Earlier this year I was asked to participate in an incident review involving a flashfire/explosion in a tote-filling process. (NOTE: the official term is an Intermediate Bulk Container or IBC) As I have written about many times, the #1 cause of these flash fires/explosions inside a container/vessel is the generation of static electricity; but this incident occurred in a tote was that “purged with Nitrogen” (N2). So how can a container/vessel that is purged with N2 have a flashfire/explosion? Simple… the N2 purge was NOT designed to ensure the atmosphere inside the container was lower than the flammable’s Limiting Oxygen Concentration (LOC), also called Limiting Oxidizer Concentration and Minumum Oxygen Concentration (MOC). Over the past several years we have come across a number of facilities who were using Nitrogen as their “sole savior” against all flammable liquid container/vessel fires/explosion. Be damned to good engineering practices/design – let’s splash fill or fill the container faster to improve the efficiency of the process… after all, nothing can happen because we have an N2 purge. This line of thinking and actions cost a business over $900K in damages and a lot of lost business when their “tote filling line” erupted into flames one night. How could this happen with an N2 purge?
We sat down at the table and having already known what had happened and the supposed “safeguards” at play, I asked to see the “design basis” for the N2 purge and the room fell silent. Eventually, the question was asked… “What the H_LL (his emphasis not mine) is an N2 Purge Design Basis?“
So what is an N2 Purge Design Basis and why did their purge not work?
Nitrogen is an inert gas and it works very good at keeping atmospheres “inert” (e.g. not enough oxygen to support life); however, not having enough O2 to support life and preventing ignition of flammable gases/vapors is altogether different worlds! We, humans, need at least 19.5% Oxygen to function properly, but most flammable gases and vapors can ignite with only half that much oxygen in the atmosphere. And such, Toluene (the flammable being filled into the totes in this incident) needs only around 9-10% oxygen to burn when in an N2 inert atmosphere. So here is how this incident went down…
NOTE: this is being shared with the anonymous client’s permission in the hopes that it may prevent others such incidents.