Ignition sources and static electricity

Even with good dust collection and housekeeping, explosible clouds exist inside equipment during normal operation, and accidents can release dust into rooms. Controlling ignition sources is therefore a core defence. This lesson surveys the main ignition sources in dust-handling facilities, explains how each is controlled, and gives particular attention to static electricity, which is less obvious than a flame or a spark from a grinder but can ignite many dusts. Electrical equipment in hazardous locations is covered separately in Lesson 10.

HSE-216 covers hot work safety and HSE-221 covers permit-to-work systems. HSE-312 covered bonding and grounding for flammable liquid transfers. This lesson builds on those courses and focuses on what is different when the fuel is a dust.

The main ignition sources

Hot work

Welding, cutting, brazing and grinding produce sparks and molten particles that can travel many metres and ignite settled dust or a cloud. Hot work has been a factor in many dust fires and explosions. In a combustible dust area, hot work should be avoided where practical, for example by moving the work to a safe location. When it must be done, a hot work permit should require that dust be cleaned from the area, including elevated surfaces within the spark travel range, that dust-producing equipment be shut down, that openings to ducts and equipment be covered, that the work be watched by a trained fire watch during the work and for a period afterward, and that suitable extinguishers be at hand. NFPA 51B provides the general requirements for hot work, and NFPA 660 adds requirements specific to dust.

Hot surfaces and smouldering

Lesson 3 explained that dust layers can ignite on hot surfaces at temperatures lower than those needed to ignite a cloud. Motors, bearings, lights, dryers, steam pipes, heaters and process equipment can all reach such temperatures, especially if covered by an insulating layer of dust. A smouldering layer can then become an ignition source for a cloud. Controls include keeping surfaces below the temperatures set by the DHA and by equipment marking requirements, keeping dust off hot surfaces through housekeeping, and inspecting dryers and ovens for residue after shutdowns.

Mechanical sparks and friction

Pieces of metal such as bolts, nuts, wire and tools, known as tramp metal, can enter process streams and cause sparks or hot spots in mills, grinders and fans. Magnets, magnetic separators and metal detectors upstream of size-reduction equipment remove much of this metal, and they must be cleaned and checked regularly. Bucket elevators and conveyors can create friction heat if belts slip, buckets rub against the casing or bearings fail. Monitoring for bearing temperature, vibration, belt alignment and speed, with alarms or automatic shutdown, helps detect problems early. The OSHA grain handling standard, 29 CFR 1910.272, includes requirements for such monitoring and maintenance at grain facilities.

Static electricity

Static electricity builds up when materials move against each other and are separated. Powder sliding down a chute, flowing through a pneumatic conveying pipe or pouring from a bag can generate large amounts of charge. If the charge collects on an isolated conductor, such as an ungrounded metal drum, a funnel, a section of duct isolated by a rubber gasket, or a person wearing insulating footwear, the voltage rises until it discharges as a spark. If the energy in that spark is greater than the minimum ignition energy of the dust, the cloud can ignite.

The energy stored in a charged conductor can be estimated with a simple formula:

E = ½ C V²

where E is the energy in joules, C is the capacitance of the conductor in farads and V is its voltage in volts. Capacitance depends on the size of the object and how close it is to ground. A person might have a capacitance of a few hundred picofarads, and a metal drum or a section of duct a few tens to a few hundred picofarads. Voltages of 10,000 volts or more are common on isolated conductors in powder handling. The values in the example below are illustrative.

def spark_energy_mj(capacitance_pf, voltage_kv):
    """E = 1/2 C V^2, returned in millijoules (illustrative)."""
    c = capacitance_pf * 1e-12
    v = voltage_kv * 1e3
    return 0.5 * c * v * v * 1e3

conductors = [
    ("Small funnel", 50, 15),
    ("Person on insulating floor", 200, 10),
    ("Metal drum, ungrounded", 100, 20),
    ("Isolated duct section", 300, 25),
]
mie_mj = 20
for name, c_pf, v_kv in conductors:
    e = spark_energy_mj(c_pf, v_kv)
    flag = "ABOVE MIE" if e >= mie_mj else "below MIE"
    print(f"{name:28s} C={c_pf:3d} pF V={v_kv:2d} kV -> {e:6.1f} mJ ({flag} of {mie_mj} mJ)")

Small funnel                 C= 50 pF V=15 kV ->    5.6 mJ (below MIE of 20 mJ)
Person on insulating floor   C=200 pF V=10 kV ->   10.0 mJ (below MIE of 20 mJ)
Metal drum, ungrounded       C=100 pF V=20 kV ->   20.0 mJ (ABOVE MIE of 20 mJ)
Isolated duct section        C=300 pF V=25 kV ->   93.8 mJ (ABOVE MIE of 20 mJ)

Bonding and grounding

The main control for static on conductors is bonding and grounding. Bonding connects conductive objects together so they stay at the same voltage. Grounding connects them to earth so charge drains away. Every conductive part of a powder handling system, including ducts, flanges isolated by gaskets, hoppers, funnels, drums, scoops, hoses and the frames of equipment, should be bonded and grounded. Grounding clamps for portable containers should be of a type that bites through paint and coatings, and some systems include a monitor that confirms the ground connection before transfer can start. People can be grounded through static-dissipative footwear and flooring. HSE-312 covers bonding and grounding for flammable liquids; the same principles apply to powders.

Insulating materials and discharge types

Plastics and other insulating materials cannot be grounded, because charge does not flow through them. Charged insulators can produce brush discharges, which can ignite flammable vapours and gases and some very sensitive dusts. A more dangerous type, the propagating brush discharge, can occur when a thin insulating layer, such as a plastic liner or coating, sits on a conductive backing and becomes highly charged. Propagating brush discharges can release energy high enough to ignite most combustible dusts. Large silos being filled with highly charged, insulating powders can also produce cone discharges, sometimes called bulking brush discharges, on the surface of the heap. For these reasons, plastic pipes, liners, containers and bags should be avoided in combustible dust service unless they are specifically designed for it. NFPA 77 gives detailed guidance on static electricity.

Flexible intermediate bulk containers

Flexible intermediate bulk containers (FIBCs), often called bulk bags or super sacks, are woven plastic bags that hold up to a tonne or more of powder. Filling and emptying them generates static, and an unsuitable bag can produce an ignition source. FIBCs are classified into four types, according to international standard IEC 61340-4-4.

Self-heating

Some materials heat up on their own through slow oxidation or biological activity. Wood pellets, grain, coal, some oily or chemically reactive powders and residues in dryers can self-heat in storage. If heat is generated faster than it can escape, the temperature rises until the material smoulders. A smouldering pocket inside a silo can ignite a dust cloud when the silo is emptied or disturbed. Temperature probes, carbon monoxide monitoring, limits on storage time and careful procedures for dealing with a smouldering silo are important. Responding to silo fires is a specialist task; Lesson 11 discusses emergency response.