January 28, 2019
As long as there have been industrial environments, dust has caused explosions. In the last 20 years alone, dust explosions have resulted in hundreds of deaths and injuries around the world, as well as massive property damage.
All it takes is one small spark to ignite dust under the right conditions, and a wide range of everyday materials can cause an explosion. Examples include sugar, corn, steel wool, aspirin, coal, aluminum, paint pigment, cornstarch, pasta, tapioca, tea, and cocoa. And almost every industry faces dust explosion hazards: from agricultural to automotive to refining.
Fortunately, explosion prevention techniques are readily available for use in industrial and agricultural applications. Pressurization provides an economical way to use electronic devices in a range of hazardous locations without putting employees and equipment in danger.
What Is a Dust Explosion?
A dust explosion occurs when a fine dust suspended in the air is ignited, causing rapid burning. In milliseconds, gaseous products are released with a subsequent pressure rise of explosive force. Dust explosions can be categorized into two phases: primary and secondary.
A primary explosion takes place in a confined atmosphere, such a as a silo or part of the manufacturing plant, with the resulting shock wave damaging and often rupturing the plant. This allows the products of the explosion (burning dust and gases) to be expelled into the surrounding area. This disturbs any settled dust and initiates a larger secondary explosion. The secondary explosion can cause severe damage to surrounding plant buildings. Most large-scale dust explosions result from chain reactions of this type.
What Conditions Are Necessary for a Dust Explosion?
How to Reduce the Hazard
One common method to reduce the hazard of dust explosions is to prevent the combustible material from reaching an explosive concentration by removing the combustible material and pressurizing the area. This prevents the accumulation of a flammable atmosphere. Other ways to minimize the risk of explosion include reducing the oxygen content and adding moisture or a dry inert material.
In coal mines, it is common practice to coat the galleries and shafts with rock dust to reduce the likelihood of coal dust explosions. The inert material adds thermal capacity without increasing the energy released by combustion. Adding the inert material also increases the amount of energy required to ignite the combustible elements of the atmosphere — much like holding a lit match to a wet log.
There are two objectives in Class II locations:
Pressurization effectively handles both of these scenarios for Division 1 and Division 2 areas.
Pressurization: What Is It?
Unlike containment or prevention protection, pressurization separates general-purpose electrical devices from the surrounding hazardous atmosphere by placing them inside a common, lightweight enclosure. This enclosure is then cleaned and pressurized with industrial-grade air, or an inert gas, and maintained at a pressure higher than the dangerous external atmosphere, preventing the combustible material from coming in contact with the internal components. Only pressurization is required in a Class II dust atmosphere. Purging is used in Class I or gas applications. If purging is used in a Class II area, the vent will be blocked and cause a dust cloud, leading to an unsafe condition.
Most pressurization enclosure applications require a minimum enclosure pressure of 0.10 inches (2.5 mm) of water. One psi is equal to 27.7 inches of water. In some circumstances, a minimum enclosure pressure of 0.5 inches (12.7 mm) of water is required to protect against the ingress of ignitable dust. But in all cases, a higher enclosure pressure should be maintained to create a reasonable safety factor. In rare circumstances, enclosure pressures as high as 2.5 inches (63.6 mm) of water may be required to offset sudden atmospheric pressure fluctuations, such as those created near missile launchings or other applications with quick atmospheric pressure changes.
For Division I applications, loss of pressurization requires disconnect of power to the enclosure. For Division 2, loss of pressurization allows power to remain on, provided an audible or visual alarm notifies the operator of the condition. Motors, transformers, and other devices subject to overload must be provided with automatic means to deenergize them if temperature exceeds the design limits. Using cooling devices in the enclosure should also be considered. Vortex coolers provide an inexpensive solution.
The need to place general purpose equipment in hazardous locations is not new, yet in the last three decades the need has intensified dramatically. Most modern electronic equipment is expensive and delicate, requiring environmental protection that cannot be provided by explosion proof enclosures or intrinsic safety barriers. Purge and pressurization technology offer the safest and most economical means of installing electrical equipment in a hazardous location, as well as protecting this delicate equipment from corrosive environments.
How to Prevent and Control the Hazard: