Process Safety · Lesson 7 of 9 · 12 min read

Dust and powder hazards

Explosion pentagon, MIE, Kst and St classes, resistivity, and the tests that characterise a powder.

Educational overview. The principles here are general and simplified, and the figures and rules of thumb are typical values, not limits for your plant. Decisions need process-specific data, the applicable codes and standards, and a qualified assessment.

The explosion pentagon

A fire needs fuel, oxygen and an ignition source (the fire triangle). A dust explosion needs two more: the dust must be dispersed as a cloud, and it must be confined (a vessel, dryer, silo, room). Remove any one of the five and there is no explosion. Many secondary explosions come from settled dust thrown into the air by a first, small event, so housekeeping is a safety measure.

What decides risk: probability and severity

Probability of ignition	Severity if it ignites
MIE: minimum ignition energy (mJ) MEC: minimum explosible concentration (g/m³) LOC: limiting oxygen concentration (for inerting) MIT / LIT: minimum ignition temperature of a cloud / of a 5 mm layer	K_St: dust deflagration index (bar·m/s) = (dP/dt)_max · V^1/3 P_max: maximum explosion pressure (bar) St classes: St 1 0–200, St 2 200–300, St 3 > 300 bar·m/s

Tests that characterise a powder

Test	Answers
Dust explosion screening (modified Hartmann tube)	Is the cloud explosible at all? (no burning / burning / explosion)
Minimum ignition energy	How easily do sparks and static ignite it?
MIT (cloud) and LIT (layer)	Maximum allowed surface temperature of dryers, motors, lights
Burning behaviour (classes 1–6)	Does a layer just char, glow, or spread fire rapidly?
Powder volume resistivity	Will the bulk powder hold electrostatic charge?
Impact sensitivity (fall hammer)	Can a blow make it decompose or detonate? (energetic materials)
Thermal stability (DSC, oven tests)	Self-heating in dryers and bulk storage

Static electricity: the usual ignition source

Powders become charged whenever they flow, slide, are pneumatically conveyed or poured from a bag. Insulating plastics (liners, FIBCs, funnels) and isolated metal parts store that charge until it discharges as a spark. A typical spark from a person is 10–30 mJ; many fine organic powders and almost all flammable vapours ignite below that.

Hybrid mixtures are the trap.

Pouring a powder into a reactor that contains flammable solvent vapour creates a hybrid mixture that ignites far more easily than either alone. Charge solids before the solvent, or through an inerted closed system, never by hand into a hot solvent.

🧮 What does this MIE mean for my plant?

Enter the minimum ignition energy and the powder resistivity from the test report.

Educational screening tool Results are illustrative and depend on the stated assumptions. They are not a substitute for a qualified process-safety assessment, the applicable design code, vendor data, laboratory testing or engineering review.

MIE (mJ) Volume resistivity (Ω·m)

✍️ Check yourself: Which pair of elements turns a fire triangle into an explosion pentagon?

A dust explosion needs the dust dispersed as a cloud and the cloud confined. Removing either (good housekeeping, venting, open areas) prevents the explosion even if the dust can burn.

✍️ Check yourself: A powder has K_St = 240 bar·m/s. Its St class is…

St 1 is up to 200, St 2 is 200–300, St 3 above 300 bar·m/s. K_St is used to size explosion vents and suppression systems.

Key takeaways

Explosion pentagon: fuel, oxygen, ignition, dispersion, confinement.
Probability: MIE, MEC, LOC, MIT/LIT. Severity: K_St, P_max.
Static is the common ignition source. Earth and bond, avoid insulating plastics, beware hybrid mixtures with solvent vapour.