Q&A - Dust Explosions - with William J. Stevenson

William J. Stevenson, Vice President Engineering, CV Technology Inc., West Palm Beach, Florida, is a recognized authority in the field of dust explosion protection in the process industries. Mr. Stevenson has over 39 years of engineering experience and is responsible for activities principally regarding field consulting engineering for CV Technology Incorporated. He is a Senior Member of the American Institute of Chemical Engineers, and also a member of the National Fire Protection Association. Mr. Stevenson is on the NFPA Technical Committee on Explosion Protection Systems, which encompasses NFPA 68 Guide for Venting Deflagrations; and NFPA 69 Standard on Explosion Prevention Systems. He is also on the NFPA Technical Committee on Handling and Conveying of Dusts, Vapors, and Gases.

Can you discuss the essential components of a complete explosion prevention strategy?

First off, for a dust explosion to occur five things must happen simultaneously:

1) The dust must be combustible

2) The dust must be suspended in a cloud of sufficient concentration to support a chain reaction

3) There must be an enclosed area or vessel

4) There must be sufficient air

5) There must be an ignition source

So, the essential component of a complete explosion prevention strategy is to eliminate one or more of these five from being possible.

Do you feel that the risk of dust explosions is underestimated in the food industry?

Getting an accurate picture of the true risk of loss due to dust explosions is extremely difficult because companies often sustain losses that are below deductibles, and in all cases there is a reluctance to go public with what is clearly a negative experience.?According to FM Global, the largest industrial insurer, the food industry suffers the second highest loss rate of any industrial segment in North America. It should be noted, too, that food materials are far from the most explosible dusts commonly handled in industry.

What is fugitive dust and what risks does it pose?

Fugitive dust is simply dust that escapes from inside process equipment and settles on horizontal surfaces. In a typical event, a primary explosion occurs inside a vessel that results in breach of the vessel. The escaping pressure wave hits fugitive dust and lifts it up into a cloud. The flame ball that emerges through the breach then ignites the suspended dust cloud in the room and a secondary explosion ensues. A layer of dust the thickness of a paper clip is sufficient to knock down walls and lift the roof of a plant built to meet the national building codes. The losses from fugitive dust ignitions constitute the largest losses by far for both life and property.

How should decision makers at a food plant determine if they are at risk?

One way is to test the dust to determine if it is explosible. Plants should perform a qualified risk analysis to identify the risks and to determine the options for protection. If you remain unsure of your situation, then it would be better to err on the side of safety and hire a qualified explosion consultant to evaluate your process, the inherent risks, potential consequences, and make appropriate recommendations.

Which types of ingredients pose the highest risk?

Very fine particles are the most reactive. In a broad sense any organic dust, handled in large quantities, posses a risk. Some examples of well known agricultural dusts that have fueled dust explosions in the food processing industry include: milk powder, sugar, starch, fructose, flour, whey, cocoa, and malt. Organic-based spices and flavorings can also be reactive. Some food additives such as threonine are particularly ignition sensitive.