Prevention through design
The easiest way to mitigate a combustible dust explosion is to prevent it from ever happening. The engineering design of pulverised solid fuel systems in a cement plant is the best place to start. Every component within the fuel system should be addressed: the mill fan, cyclones, baghouse, ducting, conveying pipes and valves, and storage bins. In the US, NFPA 85, Boiler and Combustion Systems Hazards Code, is the primary source for design, installation, operation, maintenance, and training regarding all pulverised fuel systems. NFPA 69, Standard on Explosion Protection Systems, addresses methods such as: controlling oxidation, combustible concentrations and ignition sources, active and passive isolation, pressure containment, and passive explosion protection. NFPA 68, Standard on Explosion Protection by Deflagration Venting, is solely focused on the design, installation, operation, maintenance, and use of devices intended to vent combustion gases and pressures. NFPA 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, addresses prevention and mitigation measures.6,7,8,9 These NFPA codes and standards are there to prevent combustible dust incidents, lessen their effects if they do occur, and protect workers.
FM Global Property Loss Prevention Data Sheets 7-76, Prevention and Mitigation of Combustible Dust Explosion and Fire, and FM Global Property Loss Prevention Data Sheets 6-24, Coal Pulverizers and Pulverising Systems, provide keen insights into specific loss prevention recommendations.10,11
The FM references are particularly instructive because they address losses that occurred in pulverizers during a 12-year period: a veritable catalogue of ‘what went wrong.’ In many cases all the signs of an impending disaster were present but workers did not take the proper actions. Simple things like inerting a system, shutting down the primary air fan, and isolating the pulveriser were neglected. Startups and shutdowns were particularly problematic because of the accumulation of coal dust in pulverisers.
In Europe, ATEX provides for conformity marking for products that are certified by the manufacturer to meet the requirements of Directive 94/9/EC on equipment and protective systems intended for use in potentially explosive atmospheres.12 The key difference between ATEX and consensus standards like NFPA is that ATEX relies upon manufacturers to certify that their equipment is suitable for use in potentially explosive atmospheres.
Another design measure is the use of explosion, venting.8 By judiciously placing explosion vents throughout a solid fuel system, we can isolate the damage caused by a combustible dust explosion. The isolation won’t prevent a combustible dust explosion but it can significantly limit its destructive effects to people and property.
Lessons learned for the cement industry
All the combustible dust incidents investigated either by the US Chemical Safety Board or OSHA point to two major solutions for the cement industry and both solutions are noted in NFPA 654.9 The first solution is to control the dust. Dust control measures include:
- Minimise the escape of dust from process equipment or ventilation systems.
- Use dust collection systems and filters.
- Utilise surfaces that minimise dust accumulation and facilitate cleaning.
- Provide access to all hidden areas to permit inspection.
- Inspect for dust residues in open and hidden areas, at regular intervals.
- Clean dust residues at regular intervals.
- Use cleaning methods that do not generate dust clouds, if ignition sources are present.
- Only use vacuum cleaners approved for dust collection.
- Locate relief valves away from dust hazard areas.
- Develop and implement a hazardous dust inspection, testing, housekeeping and control programme (preferably in writing with established frequency and methods).
The second major solution for the cement industry is to control ignition sources. Ignition control measures include:
- Use appropriate electrical equipment and wiring methods.
- Control static electricity, including bonding of equipment to ground.
- Control smoking, open flames and sparks.
- Control mechanical sparks and friction.
- Use separator devices to remove foreign materials capable of igniting combustibles from process materials.
- Separate heated surfaces from dusts.
- Separate heating systems from dusts.
- Proper use and type of industrial trucks.
- Proper use of cartridge activated tools.
- Adequately maintain all the above equipment.
The final link
All the codes, standards, and guidance in the world can help, but ultimately it takes leadership to ensure that combustible dust incidents don’t happen and that fuel systems which rely on pulverised solid fuel operate safely. Good leadership can ensure that the appropriate level of training, best education, and clearest communication takes place 24/7.
6. NFPA 85 2011 Edition, Boiler and Combustion Systems Hazards Code, Quincy, MA.
7. NFPA 69 2014 Edition, Standard on Explosion Protection Systems, Quincy, MA.
8. NFPA 68 2013 Edition, Standard on Explosion Protection by Deflagration Venting, Quincy, MA.
9. NFPA 654 2013 Edition, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, Quincy, MA.
10. FM Global Property Loss Prevention Data Sheets 7-76 May 2008 Interim Revision October 2014, Prevention and Mitigation of Combustible Dust Explosion and Fire, Factory Mutual Insurance Company.
11. FM Global Property Loss Prevention Data Sheets 6-24 May 2003 Appended July 2012, Coal Pulverizers and Pulverizing Systems, Factory Mutual Insurance Company.
12. European Commission, Directive 94/9/EC of the European Parliament and the Council of 23 March 1994 on the approximation of the laws of the Member States concerning equipment and protective systems intended for use in potentially explosive atmospheres, 1994.
13. US Chemical Safety Board, US Chemical Safety Board Determines OSHA Response to Seven Open CSB Recommendations on Dust, Fuel Gas, and Process Safety Management to be “Unacceptable;” Board Votes to Designate a Combustible Dust Standard as “Most Wanted,” July 25, 2013 Press Release.
14. US Department of Labor, Occupational Safety and Health Administration, Directorate of Standards and Guidance, Office of Safety Systems, Combustible Dust in Industry: Preventing and Mitigating the Effects of Fire and Explosions, Safety and Health Information Bulletin, SHIB 07-31-2005; updated 11-12-2014.
This is part three of a three-part article written by Rick Bohan for World Cement’s April 2015 issue and abridged for the website. Subscribers can read the full issue by signing in, and can also catch up on-the-go via our new app for Apple and Android.
Read the article online at: https://www.worldcement.com/the-americas/28122015/the-dangers-of-dust-part-three-5/