By John Constance, Professional Engineer (P.E.)
The Engineers Collaborative.
"Chemical Safety Board Reports Chemical Dust Explosions are a "Serious Problem" - Nearly 200 Accidents took 100 Lives; Data, Testimony Noted at Public Hearing Today."
Washington, DC, June 22, 2005 - Carolyn Merritt, Chairman of the U.S. Chemical Safety and Hazard Investigation Board (CSB) declared that chemical dust explosions in the United States are a "serious industrial safety problem" in opening a day-long hearing into the hazards of such accidents. Chairman Merritt noted the CSB's preliminary research reveals that nearly 200 dust fires and explosions have occurred in U.S. industrial facilities over the past 25 years, resulting in approximately 100 fatalities and 600 injuries. Go to www.csb.gov to learn more.
Pneumatic conveying equipment and dust control exhaust systems that transport combustible particulate solids need to be protected from fire and dust explosions. Combustible particulate solids that have settled onto surfaces such as floors, platforms, suspended ceilings and building structural members as well as inside pipes and ducts can burn if exposed to a source of ignition. If combustible particulate solids are thrown into the workplace air during cleaning or by excessive drafts in the plant, the resulting combustible dust can present a fire or deflagration hazard if exposed to an ignition source.
If the concentration of suspended combustible solids (a.k.a. combustible dust) is above the Minimum Explosible Concentration (MEC) and the source of ignition produces energy above the Minimum Ignition Energy (MIE), the dust will ignite. If the combustible dust is in a confined space such as in a room, silo, bin, filter-receiver, dust collector or cyclone, the burning dust can produce enough pressure for a deflagration to occur.
Everyone wants to assure that their plant is safe to work in. But how do you begin to evaluate the safety conditions in your plant if it is processing combustible particulate solids?
The National Fire Protection Association (NFPA) publishes NFPA 654 Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids. This document provides standards for facility and system design and process equipment including pneumatic conveying and dust control systems. NFPA 654 covers other related topics such as building construction, deflagration venting, ignition sources, fugitive dust control and housekeeping, fire protection, training, inspection, and maintenance.
The 2006 edition of NFPA 654 defines a combustible particulate solid as "any combustible solid material, composed of distinct particles or pieces, regardless of size, shape, or chemical composition."
The 2006 edition of NFPA 654 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."
Why the distinction between combustible particulate solid and combustible dust? Even though a combustible particulate solid might not ignite readily or be capable of being suspended in air in its particulate form (the pieces could be too large), the material can break down during such activities as shipping, handling, conveying, mixing, and pulverizing. It will then become an immediate hazard in its dust form if a source of ignition is available.
NFPA 654 HIGHLIGHTS
Here are some key topics that you should be aware of with respect to pneumatic conveying and dust control exhaust systems. These topics and others are covered in more detail in NFPA 654:
- Returning cleaned exhaust air from filter receivers and dust collectors to the work area needs to be done carefully. Filtering efficiency must be at least 99.9% at 10 microns. Most importantly, the transmission of energy from a fire or explosion shall be prevented. Refer to NFPA 68, Guide for Venting of Deflagrations and NFPA 69, Standard on Explosion Prevention Systems for more details.
- Fire and explosion protection for process equipment shall include one or more of the following methods of protection:
a) Oxidant concentration reduction in accordance with NFPA 69.
b) Deflagration venting in accordance with NFPA 68.
c) Deflagration pressure containment in accordance with NFPA 69.
d) Deflagration suppression system in accordance with NFPA 69.
e) Dilution with a non-combustible dust to render the mixture non-combustible.
- Engineering, design and installation parameters for pneumatic conveying systems including fugitive dust control systems shall be documented.
- Additions of dust control system branch lines shall not be made to an existing system without redesigning the entire system. Branch lines shall not be disconnected nor shall unused portions of the system be blanked off without providing a means to maintain required and balanced airflow.
- Where an explosion hazard exists, systems shall be designed in such a manner that combustible material does not pass through the fan.
- Regular cleaning frequencies shall be established for floors and horizontal surfaces, such as ducts, pipes, hoods, ledges and beams to minimize dust accumulations within occupied and unoccupied areas of the facility.
- Surfaces shall be cleaned in a manner that minimizes the generation of dust clouds.
- All system components shall be conductive to dissipate static electricity.
Inspection and Maintenance
NFPA 654 requires an inspection, testing and maintenance program to ensure that the fire and explosion protection systems and related process controls and equipment perform as designed. Here are some areas to be included in the program:
- Fire and explosion protection and prevention equipment in accordance with applicable NFPA standards. While most are familiar with the required testing of explosion suppression systems, few plants realize that a periodic check of the operation of mechanical explosion vents needs to be done.
- Pneumatic conveying and dust control equipment, is this equipment operating as designed?
- Safe housekeeping methods to clean combustible dust that has settled onto the various surfaces in the plant. Dust layers 1/32" thick can be sufficient to require immediate cleaning of the area.
- Potential ignition sources such as static electricity. Bonded and grounded piping, ductwork and other system components and production equipment need to be tested, not just looked at.
- Electrical, process and mechanical equipment, including process interlocks.
- Process changes. For example, if the process has changed so that the particle size or shape of the dust has changed, dust explosibility may be affected. If the chemistry of the processed product has changed, dust explosibility may again be affected. Periodic explosibility testing of your solids and particulate is a must.
- Lubrication of bearings. If bearings are not lubricated and worn, heat can be generated and ignite a combustible dust.
NFPA 654 requires that records be kept of maintenance and repairs performed. It is interesting to note that the inspection and maintenance requirements required by NFPA 654 for safety reasons have additional benefits. These benefits can include:
- Less process downtime.
- Greater plant morale and productivity.
- Reduced maintenance costs.
- More effective dust control for a healthier environment and improved product quality.
- Good public relations.
The Engineers Collaborative is the dust control and industrial ventilation expert that you need on your project!
Contact us today at (215) 300-9563, to find out more about how we can help you with your industrial ventilation or dust control review, analysis, or design.