Combustible Dust: How to Safely Handle It
Combustible dusts pose a deflagration or other fire hazard when suspended in air. Deflagrations can lead to explosions. Combustible dusts are often either organic or metal dusts that are finely ground into very small particles. Types of dusts include, but are not limited to: metal dust, such as aluminum and magnesium; wood dust; plastic dust; biosolids; organic dust, such as sugar, and paper. Some industries that handle combustible dusts include: agriculture, chemicals, textiles, forest and furniture products, wastewater treatment, metal processing, paper products, pharmaceuticals, and recycling operations (metal, paper, and plastic).
Even if your plant air is below OSHA’s PEL (permissible exposure limit for personnel respiratory safety) for process generated dust, over time enough dust may be generated and settle on horizontal surfaces to become dangerous. If an incident such as excessive drafts or building vibrations throws the settled dust into the air near an ignition source, a fire and dust explosion may occur if the dust is combustible. Before a dangerous condition occurs in your plant, housekeeping action should be taken to safely clean up and remove these accumulations. Then, action needs to be taken to minimize dust generation before combustible dust again accumulates to unsafe levels in your plant.
Minimizing Dust Accumulations
One way to minimize process generated dust from accumulating in the plant is to install an effective and safe dust collection exhaust system or upgrade an existing one. Pneumatic conveying equipment and dust collection exhaust systems that transport combustible solids and particulate need to be protected from fire and dust explosions. If the concentration of combustible dust suspended in air is at or above the minimum explosible concentration (MEC) and there is a source of ignition, an explosion may occur within the confines of dust collection system components. In general, the concentration of dust is highest within the system’s cyclone and dust collector. Other system components such as the ductwork and fan need to be considered as well. Following are some points to consider when evaluating or designing a dust collection or pneumatic conveying system handling combustible dust and solids.
Dust Collector Explosion Relief Vents
Designing explosion vents for dust collectors and filter receivers involves a number of factors. These factors include the explosion pressure and rate of pressure rise of the combustible dust, the strength of the dust collector housing and the available area on the dust collector for venting. The dust can be tested to determine explosion pressure and rate of pressure rise. The stronger the dust collector housing, the fewer and/or smaller the vents need to be. The vents typically need to be installed on the dirty air side of the dust collector. Refer to the National Fire Protection Association NFPA 68, Explosion Protection by Deflagration Venting. Knowing the dust explosivity characteristics such as the maximum explosion pressure and the rate of pressure rise of your combustible dust, the dust collector manufacturer and the explosion vent supplier can supply the necessary in information for the vented design.
An existing dust collector may be strong enough “out of the box” to accommodate the necessary number and size of explosion vents. If the dust collector needs to be strengthened, reinforcing steel can be welded to the exterior of the dust collector. Calculations for the reinforcing steel size and locations can be performed knowing the dust collector housing wall thickness. Installing explosion vents onto an existing dust collector can be cost effective and can minimize production downtime as compared to replacing the existing dust collector with a new one.
Exhaust Fan Safety
Negative pressure dust control and pneumatic conveying systems usually have the exhaust fan on the clean air side of the dust collector or filter receiver, especially if the dust or conveyed solids are combustible. Refer to NFPA 654, “Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids”.
The Air Movement and Control Association International, Inc., (AMCA) has publications available concerning exhaust fan safety. Refer to:
• AMCA Standard 99-0401-86, "Classifications for Spark Resistant Construction" and
• AMCA Publication 410-96, “Recommended Safety Practices for Users and Installers of Industrial and Commercial Fans”.
System Ductwork Explosion Protection
Dust collector explosion vents are not sufficient protection for connected ductwork. The inlet and outlet ductwork of a dust collector or filter receiver are, in effect, open vents. Having explosion vents on the dust collector or filter receiver does not keep fire and debris from entering this ductwork. If the inlet and outlet ductwork are not isolated during an explosion, fire and debris can exit from the dust collector and enter the plant through the connected ductwork. Refer to NFPA 654, 2020 Edition, Annex B, Explosion Protection.
Take Action Now
Identify solids that can become combustible when reduced in size by routine handling, grinding or in other ways that reduces its size. Install dust control systems where appropriate. Clean-up accumulated dust on exposed areas such as floors, beams and ductwork and in hidden areas such as above ceilings and behind equipment. Eliminate potential ignition sources. Refer to NFPA 654 for more information on how to make your plant safer
Mr. Constance has been responsible for assignments in the private sector involving plant engineering, plant management and air pollution control. More than thirty-five years’ experience in dust control consulting engineering and construction.
He has an in-depth expertise in the engineering and design of dust and vapor control exhaust systems for the food, pharmaceutical, agricultural and other chemical processing industries involving national and international projects.
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Disclaimer: While every effort has been made to ensure the accuracy of this publication, it is not intended to provide legal advice as individual situations will differ and should be discussed with an expert and/or lawyer.For specific technical or legal advice on the information provided and related topics, please contact the author.