Flash point is the lowest temperature at which a flammable liquid evolves sufficient vapor to form an ignitable mixture of those vapors in air.
Well, that’s about it. Short article. So, what’s it to you?
Consider an example you already (hopefully) know – you should never store or dispense gasoline in your basement. While regular unleaded gasoline contains chemicals like benzene, a human carcinogen, let’s ignore the toxicity hazards. The more immediate hazard (acute effect) is blowing yourself up before you can develop cancer (chronic effect).
The flash point of regular unleaded gasoline is about minus 49 degrees F. It’s a pretty safe bet your basement is warmer than that. So, if you handle gasoline in your basement, you will have a flammable vapor cloud of some magnitude – maybe a big one if you spill it on the floor. Flammable vapor clouds are very good at finding ignition sources.
OK, you don’t smoke, have all explosion proof (XP) outlets and lighting in your basement (yeah, right), and the furnace is off for the summer – besides, it’s clear across the basement from where you’re working. Uh-huh, and do you have a gas water heater? It has a pilot flame. Gasoline vapors are 3 to 4 times more dense than air, so they tend to travel (you thought they were going to stay put?) along the floor. Guess where that pilot flame is located. You probably have drains in your basement, or else a sump. Liquid gasoline floats on and is insoluble in water. So regardless where the spill occurred, you can have vapors above all your drains, including that one right by the water heater. In the case of the sump, there’s a sump pump. Did you remember to get the XP motor for that too? Sure you did.
Oh, you only spilled a little bit, a quart maybe. How big a vapor cloud could that form? Well, a quart of gasoline weighs close to 1.5 pounds (apologies to more enlightened readers, but I’ll stick with English units). Gasoline is actually a mixture of organic compounds, but its average molecular weight (you remember – High School Chemistry) is about 108. So you spilled roughly 1.5/108 = 0.0135 pound moles. Doesn’t sound like much. Well, one pound mole of liquid can form up to 359 cubic feet of vapor at STP, standard temperature & pressure as you learned it in High School. Your basement should be a bit warmer than that, so the number would be a bit bigger.
So, 0.0135 pound moles could form a small vapor cloud of almost five (4.85) cubic feet – but wait! That’s 100% gasoline vapor. The vapors are mixing with air. The explosive concentration for gasoline in air is about 1.4 to 7.6%. At 7% concentration your one quart spill is enough to form an explosive mixture of 4.85/.07 = 69.3 cubic feet; at 2% one of 242.5 cubic feet. The cloud being heavy will tend to slump and spread along the floor. If it stays within two feet of the floor, it could spread over an area of 120 square feet or more. If that cloud finds an ignition source – well, how far does a quart of gasoline propel your car? About seven miles? Even if you only realize 10% explosion efficiency, that’s still a fair amount of energy.
OK, if you still insist on handling gasoline in your basement, we’ll let Darwin take over.
What about that cleaning solvent you’re planning to use on your newly installed kitchen counter? Is it flammable? Before you start wiping down a large surface area, right in the vicinity of that live wall electrical outlet, you might want to check. Flash fires have resulted from such scenarios. Open windows, a fan, and opening the appropriate circuit breaker(s) might be advisable. Maybe you want to find another solvent.
How about outdoors? You come across an overturned tanker truck. It is leaking fluid. You notice it has those red diamond shaped hazard placards indicating flammable contents. You’re about 200 yards away, upwind from the incident. Should be safe to observe from there, right? One of the emergency responders arriving on scene seems excited and starts yelling and waving you away. What gives? For one thing, the drainage ditch or sewer running along the roadway poses the same potential hazard as the drains in your basement. Unaffected by the wind, liquid and/or vapor could be migrating your direction. Also, 200 yards is not a safe distance in the event of an explosion with that quantity of material. Initial evacuation distances in such situations are typically a half-mile or more depending upon conditions.
What – you always carry a pocket size emergency response guidebook and binoculars around with you? You looked up the leaking chemical’s identity from the numbers off the red placard? You even found the flash point of the material and it’s 10 degrees warmer than today’s weather? So you think there’s no explosive hazard? Has it been sunny today? How warm does blacktop feel on a sunny day relative to air temperature? Is the truck motor still running? How warm might it be under that diesel engine? There are a slew of other considerations as well. Unless you’re a trained emergency responder participating in the incident, stay well clear of the area. Generally, emergency responders will back away from a situation if their combustible gas indicators register even 10-15% of the lower explosive limit. They may attempt vapor suppression with foam or take other measures, but that’s a whole other topic. I’m already more than 200 yards off on this tangent.
If you are now excited about flash points, where can you find such information? Some of the data for this article I obtained simply by typing “gasoline msds” into the search engine. MSDS stands for “Material Safety Data Sheet.” These sheets are available for all commercial products and contain a wealth of valuable information covering not just flammability but chemical structure, health effects, reactivity, physical properties, environmental impact and more – again, a whole other article. Chevron’s MSDS for regular unleaded gasoline was nicely done, a good format and easy to read. They’re not all that well written, but most are pretty decent.
What other flammables might you have around the house? Ethanol! The MSDS for ethanol shows a flash point of 55 degrees F. Your vodka is 80 proof? That’s only 40% alcohol, but on a warm day – do you smoke and drink? How about that rubbing alcohol, isopropanol. I get the 90% stuff; who wants to pay for water? Flash point is 54 degrees F. Nail polish remover do they still use acetone for that? Flash point is -4 degrees F.* The methyl ethyl ketone in your white board markers? 19F. The styrene solvent in that glue you’re using? 88F. Now we’re having fun. Feel free to continue without me. Heard any good exploding butane lighter stories lately?
* The “cc” you will see after some flash points stands for closed cup (vs. open cup) indicating the type of method used to measure the flash point. The closed cup value should be more conservative (lower value). However, flash point determinations are often imprecise. Don’t bet your life on them. Leave a big margin for error.
