This breakaway central vacuum can move 10,000 pounds of powder in an hour from 30 feet away if needed. (Air Cleaning Technology photo)

Combustible Dust Vacuums Save Lives and Property

Implementing a housekeeping routine to mitigate combustible dust minimizes explosion risk.

According to a 2012 report by the NFPA, there were an estimated 8,600 structural fires reported to U.S. fire departments each year at industrial or manufacturing properties between 2006 and 2010. Dust, fiber, or lint (including sawdust) accounted for 12 percent of the items first ignited, just behind flammable or combustible liquids and gases, which topped the list at 13 percent. When fugitive dust is unchecked, these fires can quickly escalate into catastrophic secondary dust explosions, causing devastating injury, death, and property damage.

Shocking cases like the Imperial Sugar dust explosion that injured 42 and killed 14 and the Kunshan Zhongrong Metal Products explosion in China last August that killed 146 people stand out most in the public eye, but there have been 57 combustible dust incidents in the United States from 2009 to 2013 in which 26 people died and 129 were injured—and many of those who do survive suffer catastrophic injuries.

While some explosions are caused by a blatant disregard for human health and safety, like the three Hoeganaes explosions in a year1 that killed five workers and injured three, the NFPA asserts that many dust explosions occur due to a lack of knowledge, concluding from investigative findings that "owners/operators appear to be unaware of the hazards posed by combustible particulate solids that have the potential to form combustible dusts," as reported in the March April 2015 NFPA Journal.2

Compliance
There is a noted lack of solid regulation, and thus awareness, regarding the handling of fugitive dust for general industry, including food products, rubbers, metal, wood, pharmaceuticals, plastics, paint, coatings, and synthetic organic chemicals, which is what the soon-to-be-released NFPA 652, Fundamentals on Combustible Dust, hopes to address.

In addition, the NFPA is also in the process of reviewing and modifying standards NFPA 61, Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities, NFPA 484, Standard for Combustible Metals, and NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids.

The current NFPA standards included in OSHA's Combustible Dust National Emphasis Program (NEP) are NFPA 654, 61,484,664, Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities and 655, Standard for Prevention of Sulfur Fires and Explosions. Except for NFPA 61 and 664, which address combustible metals and food/agriculture products respectively, fugitive dust control and housekeeping standards are generally the same for manufacturing, processing, and handling of combustible particulate solids, wood processing and woodworking facilities, and also for sulfur.

Housekeeping standards for combustible dust call for the use of industrial vacuum cleaners designed for Class II Division 2 environments to regularly remove and minimize dust accumulation on walls, floors, and horizontal surfaces such as equipment ledges, above suspended ceilings, and other concealed surfaces. When disturbed, these accumulations of dust become airborne, forming a dust cloud with devastating potential.

Controlling the Explosion Pentagon
The explosion pentagon includes the three elements of the fire triangle, fuel (combustible dust), ignition source (heat), and an oxidizer (air), but it needs two additional elements: dispersion of dust particles (in sufficient quantity and concentration) and the confinement of the dust cloud (vessel, area, or building).

If one of the elements is missing, a fire or explosion cannot occur. While it is difficult to remove air and fuel from the triangle, the first rule of fire prevention, and therefore explosion prevention, is to eliminate the ignition source. While most machinery manufacturers design equipment with safety in mind, mechanical equipment is capable of malfunctioning, heating up, and causing ignitions.

In some plants, every precaution is taken to eliminate ignition sources to prevent fires and dust collection equipment is implemented to safely contain most of the dust in the plant; however, fugitive dust is often overlooked, and the regular removal of accumulations that can form into dust clouds must occur to prevent dust explosions.

Controlling Fugitive Dust
According to most NFPA standards regarding combustible dust, vacuuming is the preferred method for removing the dust. Whether they are central vacs or portable vacuums, they also need to meet NFPA 77 requirements for grounding and bonding. These vacuums also meet the definition of an "intrinsically-safe system."

Industrial vacuum cleaners to control fugitive combustible dust should be suitable for use in Class II Division 2 areas. Vacuum cleaners in particular can be vulnerable to ignition, and that is why choosing a manufacturer that designs and builds its vacuum cleaners from the ground up to be explosion proof, ensuring there is no chance for the product to come into contact with anything ignitable, is essential. Any time there is powder flowing in one direction through a plastic vacuum-cleaning hose, it can create a significant static electric charge. In addition, there is the possibility that there may be static electricity buildup on individual dust particles. If a charged, ungrounded hose used to vacuum combustible dust powder were to contact an object that was grounded, the static electricity could then arc and trigger a violent explosion. This is why OSHA has issued numerous citations for using standard vacuum cleaners where Class II Division 2 equipment is required.

Employing an industrial vacuum cleaner redundantly grounded in five different ways eliminates the possibility of any kind of explosion from the vacuum. The first of the five ways that vacuums should be grounded begins with the air line that supplies the compressed air to the units. Because most plants have compressed air lines made from iron that conducts electricity, air-operated vacuums use static conductive high-pressure compressed air lines. In addition to the static conductive air lines, static conductive hoses, filters, and casters further reduce risk. A grounding lug and strap that travels from the vacuum head down to the storage container eliminates the potential for arcing.

When implementing a central vacuum system, NFPA standards call for vacuum cleaners to be fixed pipe suction systems with a remotely located exhauster and dust collector. When flammable gases are present, vacuum cleaners need to be listed for Class I and Class II hazardous locations.

It is best to work with a central vacuum cleaner manufacturer that has extensive experience with combustible dusts and will perform tests on your materials to ensure the system will work as specified.

Portable Combustible Dust Vacuums
For many facilities, central vacuum systems are cost prohibitive; however, there are several portable vacuum cleaners designed for use in Class II Division 2 areas that are excellent solutions to removing fugitive dust. NFPA 654 allows bulk storage containers to remain inside as long as they are less than 8cubic feet.

The most economical solution for cleaning combustible fugitive dust is with air-operated vacuums. Beyond the fact that air-operated vacuums use no electricity and have no moving parts, air-operated industrial vacuums for combustible dust are safer in terms of grounding. They also work more efficiently in the industrial environment. One thing to look for when choosing a vacuum for combustible dust is ease of use and sufficient sucking power.

If filters bind, suction diminishes. Some designs require removal of the vacuum head to tap caked material off the filter before the unit will regain suction. If equipment isn't easy to use, operators may not take the task to completion, leaving enough material for a potential dust explosion. Look for units that have pulse jet filter cleaners that with the push of a button releases dust from the filter and cleaning can resume immediately.

In larger facilities, such as feed mills, a breakaway central vacuum system that meets OSHA's requirements for a combustible dust vacuum could reduce costs significantly. Breakaway central vacuum systems are still portable and have collection containers under 8 cubic feet. However, these systems can be hooked up to individual tubing networks around the facility to reach other floors or areas, breaking away from one tubing network, rolling to the next network, and so on.

The 15HP model can move 10,000 pounds of powder in an hour from 30 feet away if needed. The cause for the higher suction is a result of a positive displacement pump (PD pump) vacuum producer. PD pumps are capable of generating high vacuum and excellent airflow, so they have the ability to pull massive amounts of material over distances.

In one application with a breakaway system, the user was able to clean elevator pits, which are confined spaces where dust tends to accumulate because they are out of sight, without having to enter the confined space. Extension wands rated for Class II Division 2 areas eliminated the need for a three-person team to monitor the air.

For cleanup of truly explosive materials, such as gunpowder, rocket propellant, sodium azide, aluminum powder, and others that can explode if collected in dry form, Submerged Recovery Vacuum Cleaners are available and designed specifically to pick up explosive powders safely. The explosive or hazardous material is submerged under fluid to render it inert. The unique design includes not only a high liquid level safety shut-off, but also a low liquid safety shut-off to prevent vacuum operation if insufficient liquid is in the drum.

Most NFPA guidelines for combustible dust state that a layer of dust the thickness of a paperclip is enough dust to cause a significant secondary explosion. The problem is that it doesn't account for the different values between different dusts--some are more reactive than others, some are more easily suspended into a cloud, and some may be hazardous at half the thickness of a paperclip.

Regardless of how dust tight a process is, fugitive combustible dust will end up in places that it doesn’t belong. One of the main actions a facility can take to minimize the chance of a catastrophic dust explosion is to follow a regular housekeeping routine that eliminates dust buildup before it becomes a threat.

References
1. http://www.csb.gov/csb-releases-final-investigation-report-on-three-accidents-at-the-hoeganaes-iron-powder-facility-in-gallatin-tennessee/
2. http://www.nfpa.org/newsandpublications/nfpa-journal/2015/march-april-2015/features/dust

This article originally appeared in the May 2015 issue of Occupational Health & Safety.

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