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Metal Dust Explosion Protection for Safety Managers

Metal dust explosions are different from typical combustible dust explosions and can be more difficult to control. The flame temperature of some metal dusts can exceed 3,300°C which can limit the possible mitigation strategies. The flame speed and combustible characteristics (Pmax and Kst) can also be much higher for metal dusts compared to organic materials. See the Pmax and KSt values of metal dust, offered by NFPA 68.

Metal Dusts Properties

Material Mass Median
Diameter (μm)
Minimum
Flammable
Concentration
(g/m3
)
Pmax (bar) KSt (bar-m/sec)
Aluminium 29 30 12.4 415
Bronze 18 750 4.1 31
Iron carbonyl <10 125 6.1 111
Magnesium 28 30 17.5 508
Zinc 10 250 6.7 125
Zinc <10 125 7.3 176

Metal Dusts Hazards

The finest metal powders are referred to as ultrafine metal powders, which can be prone to spontaneous ignition when exposed to air due to their extremely small particle size and surface area.

Metal dust deflagrations are often associated with more severe consequences in terms of human loss and financial impact. Aluminium dust deflagrations, especially, produce high temperatures and elevated explosion pressures which can cause severe burns and heavy damage. Metal dusts can also react with water to produce hydrogen and create a very reactive hybrid gas-dust mixture.

Dust explosion hazards must be assessed and controlled to protect people, processes, and operations. Because of the greater risks and the growing number of incidents, standards are becoming more stringent for plants handling metal dusts. In the United States, dust explosion prevention and protection are addressed in a few of the NPFA Standards:

  • NFPA 68 “Standard on Explosion Protection by Deflagration Venting”
  • NFPA 69 “Standard on Explosion Prevention Systems”
  • NFPA 484 “Standard for Combustible Metals”

Metal processing facilities generate metal dust during a wide range of operations including cutting, welding, milling, and grinding. Hazards in these process areas should be addressed through a dust hazard analysis.

After identifying the hazard, having the specific dust tested, is critical to understanding the risk of handling the metal material. Both prevention and mitigation methods may be needed to provide safeguards to manage the metal dust hazard. Common mitigation methods for metal dust deflagrations are venting, suppression, and isolation.

Metal Properties – Key Temperatures

Material Melting Point (°C) Boiling Point (°C) Solid Metal Ignition (°C) Max. Adiabatic Flame
Temperature Melting Point (°C)
Aluminium 660 2452 555 3790
Barium 725 1140 175
Boron 2300 2550 3030
Calcium 824 1440 704
Chromium 1857 2672 2900
Copper 1085 2567 1250
Hafnium 2223 5399 4580
Iron 1535 3000 930 2220
Lithium 186 1336 180
Magnesium 650 1110 623 3340
Manganese 1246 1962
Molybdenum 2617 4612 2390
Nickel 1453 2732 2130
Niobium 186 4927 3270
Plutonium 650 3315 600
Potassium 1246 760 69
Silicon 1410 2355 2970
Sodium 98 880 115
Strontium 774 1150 720 1980
Tantalum 2996 5425 3490
Thorium 1845 4500 500
Titanium 1727 3260 1593 3720
Tungsten 3422 5660 2830
Uranium 1132 3815 3815
Zinc 419 900 900 1800
Zirconium 1830 3577 1400 4690

Venting

Explosion venting is an effective and simple method to mitigate a dust explosion. Venting enables pressure that has developed during a deflagration to be released safely, which in effect prevents the process enclosure from rupturing.

Explosion venting for metal dust applications offer several features:

  • A low static burst pressure allows the vent to open in the earliest stages of a deflagration to limit pressure generation and exhaust the flame
  • The strong design prevents fragmentation during the venting process
  • Domed and composite explosion vents provide a reliable solution for different types of process equipment

Large scale testing under realistic deflagration conditions shows venting is a reliable solution.

Suppression

Explosion suppression helps control deflagrations, by absorbing the energy created during the combustion reaction. Explosion suppression systems include a pressure sensor, a control panel, and a high rate discharge (HRD) suppressor. When pressure waves created by a deflagration are detected, the suppressant discharge is initiated to extinguish the fireball. The temperature of the combustible dust is reduced.

Isolation

Explosion isolation is used to prevent flame and/or pressure from propagating from one piece of equipment to other parts of the process. Isolation is difficult to achieve when explosion pressure and flame speed are high, which is common in metal dusts. Isolation can be accomplished with mechanical systems that interrupt or block the passage of flame, such as with the  Interceptor®-FV®, and also with suppression methods that extinguish the flame and prevent ignition of the unburned fuel. Isolation system placement is essential to success; equipment needs to be in the correct proximity to the initial deflagration to be effective.

Expert Understanding of Dust Explosibility

Understanding dust explosibility is vital to properly characterize the metal dust risk and to offer adequate explosion protection solutions. The type of metal, particle size and shape, and the degree of oxidation are characteristics that affect the risk a plant or process is undertaking. CV Technology has managed industrial processes handling metal dusts and has a total solution against fires and explosions, including venting, suppressing, and isolation. CV Technology is a proven leader, equipped to help customers protect their people and process from catastrophic losses.