4.3.2 Characterization of Flammable Vapor Hazards

2

Hypothetical ExperimentImagine the

following experiment (DON’T ATTEMPT THIS!):

Roll up some newspaper, light one end, and quickly plunge the lit end into the opening of a car gas tank.

What do you expect to happen?

Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment

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Hypothetical ExperimentWhen the lit

newspaper torch is plunged into the tank, it should go out!

Why does this happen?◦ Gasoline is a highly

flammable hydrocarbon, so it ignites easily, thus making it a good fuel.

Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment

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Hypothetical Experiment For a material to burn,

there needs to be oxygen present – burning is a chemical reaction under which oxygen combines chemically with some fuel to give off a substantial amount of heat.

If there is enough oxygen, this burning process will continue until the fuel is depleted.

However, if there isn’t enough oxygen, as is the case in the gas tank’s vapor space, the burning process will stop!

Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment

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Hypothetical Experiment A natural question to ask

is – what about putting the torch near the entrance to the gas tank – could there be more oxygen there?

The answer is yes, in fact there may be enough to cause an explosion (this is how a car engine works – enough oxygen and gasoline vapor are mixed to cause an explosion, driving a piston).

This is why the experiment we are discussing is hypothetical!

Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment

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Flammable LimitsLet’s look at this type of

situation in a more scientific fashion!

Consider the situation we discussed earlier of a beaker containing a chemical, placed under a glass cover.

After evaporation has taken place to the point at which equilibrium is reached, there will be a mixture of both chemical and inert gas molecules in the vapor space.

Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment

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Flammable LimitsFor a certain

range of percentages of chemical molecules in the vapor space, burning can take place!

Outside of this range of percentages, burning will not be able to occur!

Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment

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Flammable LimitsDefinition:

◦ The maximum percentage of chemical within the vapor space that allows burning is called the upper flammable limit (UFL).

◦ The minimum percentage of chemical within the vapor space that allows burning is called the lower flammable limit (LFL).

Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment

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Flammable Limits For our gasoline

experiment, if may be the case that at the gas tank opening, the concentration of oxygen and gasoline may be within the flammable limits.

Of these two percentages, the one that is more crucial for safety is a chemical vapor mixture’s LFL – as long as the vapor mixture is below the LFL, no burning can occur!

What about being above the UFL?

Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment

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Flammable Limits Examples

Chemical Lower Flammable Limit (LFL),

Percent

Upper Flammable Limit (UFL),

Percent

Acetone 2 13

Butane 1.8 8.4

Gasoline 1.4 7.5

Methanol 7.3 36

Vinyl Acetate Monomer

2.6 13.4

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Flash PointIt turns out that just as

in the case of vapor pressure, the percentage of chemical vapor in the vapor space depends on temperature!

As the temperature of a chemical is raised, it’s propensity to evaporate increases, increasing the number of molecules in the vapor space.

Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment

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Flash Point Thus, if the temperature is

kept low enough, the percentage of chemical in the vapor space will stay below the LFL, so there won’t be a flammable vapor hazard!

For any given flammable chemical, there must be a critical temperature, below which the amount of vapor is below LFL and above which the amount of vapor reaches or exceed LFL.

This temperature is called the flash point.

Image Courtesy Charles Hadlock: Mathematical Modeling in the Environment

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Flash Point ExamplesChemical Flash Point (degrees

Fahrenheit)

Acetone 0

Butane -101

Gasoline* -45

Methanol 51

Vinyl Acetate Monomer 18

*Gasoline’s flash point can vary, depending on its blend.

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Flash PointAll other factors being equal,

◦ It is preferable to handle a flammable material at a temperature below its flash point.

◦Materials with a higher flash point are “safer.”The term “flammable” usually refers to

chemicals with a flash point below some threshold, such as 73 degrees Fahrenheit or 100 degrees Fahrenheit (this limit varies and is somewhat arbitrary).

Materials that burn, but have a higher flash point are called “combustible.”

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Volatile MaterialsThe term volatile is used to

describe how “easily” a material evaporates.

For example, gasoline is more volatile than water.

Usually, a material with a lower flash point tends to be more volatile and have a higher vapor pressure at any fixed temperature.

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Explosive LimitsIn some sources, upper and lower

flammable limits are referred to as upper and lower “explosive” limits – they mean the same thing.

Thus, instead of LFL or UFL, one may see LEL or UEL.

Just like flammable limits, explosive limits refer to fire, not explosions.

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Material Safety Data SheetsFor a given chemical, properties such as LFL,

UFL, flash point, density, boiling point, etc. can be found in handbooks or Material Safety Data Sheets (MSDS).

Almost any organization that uses, buys, sells, or handles a chemical or potentially hazardous material will have on file an MSDS for the material.

For example, here are two MSDS’s for acetone:◦ http://www.sciencelab.com/msds.php?msdsId=99

27062◦ http://blogs.nwic.edu/rocketteam/files/2011/09/Ac

etone.pdf

4.3.2 Characterization of Toxicity Hazards

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Toxic HazardsIn this section we will look at

toxic hazards.It turns out, as we will see that

these types of hazards can occur at much lower concentration levels than flammable hazards.

There are two types of toxic hazards – acute toxic hazards and chronic toxic hazards.

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Acute Toxic HazardsAn example of an acute toxic

hazard is a poisonous gas that causes severe injury or death almost immediately, such as◦A strongly acidic gas that damages

the respiratory tract so much one can no longer breathe effectively, or

◦A gas such as carbon monoxide that attaches to the hemoglobin in blood, making it impossible to transfer oxygen to vital organs.

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Chronic Toxic HazardsAn example of a chronic toxic

hazard might be when long term exposure to a gaseous chemical causes gradual organ deterioration or some type of cancer.

In such cases, short term exposure may have no noticeable effect.

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Threshold Limit ValuesLevels of exposure that may be hazardous

are determined by a combination of both concentration and time.

An example of such an exposure level is the threshold limit value (TLV).

TLV’s represent levels of acceptible exposures under normal working conditions.

TLV’s may also be given as daily time weighted averages or limits for short term (15 minute) exposures.

TLV’s are issued by the American Conference of Government Industrial Hygienists (ACGIH).

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Threshold Limit ValuesWhat is a drawback to TLV’s?They are not much use for determining

acute risk levels associated with one-time or sporadic events, such as a hazardous spill.

If the TLV’s are permitted on a daily basis, would a one-time higher level of exposure be tolerable?

If so, how much higher than the normal TLV would be considered “safe”?

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IDLH ValuesAnother measure used that is more relevant to an

emergency situation or one-time accident are the immediately dangerous to life or health (IDLH) values.

IDLH values are published by the National Institute for Occupational Safety and Health (NIOSH).

These levels have been developed to indicate when occupational workers should wear respirators to protect against airborne toxins.

While not specifically designed for emergency situations, IDLH values can be used for practice or rough emergency planning calculations.

For our purposes (mathematical modeling of hazardous situations), IDLH values and information from MSDS’s will be sufficient!

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Parts per Million (PPM)Unlike LFL’s and UFL’s, which are

specified in percentages, toxic levels are usually specified in parts per million (ppm).

As an example, a 5% concentration in terms of ppm would correspond to 50,000 molecules of a chemical for every one million molecules in the vapor space, since 50,000/1,000,000 = 0.05 = 5%.

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Parts per Million (PPM)One reason that ppm are used is

that toxic concentrations that appear in the IDLH are measured in tens, hundreds, or even lower ppm!

Note that unlike ppm for water contamination in which ppm are based on weight or mass percentages, chemical ppm are measured in terms of molecular or volume percentages.

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Fundamental Chemistry PrinciplesSuppose we know that a toxic

concentration level in the air for the chemical ethylene glycol (C2H6O2, commonly known as antifreeze) is 110 mg/m^3.

What is this concentration in ppm?

To answer this we need a few fundamental principles from chemistry!

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Fundamental Chemistry Principles(a) (Avogadro’s principle – 1811): Under

fixed conditions of pressure and temperature, a given volume of gas will contain the same number of molecules of any gaseous substance whether the molecules are small, light molecules or larger, heavier ones.

Molecules are sufficiently far apart in a gas that their individual size has no impact on how much volume the gas takes up at a given pressure and temperature!

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Fundamental Chemistry Principles(b) If M is the molecular weight of a gaseous

substance, then M grams of that substance should take up the same space and contain the same number of molecules as N grams of a substance whose molecular weight is N.

Thus, since nitrogen gas (N2) has a molecular weight of about 28 and hydrogen gas (H2) has a molecular weight of about 2, it follows that 28 grams of nitrogen gas should occupy the same space and have the same number of molecules as 2 grams of hydrogen gas!

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Fundamental Chemistry Principles(c) The actual number of molecules contained in M

grams of a substance with molecular weight M is about 6.022 x 1023 molecules.

The number 6.022 x 1023 is known as Avogadro’s number.

This quantity is called one mole of the substance.By part (b) of these principles, we know this

number is the same no matter what substance we have.

Thus, one mole is M grams of a material with molecular weight M, and the number of molecules in this amount of material is Avogadro’s number.

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Fundamental Chemistry Principles(d) One mole of a gas under

“standard temperature and pressure conditions” occupies a volume of 22.4 liters.

This fact has been determined by chemists, both experimentally and theoretically!

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Fundamental Chemistry Principles(e) If we have x grams of a gas and if

its molecular weight is M, then we would have x/M moles of the gas and the number of molecules would be (x/M) x 6.022 x 1023.

As an example, since nitrogen gas has a molecular weight of about 28,

56 grams of nitrogen gas = 56/28 = 2 moles of nitrogen gas = 2 x 6.022 X 1023 molecules of nitrogen gas.

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Back to the C2H6O2 Example!Suppose an emergency level of

concentration for ethylene glycol (C2H6O2) is published as 110 mg/m^3.

What does this correspond to in ppm?

To answer this, we need to know the molecular weight of ethylene glycol.

M = 2(12.011) + 6(1.008) + 2(15.9994) = 62.0688

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Back to the C2H6O2 Example!Thus, we have

(110 mg glycol)/(m^3 air)= (110 mg glycol)/(m^3 air)*(1 m^3)/(100 cm)^3*(1000 cm^3)/(1 liter)*(22.4 liter air)/(1 mole air)*(1 mole air)/(6.022 x1023 molecules air)*(10^6 molecules)/(1 million molecules)* …

35

Back to the C2H6O2 Example!

*(1 g)/(1000 mg)*(1 mole glycol)/(62.0688 g glycol)*(6.022 x1023 molecules glycol)/(1 mole

glycol)= (39.6979 molecules of glycol)(million

molecules of air)= 39.6979 ppm

Note that the red calculations converted the denominator to million molecules of air and the blue calculations converted the numerator to molecules of glycol.

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Resourceshttp

://www.sciencelab.com/msds.php?msdsId=9927062

http://blogs.nwic.edu/rocketteam/files/2011/09/Acetone.pdf

Charles Hadlock – Mathematical Modeling in the Environment, Section 4.3