Deflagration Prevention and Protection in Biomass Facilities

Explosions in the biomass industry are constant threats that have the potential to harm employees, stop production or destroy an entire facility
By Geffrey Mitchell | July 10, 2018

Explosions in the biomass industry are constant threats that have the potential to harm employees, stop production or destroy an entire facility. Combustible dust-fueled deflagrations have caused hundreds of injuries, years of down time and major reconstruction at multiple facilities, adding up to millions of dollars in injury claims, lost production and construction costs. In many of the facilities affected by these incidents, the threat was known but not addressed, since an incident had not yet happened. Nearly all of these tragedies could have been prevented with a comprehensive combustible dust prevention and protection plan. With one in place, a facility will be prepared to minimize the risk of a deflagration incident happening, or, if one does, be able to mitigate its effect on the safety of personnel, damage to the facility and costly downtime. 

The mills, hoppers, bins, dust collectors, cyclones and driers that are used in the biomass industry often contain, create or convey combustible dusts in sufficient quantities to be considered a hazard. If an ignition source is introduced into that volume, all of the ingredients for a deflagration are present. Once a deflagration is started, it can rupture the vessel it originated in, sending pressure and flame fronts through the duct work and into the greater building volume. The initial explosion will also disrupt nuisance dust settled onto horizontal surfaces such as rafters, ducts or pipes. Once the flame front and the airborne nuisance dust meet, an extremely hazardous secondary explosion can occur in the facility.

The first line of defense in protecting against combustible dust hazards are incorporating prevention methods, a mix of actions and solutions that can help diminish the chance of a fire or deflagration from happening. Below are the common methods of deflagration prevention used in biomass facilities.

Dust Hazard Analysis: The National Fire Protection Association Standard 652, Fundamentals of Combustible Dusts, discusses how and why performing a DHA for biomass facilities is crucial in determining where the risks are located, and where a prevention and protection plan should focus.

NFPA 652, as well as other industry-specific dust standards require facilities to conduct both an initial DHA to identify these threats, as well as follow-up assessments on either a 3- or 5-year cycle. 

Housekeeping: Keeping the facility free from nuisance dust on all horizontal surfaces can dramatically decrease the risk of secondary explosions occurring outside of process vessels. NFPA 652 states that any dust accumulation that exceeds one-eighth inch, or prevents one from discerning the color of the surface below the dust, is an immediate hazard and shall be cleaned immediately.

Management of Change: Whenever a change is made within a process or facility, it is crucial to perform an assessment to ensure that the change does not affect the current prevention and protection strategy. 

Ignition Control: Minimizing ignition risk is crucial to any plant deflagration prevention strategy. Some potential ignition sources inside a biomass facility include electrostatic discharge, overheat conditions from mechanical failure, conveyed sparks or hot particles through ducts, tramp metal, and improperly-rated electrical devices in areas prone to refuge dust. Proper hot work procedures and operator hazard awareness training are a must to minimize the risk of a deflagration from human error.

Deflagration protection is the next line of defense in case prevention methods do not work, due to an upset condition or atypical situation. Installing deflagration protection will allow a facility to start back up after an event much more quickly than if there were no protection. It is much quicker and easier to replace or refill protection components than it is to rebuild a manufacturing process or facility. The following are typical means of deflagration protection in biomass facilities.

Deflagration Venting: Venting a deflagration from a process vessel allows the pressures and flames created during a deflagration to exit the vessel before the vessel strength is exceeded, leading to an explosion. Venting of process vessels is discussed in NFPA 68.

Deflagration Suppression: Suppressing a deflagration in a process vessel works by detecting the very early pressure increase during a deflagration and using high-rate discharge extinguishers to introduce a calculated amount of dry chemical agent into the volume, to stop the flame front from growing. This extinguishes the deflagration in milliseconds, before it overcomes the strength of the vessel. NFPA 69 Chapter 10 discusses Deflagration Suppression.

Deflagration Isolation: Using deflagration isolation on interconnecting duct work is a crucial part of any deflagration protection strategy. If a deflagration happens in one vessel, it can easily travel through the interconnecting ductwork to other vessels or areas of the plant, and cause secondary explosions that are much more energetic that the original deflagration. Deflagration isolation can be achieved using passive explosion isolation valves, dry chemical isolation, or fast-acting knife gate valves, depending on the process being protected. NFPA 69 Chapters 11 and 12 discuss deflagration isolation.

Although the presence of combustible dust is always going to be common in a biomass facility, dust explosions do not have to happen. Adapting comprehensive deflagration prevention strategies to minimize the risk of an explosion starting, and implementing explosion protection in higher-risk vessels, will help minimize the threat that an explosion will happen at your biomass facility.

Author: Geffrey Mitchell
IEP Technologies, Western Regional Sales Manager