I want to touch a bit more on two of the elements I mentioned in my original post…Vapor Density and Lower Explosive Limit (sometimes called Lower Flammable Limit). Vapor Density is basically the weight of the vapor or gas in air. It tells us whether the gas/vapor will rise or fall when it flashes off the liquid surface. As it is flashing off, the energy created from the liquid reaching it’s flash point will push the vapor up off the surface of the liquid. Depending on the FP and the Boiling Point of the material, this energy is usually low, but just enough to raise the vapor. Once the vapor gets out over the edge of the liquid surface or high enough off the liquid surface, the Vapor Density will then determine where that vapor will go. Generally, most hydrocarbons will have a VD of 2-3 times heavier than air, meaning they will settle into low lying areas (trenches, ditches, sumps, pits, dikes, etc.) which is VERY VERY BAD. This allows the vapor to increase in concentration, thus reaching it’s Lower Explosive Limit (LEL). Most of the gases (yes there is technically a difference between a gas and vapor) in common use today like Hydrogen and Acetylene are MUCH lighter than air. If their release is outdoors, we like this, but inside a structure, this is VERY VERY BAD. Outside these gases continue to rise and dissipate (dilute) as they rise, thus not being able to achieve their LEL. Of course, they are probably ABOVE THEIR UEL at the point of release due to their EXTREMELY LOW BOILING POINT, so at some time as they rise they WILL PASS THROUGH THEIR FLAMMABLE RANGE!!! If you have materials with EXTREMELY LOW BOILING POINTS you should really read my BLEVE posting earlier this year. NOTE: You can look up Molecular Weight of a flammable compound and compare it to air to see if it will rise or sink. The Molecular weight of air is 29.
Now since we have a basic understanding of Vapor Density, let’s discuss the next element of a fire/explosion and that is Lower Explosive Limit (LEL). LEL is defined as the minimum concentration of a gas/vapor needed to ignite in the presence of an ignition source. This is NOT the technical definition, but rather the common sense definition I will use in my postings. When an atmosphere is BELOW the LEL we say it is “too lean” to ignite. When an atmosphere is ABOVE the UEL (Upper Explosive Limit) we say it is TOO RICH to ignite – more on UEL in a future discussion. Vapors that have a VD greater than 1 and will collect in low lying areas are VERY DANGEROUS as they will eventually reach their LEL. Look at it this way, take out a yard stick and stand it up on the end. It is 36″ tall. Now imagine you are measuring the vapor cloud’s depth using this stick (DO NOT DO THIS AT HOME!!!). The vapor cloud is 36″ tall/deep. MOST LIKELY the top of the cloud will be BELOW the LEL, but was we travel down the yard stick the concentration INCREASES and begins to reach the LEL. Let’s say for discussion, we reach the LEL at the 20″ mark on the stick. This means at 20″ there is the right mixture of the vapor/gas and Oxygen to ignite. This 20″ mark is called 100% of the LEL, meaning we have reached the point where it will go BOOM! as we continue to travel down the stick we will eventually reach a point where the vapor/gas concentration will get so concentrated that we have displaced the oxygen to a point where the gas is TOO RICH to burn and we call this the UEL. Between the LEL and UEL is the flammable range. THIS IS NOT AN EXACT SCIENCE and should never be looked at as such!!!!!! So for a more visual example, a spark/ember is falling to earth and as it passes through this vapor/gas cloud IT WILL PASS through the flammable range AND if the spark/ember has more energy than the Minimum Ignition Energy (MIE) needed, the cloud will IGNITE and burn rapidly!!!! We will discuss UEL and MIE in a future post.