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Class D refers to flammable metals such as lithium. Lithium-ion batteries are a rechargeable battery technology that is growing in the industry. But they are not safe at excessively high temperatures and are vulnerable to catching fire. Class F is for deep fat fryers (fats, cooking oil). Class F is sometimes considered to be a sub-class of Class B (liquid) and Class C (gas) fires, but Class F is more applicable to kitchen fires that can occur at dangerously high temperatures. Class Electrical is, of course, for electrical equipment (the letter "E" is not actually used); electrical fires can fall under any of the other five (5) classes since electrical current (e.g., spark) is causing the ignition.

Section 5 of a substance's Material Safety Data Sheet/Safety Data Sheet (MSDS/SDS) lists fire-fighting measures (FE Reference Handbook, v 10.0.1, p. 18). The FE Reference Handbook also describes flammability limits (v 10.0.1, p. 19-20). The Fire Triangle consists of the three (3) components needed to start a fire and continue burning; they are heat, fuel, and an oxidizing agent (oxygen). Flash Point refers to the lowest temperature that material vapor will ignite if exposed to an ignition source. This is why liquids with a flash point below 100^° F (38^° C) are flagged as flammable by both the NFPA and United States Department of Transportation (USDOT). A material with a lower flash point material is more volatile and dangerous if near ignition sources. Room temperature (68-74^° F) is well below the 100^° F threshold, so materials at room temperature are generally non-flammable since it is unlikely that a material would catch fire under standard temperature conditions.

The National Transportation Safety Board (NTSB) is the U.S. government agency that oversees the investigation of transport accidents and incidents. This coincides with the USDOT since both government agencies focus on transportation. Materials such as liquefied natural gas (LNG) and liquefied petroleum gas (LPG) require oversight since they are often transported by ground vehicles. It is crucial to identify any materials that may be volatile at temperatures below 100^° F; vehicular accidents can lead to devastating fires and explosions that may cause human fatalities and damage to roads, bridges, and other highway infrastructure. Flash point is usually seen in laboratory environments, but you should certainly be mindful if you are driving near trucks and tractor trailers that are transporting volatile fuels.

Flammability applies to all material states, including solids, liquids, and gases (vapor). Material flammability can also be defined by the upper and lower limits. The limits are lower flammability limit (LFL) and upper flammability limit (UFL); these limits are also known as lower explosive limit (LEL) and upper explosive limit (UEL). Both "flammability" and "explosive" are interchangeable terms when discussing the limits. The limits apply to vapor-air mixtures (volume % in air); if the concentration is above the UEL, then the mixture is too rich to burn; if the concentration is below the LEL, then the mixture is too lean to burn. The in-between range is ignitable and should be carefully monitored if these conditions occur.

A combustible gas indicator (CGI) is an instrument that can be used to measure vapor-air mixture concentrations in a room or area. CGIs can be calibrated for different gases; methane (natural gas) is the most common since many homes and businesses utilize natural gas for heating and cooking appliances. A CGI is a good tool, and I would recommend purchasing one for your own household since it can function like a smoke detector and carbon monoxide (CO) detector. While propane has a lower concentration range compared to natural gas, propane still poses a more dangerous explosion risk. In my opinion, a lower LEL (2.1% for propane) is also more hazardous since you do not need a large concentration to enter the explosive range. You are more likely to cross beyond the LEL range than surpass the UEL range; propane explosions and fatalities have occurred at homes and businesses due to an accidental leak that did not have a large concentration needed to ignite.

It should also be noted that heavier hydrocarbons (e.g., butane, pentane, octane) have lower LEL values; as a rule of thumb, heavier hydrocarbons (more carbon atoms) have decreasing LEL values. UEL values decrease with heavier hydrocarbons too. But I would more closely study the LEL values for different volatile materials since those are more probable. For example, gasoline is a common fuel source and has a low LEL value, so it is generally considered flammable. Heat and other ignition sources should be avoided at gas stations and other fuel areas.

Propane is denser than air, so it does not rise to vent like natural gas in the case of a gas leak. Because propane settles, it creates a more hazardous environment compared to natural gas, so natural gas is more widely used for utility appliances (but I think propane having a lower LEL value is another reason natural gas is more favorable). Since propane (C3H8) is also a hydrocarbon, it is non-polar; you cannot spray water to disperse a propane gas release like you could with a more polar substance like ammonia (NH3). Polarity is due to differences in electronegativities and is a periodic table trend. Fluorine is the most electronegative element. Non-metals (Right Side) are more electronegative whereas metals (Left Side) are electropositive and therefore have lower electronegativities; this creates the difference in polarity. Ionic compounds such as table salt (NaCl) are perfectly polar and dissolve readily in water (H2O). You can also refer to the periodic table in the FE Reference Handbook (v 10.0.1, p. 88).

Carbon monoxide (CO) is slightly less dense than air. However, it is not enough to rise quickly, so you must evacuate in the case of a spike in CO concentration that would alert a CO detector. The CO concentration will displace the oxygen in your body faster than it can rise to vent outside; this is how death by asphyxiation occurs. You should become more familiar with the topic of flammability and understand the science, not just for your engineering career, but also as personal advice. I described many different household materials (e.g., natural gas, propane, cooking oil, gasoline) that are used in everyday life to help you stay safe and educate others. There are always ongoing HSE issues in the industry; you can read more online on the Occupational Safety and Health Administration (OSHA) website and check back with School of PE for more HSE blog posts.

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