It is critical that all the systems and components used as a part of an explosion protection system should be third party ATEX approved. Explosion protection and prevention systems are life critical systems thus should be designed by system supplier’s qualified design engineers as well as should be maintained only by manufacturer approved service personnel.    

Dust explosions are one of the lesser-known industrial accidents when considering industrial explosions. Ignitability and explosibility parameters of grain and cereal dusts are sometimes not well understood by the operators of such plants. However, dust explosions often occur in the grain handling facilities due to large amounts of combustible dust formed during operations. In general, if a solid material is combustible, then in its dust form (particle size < 500 microns), it can become explosible if certain conditions are met. In the feed and grain industry, most of the material used throughout the processes are combustible organic material. Thus, use of explosion and prevention methods would help to minimize such risks while providing a safer work environment.

What is a dust explosion?

For a dust explosion to occur, five elements should be present simultaneously: 1) enough amount of combustible dust (particle size < 500 micron), 2) Suspension of the dust in air creating a dense dust cloud, 3) Confined space (process vessels and/or closed buildings), 4) Ignition source sufficient to ignite the dust cloud, 5) Oxygen source (such as air). When all five of these elements are present, then a dust explosion would occur. In many cases, explosion starts inside a process equipment, then quickly propagates to interconnected vessels or to production floor via connecting pipes, or process openings, creating a Secondary Explosion. Secondary explosions are known to be much more destructive to overall production plant. System owners would need to take measures to minimize explosion propagation risks. 

Which processes are most vulnerable to dust explosions?

According to industrial dust explosion statistics1, dust collectors, mills and dust conveying systems are the top 3 processes prone to explosions. However based on the US Grain Industry dust explosion statistics 64% of all dust explosions occurred inside bucket elevators, 48% in the milling processes,  and 17% in the silos.

What types of materials are susceptible to feed and grain dust explosions?

Most materials used in the feed and grain industry are combustible. Their dust clouds are considered as explosible dusts. Particle size and moisture content are major determining factors on the explosibility of such dusts. As the moisture content and particle size gets smaller, the ignitibility of the dust becomes much easier, becoming more explosible. To understand the ignition sensitivity and explosion severity of dusts, it is important to understand their characteristics. Typical explosibility values that need to be determined via testing3 are provided in the table below. 

Explosion pressures expected from a grain dust explosion can reach up to 8 barg which is 10 -12 times higher than the typical mechanical resistance of most process equipment.

What are some of the probable ignition sources initiating such events? 

According to EN1127-1, there are potentially 14 different ignition sources in an industrial process. It is crucial for plant operators to conduct ignition source assessments to ensure they are either not present or there are measures taken to control such sources.

Feed and grain operations include many moving parts such as mills, elevators which can potentially generate sparks, frictional heating as well as hot surfaces. 

What are some of the prevention and protection measures against such explosions?

There are seven industrially recognized methods for dust explosion prevention and protection. In many cases multiple measures are taken together in order to minimize risks to an acceptable level.  Most commonly utilized methods are: 1) Control of ignition sources (prevention), 2) Combustible dust reduction (e.g. cleaning, dedusting, replacement of the material with coarse ones), 3) Inerting (oxygen reduction), 4) Containment (pressure shock resistant process design), 5) Explosion venting, 6) explosion suppression, and in addition to 4, 5 and 6; 6) Explosion isolation. Choice of the appropriate risk reduction measure should be based on explosion risk assessment and by expert advice from the system suppliers. Below you will see several commonly used explosion prevention and protection examples used in the feed and grain handling processes.

Spark Detection Systems (prevention) – the goal is to identify sparks, hot particles or ambers within a pneumatic conveying system or within a chute, and extinguish it via atomized water extinguishers before ignition source reaches a combustible volume thus reducing the risk of fires and explosions. Typical applications in the feed and grain industries includes dust collector/aspiration duct spark detection and extinguishment, mill outlet spark detection and extinguishment, bucket elevator inlet / outlet chute spark detection and extinguishment. 

Example 1 – Dust collector inlet pipe spark detection and extinguishment application. Main components are 1 : pair of IR detectors (for detecting sparks and hot particles), 2: water extinguisher at high pressure,3 : signal router/local controller,4: control panel, 5: alarm and flash light, 6: programmable relay, 7: linear heat detector, 8: pressure booster controller, 9: pressure booster.

Example 2 – Animal feed manufacturing plant spark detector and extinguishment. The goal here is to detect sparks and hot particles created after the mill, elevator and pellet press and extinguish it before these ignition sources reaches an explosible zone (process vessel). 

Explosion Venting and Isolation (protection)- The goal is to relieve the explosion pressure to a safe outside location and simultaneously initiate explosion isolation system to prevent propagation of dust explosion to connected vessels. IN feed mills or grain handling facilities, equipment is located inside a production building thus use of standard explosion relief vents is not possible. Thus cases, flameless vents are used which the flame arresting material surrounding the vent panel absorbs the heat from the flame in milliseconds and extinguishes the flame while relieving the reduced pressure to production area. Note that flameless vents can come in multiple forms, the one shown below is an EVN type flameless vent which is a multi-use spring loaded panel design.

Example 3 – Explosion Protection of a Feed mill. Hammermill with integrated bunker and dust collector are protected via EVN type flameless vents. Explosion detection is done by SmartDS rate of rise type explosion pressure detectors along with IR-13 flame detectors. System owner2s risk assessment indicates that explosion is expected either in the filter or in the mill. This protection covers both scenarios. An ATEX approved rotary valve which would not allow for flame propagation is also used at the mill-bunker screw conveyor outlet by the system owner.

Example 4 – Explosion venting and isolation of a dual-leg bucket elevator. This design assumes elevator is located outdoors or close to an outside wall. Explosion isolation is provided using EHRD type suppressors. Explosion detection is done by SmartDS rate of rise explosion pressure detectors along with IR flame detectors. Explosion propagation risks to interconnected vessels are minimized with the use of the isolation system.  

It is critical that all the systems and components used as a part of an explosion protection system should be third party ATEX approved. Explosion protection and prevention systems are life critical systems thus should be designed by system supplier’s qualified design engineers as well as should be maintained only by manufacturer approved service personnel.   

Properly designed and protected feed mill or a grain handling facility would provide a safer work environment to the employees while minimizing unplanned downtime due to dust explosions.