Anyone working in a grain elevator, processing facility, or flour mill understands the dangers associated with combustible dust. Many workers have experienced the power of these tiny particles firsthand or have seen pictures of the aftermath of dust explosions. This article will provide a review of the elements that determine the combustibility of dust.
The National Fire Protection Association (NFPA) defines combustible dust as a “combustible particulate solid that presents a fire or deflagration hazard when suspended in air or some other oxidizing medium, over a range of concentrations, regardless of particle size or shape.”
This definition encompasses almost any dust associated with agricultural products. NFPA further defines combustible dust as a finely divided organic or metal material, typically 420 microns or smaller. A micron is a unit of length equal to one millionth of a meter, used in many technological and scientific fields. These particles are capable of causing an explosion when suspended in air and exposed to a sufficient source of ignition.
The word explosion means a violent and destructive shattering or blowing apart of something. This requires that the dust is confined within a structure or piece of equipment.
Particles of dry grain dust from wheat, milo, oats, barley, wheat, or oat flour and corn starch can create an explosion under the right conditions. In addition to these materials, dust from other agricultural materials is also explosive. These materials include cottonseed, sugar, rice starch, potato flour, rye flour, corn meal, semolina, soybeans, hops (malted), and rice. These types of materials generally are handled in facilities that process products for human or animal consumption.
Three key components determine the combustibility of dust. These include the minimum ignition energy (MIE), minimum explosive concentration (MEC), and the deflagration index, commonly referred to as the Kst value. MIE is the smallest amount of heat or electrical energy that is needed to ignite a dust-in-air mixture. The MIE is measured in millijoules. The table above indicates the millijoules required to ignite a dust-in-air mixture based on the commodity or byproduct listed. The lower the MIE the more volatile or explosive material is.
MEC is the minimum concentration of combustible dust suspended in air, measured in mass per unit volume that will support deflagration. Deflagration is defined as combustion that propagates through a gas or across a surface of an explosive at subsonic speeds, driven by the transfer of heat. Put simply, deflagration is the heating of a substance until it burns away rapidly.
The MEC depends on the size of the particle, the ignition temperature, and the amount of oxygen in the air. In order for a dust explosion to occur, the dust concentration must be between the lower explosive limit and upper explosive limit. Research suggests that the MEC range for grain dust is approximately 50 to 150 grams per cubic meter, depending on the type of the dust and the size of the particles.
The deflagration index (Kst value) indicates how large an explosion would be by measuring the maximum rate of pressure rise for the most favorable dust-in-air mixtures. The larger the Kst value, the more violent the explosion. For example, the generally accepted Kst value for grain dust is 89 bar-meters per second, and the Kst value for corn starch is 202 bar-meters per second. Particle size has a direct relationship to the Kst value in this example.
Testing dust to determine the Kst value is essential for equipment design. Kst values are used by manufacturers and engineers to validate the design of protection systems such as spark detection, deflagration venting, explosion suppression, and explosion containment. Understanding the Kst value is particularly important in processing facilities, where commodities are processed or milled creating smaller particle sizes that directly correlate to explosion severity.
Understanding the components that influence the combustibility of dust allows facility operators to make smart decisions prior to retrofitting or installing new equipment. Understanding the Kst values for the agricultural products handled or processed at your facility is a critical element in preventing combustible dust explosions.
This article was written by Joe Mlynek, president and safety and loss control consultant for Progressive Safety Services LLC, Gates Mills, OH; 216-403-9669; and content creation expert for Safety Made Simple, Olathe, KS.