Combustible Dusts and Dust Explosions

Summary

This study has been prepared to explain the dynamics of dust explosions that may occur in industry, to emphasize the measures to be taken, and to inform practitioners about an example risk assessment in a section of a flour mill.

1. Introduction

Many dusts are combustible, meaning they can burn and cause fire and explosion. These are dusts of substances with organic composition and combustible metals. When fine dust particles are dispersed in the air in cloud form, they can burn rapidly and even cause explosion if an ignition source is present in the environment.

How Are Workers Affected?

In almost every line of work, workers engaged in production, particularly those carrying, processing, polishing, clumping or shaping various workplace materials, may be exposed to combustible dust. Workers may also generate combustible dust when they are involved in abrasive blasting, cutting, crushing, mixing, sweeping or sieving dry materials.

The Most Common Combustible Dust Sources Are:

• Agriculture. Fertilizer, grain, etc. • Food products and beverages. Grain, sugar, flour, etc. • Mechanized production. Woodworking, textiles, metals, plastics, auto tyres, etc. • Coal mining and processing. Coal. • Chemical production. Pharmaceutical industry, etc.

Risks

When dusts are dispersed in cloud form in the atmosphere and encounter an ignition source in the environment, they can burn and explode. Due to unsafe personnel movements, an initial explosion, etc., dusts accumulated on the ground or horizontal surfaces are dispersed in cloud form into the enclosed space atmosphere, and the presence of an ignition source is sufficient to trigger a combustion and explosion reaction. For an explosion to occur, the following five factors must be present in the environment: fuel, dispersion (dust entering the air), oxygen, ignition, and an enclosed space.

Figure 1. Dust explosion pentagon

The simultaneous presence of all five factors above can cause combustion and explosion. Following the occurrence of a combustible dust explosion, one or more explosions follow. The initial explosion aerosolizes dusts accumulated on surrounding surfaces and creates a dust cloud. This dust cloud can cause a second explosion; this typically begins almost immediately after the first and subsequently the process continues in a chain of explosions.

A Warning on Combustible Dust Fires

Unless workers are properly trained, they should not attempt to combat a combustible dust fire. If fire-fighting is applied improperly, accumulated combustible dust is also dispersed into the atmosphere and subsequently explosion and fireball occur. Instead, an alarm is sounded and pre-established emergency and evacuation procedures are followed.

How Are Risks Reduced?

Dust control is the most important factor in preventing fire and explosions in any dust-generating workplace. Effective dust control can reduce the risk of catastrophic fire and explosion in the workplace. However, if a dust control system is not properly designed or is not functioning, it can create conditions that support dangerous fire and explosion. The most effective way to reduce risks related to combustible dust is to eliminate the source. If this cannot be achieved, you can use other risk controls. When selecting risk controls, begin by asking yourself the questions that appear in the following steps, listed in order of decreasing effectiveness.

1- Elimination or Substitution

The safest control possible is to eliminate the hazard by using a safer process or material. Some questions to consider: Can a different material that does not produce combustible dust be used? Can a different process that does not produce dust be used?

2- Engineering Controls

Physical changes are made in facilities, equipment and processes to reduce exposure. Some questions to consider: • Was your dust collection system properly designed by an engineer? • Are dusts being captured at the source? • Are dust collectors placed outdoors? • Do dust collectors have appropriate explosion venting? • Can ignition sources be minimized and can sparks or flames be contained? • Are dust and product conveying and transport systems designed to minimize dust dispersal? • Can steam be used to prevent dust dispersion? • Are appropriate local and general ventilation applied? • Are explosion pressure relief dampers positioned and installed correctly?

3- Administrative Controls

Work practices and policies should be modified and awareness tools and training should be provided to limit combustible dust risk. Some questions to consider: • Has a dust management program been developed that includes a risk assessment of all work areas where combustible dusts are generated or may accumulate, all possible ignition sources, and all dust dispersal means? • Are dust accumulations properly controlled, measured and monitored? • Are there effective cleaning and housekeeping programs? • Have workers received proper induction training and on-the-job training? • Are emergency preparedness plans in place? • Have safe work procedures been developed and implemented to minimize fire and explosion risks?

4- Personal Protective Equipment

Personal protective equipment is not an effective control measure for combustible dust explosions. However, where dust is generated, there may be other hazards to evaluate. Some questions to consider: • Do workers have appropriate respiratory devices, eye protection and protective clothing? • Have personal protective equipment been tested to ensure proper function? 5- Standards Related to Dust Explosions (a) NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids (b) NFPA 61, Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities. (c) NFPA 484, Standard for Combustible Metals. (ç) NFPA 664, Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities. (d) NFPA 655, Standard for the Prevention of Sulfur Fires and Explosions. (e) BS IEC EN ISO 60079-10-2, Classification of areas – Explosive dust atmospheres. İlker Erdoğan Chemical Engineer İGU (B), Risk Assessment Document Preparer Technical Protection   Mustafa Cüneyt Gezen Lecturer Üsküdar University Textile Chemical Engineer Occupational Health and Safety, Textile Auxiliary and Dye Group