Dust Explosion Injury Investigations Expert Article

In this article, Industrial Hygienist and Workplace Safety Expert, Ronald Schaible, CIH, CSP, CPE provides an introduction to the hazards of manually transferring powders into industrial process vessels. His discussion includes an explanation of the hazards as well as industry safety practices for protecting workers.

Hazards Associated With Manually Transferring Powders

Manual material handling in industry is a common practice. In many processes, raw materials can be in the form of “fine” (i.e., very small particle size) powders that that are scooped or ladled, or poured or “tipped” from bulk containers such as bags (made of plastic, paper, fiberboard or other materials), drums (made of plastic, metal or other materials, with or without plastic liners), and carboys. Bulk powder types that may be handled/transferred manually include:

• Coke dust
• Coal dust
• Cotton dust
• Food & Beverage Industry products – garlic powder, sugar, grains, dry flavorings
• Powdered metals, plastics, and resins

If the process is not carefully controlled and the powder has combustible characteristics, ignition sources such as static electricity (among others) can be generated from material flowing against a surface in a manner that can cause flammable atmospheres, burn injuries, or explosions, which under the right conditions can injure workers and/or cause property damage.

Dust Explosions

The Dust Explosion Pentagon (shown below) illustrates the 5 key ingredients that need to concurrently exist in an industrial facility for a combustible dust explosion to occur.

Though some incidents involve a single explosion, it is more common for a series of deflagrations to occur. The initial explosion can dislodge ignitable dust hidden on overhead surfaces or other areas over a large area and trigger secondary explosions that can be ignited from the initial explosion or from other ignition sources. It is these secondary explosions that have historically caused the majority of injuries and damage to property.

Focused and Informed Forensic Analysis

In the process of investigating a dust explosion, forensic investigators will seek to determine if facility operators followed applicable industry standards and best practices including:

• Explosion venting: A designed “weak link” in a dust collector that vents excess pressure & the flame front to a safe area.
• Flameless venting: Installed over a standard explosion vent to extinguish the flame exiting the vented area, preventing it from exiting the device.
• Passive float valves: Installed in the outlet ducting of a dust collector it prevents flames from propagating further upstream through the ducting.
• Back draft dampers: Held open by process air, this mechanical barrier slams shut by the force of an explosion’s pressure & prevents flame propagation further upstream through the ducting.
• Flame front diverters: These devices divert the flame front to the atmosphere and away from the downstream piping.
• Chemical isolation: A system that detects and suppresses flame propagation using chemical agents.
• Chemical suppression: A system that detects and suppresses explosions in milliseconds using chemical agents.
• Fast acting valves: Mechanical barriers in ductwork that react within milliseconds to isolate pressure and flame fronts from propagating further through the process.

Hazard Control

A well-established and accepted approach for the control of hazards is the “safety hierarchy.” This thought process provides the basic measures for preventing accidental injury, in order of effectiveness and preference:

1. Eliminate the hazard from the machine, method, material, or plant structure.
2. Control the hazard by enclosing or guarding it at its source.
3. Train personnel to be aware of the hazard and to follow safe job procedures to avoid it.
4. Prescribe personal protective equipment for personnel to shield them against the hazard.

In forensic investigations, notice can sometimes be evaluated through the application of the safety hierarchy. In examining the hazard of a dust fire or explosion, all four (4) of these measures require careful attention. The act of transferring fine powders into a process vessel frequently results in the formation of a dust cloud, which is a well-recognized hazard.

Elimination or minimization of the dust hazard and flammable atmosphere is the preferred approach. In addition to knowing the characteristics of the powder being used, potential ignition sources and static electricity must be identified and adequately controlled. This can be done by bonding and grounding conductive buildings and equipment. Often forgotten is the need to periodically evaluate (“test”) the reliability of bonding and grounding systems over time. It may also be possible to use local exhaust ventilation to remove dust from the process or to use inert gas in a process vessel.

The use of proper conductive flooring materials is a useful control measure. However, if shrink wrap or other nonconductive packaging materials come between a worker’s feet and the flooring then static electricity charges can accumulate and an explosion may occur.

The use of appropriate personal protective equipment (PPE) such as static dissipative footwear, gloves and full-body garments worn during the process are also variables to be evaluated and used as appropriate to the process. The importance of worker training to appreciate and understand combustible dust hazards during manual pouring/handling tasks must also be part of the process hazard control program.

Although a common and seemingly simple task that occurs in many industries, facility operators and other safety professionals know or should know that manual transfer of fine powders can be potentially hazardous. A post-incident investigation by a qualified forensic expert(s) can determine if industrial processes and systems incorporated reasonable and required measures to avoid flammable atmospheres and eliminate potential ignition sources.

Workplace Safety Investigations

The multidisciplinary workplace safety group at Robson Forensic includes professional engineers, hazardous material management professionals, occupational safety specialists, toxicologists, and certified industrial hygienists. We evaluate complex human health and safety issues including chemical and biological exposures, industrial machine mishaps, and a variety of other workplace safety issues.

For more information, visit our workplace safety practice page.

Featured Expert

Ronald D. Schaible, CIH, CSP, CPE

Industrial Hygienist, Workplace Safety & Ergonomics Expert

How do you match the experience of an individual who developed and deployed the global health and safety management system for a Fortune 500 company with 250 locations in 50 countries? Or one that has… read more.