Post From Safety 2013 Guest Blogger Frank D'Orsi
"Imagine trying to start a log on fire. By itself, it will be difficult to start, and require a lot of energy to bring it to a sustainable burn. It will take hours to burn to ash. If the log is split into several smaller firewood pieces, it will be easier to ignite, and require much less energy to bring it to a sustainable burn, will burn with a higher rate of heat release, and will burn to ash more quickly than the same amount of wood as a log.
"Taking those firewood size pieces, and further splitting them into small kindling wood, and arranging them in a fashion that provides good air circulation will further change the ignition requirements. The energy required to ignite and bring a pile of kindling wood to a sustainable burn is even further reduced. The kindling will burn with a higher rate of heat release, and will burn to ash very quickly. This is due to a number of factors, but chiefly because the surface area of the wood has been greatly increased, the configuration of the wood in the pile is more conducive to ignition, and the air around the wood supplies enough oxygen to readily support combustion.
"Taking this analogy another step further, if the log is reduced to dust in fine particle size, and suspended in a cloud with air around all of the many particles, the dust cloud will burn so violently, a flash fire will occur. The energy released within the few seconds that the cloud takes to burn is dependent upon the size of the particles and the specific properties of the wood used. Think of the log taking hours to burn all the wood to ash. That same wood, and all the energy needed to convert the log to ash is all released in the matter of a few seconds.
"Imagine that dust in a cloud with perfect density in air and igniting in a open field with nothing around it. The sight of this burning is spectacular, and there is limited or no resultant damage other than the loss of the wood dust. This is often called a fireball or deflagration. Now, imagine that same energy release in a confined space such as a machine or a building which act like a vessel to contain the deflagration. The pressure wave from the rapid burning of the dust cloud tries to expand very quickly. The vessel contains the pressure and allows it to build until it bursts out of its confinement in a very rapid fashion. This is a dust explosion! In the wood dust cloud scenario, there can be injury or death to people nearby, and significant property loss to the machinery or building involved."
Walt delivered an excellent presentation with respect to the basic elements to prepare, assess, evaluate and investigate combutible dust exposures and incidents, respectively. He covered everything from the basics of combustion and failures that cause combustible dust exposures to risk assessment for exposures, and the various methods and available tools and resources to prepare for prevention and investigate mishaps.
Topics included:
- History of dust esplosions;
- NFPA 654 changes;
- Identification of probability and severity;
- Elements of combustible dust assessments;
- Investigation team elements;
- Managment of change;
- Investigative techniques including what if scenarios, HazOps, fault tree analysis, failure modes and effects analysis;
- Critical documentaion of events;
- Resources to help SH&E professionals in all phases of combustible dust exposures and events.
If you weren't able to attend this session at PDC, be sure to look for Walt's paper on the proceedings CD or look in the July 2013 issue of Professional Safety for information on ordering recorded sessions.
This Safety 2013 session was sponsored by the Chubb Group of Insurance Cos. and ASSE's Fire Protection Practice Specialty.