STL-1168-2008 D-5191-2009 a Dispersion Gas NGSDETECTION GAS IN COMPETENCE 02 | RISK MANAGEMENT PROGRAM GAS DISPERSION

Risk Management Program

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

Learning from history escape, or where the release is explosive, accidents occur and the consequences of Recognising that accidents do happen, to automatically switch off ignition sources. such accidents. There are three main Governments worldwide publish Directives Other countermeasures may include the reasons for why accidents occur. and enforce these Directives through activation of local and remote alarms, legislation to mitigate against accidents. isolation of process fluids by automatically Majoraccidentsdonotjusthappen,they In Europe the Seveso II Directive closing valves or initiating additional are a combination of minor events, (98/82/EC) is an example of one such emergency response services. including small gas releases, which Directive. In the USA, the Risk and Gas detection systems do not prevent the collectively result in a major accident or Management Program (RMP) is another initial release. Gas detection systems only incident. The impact of large scale accidents example of how Governments are trying to respond to a release event. can be reduced or even avoided by early protect the public and environment. detection or a gas or substance release, In order to determine the magnitude of any with appropriate action being initiated. A Government Directives require that Emergency Response Plan it is necessary well designed gas detection system, where companies, who hold excess amounts of to simulate a large scale gas release. This the position of the gas detectors are dangerous substances, perform a Hazard simulation will determine the boundaries or optimised will act as a first line of defense Assessment. The main demands of most end point where a release of a toxic against the dangerous release of toxic or Hazard Assessments are to detail substance will no longer be harmful to people explosive substances. A prompt detection Prevention, Preparedness and Emergency or where explosive gas/air mixtures will no system is a valuable asset. Response Plans. longer ignite. Knowledge of gas dispersion is fundamental to conduct a simulation. Thefigureonthebottomdetailsthemost Gas detection systems play a major role in common substances involved with the prevention or mitigation of a major Analysing accidents industrial leakages. It shows that there are accident or incident. Prompt detection of a The gathering of data from all accidents is a handful of substances which are most toxic or flammable release allows people to very important. Data gathered from accidents difficult to handle safely. be informed early enough to make their enables investigators to give reasons of why RISK MANAGEMENT PROGRAM GAS DISPERSION | 03

The role of gas detection systems process pressures etc, which establishes much larger area between two points. Each A gas detection system is not simply a the correct location of gas detectors. It is type of gas detector has its own strengths handful of gas detectors spread across an not practical to saturate an industrial plant and weaknesses. industrial plant. The choice of detection with gas detectors, nor is it sensible to technology, quantity of detectors and routine install only a single gas detector in a large It is also important to choose the correct service and maintenance of the entire gas area. There must be a compromise measuring technology as some gas detection system are all important. However, between cost and risk reduction. detectors may be poisoned by other the real challenge is to identify the There are two main types of gas detectors; chemicals in use, or be susceptible to high possible migration path of any gas release a point gas detector which monitors the humidity or give invalid readings due to based on a variety of factors wind direction, immediate vicinity of the gas detector, or an cross sensitivities. In all applications it is ambient temperatures, terrain, open path gas detector which monitors a important to avoid spurious gas alarms.

Reason for accidents

35 % Wear and tear of material and equipment 30 % Human error 30 % Process out of control 5% Others

Accident consequences

50 % Release of a substance 30 % Explosion 20 % Fire ST-1173-2008 ST-1171-2008

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

Ammonia Chlorine Hydrogen chloride Hydrogen Propane/Butane Natural gas Hydrogen fluoride Methanol Fluorine Hydrogen sulfide

ST-1172-2008 Sulfur dioxide 04 | ACCIDENTAL RELEASE GAS DISPERSION

Accidental release of toxic and flammable matter

Industrial plants are not designed to leak toxic or flammable gases. However, each piece of equipment used in a process line does have a potential to leak, especially if service routines are not maintained. In each 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 of release. The goal is to detect an accidental release before it develops into a major hazard.

Invisible gas hazard dispersion model. Near-source dispersion Many solids and liquids below their boiling Unfortunately, many gas releases are models must therefore rely more on the point have a tendency to evaporate to a invisible to the human eye. This means that thermodynamic conditions and physical gaseousstatewhichiscalledvapour.There we have little experience about how gas properties of the gas components released is a limit to the amount of vapour which can releases behave and where the released gas than on meteorology or site-specific be in the gaseous state. This limitation is a eventual disperses. We can base our conditions. Simply using gas density as the function of temperature and pressure. The judgement on the behavior of gas releases only parameter may lead to dangerous vapour concentration can never exceed the onsmokecloudsfromfireswhichwehave misinterpretation of the predicted gas substance’s specified value called the good experience of. However, this dispersion. saturation vapour pressure. With changing technique has its limitations because fires ambient conditions, saturated vapours can generate their own thermal energy which is Stages of matter condense. different to the motive force behind a gas To fully understand the characteristics of release. What we do understand is that gas any gas cloud we must first understand Condensation can either form small releases have a high concentration within matter.Thethreestagesofmatterare droplets suspended in the air or form a their initial central core and with turbulence –Gas liquid layer on cold surfaces. Small droplets at their fringes a lower gas concentration can –Liquid in the air move freely and are called be measured. Distance dilutes a gas release –Solid aerosols. When aerosols become too until the release reaches a boundary or end heavy or large they fall to the ground like point where harmful effects are unlikely. If the temperature of a substance is greater rain. Aerosols are not a gas but the liquid than its boiling point, then the substance is phase of a substance. Software simulation in its gas phase. The volume of a gas will In recent years gas dispersion software to change as temperature and pressure Although pure water is not a gas at room predict the movement and dispersion changes. An increase in temperature will temperature, water in the vapour phase characteristics of gas releases over large expand any gas increasing the volume and mixes readily with dry air, until the partial areas has become available. These soft- decreasing the density. The result of this is pressure of the water vapour reaches the ware packages are not accurate at that the warm gas cloud will rise. Gases saturation water vapour pressure at the characterising gas releases within a few which are stored and compressed in a current temperature. Such moist air is metres of the release source because there vessel have a higher density. At higher lighter than dry air at the same temperature, is simply insufficient information available densitiesorpressuremanygaseschange because the molecular mass of water is close to the source to generate an accurate state and take up a liquid state. lower than the average molecular mass of ACCIDENTAL RELEASE GAS DISPERSION | 05

dry air. The maximum concentration of Property of gaseous matter with air. Substances which are lighter than water vapour (saturated vapour pressure) The density of a substance is the ratio air show buoyant characteristics and in air at 20 °C / 68 °F is 23000 ppm. At between its mass and the volume it generally float upwards; such as hydrogen 10°C/50°Fitisonly12000ppmandat occupies. All substances in a pure form and helium which are used in balloons. freezing point only 6000 ppm. It almost have a different but specific density. The Gases which are heavier than air are doubles every 10 °C / 18 °F temperature weight of a substance is related to the affected by gravity, thus flow towards and increase. When cooling down the surplus substance’s density. If the densities of all along ground level. will condense as clouds, fog or rain. individual substances are compared with air then we find that there are substances If the density of a gas is neutral with air, heavier than air, lighter than air or neutral then this gas will move with air and be pushed and pulled by air currents. Neutral gaseshavenoselfmotiveforce. DENSITY When comparing relative densities it is important to remember that the information Substance Formula Rel density Boiling temperature given in reference tables supports pure Hydrogen H2 0,07 gas – 253 °C – 423 °F substances which are at the same tem- Helium He 0,14 gas – 269 °C – 452 °F perature as the surrounding air e.g. 20 °C / Methane CH4 0,55 gas – 162 °C – 260 °F 68 °F. If a gas is heated then its density Ammonia NH3 0,59 gas – 33 °C – 27 °F decreases and the motive force dictates Hydrogen fluoride HF 0,69 gas 19 °C 66 °F that the gas cloud will rise; this is why hot Acetylene C H 0,90 gas – 84 °C – 119 °F 2 2 air balloons float. Whereas, if a gas is Hydrogen cyanide HCN 0,93 vapour 26 °C 79 °F cooled its density increases resulting in the Carbon monoxide CO 0,97 gas – 192 °C – 314 °F gas cloud falling naturally towards the Nitrogen N2 0,97 gas – 196 °C – 321 °F ground; the motive force being the pull of Ethylene C2H4 0,97 gas – 104 °C – 155 °F Air at 20 °C / 68 °F – 1,00 gas gravity. A rule of thumb is that the density Formaldehyde HCHO 1,04 gas – 21 °C – 6 °F of a substance can change by 3 % for every Nitrogenmonoxide NO 1,04 gas –152°C –242°F 10 °C / 18 °F change in temperature. Ethane C2H6 1,04 gas – 89 °C – 128 °F Air at 0 °C / 32 °F – 1,07 gas At room temperature 100 vol.% Methane Methanol CH3OH 1,10 vapour 65 °C 149 °F has a relative density of 0.55 compared with Oxygen O2 1,10 gas – 183 °C – 297 °F air.Thismeans,itisalightgasandwillrise. Phosphine PH3 1,17 gas – 88 °C – 126 °F For transportation and storage Methane is Hydrogen sulfide H2S 1,18 gas –60°C –76°F liquefied at – 162 °C / – 260 °F (LNG). If Hydrogen chloride HCl 1,26 gas – 85 °C – 121 °F Methane leaks a very cold gas cloud is Fluorine F 1,31 gas – 188 °C – 306 °F 2 formed, which, due to the high density of Propylene C H 1,45 gas – 48 °C – 54 °F 3 6 cold Methane, is heavier than air. At – Ethylene oxide C H O 1,52 gas 11 °C 52 °F 2 4 112 °C / – 170 °F Methane density will be Carbon dioxide CO2 1,52 gas – 79 °C – 110 °F equal to air and when warmed further, the Propane C3H8 1,52 gas – 42 °C – 44 °F light gas properties will dominate. 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 As a gas cloud disperses two events occur; Acrolein (Acrylaldehyde) C2H3CHO 1,94 vapour 57 °C 135 °F the concentration of the gas cloud n-Butane C4H10 2,01 gas –1°C 30°F decreases and with this change the gas Sulfur dioxide SO2 2,21 gas – 10 °C 14 °F cloud density will approach that of air. Chlorine Cl2 2,45 gas – 34 °C – 29 °F Therefore, as a gas cloud disperses its Benzene C6H6 2,70 vapour 80 °C 176 °F behaviour changes and finally becomes Hydrogen bromide HBr 2,79 gas – 67 °C – 89 °F neutral with air. A diluted gas will never Phosgene COCl2 3,41 gas 8 °C 46 °F separate again from air to produce higher Bromine Br 5,52 vapour 58 °C 136 °F 2 concentrations.

Buoyant gases Neutral gases List of substances sorted by Dense gases relative density compared to air. Vapours Reference 06 | ATMOSPHERIC DISPERSION GAS DISPERSION

Atmospheric dispersion

When a gas leaks from a process there is a boundary between when the gas is influenced by its process characteristics or thermodynamics (i.e. pressure, temperature, etc.) and the point where it becomes influenced by the ambient conditions (i.e. wind speed, terrain, temperature, etc.) It is extremely complicated to model a gas release due to the number of variables acting upon the released gas. It is not accurate to base 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.

Depending upon the temperature of the point, keeping the predominant wind range, they have reached density leaking gas additional thermal effect may direction in mind. Additional baffles or equilibrium with air and will behave be experienced. A hot gas will tend to collecting cones can be used to direct the neutrally buoyant. warm the surrounding air and create buoyant gas towards the gas detector. Due thermal currents which rise. Alternatively, to the characteristics of a buoyant gas To detect such a release the recommended cold gases will generate downward release it is often found that gas location for a gas detector is level with any thermals. measurements are unstable due to the potential release point, keeping the frequently changing gas concentrations predominant wind direction in mind. Buoyant gas within the gas cloud. If a gas release has a steady state then it immediately generates a cloud or a plume. Typical examples of buoyant gases are Dense gas If this cloud or plume is more than 2 % less Methane, Ammonia and Hydrogen. Dense gases and vapours are much dense than air then the release is said to Statistically, only 20 % of all gas releases heavier than air and collectively form the be buoyant and will want to rise naturally. are of a buoyant nature. Like all gas largest group of all dispersing substances. Hot gases react in an identical manner. The releases the final stage of the gas cloud is This group of dispersing substances relative motion of a rising gas cloud when it becomes neutral with air. includes heavier than air gases, vapours generates turbulence at its fringes, from evaporating liquids and cold gas resulting in rapid mixing of the gas and air. Neutrally buoyant gas clouds. The motive force for the migration This mixing expands the rising gas clouds A neutrally buoyant gas has almost the of dense gases is gravity, therefore the laterally. As the gas clouds rises, dilutes same density as air. Typical gases with dispersion normally follows the gradient of and expands laterally its density decreases, neutral buoyancy are Ethylene, Ethane, the terrain. A dense gas cloud will fall like resulting in the gas cloud becoming Carbon monoxide and Ethanol. Neutrally a waterfall, flow along the surface like water neutral with respect to air. Once the gas buoyant gases do not have any intrinsic and can travel long distances before natural cloud looses its buoyancy, then natural movement of either up or down. Gas dilution occurs or turbulence disperses the ambient conditions become dominant and clouds are driven by wind or artificial air cloud altogether. Long dispersion distances the gas cloud can move anywhere. streams. Neutrally buoyant gas clouds mix create greater areas of danger. Dense gas extremely quickly with the surrounding clouds are not easily distorted by wind, To detect such a release, the recom- atmosphere due to turbulence and however structures, walls and dikes can mended location for a gas detector is vortexes. When toxic gases mix with air alter or control the flow of the moving gas above and close to any potential release down to their work-place limits in the ppm cloud. Dense gas clouds are extremely ATMOSPHERIC DISPERSION GAS DISPERSION | 07 STL-1169-2008

dangerous as the can disappear by The dispersion of an aerosol may vary vaporises the temperature of the liquid entering basements, tunnels, wells etc, between the behaviour of a dense gas or decreases resulting in a cooling effect, which makes countermeasures very a neutrally buoyant gas. However, as the which in turn slows down the evaporation difficult. However, as their migration paths aerosol droplets absorb temperature from process. The temperature of the vapour is are very predictable the location of gas the surrounding environment they will lower than the original liquid, which detectors is relatively simple and straight evaporate and generate a gas/vapour causes the vapour to act like a dense gas. forward. All gas detectors should be cloud. The concentration of the liberated vapour mounted close to the ground and in the is not easy to predict as it is a function of presumed pathway of the gas cloud. If the Substances with a hygroscopic property the evaporation rate, temperature of the gas cloud is made up from cold gases canformaerosolsbyabsorbingmoisture liquid and the surrounding air flow. Gas which are normally buoyant at room from the surrounding atmosphere. detectors should be located close to the temperature then the gas cloud will act a Substances like HCl, HF and SO3 are typical ground in the presumed pathway of the little differently. Initially the temperature of examples. The gas SO3 which has a very vapour cloud. the gas cloud will make it behave as a strong hygroscopic property will absorb dense gas cloud. As the gas cloud heats moisture from the air immediately upon up, its characteristics will change from release. The reaction between SO3 and ‘dense’ to ‘buoyant’. Dense, cold gas water produces droplets of sulfuric acid. clouds are sometimes easy to see as they As the weight of the sulfuric acid aerosol condense water vapour from the surrounding increases droplets will fall to the ground to atmosphere to produce visible fog. produce pools of acidic liquid. For the detection of acid gases it is Aerosols recommended to position the gas detector An aerosol is not a gas, but a liquid made low to the ground. from small droplets which are suspended in air. The droplets are formed from Vapour vapours or gases under certain thermo- Liquids naturally have a vapour pressure dynamic conditions or by flash evaporation which is a function of the temperature. of pressurised liquids. The scattering of The process of evaporation requires energy light within an aerosol cloud frequently which normally comes from the liquid and

makes the cloud visible to the naked eye. the surrounding environment. As a liquid STL-1170-2008 08 | SOURCE CHARACTERISTICS GAS DISPERSION

Source characteristics

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

The characteristics of any substance leak instant when the process leak becomes an As the gas disperses from its point of depends upon the following main process accidental discharge. release a gas plume is formed. The conditions concentration within the plume changes Gaseous release constantly: it may rise and fall depending – Gas at different temperatures and An over pressure gas release will produce upon the movement of the plume. A gas pressures a gas cloud (concentration at the point of plume can be compared with the charac- – Gas liquefied under pressure release equals the process concentration) teristics of visible smoke from a chimney. – Gas liquefied by refrigeration at the release point e.g. pump seal. The – Liquids or solvents density of the gas and the prevailing wind Plumes of gas moving across a gas detector willmovethegascloudinapredetermined will generate random gas measurements. Most leakages occur slowly. Process lines manner. Light gases or hot gases having a Sometimes the gas detector will measure corrode, pumps seals age and pressure density lower than air will tend to float, high concentrations, and as the plume valves weep slowly. Where leakages of this whereas heavy gases or cold gases having meanders, the gas detector may measure size are continuous it may be possible to a density higher than air will tend to sink. low concentrations – random gas values detect reasonable concentrations with Irrespective of whether the gas cloud is are typical. This type of gas dispersion localised gas detectors as the gas disperses buoyant or dense the uniformity of the gas model is called the Gaussian dispersion. with air movement. When leak sources cloud is not constant and changes drama- Eventually, the gas plume will completely continue without repair, there may be an tically with distance and time. disperse and its contents become part of the ambient atmosphere.

Medium pressure gas release reacts diffe- High pressure gas jet displaying gas concentration by different colours. rently to an over pressure gas release. A thermodynamic effect called adiabatic expansion comes into play. As the high pressure gas escapes, it expands and cools down. The resulting cold gas cloud therefore acts likes a dense gas.

High pressure gas releases will initially produce jets of released gas. The shape and distance of the jet varies, however the format of the jet is reasonably stable, having a high concentration along its axis, with concentrations decreasing towards the edges. The shape of the jet release is ST-1167-2008 SOURCE CHARACTERISTICS GAS DISPERSION | 09 ST-1166-2008

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

tapered outwards with distance. Due to the called ‘flash’. Second the evaporating walls can be used efficiently to direct or high gas velocities experienced the liquid pulls energy from itself and the hold the flow of all leakages. The most released gas quickly mixes with air due to surrounding atmosphere and in turn cools effective place to mount a gas detector turbulence. Quick dilution creates a natu- down the leaking fluid. The cooling of the would be inside of the dike or bund wall. rally buoyant gas cloud. Due to the con- fluid prevents total evaporation therefore an centrated direction of gas jets the positio- aerosol is produced. If the leak is large Liquid spillage ning of gas detectors is quite difficult, enough then cold pools of fluid can The spillage of liquids will always form a because jets can form in any direction. accumulate on the ground which will pool on the ground unless the surface is Baffles located around possible leak evaporate to produce a gas release. The absorbent. The vapour pressure and points help to destroy a jet release, thus a cold aerosol cloud will act like a dense gas. evaporation rate of the liquid will form a more turbulent release occurs which easily A pressurised liquid release can often be vapour cloud at the liquid’s surface. The disperses across gas detectors. seen by the naked eye as the cooling effect maximum concentration of the vapour of evaporation will condense ambient cloud is determined by the vapour pressure Liquefied under pressure humidity to produce a vapour cloud. of the specific liquid and temperature. At The storage of gases in their liquefied higher temperatures, higher gasconcen- phase is very common in industry. The Liquefied by refrigeration trations will be experienced. liquefied phase of a gas reduces the total When a gas is refrigerated below its space required for storage and makes boiling point it will become a liquid. If a The rate of evaporation of any substance transportation of the gas easier. The volume refrigerated gas leaks, a cold pool of liquid is fixed, therefore the concentration taken by the liquid phase of a gas is reduced is formed at ground level. As the cold liquid build-up is a function of time. If the gas is by a factor of 100 to 500 depending upon pulls energy from the surrounding dispersed by air currents, then only low the gas. There are two ways to liquefy gases; atmosphere the liquid will boil naturally. concentrations will be experienced. the first way is to increase the This is a self controlling process. Limited Vapours liberated by non-boiling liquids pressure and the second way is to reduce heat transfer will only allow a certain rate will act like a dense gas e.g. flows along the temperature. of gas to be generated. The temperature of the ground where walls and dikes can the liquid and the evolving gases are low, effectively control their direction. When a pressurised liquid escapes, there therefore both the liquid and gases will act are two phases associated with the leak. like a dense gas. Gravity and wind flow will First, a jet of liquid will be released which dictate the direction of liquid and gas flow. instantly evaporates. This evaporation is Like dense gases, the use of dikes or bund 10 | GASEOUS RELEASE INSIDE BUILDINGS GAS DISPERSION

Gaseous release inside buildings

The behaviour of a gas release inside a building is completely different to the known behaviour of the same release occurring outdoors. Not only do you have the containment of the gas, resulting in an 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´s air flow is known. This can be achieved by using smoke tubes.

The indoor difference An additional hazard, in buildings, is the remembered that leaks occur in all For indoor applications, the prevailing wind replacement of ambient Oxygen by a directions. This would mean many sensors for an outdoor application is replaced by leaking gaseous substance. Gases which across a three dimensional grid. Installing internal air currents. Air currents within a displace Oxygen are not limited to gases a grid format of sensors in a room may be building can be forced e.g. heating and which are either explosive or toxic, but more practical and economic, however this ventilation systems, by convection from hot include nonflammable and non toxic gases type of installation stretches the alarm process equipment or by thermal effects such as Nitrogen, Helium, Argon or Sulfur response time as the gas must travel to the on the walls and roof from external hexafluoride (SF6). sensor. sunlight. There are many other factors At a concentration of 10 vol.% Argon in which can affect internal air currents e.g. the air the Oxygen concentration drops by The correct location of a gas detector can the movement of people or product. 2 vol.% from 20.9 vol.% to 18.9 vol.%. only be determined if the air movement wit- hin the building/room is analysed. The first In an open plant, a gas cloud can spread Where there is a risk of Oxygen depletion, step is to identify all of the possible in all directions and has no boundary. Wit- additional gas detectors must be installed. sources which generate air movement e.g. hin a building, a gas cloud can only ventilation systems, hot surfaces, movement consume a fixed and finite volume, thus the The location of gas detectors inside a of people and product, thermal radiation concentration of the leaked gas will building is not a simple and straight etc. The second step is to simulate the air increase. Given sufficient time, even the forward task. It may be sensible to locate movement by using smoke tubes, taking smallest of leaks can exceed the LEL a gas detector adjacent to a potential care to monitor dead spaces like corners, (explosive) or TLV (toxic) levels within the source of release. This would give the roof voids and sumps. entire room volume. earliest warning. However, it must be SOURCE CHARACTERISTICS GAS DISPERSION | 11 ST-3734-2003

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, Duct openings ST-3734-2003 12 | GASEOUS RELEASE INSIDE BUILDINGS GAS DISPERSION

Gaseous release inside buildings

Different scenarios which are typical for gas leaks within a building

, Air tight rooms An air tight room means that there is very little air exchange between the volume of the room and the outside. This is typical for large warehouses which store chemicals, gases and other goods. Within such a room, air movement does exist, however it’s so small that it has no influence or motive Air tight room; light forcetomoveordiluteagascloud.The gas„blue”,densegas exchange of air with the outside can be as “yellow”. low as 0.1/h. This means that 10 % of the air volume within the room will be exchan- ged with the outside air through natural gaps in the room. This exchange is not enough to dilute any gas release, therefore insufficient 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. Thermal source devel- oping convection cur- rents. ST-1175-2008Examples of gas releases ST-1174-2008 in an air tight room a) An evaporating pool of solvent will create a dense cloud at floor level b) Carbon dioxide will fall to the ground and accumulate at the lowest point c) Methane will rise to the ceiling d) Ammonia will disperse and rise to the ceiling

Gas mixtures have different dispersion Room with ventilation characteristics with respect to their

system. ST-1176-2008 particular contents. GASEOUS RELEASE INSIDE BUILDINGS GAS DISPERSION | 13 ST-64-98

Hydrogen does not always rise just becau- irrespective of their density. The turbulent gas emissions. In the case of a gas se it is lighter than air. In a galvanic characteristics of the circular air currents emission the ventilation system may be process Oxygen and Hydrogen can be encourage any leaking gas to mix with the required to shutdown to contain the gas. generated at the same time and at the air resulting in dilution of the concentration. same location. Oxygen is heavier than air The concentration will increase with time. (relative density 1.1) and Hydrogen is lighter Air flow than air (relative density 0.07). An Gas detectors should be installed at Within any controlled environment e.g. inside undisturbed mixture of Oxygen and ceiling height above the heat source, or of a building, it is recommended that an air Hydrogen, up to a concentration of 10 where the air current can be defined. flow check is performed on a regular basis. vol.% Hydrogen remains denser than air The Dräger Flow Check is a device which and will fall to the ground. The LEL of Active ventilation makes air streams visible by generating a Hydrogen is 4 vol.%, therefore at floor level For safety reasons it is common to find visible aerosol of a neutral buoyant there exists an explosive level of Hydrogen. rooms where forced ventilation is installed. property. The visible aerosol can be This is to ensure that there is a constant air released adjacent to any potential leak The location of a gas detector in an air tight exchange to guard against any build up of source and the aerosol can be watched to room is totally dependent on the relative toxic or explosive gases. The air movement determine the surrounding air currents. density of the leaking substance, including within the room is calculated (rate of change) This method of determining air currents is the combined influences of gases in and the profile of air currents is known, the- best suited to areas where there are mixtures. refore the location of the gas detectors is thermal sources or forced ventilation. If a made easier. There are two options for the room is totally stagnant, then the Flow Thermal sources location of gas detectors Check will only simulate the characteristics There are many heat dissipating sources of a neutrally buoyant gas. within a room, especially machinery rooms. a) The gas detector can be located in the The energy from the heat source heats up known air stream within the room, or Air flow within a room will be disturbed by the surrounding air which rises to the b) The gas detector can be located in the high pressure releases. A high pressure ceiling. This warm air will spread sideways ventilation system (outlet) release will increase the gas concentration towards the walls. As the air cools it begins Gas detectors with a lower measuring within a room rapidly. Mechanical baffle to fall down towards floor level, thus range will be needed as the constant air plates can be used to disturb any jet creating a circular motion. This circular air exchange will dilute any gas leak. Gas release. current is so strong that it carries gases detectors may be required to monitor for 14 |

SUBSTANCEOCCURRENCEINOPERATIONALFAULTSITUATION

liquefied by liquefied under pure gas cloud refrigeration pressure highly pressurized Toxic gases mass boiling point vapour pressure g/mol comment °C °F bar* (20 °C / 68 °F) jet aerosol puddle Carbon monoxide CO 28 – 191 – 312 35 x Fluorine F2 38 – 188 – 306 – x Nitrogen monoxide NO 30 – 150 – 238 35 x Hydrogen chloride HCl 36 fuming – 85 – 121 43 x x x Hydrogen sulfide H2S34–60–7618xx Hydrogen bromide HBr 81 fuming – 66 – 87 21 x x Chlorine Cl2 71 – 34 – 29 x 6.8 x x Ammonia NH3 17 – 33 – 27 x 8.6 x x x x Sulfur dioxide SO2 64–1014x3.3xxxx Phosgene COCl2 99 8 46 x 1.6 x Hydrogen fluoride (HF)n (20)*n fuming 19 66 x 1 x x x Nitrogen dioxide NO2/N2O4 46/92 visible 21 70 x 1 x x x Flammable gases Hydrogen H2 2–252–422– x Methane, LNG CH4 16 –162 –260 x – x Ethylene, Ethene C2H4 28 –104 –155 x 41.0 x Acetylene, Ethine C2H2 26 in Aceton – 83 – 171 43.0 x Propylene, Propene C3H6 42–50–5810.0xxxx Propane C3H8 44–42–448.4xxxx Formaldehyde HCHO 30 – 21 – 6 x Vinyl chloride C2H3Cl62–147x3.4xxxx 1,3-Butadiene C4H6 54–425x2.4xxxx LPG, Propane/Butane 50 – 1 30 8.0 x x x x n-Butane C4H10 58–1302.1xxxx Ethylene oxide C2H4O4411521.5xxxx Vapours from liquids Hydrogen cyanide HCN 27 26 79 0.82 x Propylene oxide C3H6O58 34930.57 x Carbon disulfide CS2 76 46 115 0.40 x Acrolein C3H4O56 571350.30 x Bromine Br2 150 visible 59 138 0.25 x Methanol CH3OH 32 65 149 0.13 x Ethanol C2H5OH 46 78 172 0.06 x Acrylonitrile C3H3N53 771710.12 x Benzene C6H6 78 80 176 0.11 x Off-gasing solutions Hydrocyanic acid HCN 27 x Ammonia solution 32 % NH3 17 25 77 0.80 x Formalin 55 % HCHO 30 0.002 x Acetyl chloride HCl 36 fuming 52 126 0.32 x x Hydrochloric acid 32 % HCl fuming 57 135 0.038 x x Carbonic acid CO2 44 x Hydrofluoric acid HF > 70 % fuming 106 223 x x Nitric acid 65 % NO2/N2O4 visible 122 252 0.06 x x Phosphoric acid 85 % 213 415 0 x Sulfuric acid < 96 % > 96 % fuming 280 536 0 x SO3 (Oleum 24 %) H2SO4 fuming x Other gas Helium He 4 Water vapour H2O18 Air (dry) air 28.9 Buoyant Neutrally buoyant Oxygen O2 32 Nitrogen N 28 Dense gas 2 Not applicable Carbon dioxide CO 44 2 Sorted by boiling temperature | 15

RECOMMENDATION FOR TRANSMITTER PLACEMENT

Decision tree for deriving the most likely dispersion. Thearrowsindicate buoyant gas buoyant if a detector should be installed above (arrow up) or in the same height (arrow straight on) or on the floor (arrow down). low neutrally pressure neutral buoyant gas leak

dense gas heavy

high gaseous system pressure

hot gas buoyant

cold gas heavy

liquefied by pressure

gas jet diluted neutral

source

cold gas heavy

liquefied flash under evaporation refrigeration

aerosol heavy

condensation

liquid

heat boiling pool controlled cold gas heavy evaporation

heat controlled vapour heavy evaporation

non boiling pool

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