Dust Explosion Venting Reconsidered, Part 1

(West Palm Beach, Fl) January 4, 2006 — Explosion venting, while simple in concept, is quite complex in implementation. This will be the first in a series of short articles that will discuss the issues and provide guidance for safe explosion protection via venting.

Think of an explosion vent as a window in a room. God forbid it should happen to you, but if an explosion were to occur in the room, the window would blow out first. If the window is large enough and weak enough relative to the walls, floor, and ceiling, the room would still be standing after the blast. You can visualize that there would be some residual damage. The window had nothing to do with the explosion and could not have prevented it. Through this simple analogy you now have a useful description of what an explosion vent is. The definition in NFPA 68, Guide for Venting of Deflagrations, is “An opening in an enclosure to relieve the developing pressure from a deflagration.” An explosion vent is a damage limiting technique. Such a device does not prevent occurrence, rather it mitigates damage. It is a passive device and requires little if any routine maintenance.

It should be easy to appreciate the elegance of such a simple and effective solution to the problem of providing explosion protection. It is no wonder that explosion venting is so frequently chosen. There are some pitfalls and even unknown factors involving explosion venting that can move from safe protection to complete disaster. It is my purpose to make you aware of these factors to the best of my ability and at the current state of the art. Be aware, however, that although I consider myself a serious student of the subject, this is an evolving science, and no one is omniscient.

The NFPA Technical Committee on Explosion Protection Systems, of which I am privileged to be a member, has expertise in a variety of disciplines and is global in terms of representation. I think every member of the committee would say much the same thing. We don’t know all there is to know yet, the science of explosion venting is evolving with more testing and better communication providing a foundation for constant learning and improved safety.

So what are the pitfalls that render this seemingly simple concept so profoundly complex? Well it starts and ends with trying to determine the appropriate size and placement of an explosion venting device. For many years it has been typical that vent sizing was quite conservative. Bigger would be better, or at least safer, or so the thinking went. Unfortunately, there are problems with that approach. If, for example, the sizing calculation method results in larger vent areas than can be fit onto the enclosure there is a dilemma. In a situation where a duct is used to direct a flameball outside of building, a too low pressure discharge from the vent into the duct can result in a large quantity of unburned, suspended dust entering the duct where ignition would result in a far larger and more severe secondary explosion. In other words, bigger is not necessarily better. Ducts are complex and will be treated in more detail later.

If there are practical restrictions on the vent area; for example, there is no advantage to providing more vent area than the cross sectional area on elongated vessels, then the maximum reduced explosion pressure (Pred) for the protected structure must be carefully considered. If Pred were to exceed the ultimate yield strength of the vessel during a deflagration, then breach would occur and vent effectiveness would be obviated. Another way of saying this is that were such an event to occur, the explosion vent would not limit damage.

In the next article in this series I will discuss the latest basic vent sizing methodology that upon adoption will become available with the next edition of NFPA 68. Then in subsequent articles all of the vent size modifiers will be discussed such as ducts, length to diameter ratios, the properties of combustible materials, and so forth.

CV Technology combines a legacy of experienced explosion consulting with revolutionary and completely unique explosion prevention and explosion protection technologies to specialize in the prevention, protection, and elimination of dust explosion hazards in all industries which process powders and dry bulk materials.

More Information:
www.cvtechnology.com/Dust-Explosion.htm
www.cvtechnology.com/Explosion-Protection.htm
www.cvtechnology.com/Explosion-Prevention.htm
www.cvtechnology.com/Explosion-Consulting.htm

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