In today’s episode of the Dust Safety Science podcast, we discuss the different methods used to prevent and mitigate dust explosions.
This topic was inspired by Chapter 1.4 of Dust Explosions in the Process Industries (Third Edition) by Dr. Rolf K. Eckhoff. Titled ‘Means for Preventing and Mitigating Dust Explosions,’ it contains a high-level summary of options that can be applied in various settings.
Dr. Eckhoff broke these strategies into three categories:
- Prevention of ignition sources like smouldering combustion, open flames, hot surfaces, electric sparks, arcs, electrostatic discharges and heat from mechanical impact.
- Prevention of explosible dust clouds through inerting gases and dust that keep dust concentration outside the explosible range.
- Mitigation strategies such as inerting, isolation, venting, pressure-resistant construction, explosion suppression and housekeeping.
Prevention of Ignition Sources
This category is divided into two subcategories: organizational and operational.
Organizational Ignition Sources
Organizational ignition sources cover common hot work activities like smoking, open flames, open lights, welding, cutting and grinding.
Operational Ignition Sources
Operational ignition sources include hot surfaces, self-heating and smouldering nests, exothermic decomposition, mechanical impact, electrostatic sparks, arcs and discharges.
It’s critical to understand the properties of the material in your facility and how it heats, smoulders, decomposes and burns. The moisture content of the material and the temperature at which it is stored need to be controlled, along with the cycling and cooling processes. Temperature monitoring and, in some cases, inerting of storage areas or removing a foreign object can all prevent or control smouldering combustion.
The second ignition source Dr. Eckhoff mentions is open flames and hot gases. A lot of these are going to be organizational (hot work, torch-cutting, welding) but there are some operational ones too. For example, your plant layout should not have an open burner next to a grain dust dump station.
The next ignition source is hot surfaces like motor casings, internal components, bearings, anywhere where there is potential for mechanical rubbing. With these potential hazards, you need to understand the minimum layer ignition temperature of the materials being worked with and keep those hot surfaces below those temperatures. Thermal imaging cameras, videos, and guns can check heat levels and indicate when temperatures may be too high.
Three more ignition sources mentioned in the chapter are:
- Conveyance of Smouldering Nests. This happens when material is, for example, stuck on a dryer and not transported. It becomes overheated and, when finally released, moves downstream as a hot nest that can ignite an explosion. With these hazards, you should address any design issues that cause material to get stuck and have a prevention protection system downstream as well.
- Moving Equipment. Rotating drums, elevator bucket legs, and similar equipment can rub against the side of a casing, so look into adding alignment sensors and temperature sensors. It’s also a good idea to implement systems that remove foreign objects like tramp metal or broken screws.
- Electrostatic Sparks, Arcs and Discharges. Any material has minimum ignition energy when dispersed as a cloud. It’s important to test your dust, understand its minimum ignition energy, and determine whether or not electrostatic sources like sparks, arcs and discharges, can ignite that material. If they can, you will have to determine how to best minimize the danger. Options may include grounding and bonding and similar electrostatic safety systems.
Prevention of Explosible Clouds
Dr. Eckhoff talks about preventing explosible dust clouds through inserting systems like gas and dust. Common inerting gases include carbon dioxide, water vapour, flue gases, and nitrogen while coal dust is commonly used in mining applications.
Having an inerting system requires you to be careful about ignition sources because if they are present and active when your inerting system goes down, they will ignite the surrounding material.
Other prevention strategies include keeping the dust concentration outside the explosible range. Systems like ducting can keep flow velocities high enough that there won’t be enough dust accumulation to cause or lead to an explosion.
Mitigation
Mitigation strategies kick in when an explosion actually starts. There are a number of options that include:
- Preventing explosion transfer between processing units, otherwise known as isolation. This system can prevent the pressure piling that leads to violent explosions. Options include passive devices like abort gates and flap valves and active devices for stopping explosion propagation.
- Explosion-pressure resistant equipment. A good estimation of the equipment’s strength is important to avoid deformation or damage when an explosion occurs and to prevent the incident from spreading to other pieces of equipment.
- Explosion venting. Vent areas must be designed to allow the pressure from the explosion to be vented, preventing it from damaging, deforming, or knocking over the vessel.
- Quenching tubes. A preliminary form of flameless venting, quenching tubes may apply protection to applications where vent ducts are difficult to implement.
- Automatic detection and suppression system. This is a sensor that detects when a flame is passing and activates an extinguishing agent.
- Good housekeeping practices. Keeping control over fugitive dust can avoid a secondary explosion outside the processing facility. Cleaning schedules and good clean-up practices can minimize the hazard.
- Construction and layout of buildings. The ideal factory should be located at a safe distance from other buildings. If the building has multiple levels, any hazard risk should be as high as possible- preferably on the roof. This way, the entire building doesn’t collapse due to an explosion on the bottom floors.
Conclusion
Dust explosions continue to be a risk in multiple industries. Understanding your risks and applying the right prevention and mitigation strategies can result in a safer and more productive facility.
If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.
Resources mentioned
Dust Safety Science
Combustible Dust Incident Database
Dust Safety Science Podcast
Questions from the Community
Dust Safety Academy
Dust Safety Professionals
Publications
Eckhoff, R. (2016). Dust explosions in the process industries: Identification, assessment, prevention and mitigation. Elsevier Butterworth-Hein.
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