Preventing Dust Explosions
Dust in the air creates a harsh and dangerous environment for both workers and equipment. Following the proper guidelines and regulations can ensure a high degree of safety by preventing unexpected dust explosions.
BY: SIMON FRIDLYAND
One of many types of severe-service environments in which machinery and workers must operate is where dust, powder or contamination fill the air. But there’s more to it than taking steps to keep workers healthy and machinery clean. In such environments, a dust explosion can be catastrophic and cause employee deaths, injuries, and destruction of entire buildings.
Recently — on February 7, 2008 — 13 workers were killed and more than 60 seriously injured in a catastrophic dust explosion at Imperial Sugar in Port Wentworth, GA. In many combustible dust accidents such as this, employers and employees are unaware that a hazard even exists. It is important to determine if your company has this hazard, and if it does, you must take action now to prevent tragic consequences.
Combustible dust (fine particles that present an explosion hazard when suspended in air in certain conditions) creates explosion hazards in a variety of industries. These include agriculture, chemicals, food (e. g., candy, sugar, spice, starch, flour and feed), grain, fertilizer, tobacco, plastics, wood, forest products, paper, pulp, rubber, furniture, textiles, pesticides, pharmaceuticals, tire and rubber manufacturing, dyes, coal, metal processing (e. g., aluminum, chromium, iron, magnesium and zinc), recycling operations, and fossil fuel power generation (coal).
Let’s begin by understanding how explosions occur.
In addition to the familiar fire triangle of oxygen, heat and fuel (in this case, the dust), dispersion of dust particles — in sufficient quantity and concentration — can cause rapid combustion known as a deflagration.
If the event is confined by an enclosure such as a building, room, vessel or process equipment, the resulting pressure rise may cause an explosion. The five factors of oxygen, heat, fuel, dispersion and confinement are known as the ‘dust explosion pentagon’. If one element of the pentagon is missing, an explosion cannot occur.
An initial (primary) explosion in processing equipment, or in an area where fugitive dust has accumulated, may dislodge more accumulated dust into the air, or damage a containment system such as a duct, vessel or collector. As a result, if ignited, the additional dust into the air may cause one or more secondary explosions. These can be far more destructive than the primary explosion due to the increased quantity and concentration of dispersed combustible dust. Many deaths in past accidents, as well as other damage, have been caused by secondary explosions.
Usually, the secondary explosion occurs because there is enough dust accumulation on the floor, on structural elements or on top of equipment.
An immediate cleaning is warranted whenever a dust layer of 1/32-in. thickness (approximately equal to the thickness of a typical paper clip) accumulates over a surface area of at least 5% of the floor area of a facility or any given room. Rough calculations show that the available surface area of bar joists is approximately 5% of the floor area and the equivalent surface area for steel beams can be as high as 10%.
Dust collection systems
The best way to capture dust is to use a dust collection system. A dust collection system is to be used for processes where dust is generated, as well as for dust removal from surfaces. The use of compressed air for cleaning purposes is not permitted under industry regulations.
Special attention must be paid to dust collectors, duct work and other containers, such as mixers or storage silos, because they maintain a cloud of finely divided particles suspended in the air. Because they maintain a cloud of combustible dust, a sound ignition control program that prevents introduction of ignition sources into them (including sparks from electrostatic discharge, open flames or other similar sources) must be maintained by plant personnel.
Additionally, housekeeping problems may be exacerbated by the inefficient operation of dust collectors. Dust collectors generally operate most effectively between limited pressure drops of 3 in. to 5 in. of water.
Since, in many instances, dust collectors, storage bins, etc., are connected to other pieces of equipment as integral parts of a process, there is a danger of explosion propagation into the next piece of equipment. Therefore, explosion detection, isolation and suppression techniques must be used.
There are several examples of explosion isolation systems, including flame arresters, flame front diverters, spark detection devices, spark extinguishing equipment, and rotary valves or chemical types of explosion isolation. Explosion detection and suppression systems must be tested and certified by a third-party certification organization recognized by the Standards Council of Canada.
It takes only 50 milliseconds for a dust explosion to become fully developed. Systems must be capable of detection, suppression and isolation within a 50-millisecond period.
As an alternative, once the explosion is isolated, it can also be safely vented outside by using explosion venting techniques. These explosion venting techniques may apply to a building, or equipment such as dust collectors, silos, etc.
Since the consequences of handling combustible dusts/powders could be deadly, the Ontario Occupational Health and Safety Act mandates that the owner or lessee of the processes where dusts/ powders are handled obtain a Pre-Start Health and Safety Report bearing the seal and signature of an Ontario Professional Engineer, stating compliance to current and applicable standards.
When designing or reviewing combustible dust/powder-related applications, a professional engineer must be knowledgeable in the following areas: Canadian electrical code, including hazardous location classification issues; industrial hygiene; heating and ventilation; dust collection systems and pneumatic transfer; building and fire codes; NFPA standards; and Factory Mutual documents.
With this knowledge and the application of the appropriate standards, disasters can be prevented in most industrial facilities.
Simon Fridlyand, P. Eng., is president of S. A. F. E. Engineering Inc., a Toronto-based company specializing in industrial health and safety issues and PSR compliance. For more information, visitwww.safeengineering.ca.