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01 April 2003

Dust busters: Don't let your plant go up in smoke

Like flammable gas, flammable dust needs to mix with air and encounter a source of ignition to explode. . . . A dust cloud ignites when a door is opened and a breeze stirs up the dust and burning embers at the same time.

By Al Engler

The January disaster at the West Pharmaceuticals Plant in Kinston, N.C., is potent testimony to the tremendous destructive power a major dust explosion can unleash. In a matter of seconds, a large manufacturing plant transformed into a blazing inferno, with a blast that blew in doors on houses a mile away.

Operators of grain elevators, food processing plants, and coal-burning power plants have too often learned the hard way about the danger of flammable dust. What causes dust explosions? How can a dust explosion destroy a whole plant so rapidly? How can a company prevent them? To answer these questions, we should understand dust explosion dynamics—how explosions ignite, how they propagate, and what conditions lead up to them.

WHAT CAUSES A DUST EXPLOSION?

Like flammable gas, flammable dust needs to mix with air and encounter a source of ignition to explode. Flammable dust has been a problem in underground coal mining for many years. Mining experts did much of the research on dust explosion hazards, as well as much of the early work on flammable gas hazards. The U.S. Bureau of Mines performed research on dust ignition properties. It even built a simulated coal mine tunnel it could fill with dust and air and ignited it to study the properties of dust explosions. This research helped determine how much energy it took to ignite a cloud of a particular dust mixed with air and provided data on how a dust explosion propagates.

Ignition can happen two ways: by spark energy or by a hot surface. Each type of dust has a particular ignition energy and an autoignition temperature—the temperature a hot surface has to reach to set off a dust/air cloud. The National Fire Protection Association (NFPA) publishes a standard, NFPA 499, that lists these properties for different types of flammable dust.

Flammable dust is classified into groups that put dusts with similar ignition properties together. In the U.S., flammable dust is called a Class II hazard (flammable gas is a Class I hazard, and flyings—dust particles too big to be called dust—are a Class III hazard) and is divided into three groups: E, F, and G. Group E is metal dust, Group F is coal and similar dusts, and Group G is grain dust. Electrical equipment can be built for use in particular types of flammable dust atmospheres and not cause ignition of a dust cloud. The National Electrical Code (NFPA 70) has rules in Articles 502 and 503 for installing electrical equipment in areas where flammable dust or flyings may be present.

If we know the properties of flammable dust and we know how to build and install electrical equipment that will not set it off, why do we have explosions? Why are these explosions often so destructive?

The answer to the first question is complex because there are many things other than electrical equipment, such as human error, that can set off a dust cloud. A buried hot surface can cause a smoldering fire. A dust cloud ignites when a door is opened and a breeze stirs up the dust and burning embers at the same time. The answer to the second question is often very simple: bad housekeeping.

An ignition-capable dust cloud is dense. It usually exists only inside limited areas because it would blind and choke a person. An ignition usually sets off only a small amount. The dust that brings the building down is piled in heaps on the floor or, worse, is caked on the walls and ceiling. A small explosion air blast knocks loose the dust on the walls, floors, and ceilings and forms a cloud. And the second explosion is the one that brings the house down.

How can you avoid catastrophic dust explosions? Use equipment rated for the area, install and maintain it properly, avoid mechanical ignition hazards, and keep the place cleaned up. Don't let a small event turn into one that takes the entire building with it. IT

Al Engler is quality director at EGS Electrical Group in Skokie, Ill.