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April 30, 2020

Disaster doesn’t wait: How to protect mills from combustible dust hazards

by Caroline Margolis, FIKE, USA

Milling, mixing, conveying and packaging often produce significant amounts of dust in the air inside mills and in their processing equipment. Because flour is a highly combustible dust, safety measures must be taken to ensure the facility and its employees are fully protected from potential hazards involving combustible dust.

Certain inherent characteristics make mills particularly susceptible to dust explosions.
 

Firstly, the process of milling is to separate wheat grain from its constituents in the form of a fine powder. The finer the dust, the easier combustion can be ignited and the higher its “rate of pressure rise (KST),” which is the velocity at which the pressure in a device increases if combustion occurs. Furthermore, this fine dust has a greater tendency to collect in the mill’s infeed and outfeed hoppers, and to find its way into other areas of production.

Secondly, grinding grain requires many fast-rotating operations and parts, which can create hot surfaces and sparks due to friction. This process, coupled with the transportation of fine powder via conveyors or air jets, produce various potential ignition sources that must be considered.

It’s important to remember that even if an incident hasn’t occurred in the past, that doesn’t mean the process is immune from a combustible dust hazard.

Recommended solutions
In basic terms, a comprehensive explosion protection system includes protecting vessels from a primary explosion (via deflagration venting or chemical suppression) and isolating interconnected ducts, tubes, screws or pipes from secondary explosions (via mechanical or chemical isolation devices).

Mills’ infeed and outfeed hoppers often are protected with these traditional venting and suppressing methods. However, the mills themselves pose unique challenges that must be addressed in the explosion protection system design.

Protecting the vessel (mill) from primary explosions
Mills’ size limitations often make traditional methods of deflagrations venting and suppressing unobtainable. When venting and suppressing isn’t possible, the often-recommended strategy involves a mill “containing” the primary deflagration and pressure wave.

In other words, a vessel with a Maximum Allowable Working Pressure (MAWP) higher than the dust’s PMAX (the maximum pressure developed in an enclosed deflagration), will be strong enough to withstand the pressure.

Therefore, if the mill or other processing equipment is built to contain the initial pressure, it’s the secondary deflagrations into upstream and downstream equipment that are of most concern in the milling process.


Read more HERE.
 

The Global Miller
This blog is maintained by The Global Miller staff and is supported by the magazine Milling and Grain
which is published by Perendale Publishers Limited.


For additional daily news from milling around the world: global-milling.com

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