User`s Manual ca eo®
Contents
1. User s Manual ON PR pica S 9 A a N CS A E 2 3 i gt x WAGENC ano ES Ca eV Contents Contents 1 Welcome to ALOHA a ee 1 Abot ALOHA er 1 Program purposes lira 1 Basic program organization pap ilaele 1 ALOHA sSminenibatiio lalui 2 G tting Neal 3 TWIN libre eee ait ieee ws 5 Ona Macintosh a tines 5 How to use this mianuals nee en 5 Introduction to air Modelitifi ira dodo jar aaa 6 Dispersi n mod nr 6 A A ne 7 The Gaussian equation le 7 A E aaa a 8 Classification of heavy gases i 9 Flash boiling and two phase flow 9 ALOHA s heavy gas calculations 9 ALOHA can choose a dispersion model for you 10 ALORA simitations anna ai 10 Very low wind A le 11 Very stable atmospheric conditions 11 Wind shifts and terrain steering effects 12 Concentration patchiness id 13 ALOHA does not account for the effects of 14 c MITES or chemical te actions pali a 14 Particulas manantial 15 Ms odia ideata ea cnet 15 A See ESE NNT 15 Tastallino ALOHA ut aisi ili liana 17 Before yowinstall ALOHA ee aha 17 Memory and hard disk space requirements ennnnennenenen 17 Windows O E E E 17 Ona Macmlosh nassen as 17 If you already have a previous version of ALOHA 5 2 3 or older 17 If you already have2. cane avec va aT as 167 Example 5 A Chemical Solution tii een 175 Choosing a location and a chemical solution i 175 Entering weather information ern 178 Deseribing therelease an ae 180 Choosing a LOC and plotting a footprint i 182 A sinne 187 Bibliography 193 OA 195 Chapter 1 Welcome to ALOHA Welcome to ALOHA This chapter contains an overview of ALOHA an explanation of how to use this manual and ALOHA s online help and a discussion of basic air dispersion modeling concepts About ALOHA Program purpose ALOHA Areal Locations of Hazardous Atmospheres is a computer program designed especially for use by people responding to chemical accidents as well as for emergency planning and training ALOHA can predict the rates at which chemical vapors may escape into the atmosphere from broken gas pipes leaking tanks and evaporating puddles It can then predict how a hazardous gas cloud might disperse in the atmosphere after an accidental chemical release ALOHA runs quickly on small computers Windows or Macintosh that are easily transportable and affordable for most people It is designed to be easy to use so that you can operate it successfully during high pressure situations Its chemical library contains information about the physical properties of about 1 000 common hazardous chemicals Its computations represent a compromise between accuracy and speed it has been designed to produ
3. of a jet release ALOHA DEGADIS assumes that all heavy gas releases originate at ground level e the mathematical approximation procedures used for solving the model s equations are faster but less accurate than those used in DEGADIS and e ALOHA DEGADIS models sources for which the release rate changes over time as a series of short steady releases rather than as a number of individual point source puffs Throughout the creation of ALOHA DEGADIS NOAA worked closely with the original authors of DEGADIS to ensure a faithful representation of DEGADIS model dynamics ALOHA DEGADIS was checked against DEGADIS to ensure that only minor differences existed in results obtained from both models Considering the typical inaccuracies common in emergency response these differences are probably not significant In cases where technical accuracy is required you should obtain the original DEGADIS model and use it to investigate the scenarios of interest ALOHA can automatically choose whether to predict the dispersion of a chemical as a Gaussian or heavy gas release it bases this choice mainly on molecular weight size of the release and temperature of the gas cloud But sometimes you may want to specify the model to use rather than letting ALOHA choose In particular when a chemical with a molecular weight less than that of air has been stored at a low temperature or under high pressure it can behave like a heavy gas ammonia is an example of such
4. tile or stack ALOHA s windows and choose whether to see Chapter 1 Welcome to ALOHA ALOHA s results displayed in English or metric units Select a Level of Concern LOC for the footprint this is the threshold concentration of an airborne pollutant usually the concentration above which a hazard may exist ALOHA s footprint represents the zone where the ground level pollutant concentration may exceed your LOC at some time after a release begins Choose to see the footprint either plotted on a grid at a scale automatically selected by ALOHA or displayed at a scale that you specify Sharing Choose items from this menu a to display an ALOHA footprint on a background map using MARPLOT the CAMEO mapping module or b to see detailed information about the chemical you ve selected displayed in CAMEO s Response Information Data Sheets RIDS module SiteData SetUp Display Sharing Location Chemical 36H Tile Windows About Shared Menus Atmospheric Stack Windows Edit Shared Menus SEN 0 Undo Cut New Open ser i UES ns ue IES AS Pra jo Figure 1 1 ALOHA s menus on a Macintosh ALOHA s menus in Windows are nearly identical Getting help Online help is available at any time when ALOHA is running If you re running ALOHA the right most menu is the Help menu Select Topics from the Help menu and a Help Index will appear Help Abo
5. travel upwind of its release point Farther downwind as the cloud becomes more diluted and its density approaches that of air it begins behaving like a neutrally buoyant gas This takes place when the concentration of heavy gas in the surrounding air drops below about percent For many small releases this will occur in the first few yards meters For large releases this may happen much further downwind Chapter 1 Welcome to ALOHA Figure 1 9 Cloud spread as a result of gravity A gas that has a molecular weight greater than that of air the average molecular weight of air is about 29 kilograms per kilomole will form a heavy gas cloud if enough is released Gases such as anhydrous ammonia that are lighter than air at room temperature but that are stored in a cryogenic low temperature state can also form heavy gas clouds Ifthe density of a gas cloud is substantially greater than the density of the air the density of air is about 1 1 kilograms per cubic meter ALOHA considers the gas to be heavy Many substances that are gases under normal pressures and temperatures are stored under high enough pressures to liquefy them For example propane is a gas at normal pressures and temperatures but is often stored under pressure as a liquid When a tank rupture or broken valve causes a sudden pressure loss in a tank of liquefied gas the liquid boils violently the tank contents foam up and the tank fills with a mixture of gas and fin
6. 4 contains descriptions of the main features of ALOHA Sections in this chapter include explanations of each of ALOHA s menus along with background information to help you to better understand ALOHA s computations Turn to Chapter 5 for some ALOHA example problems and to Chapter 6 for trouble shooting advice At the back of the manual you ll find a bibliography a glossary of air modeling terms and an index Chapter 1 Welcome to ALOHA Introduction to air modeling ALOHA is an air dispersion model which you can use as a tool for predicting the movement and dispersion of gases It predicts pollutant concentrations downwind from the source of a spill taking into consideration the physical characteristics of the spilled material ALOHA also accounts for some of the physical characteristics of the release site weather conditions and the circumstances of the release Like many computer programs it can solve problems rapidly and provide results in a graphic easy to use format This can be helpful during an emergency response or planning for such a response Keep in mind that ALOHA is only a tool Its usefulness depends on your accurate interpretation of the data ALOHA originated as a tool to aid in emergency response It has evolved over the years into a tool used for a wide range of response planning and academic purposes However you must still rely on your own common sense and experience when deciding how to respond to a particular i
7. HA operations by moving left to right through the six menus in its menu bar File and Edit Choose items from these two menus to perform basic Macintosh and Microsoft Windows operations such as opening closing and saving files printing the contents of ALOHA s windows and copying text and graphics displayed in ALOHA SiteData Choose items from the SiteData menu to enter information about a the date and time and b location of an accidental release and c the type of buildings downwind of the release SetUp Choose items from the SetUp menu to a select a chemical from ALOHA s chemical library or to add a chemical to the library if you need to b indicate weather conditions you can do this either manually or by connecting your computer to a portable meteorological station c set the source describe how the chemical is escaping from containment into the atmosphere d choose the type of dispersion calculations for ALOHA to make ALOHA can predict the movement of either neutrally buoyant clouds that are about as dense as air and heavy gas clouds which are denser than air and e adjust the exponent in ALOHA s dose equation dose is the accumulated amount of the chemical to which a person is exposed at a particular location Display Choose items from the Display menu to indicate the ALOHA results that you would like to see and to choose how you would like the information to be displayed Choose to
8. a chemical If you have chosen one of these chemicals depending on how you model its release ALOHA may not have enough information about the release to determine whether a heavy gas could be formed In such a case ALOHA will make Gaussian calculations but will alert you that you should try running the heavy gas model as well In such cases you should re run ALOHA using the heavy gas calculations and compare the two footprint estimates ALOHA s limitations Like any model ALOHA cannot be more accurate than the information you give it to work with But even when you provide the best input values possible ALOHA like any 10 Chapter 1 Welcome to ALOHA model can be unreliable in certain situations and it cannot model some types of releases at all Even when you can provide accurate input information ALOHA s results can be unreliable when the following conditions exist e very low wind speeds e very stable atmospheric conditions e wind shifts and terrain steering effects e concentration patchiness particularly near the spill source ALOHA doesn t account for the effects of e fires or chemical reactions e particulates e chemical mixtures e terrain ALOHA s footprint accurately depicts a pollutant cloud s location only if the wind direction does not change from the value that you entered Generally wind direction is least predictable when wind speed is low To show how much the cloud s position could change
9. a previous version of ALOHA 5 3 18 Installing ALOHA WindowS ie 18 Making the installation rn nn vita siale Aghodeniplass 18 Your new Aloha menu will include 19 Installing ALOHA on a Macintosh 19 Making the installation han aan 19 Learning the Basics 21 Guidedid UL Pa ss 21 Chapter 1 Welcome to ALOHA Describing the Time and Place a BR abe 22 Cho sing a Chemical sanken a a e i aaan 24 Describing th Weather sanieren 25 Describing the Release a Sn elia 27 Checking the Computational Setting nal aa 32 Plotting Footprint an ernsten 32 Checking Concentrations at a Location of Concern 36 Exiting Quitting ALOHA zn eine 39 References leali dee 41 TheFil and EMOS ae 41 Theile Mens a a eee 41 A as ee 41 A sical a Ga toda vaca ag cates ads OO wate celta teensy eaten ah hagas 41 Response Mode ta dadsdauion nara Ea aE 41 Planning Moden a Na nr ai 42 U Vo RO 42 S ve and Save Asien 42 Pira ER 42 A Ne Obes 43 DITUT ap D S AMOS RER Aiea IRA Re BER SR OA ACR RER WR RTS 43 The Edit Mental ea ee 43 O 43 TSE MU ias 43 A ea a a oa er sen 44 Selectins a location alain 44 Adding modifying and deleting location information 45 Adding information about a U S city 45 Adding a location outside the U S n 47 Modifying a loeation au a men 48 Deletine a locations rare 48 A 49 Date amp WATS alin Bai DER t
10. all avian A nia nee eco aaah 51 Thesetllp Meile een einen 52 A ae an i i aH a E te ron 53 Selecting a Chemical energie 54 dl ee 54 Chemical information in the Text Summary window 56 Chemical Data eu een 57 Adding modifying or deleting chemicals 59 How to add a chemical to the library 59 How to modify information about a chemical 60 Howto delete a Chemicals aus e e 61 Almospherie as ee 61 User Input een a ls as 61 Wind speed direction and measurement height 62 Ground TOUGHNESS ann ae 65 Cloud CUP Ense 68 Adr temperature ers 68 Stability class aueh ee aa 68 A eisen 70 HIM seen 71 SAM SAO A ri 71 Cho sing a SAM illude 71 Transmitting SAM data to ALOHA ii 72 Using a SAM during an incident 73 Choosing the correct port for receiving SAM data 73 Choosing a radio frequency scrl 73 Setting up ALOHA when you re using a SAM 74 SAMP idos 75 ACID nd 76 ENDE A A RN RT SAP POY ero e ni 77 Processed Dat aE mica atv ara iota 77 Wind Rosi boi 78 Monitoring a lons tern rekase A 79 Cheek the time and date a 79 SO O E i bo i e Lul 80 ALOHA s duration limits ann 80 ALOHA reports release rate ini 81 When you re using a SAM nenn 81 O I ae 82 Des
11. ce good results quickly enough to be of use to responders ALOHA is designed to minimize operator error It checks information that you enter and warns you when you make a mistake ALOHA s online help offers you quick access to explanations of ALOHA s features and computations as well as background information to help you interpret its output Basic program organization To use ALOHA you ll typically perform several basic steps e indicate the city where an accidental chemical release is occurring and the time and date of the accident e choose the chemical of concern from ALOHA s library of chemical information e enter information about current weather conditions Chapter 1 Welcome to ALOHA e describe how the chemical is escaping from containment e request ALOHA to display a footprint showing the area where chemical concentrations in the air may become high enough to pose a hazard to people ALOHA can display this footprint on an electronic map of your city You can also view graphs showing predicted indoor and outdoor chemical concentrations at any location of special concern downwind of a release such as a school or hospital and the dose of chemical to which people at that location may be exposed You can save ALOHA results as archive files and you can copy and paste graphs plots and text information from ALOHA into documents or reports in word processing or graphics programs ALOHA s menu bar Perform basic ALO
12. cribing a direetrelease carica nali 82 Source MEINE she een 83 A EA 83 Entering information about a puddle 85 Watch for changing weather conditions 87 Tank ee area 87 Pressufized hguids ns ee 88 Tank size and orientation unse 89 Chemical state asii epin neei ina Dekra 89 Liquid in a A esse 91 Genoani 92 Chemical of unknown state in a tank 93 Atea and type Ol alo iero lati 93 Leak height on the tank wall ii 94 Puddle 95 Pre atei ed 96 Pipe Source ANUS an ae rast 97 Source strength information in the Text Summary 98 Computational ee ae ae 99 Dispersion Options a nee lee 99 Deine de ae es 100 TheDisplay Met ne Bes us are 101 Chapter 1 Welcome to ALOHA Tile and Stack Windows uns Ri 102 OPIO ns RA ee 102 Footprint OPTIONS usa a E N Ee 103 A A A IRR 104 Text Summary O 104 Footprint ee a e gia EAEE ee 105 Level OE Conter are 105 Int rpr ting a footprint ans ei a 107 Concentration aa aid 108 Designating a location a ss 109 Choosing coordinate ssaa eis 110 Using fixed east west and north south coordinates 110 Using relative downwind and crosswind coordinates 111 Dose lalla alain dai 111 ALOHA definition OF dos ars eo eave 112 Adjusting the doseexponent r sailor passa 112 Obtaining a dose graph grano aan an 112 DOUECE STEN Lilia 113 Constan
13. cts when it produces a footprint plot the footprint will appear to go right over or through obstacles such as buildings Consider the effects of terrain on wind flow whenever you are interpreting ALOHA results Figure 1 11 Small scale variations in wind direction Because the wind is likely to shift in direction and change speed over both distance and time limits have been placed on ALOHA s output ALOHA will not make predictions for more than an hour after a release begins or for distances more than 10 kilometers 6 2 miles from the release point it truncates footprints that are longer than 10 kilometers ALOHA s 1 hour time cutoff exists because wind shifts direction and changes speed frequently One reason for the 10 kilometer cutoff for ALOHA footprint length is that we don t know what the wind speed and direction are 10 kilometers away and can t assume that they are the same as those we re experiencing at the point where a pollutant is being released If ALOHA has incorrect values for wind speed and direction it can t correctly estimate footprint size or location No one can predict gas concentrations at any particular instant downwind of a release with certainty because they result partly from random chance Instead ALOHA shows you concentrations that represent averages for time periods of several minutes it uses the laws of probability as well as meteorologists knowledge of the atmosphere to do this ALOHA predic
14. e density as air According to this model wind and atmospheric turbulence are the forces that move the molecules of a released gas through the air so as an escaped cloud is blown downwind turbulent mixing causes it to spread out in the crosswind and upward directions According to the Gaussian model a graph of gas concentration within any crosswind slice of a moving pollutant cloud looks like a bell shaped curve high in the center where concentration is highest and lower on the sides where concentration is lower Right at the point of a release the pollutant gas concentration is very high and the gas has not diffused very far in the crosswind and upward directions so a graph of concentration in a crosswind slice of the cloud close to the source looks like a spike As the pollutant cloud drifts farther downwind it spreads out and the bell shape becomes wider and flatter Chapter 1 Welcome to ALOHA 100 80 60 40 CONCENTRATION ppm 20 0 CROSSWIND DISTANCE Figure 1 7 Gaussian distribution Y gt CONCENTRATION oot CROSSWIND SOURCE of SPILL Figure 1 8 Gaussian spread When a gas that is heavier than air is released it initially behaves very differently from a neutrally buoyant gas The heavy gas will first slump or sink because it is heavier than the surrounding air As the gas cloud moves downwind gravity makes it spread this can cause some of the vapor to
15. e liquid droplets called aerosol Flash boiling is the sudden vaporization of a liquid caused by a loss of pressure When the liquid and gas phases of a chemical escape together from a ruptured tank the release is called a two phase flow When such a two phase mixture escapes from storage the release rate can be significantly greater than that for a release of pure gas When liquefied propane or a similar chemical escapes from storage as a two phase release it can form a heavy gas cloud The cloud is heavy in part because it is initially cold and therefore dense and also because it consists of a two phase mixture The tiny aerosol droplets mixed into the cloud act to weigh the cloud down and make it more dense and their evaporation acts to cool the cloud The heavy gas dispersion calculations that are used in ALOHA are based on those used in the DEGADIS model Spicer and Havens 1989 one of several well known heavy gas models This model was selected because of its general acceptance and the extensive testing that was carried out by its authors In order to speed up the computational procedures and reduce the requirement for input data that would typically be difficult to Chapter 1 Welcome to ALOHA obtain during an accidental release a few simplifications were introduced into ALOHA DEGADIS making it different from the DEGADIS model These simplifications include e ALOHA does not use elevated sources to account for the initial momentum
16. if the wind were to shift direction under the particular weather conditions that you enter ALOHA draws two dashed lines one along each side of the footprint ALOHA predicts that about 95 percent of the time the wind will not shift direction enough to steadily blow the pollutant cloud outside of either line The wider the zone between the lines the less predictable is the wind direction and the more likely it is to change substantially At the lowest wind speeds acceptable to ALOHA about 2 knots or 1 meter per second at a height of 10 meters these lines form a circle to indicate that the wind could blow from any direction Under the most stable atmospheric conditions most common late at night or very early in the morning there is usually very little wind and almost no mixing of the pollutant cloud with the surrounding air Gas concentrations within the cloud can remain high far from the source The accidental release of methyl isocyanate gas at Bhopal India in 1984 is an example of what can happen under Chapter 1 Welcome to ALOHA people died including many who were far from the release In a very stable atmosphere a chemical cloud will spread out in the same manner as cream poured into a coffee cup The cream will dilute and spread slowly into the coffee but until you stir it will take a very long time to mix completely into the coffee Similarly the cloud will spread slowly and high gas concentrations may build up in small val
17. l s release rate and dispersion estimates will not be valid ALOHA expects the ground below a leaking tank or puddle to be flat so that the liquid spreads out evenly in all directions It does not account for pooling within depressions or the flow of liquid across sloping ground 15
18. leys or depressions and remain for long periods of time even at distances far from the release point ALOHA does not account for buildup of high gas concentrations in low lying areas First responders should be aware that very stable atmospheric conditions create a dangerous situation in which models like ALOHA are not very reliable In this situation think about whether the chemical will behave as a heavy gas and look for physical depressions and topographic features that may trap or steer the dispersing cloud ALOHA allows you to enter only single values for wind speed and wind direction It then assumes that wind speed and direction remain constant at any given height throughout the area downwind of a chemical release ALOHA also expects the ground below a dispersing cloud to be flat and free of obstacles In reality though the wind typically shifts speed and direction as it flows up or down slopes between hills or down into valleys turning where terrain features turn The way in which land features modify patterns of air flow is called terrain steering Figure 1 10 Wind shifts In urban areas wind flowing around large buildings forms eddies and changes direction and speed significantly altering a cloud s shape and movement Through streets bordered 12 Chapter 1 Welcome to ALOHA by large buildings can generate a street canyon wind pattern that constrains and funnels a dispersing cloud ALOHA ignores these effe
19. lick Select to view a discussion of that topic When you ve finished reading the topic discussion click either Topies or Cancel to return to the Help index to select another topic Print to print the Help screen or Copy to copy the discussion text to the clipboard ALOHA S LIMITATIONS ALOHA cannot be more accurate than the information you give it to work with But even when you provide the best input values possible ALOHA like any model can be unreliable in certain situations and it cannot model some types of releases at all ALOHA s results can be unreliable when the following conditions exist a Very low wind speeds ALOHA s footprint accurately depicts a pollutant cloud s location only if the wind direction does not change from the value that you entered Generally wind direction is least predictable when wind speed is low To show how much the cloud s position could change if the wind were to shift direction under the particular weather conditions that you enter 4LOHA draws two dashed lines one along each side of the footprint ALOHA predicts that about 95 percent of the time the wind will tance Cs Figure 1 6 Sample Help text on a Macintosh How to use this manual This manual includes six chapters Begin here in Chapter 1 by reviewing a discussion of basic air modeling concepts Turn to Chapter 2 for instructions on installing ALOHA and to Chapter 3 for a step by step ALOHA tutorial Chapter
20. ncident There are some features that would be useful in a dispersion model for example equations accounting for site topography that have not been included in ALOHA because they would require extensive input and computational time ALOHA s most important limitations are discussed in the following pages Dispersion modeling Many different types of air dispersion models exist They range from simple equations that can be solved by hand to complex models that require massive amounts of input data and powerful computers The type of model appropriate for a particular use depends on the scale of the problem the level of detail available for input and required for output the background of the intended user and the time available to wait for the model computations to be completed ALOHA was designed with first responders in mind It is intended to be used for predicting the extent of the area downwind of a short duration chemical accident where people may be at risk of exposure to hazardous concentrations of a toxic gas It is not intended for use with accidents involving radioactive chemicals Nor is ALOHA intended to be used for permitting of stack gas or modeling chronic low level fugitive emissions Other models are designed to address larger scale and or air quality issues Turner and Bender 1986 Since most first responders do not have dispersion modeling backgrounds ALOHA has been designed to require input data that are either easil
21. r even themselves Because of these chemical reactions the chemical that disperses downwind might be very different from the chemical that originally escaped from containment In some cases this difference may be substantial enough to make ALOHA s dispersion predictions inaccurate For example if aluminum phosphide escapes from containment and comes in contact with water the reaction between the water and aluminum phosphide produces phosphine gas It is the phosphine 14 Chapter 1 Welcome to ALOHA rather than the aluminum phosphide that escapes into the atmosphere If you respond to such an accident and wish to use ALOHA to obtain a footprint estimate you need to estimate how rapidly phosphine is being generated from the reaction between water and aluminum phosphide and you need to model the incident in ALOHA as a release of phosphine rather than aluminum phosphide ALOHA does not account for the processes that affect dispersion of particulates including radioactive particles ALOHA is designed to model the release and dispersion of pure chemicals and some chemical solutions It s difficult for any model to correctly predict the behavior of a solution or a mixture of chemicals because it s difficult to accurately predict chemical properties such as vapor pressure for solutions or mixtures ALOHA s predictions are greatly affected by this and other chemical properties When an incorrect property value is used in ALOHA the mode
22. t source strength a sul Naila na 113 Variable source strength a iii 114 Source strength IVC aii 114 The shanng Menu na wear eis 115 Items in the Sharing Men een aaa 115 The CAMEO BEN a lla 116 The MARPLOT MEDU a i 116 Displaying an ALOHA footprint on a Map 116 Using MARPLOT estes tds AS 117 Examples carina rara ira 119 Example Is A Tak Souree nalen 119 Choosing a location and a chemical 119 Entering weather information iii 122 Describing the Te lease rata A ekariay a 125 Choosing a LOC and plotting a footprint 130 Example 2 Direct Input Heavy Gas can aeg 135 Choosing a location building type and chemical 135 Entering weather information ii 138 Describing thetelease innen 140 Checking concentration cara 142 Example 3 A Pipe SOUT CS e Bas E n a ESR 147 Choosing a location and a chemical iii ld 147 Entering weather information ecsuesssesssessnenssnnesnnnnsnnnsnnnensnnsnnnnsnnnnsnnen 149 Deseribingtherelease a el 152 Choosing a LOC and plotting a footprint 153 Example 4 Using ALOHA and aMARPLOT map 157 Choosing a location and a chemical 157 Entering weather information i 160 Describing the release see 162 Choosing a LOC and plotting a footprint i 164 Using MARPLO Tiaa cesta cst
23. ts that average concentrations will be highest near the release point and along the centerline of any pollutant cloud and will drop off smoothly and gradually in the downwind and crosswind directions However especially near the source of a release 13 Chapter 1 Welcome to ALOHA wind eddies push a cloud unpredictably about causing gas concentrations at any moment to be high in one location and low in another This kind of movement is familiar to anyone who has tried to toast marshmallows over a campfire no matter where you sit the smoke from the fire always seems to come straight towards you Meanwhile the average concentrations are likely to behave approximately as ALOHA predicts As the cloud moves downwind from the release point these eddies shift and spread the cloud evening out concentrations within the cloud so that they become more similar to ALOHA s predictions Figure 1 12 Concentration patchiness close to the source ALOHA does not account for the effects of The smoke from a fire because it has been heated rises rapidly before it begins to move downwind ALOHA doesn t account for this initial rise It also doesn t account for the by products of combustion or for chemical reactions of any kind ALOHA assumes that a dispersing chemical cloud does not react with the gases that make up the atmosphere such as oxygen and water vapor However many chemicals react with dry or humid air water other chemicals o
24. ut Balloon Help Show Balloons Topics Figure 1 2 Accessing ALOHA Help In Windows or on a Macintosh click Help buttons on ALOHA windows and dialog boxes Figure 1 4 to view information about particular features or required model inputs Chapter 1 Welcome to ALOHA Infiltration Building Parameters H Select building type or enter exchange parameter Q Enclosed office building E per hour Single storied building Q Double storied building O No of air changes is Select building surroundings Sheltered surroundings trees bushes etc Unsheltered surroundings Figure 1 4 Clicking an ALOHA Help button on a Macintosh In Windows or on a Macintosh the Help Index that you access from About ALOHA is arranged alphabetically Figure 1 5 ALOHAG 5 3 Developed jointly by NOAA and ERA Help Index i 11 Io ALOHA Macintosh version About help ALOHA Windows versioni Add chemical data Add location data AEGL Air temperature ALOHA Limitations Alohaspy Ambient saturation concentration Amount of chemical unknown state in tank Gaza Figure 1 5 ALOHA Help Index on a Macintosh Chapter 1 Welcome to ALOHA Click any topic name to view a discussion of that topic When you ve finished reading about that topic click Contents to return to the Help Index When you are ready to go back to using ALOHA close or minimize the Help window Highlight a topic name then c
25. y obtained or estimated at the scene of an accident ALOHA s online help can assist you in choosing inputs Chapter 1 Welcome to ALOHA What is dispersion Dispersion is a term used by modelers to include advection moving and diffusion spreading A dispersing vapor cloud will generally move advect in adownwind direction and spread diffuse in a crosswind and vertical direction crosswind is the direction perpendicular to the wind A cloud of gas that is denser or heavier than air called a heavy gas can also spread upwind to a small extent ALOHA models the dispersion of a cloud of pollutant gas in the atmosphere and displays a diagram that shows an overhead view of the area in which it predicts that gas concentrations will reach hazardous levels This diagram is called the cloud s footprint To obtain a footprint plot you first must identify a threshold concentration of an airborne pollutant usually the concentration above which the gas may pose a hazard to people This value is called the Level of Concern The footprint represents the area within which the ground level concentration of a pollutant gas is predicted to exceed your Level of Concern LOC at some time after a release begins There are really two separate dispersion models in ALOHA Gaussian and heavy gas ALOHA uses the Gaussian model to predict how gases that are about as buoyant as air will disperse in the atmosphere Such neutrally buoyant gases have about the sam
Download Pdf Manuals
Related Search