We are back for Part 3 of our Confined Space Atmospheric Hazards series. In this post, we will be discussing explosive/flammable atmospheres. For the purposes of this blog post, we will be utilizing the term explosive to keep things clear and concise.
An atmosphere is explosive when a flammable gas is present in the space in enough quantity that it can explode with the introduction of an ignition source. If there is not enough of the flammable gas, we refer to the gas as being below its lower explosive limit (LEL). If there is too much flammable gas, we refer to the gas as being above its upper explosive limit (UEL). It is the area between the LEL and UEL that is the most dangerous – the explosive range.
In the graphic above, the explosive range is shown sandwiched between the LEL and UEL. Note that the actual explosive range for every gas is different. Some gases, like acetylene have a very wide explosive range which makes them extremely hazardous. The explosive range of acetylene is 2.5 – 100% (NIOSH Pocket Guide). It takes relatively little acetylene to make a confined space atmosphere explosive, and it will stay explosive even as its concentration increases to high levels.
We monitor for explosive atmospheres using our typical four-gas detector with an LEL sensor. Most emergency services agencies use a sensor that shows them a % of LEL. What that means is that the percentage displayed on the detector is essentially telling you how close you are to reaching the LEL. The graphic above depicts the area that is analyzed by your sensor. If your sensor says 10%, which also happens to be the highest acceptable level for confined space entry, that means you are 10% of the way to being in an explosive atmosphere. Remember, every gas is different, and your sensor is only calibrated to one specific gas. Correction factors can be used to determine more accurate levels if you know your calibration gas and the gas present in the confined space.
Gases are not the only explosive hazard inside confined spaces. Combustible dust explosions occur when a fine, combustible dust is suspended in the air and ignited. Between 1980 and 2005, there were 281 combustible dust incidents that resulted in 119 workers killed and 718 injured. Emergency services agencies typically do not carry or utilize detector technology that can monitor dust concentrations. I highly recommend getting out into your community and pre-planning facilities that may have this issue!
In Part 4 of this series of posts, we will be looking at the two most used toxic gas sensors – carbon monoxide (CO) and hydrogen sulfide (H2S). Check back soon!
Owner / Lead Instructor
Elder Technical Rescue Services, LLC