NIOSH recently released an LODD Report: One Fire Fighter Dies and Another Injured in Natural Gas Line Explosion— Wisconsin
Discussion. Every so often a report pops up that should cause us pause. This report had two key statements that made me pause and I wanted to share them with you. Please don’t mistake this for a full discussion or summary of the report. Also note that I make some assumptions here as I try to apply the lessons of this event to our operational context.
“…the Captain expressed concern that the gas meter was alarming at 100%.”
What the report does not say is what gas the meter was calibrated to. In MCFRS we calibrate to methane. It used to be pentane but we switched many years ago. Why that matters so much is that, assuming your meter is properly calibrated, you are reading natural gas (methane) 1 to 1. When the meter says 100% you are at the Lower Explosive Limit for methane. That means that the environment you are in is ripe for an explosion.
How does that impact you? A couple of things.
- First, it is very unlikely that the meter went from 0% outside to 100% right away. In other words, the crew likely saw the numbers going up BUT kept going. Our meters alarm at 10%. Our hazmat team takes a pause and huddles when it hits 20%.
- DO NOT FOOL WITH AN LEL%s above10% inside of structures or confined areas. There is too much that you may not know or may not be able to control. Get a hazmat consult.
- Also remember we are calibrated at methane. If you are reading anything else the numbers are accurate but not for the other gas.
- With natural gas having a vapor density of 0.55-0.65 (air=1), natural gas will rise quickly in air. If the crew was reading 100% at waist level (where the meter is usually held) there was likely a much higher concentration above their heads.
- When running natural gas leaks it makes sense to measure high first.
- A HUGE thing to consider: Given that we don’t know what the gas readings were throughout the area of the explosion it is possible that there were areas with an LEL much higher than 100%. For every LEL there is a corresponding Upper Explosive Limit (UEL). The MOST DANGEROUS position to be in is an environment that is above the UEL because before you clear the danger you must come back through the LEL. The environment can’t explode if the concentration is above the UEL or below the LEL BUT that entire space in between is scary.
“As the Captain and the other firefighter were stepping out of the threshold on Side Delta, the building exploded”
How does that impact you? A couple of things.
- Just like with a ventilation induced flashover there is a latency when dealing with flammable gases. As the gas leaks it fills the space BUT there are pockets of higher and lower concentration. The concentration of natural gas IS NOT uniform across the space.
- It is unlikely (possible but unlikely) that the exit is what sparked the explosion. What is more likely is that the was an ignition source already present! The act of opening and closing doors (VENTILATION) along with moving through space caused mixing of air and gas. The compartment hit moved into the flammable range and the ignition source ignited the explosion.
- The takeaway here is simple. Opening doors is ventilation and ventilation causes mixing and there is a time delay between when mixing happens and when the ventilation induced event happens. Does that mean we never open a door when checking? Of course not, it means that you must move slowly, trust what your LEL sensor is telling you AND stop before you get into a bad spot.
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There is much more to learn from this report but out of respect for your time I just hit on a couple of highlights. Thanks for taking the time to read.
Thinking it Through 