Explosion Protection for Chemical Plants

Chemical powders span a far wider range of reactivity than most industries deal with. Metal powders, some organic intermediates and certain pigments have high Kst values and low minimum ignition energies, and the data is specific to the material and even to the batch.

Solvents complicate it. Where a powder is wet with solvent, or where a dryer handles both, the atmosphere is hybrid: vapour and dust together ignite at a fraction of the energy the dust alone needs, and a static discharge that the dust assessment dismissed becomes sufficient.

That is why the basis of safety in this industry is often avoidance rather than protection. Inerting a dryer or a mill circuit with nitrogen removes the oxygen rather than containing the consequence, and that choice carries its own duties: continuous monitoring, a defined action on loss of inerting, and a documented case for startup and maintenance when the inerting is not established.

And everything has to be consistent with a safety case that already exists for the gas and the process hazards.

Explosion protection as a discipline starts from one measured dust. A chemical plant rarely has one, and that is the whole of the difficulty.

Explosion characteristics measured on the material as handled, including the hybrid case where vapour can be present, and documented per product rather than per plant.

A basis of safety chosen explicitly for each enclosure: avoidance by inerting, venting, suppression or containment, with the reasoning written down.

Where inerting is the basis of safety, continuous oxygen monitoring, a defined trip action, and a documented case for the states where the inerting is not yet established.

In a food powder plant the data has to come from the dust the filter actually holds, which is finer and more severe than the product it was taken from.

Measurement comes first and it is per product. Kst, minimum ignition energy, minimum explosible concentration and, where relevant, the hybrid ignition energy with the solvent present. A campaign plant may need this for several materials, and the protection has to hold for the worst of them.

The basis of safety is then chosen per enclosure rather than for the plant. A dryer handling a solvent wet product is usually inerted; a collector on a dry duty is usually vented or suppressed; a mill may be either. What matters is that the choice is deliberate and recorded, because the one thing that fails reliably is a plant where nobody can say which approach applies to which vessel.

Inerting is then engineered as a safety system: oxygen measurement with a validated sample point, a trip action, and a procedure for startup, shutdown and maintenance. Nitrogen also brings an asphyxiation hazard that has to be managed in the same breath.

Finally the documentation is integrated with the existing process safety case so a reader starting from either side finds the other.

• Explosion characteristics measured per product, including the hybrid case with solvent present.
• Basis of safety chosen and recorded per enclosure, not per plant.
• Inerting engineered as a safety system with oxygen monitoring and a trip action.
• Startup, shutdown and maintenance states covered where inerting is the basis of safety.
• Asphyxiation hazard from inert gas managed alongside the explosion hazard.
• Dust documentation integrated with the existing process safety case.

• Measured explosion characteristics per product, with the campaign plan defining the worst case.
• Credible solvent vapour presence, which changes the ignition energy by orders of magnitude.
• Basis of safety: whether avoidance by inerting is preferable to protection.
• Vessel strength and location, which decide venting feasibility.
• Area classification, which affects every device and instrument in the concept.
• Inert gas supply reliability and the consequences of losing it.