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Thermal Levels of Concern
In ALOHA, there are two types of thermal Levels of Concern (LOCs): one that measures the threat associated with chemical releases that are on fire (thermal radiation LOCs) and one that measures the threat associated with releases that may catch on fire (flammable LOCs).
This page discusses the following topics:
What are thermal radiation Levels of Concern?
A thermal radiation LOC is a threshold level of thermal radiation (heat), usually the level above which a hazard may exist. When you run a pool fire, jet fire, or BLEVE (Boiling Liquid Expanding Vapor Explosion) scenario in ALOHA, thermal radiation is the hazard that is modeled.
Unlike toxic LOCs, no well-defined guidelines or standards exist to evaluate the thermal radiation hazard. So, ALOHA uses default thermal radiation values (in kilowatts per square meter) that are based on a review of several widely accepted sources on thermal radiation:
- 10 kW/(sq m) = potentially lethal within 60 seconds
- 5 kW/(sq m) = second-degree burns within 60 seconds
- 2 kW/(sq m) = pain within 60 seconds
The thermal radiation effects that people experience depend upon the length of time they are exposed to a specific thermal radiation level. Longer exposure durations, even at a lower thermal radiation level, can produce serious physiological effects. The threat zones displayed by ALOHA represent thermal radiation levels; the accompanying text indicates the effects on people who are exposed to those thermal radiation levels, but are able to seek shelter within one minute.
If you model a pool fire scenario in ALOHA using the default LOCs, your threat zone plot will look similar to the one pictured below. The red, orange, and yellow threat zones indicate the areas where the thermal radiation is predicted to exceed the corresponding LOC at some time after the release begins.
If you prefer, you can enter up to three thermal radiation values of your own (rather than use the default values). On the Thermal Radiation Level of Concern dialog box, simply choose User Specified from the pulldown list of LOC values and type in your own LOCs. When setting your own LOCs, you may want to refer to the table below.
Time for physiological effects (on bare skin) to occur
following exposure to specific thermal radiation levels
Radiation Intensity
(kW/m2) | Time for Severe Pain
(seconds) | Time for 2nd Degree Burns (seconds) |
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| 1 | 115 | 663 |
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| 2 | 45 | 187 |
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| 3 | 27 | 92 |
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| 4 | 18 | 57 |
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| 5 | 13 | 40 |
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| 6 | 11 | 30 |
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| 8 | 7 | 20 |
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| 10 | 5 | 14 |
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| 12 | 4 | 11 |
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| Federal Emergency Management Agency, U.S. Department of Transportation, and U.S. Environmental Protection Agency. 1988. Handbook of Chemical Hazard Analysis Procedures. Washington, D.C.: Federal Emergency Management Agency Publications Office. | ||
Note: The durations that correspond to effects like pain or second-degree burns can vary considerably, depending on circumstances. The effects above were observed on bare skin that was exposed directly to the thermal radiation. Some types of clothing can serve as a protective barrier against thermal radiation and can affect the exposure duration. However, exposure duration should be kept to a minimum, even at low levels of thermal radiation.
What are flammable Levels of Concern?
A flammable LOC is a threshold concentration of fuel in the air above which a flammability hazard may exist. When you are modeling the release of a flammable chemical that is not currently burning (but may catch on fire if exposed to an ignition source), ALOHA can predict the flammable area so that you can assess the flammability hazard.
The flammable area is the part of a flammable vapor cloud where the concentration is in the flammable range, between the Lower and Upper Explosive Limits (LEL and UEL). (These are also known as the Lower and Upper Flammability Limits.) These limits are percentages that represent the concentration of the fuel (that is, the chemical) vapor in the air. If the chemical vapor comes into contact with an ignition source (such as a spark), it will burn only if its fuel-air concentration is between the LEL and the UEL, because that portion of the cloud is already pre-mixed to the right mixture of fuel and air for burning to occur.
If the fuel-air concentration is below the LEL, there is not enough fuel in the air to sustain a fire or an explosion--it is too lean. If the fuel-air concentration is above the UEL, there is not enough oxygen to sustain a fire or an explosion because there is too much fuel--it is too rich. (This is similar to an engine that cannot start because it has been flooded with gasoline.)
When ALOHA models the flammable area, it provides two default flammable LOCs that are each a fraction of the LEL:
- 60% of the LEL = flame pockets
- 10% of the LEL
You might expect that the LEL itself could be used as the LOC to determine the areas in which a fire might occur. However, the concentration levels estimated by ALOHA are time-averaged concentrations. In an actual vapor cloud, there will be areas where the concentration is higher than the average and areas where the concentration is lower than the average. This is called concentration patchiness. Because of concentration patchiness, there will be areas (called pockets) where the chemical is in the flammable range even though the average concentration has fallen below the LEL. (ALOHA uses a shorter averaging time when estimating the flammable areas to help compensate for this effect, but it cannot completely compensate for this effect.)
Some experiments have shown that flame pockets can occur in places where the average concentration is above 60% of the LEL. ALOHA uses 60% of the LEL as the default LOC for the red threat zone. Another common threat level used by responders is 10% of the LEL. ALOHA uses this concentration as the default LOC for the yellow threat zone.
If you model a flammable area in ALOHA using the default LOCs, your threat zone plot will look similar to the one pictured below. The red and yellow threat zones indicate the areas where the fuel-air concentration is predicted to exceed that LOC at some time after a release begins.
If you prefer, you can enter up to three flammability LOCs of your own (rather than use the default values). On the Flammable Level of Concern dialog box, simply choose User Specified from the pulldown list of LOC values and type in your own LOCs.
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