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.

Protecting interconnected equipment from secondary explosions

While the primary deflagration may be contained in the mill, deflagrations must be isolated from interconnected equipment where secondary explosions are likely. Both experimental evidence and past catastrophes have proven that these secondary explosions may become increasingly more damaging due to three phenomena1:

  • Flame acceleration: Gas flow created by the primary explosion in a vessel will stretch the propagating flame into the pipes, increasing its surface area and rate of combustion, and thus leading to higher flame speeds and pressure. Eventually, the initial deflagration can become a detonation, resulting in much higher explosion pressures
  • Flame jet ignition: When the initial flame reaches the secondary enclosure, it will ignite the remaining unburned material more violently and lead to higher explosion pressures and rates of pressure rise
     
  • Pressure piling: Gas expansion from the primary explosion will increase pressure into the pipes and the secondary enclosure prior to the passage of the flame, leading to a more violent explosion than for ambient conditions

A robust explosion isolation system can keep deflagrations from reaching interconnected processes and causing secondary explosions. Two types of isolation systems may be used: active and passive.

An active explosion protection system mitigates or prevents the effects of a deflagration using a pressure and/or an infrared detector to notify a control panel to activate the respective explosion protection devices, which may include explosion isolation valves or chemical isolation units. Benefits of active systems include:

  • Cost of additional containers may be minimal (if suppression system is already installed)
  • Able to isolate on larger ducts
  • Can be used to block spark propagation
  • Reduced pressure drop over the active devices
  • Ability to view pressure curve of a deflagration event
  • Can support higher KST (relative explosion severity compared to other dusts) and Pred (maximum allowable pressure to prevent deformation of an enclosure)

A passive explosion protection system mitigates or prevents the effects of a deflagration in which the explosion isolation valves are activated in response to the deflagration pressure. Benefits of passive systems include:

  • Operates under normal conditions and activates upon pressure increase
  • Eliminates the need for electronic monitoring system to activate the device
  • Eliminates maintenance by factory-certified technician required with active systems
  • Can be inspected and reset without significant downtime
  • More cost-effective option, if primary deflagration protection is not designed with electronic monitoring e.g. with explosion venting or containment

Combustible dust protection expert Fike can perform a site survey of your facility, which will then determine the best solutions to help you reach ATEX compliance and protect people, product and processes from combustible dust hazards.

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