Explosion Protection for Food Powder Plants

A food plant has every ingredient a dust explosion needs. Fine organic dust, enclosed equipment holding it in the explosible concentration range, oxygen, and a list of credible ignition sources: a hot bearing, a tramp metal spark in a mill, a static discharge from an ungrounded liner, a smouldering particle from a dryer.

The measurement problem comes first. Explosion data taken from the product is often optimistic, because the dust that matters is the fraction that got airborne and reached the filter, and that fraction is finer, drier and more reactive than the bulk. Protection designed on the product specification can be designed on the wrong material.

The connections are the second problem. Mills, dryers, silos and the collector are joined by ducts, and an event in one of them travels. The receiving vessel then sees a flame front in an atmosphere that has already been compressed and stirred, which is more severe than the case its own protection was sized for.

And indoors there is nowhere to vent. A conventional vent panel needs a safe discharge route, and in a food factory there usually is not one.

Explosion characteristics measured on a representative sample of the dust that actually accumulates, with the collected fines as the design basis.

Each enclosure that can hold an explosible cloud identified and given a protection concept: venting where there is a safe route, flameless venting or suppression indoors, containment where the vessel can be built strong enough.

Isolation at the connections between protected vessels, and dust layers in the building controlled, since a layer is what turns a contained event into a plant wide one.

A grain plant has the same sampling problem outdoors: protection fitted to the elevator while the dust layers in the gallery remain the real hazard.

The sequence is the same in every plant and the order matters. First establish the explosion characteristics of the dust from a representative sample. Then identify every enclosure that can contain a cloud. Then choose a protection method for each one, and only then specify hardware.

The collector is usually the starting point because it holds the highest dust concentration in the plant and it is connected to everything. Its vent area or suppression requirement follows from the measured Kst, the vessel volume and the strength the vessel was actually built to, which is a number worth checking rather than assuming.

Indoors the venting question becomes a device question. Flameless venting arrests the flame and cools the gases so the discharge stays inside the building; suppression stops the event before it develops. Which one applies depends on the dust, the vessel and what is standing next to it.

Isolation then protects the connections, and housekeeping protects the building. Both are easy to leave out of a protection concept and both are the reason incidents become large.

• Explosion characteristics measured on the collected fines, not on the product.
• Every enclosure that can hold an explosible cloud listed, including small ones.
• Vessel strength verified rather than assumed, since it sets the reduced pressure target.
• Flameless venting or suppression used indoors where there is no safe discharge route.
• Isolation provided at the ducts between protected vessels.
• Dust layer control written into the protection concept with a method and an interval.

• Explosion characteristics of the collected dust, measured on a representative sample rather than on the saleable product.
• Enclosure strength, which sets the reduced explosion pressure the protection has to achieve.
• Indoor or outdoor location of each vessel and whether a safe discharge route exists.
• Duct geometry between vessels, which decides the isolation method available.
• Cleaning regime, since allergen cleaning and dust layer control can be made to serve each other.
• Ignition source inventory, including bearings, tramp metal, static and dryer carryover.