regular basis.The Dräger Flow Check is a device whichmakes air streams visible by generating avisible aerosol of a neutral buoyant property. The visible aerosol can be released adjacent to any potential leaksource and the aerosol can be watched todetermine the surrounding air currents.This method of determining air currents isbest suited to areas where there are thermal sources or forced ventilation. If aroom is totally stagnant, then the FlowCheck will only simulate the characteristicsof a neutrally buoyant gas.

Air flow within a room will be disturbed byhigh pressure releases. A high pressurerelease will increase the gas concentrationwithin a room rapidly. Mechanical baffleplates can be used to disturb any jetrelease.

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14 |

SUBSTANCE OCCURRENCE IN OPERATIONAL FAULT SITUATION

liquefied by liquefied underpure gas cloud refrigeration pressure highly pressurized

Toxic gases mass boiling point vapour pressureg/mol comment °C °F bar* (20 °C / 68 °F) jet aerosol puddle

Carbon monoxide CO 28 – 191 – 312 35 xFluorine F2 38 – 188 – 306 – xNitrogen monoxide NO 30 – 150 – 238 35 xHydrogen chloride HCl 36 fuming – 85 – 121 43 x x xHydrogen sulfide H2S 34 – 60 – 76 18 x xHydrogen bromide HBr 81 fuming – 66 – 87 21 x xChlorine Cl2 71 – 34 – 29 x 6.8 x xAmmonia NH3 17 – 33 – 27 x 8.6 x x x xSulfur dioxide SO2 64 – 10 14 x 3.3 x x x xPhosgene COCl2 99 8 46 x 1.6 xHydrogen fluoride (HF)n (20)*n fuming 19 66 x 1 x x xNitrogen dioxide NO2/N2O4 46/92 visible 21 70 x 1 x x xFlammable gasesHydrogen H2 2 – 252 – 422 – xMethane, LNG CH4 16 – 162 – 260 x – xEthylene, Ethene C2H4 28 – 104 – 155 x 41.0 xAcetylene, Ethine C2H2 26 in Aceton – 83 – 171 43.0 xPropylene, Propene C3H6 42 – 50 – 58 10.0 x x x xPropane C3H8 44 – 42 – 44 8.4 x x x xFormaldehyde HCHO 30 – 21 – 6 xVinyl chloride C2H3Cl 62 – 14 7 x 3.4 x x x x1,3-Butadiene C4H6 54 – 4 25 x 2.4 x x x xLPG, Propane/Butane 50 – 1 30 8.0 x x x xn-Butane C4H10 58 – 1 30 2.1 x x x xEthylene oxide C2H4O 44 11 52 1.5 x x x xVapours from liquidsHydrogen cyanide HCN 27 26 79 0.82 xPropylene oxide C3H6O 58 34 93 0.57 xCarbon disulfide CS2 76 46 115 0.40 xAcrolein C3H4O 56 57 135 0.30 xBromine Br2 150 visible 59 138 0.25 xMethanol CH3OH 32 65 149 0.13 xEthanol C2H5OH 46 78 172 0.06 xAcrylonitrile C3H3N 53 77 171 0.12 xBenzene C6H6 78 80 176 0.11 xOff-gasing solutionsHydrocyanic acid HCN 27 xAmmonia solution 32 % NH3 17 25 77 0.80 xFormalin 55 % HCHO 30 0.002 xAcetyl chloride HCl 36 fuming 52 126 0.32 x xHydrochloric acid 32 % HCl fuming 57 135 0.038 x xCarbonic acid CO2 44 xHydrofluoric acid HF > 70 % fuming 106 223 x xNitric acid 65 % NO2/N2O4 visible 122 252 0.06 x xPhosphoric acid 85 % 213 415 0 xSulfuric acid < 96 % > 96 % fuming 280 536 0 xSO3 (Oleum 24 %) H2SO4 fuming xOther gasHelium He 4Water vapour H2O 18Air (dry) air 28.9Oxygen O2 32Nitrogen N2 28Carbon dioxide CO2 44

BuoyantNeutrally buoyantDense gasNot applicableSorted by boiling temperature


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buoyant gasRelease scenario buoyant

lowpressure

leak

neutrally buoyant gas

neutral

gaseous high

systempressure

dense gas heavy

hot gas buoyant

cold gas heavy

gas jet diluted neutral

source

cold gas heavy

aerosol heavy

condensation

heat controlled evaporation

liquefied under

refrigeration

liquefied by

pressure

cold gas heavy

heat controlled evaporation

vapour heavy

liquid

non boilingpool

flashevaporation

boiling pool

fuming liquid aerosol heavy

Decision tree for derivingthe most likely dispersion.The arrows indicate if a detector should beinstalled above (arrowup) or in the same height(arrow straight on) or onthe floor (arrow down).

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HEADQUARTERSDräger Safety AG & Co. KGaARevalstrasse 123560 Lübeck, Germany

www.draeger.com

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Documents

Gas Dispersion - Draeger · 2016-11-04 · dispersion model. Near-source dispersion models must therefore rely more on the thermodynamic conditions and physical properties of the