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Dust explosions in the food industry

12 July 2012

Nearly all foodstuffs and ingredients including grain, sugar, artificial sweeteners, starch, and flavours will burn with difficulty as a powder layer, but they can explode violently when ignited in the form of a dust cloud. Here Mike Weaver, Principal Specialist at Chilworth Global, looks at the extent and nature of the problem within the industry, the basis of safety, and protection and prevention measures.

Dust explosions: the nature of the problem

There are many factors that affect the explosion violence or the sensitivity to ignition of a dust cloud such as particle size, cloud density or moisture content. 

The dust cloud density is quite important and it would need to resemble a dense fog before ignition could occur.  Although such concentrations are not normally expected to be present within processing buildings, explosible dust clouds are regularly formed inside the material handling/processing equipment, i.e. silos, bin filling/emptying, pneumatic conveying or dust collectors.

The particle size of the dust is another important property which influences the explosibility of the dust cloud.  Even pellets can include powder as well as granules and dust particles. The finer the particles, the greater the surface area per unit mass and thus the more explosible a given dust is likely to be.  When the cloud is composed of a series of particle sizes, ranging from fine to coarse, the fines play a prominent part in the ignition and the explosion propagation.  As dust settles, larger particles drop out of suspension, leaving the finest particles to form the dust cloud. The presence of dusts should be acknowledged in a process stream, regardless of the starting particle size of the material. For example, friable materials (granular sugar) will create very fine dust in transfer operations by means of attrition.

Moisture content will also affect the explosion risk.  A dry dust contains less than 10% moisture and the dryer it becomes the effect on ignition sensitivity and explosion violence will increase. Dry dusts, of small particle size will be more easily ignited and produce more violent explosions.  

However, it must be noted that moisture contents in the range of 12 – 18%, as found naturally in many agricultural products, are not enough to render the dust non-flammable.  Care must be taken concerning material that has been present in process equipment for a long period of time, due to poor cleaning programs, and which has now dried out.

Please also be aware that with the introduction of chemicals such as artificial sweeteners, and vitamins into the food industry in recent years, the explosion properties of products may now be totally different, with potential for increased sensitivity to ignition and greater explosion violence.

One of the most catastrophic dust explosions in recent years occurred at a sugar refinery in the USA, in 2008, resulting in 14 fatalities, multiple injuries and major facility destruction. Dust explosions in the food industry are nothing new, and the oldest recorded dust explosion occurred in Mr. Giacomelli’s Bakery in Turin on December 14, 1785. As facilities increased in size, equipment became more mechanised and sophisticated, the consequence of incidents also increased, such as the grain elevator explosion in April 1981 that killed 9, injured 30 and caused US$30 million damage. And as the aforementioned sugar refinery explosion shows, devastating dust explosions continue to occur.

Conditions Required for Dust Explosions to Occur

A number of conditions must exist simultaneously for a dust explosion to occur:

* The dust must be combustible (combustible, flammable, and explosible all have the same meaning);

* The dust must be dispersed (forming a dust cloud in air);

* The dust concentration must be within the explosible range (above the Minimum Explosive Concentration, MEC);

* The dust must have a particle size distribution capable of propagating flame;

* The atmosphere in which the dust cloud is present must be capable of supporting combustion;

* Any ignition source must have sufficient energy to initiate combustion.

Most commonly identified ignition sources in dust handling/processing plants include welding, mechanical sparks, hot bearings, hot surfaces, open flames, burning embers, self-heating, electrostatic discharges, and electrical sparks.

In order to determine the sensitivity of a dust cloud to ignition, appropriate laboratory tests should be performed.

Assessment of dust explosion hazards in your facility

A systematic approach to dust cloud explosion hazards involves:

* Identifying areas where combustible dust clouds could occur under normal and abnormal conditions;

* Preventing the formation of explosible dust clouds

* Determination of ignition sensitivity and explosion severity characteristics through appropriate laboratory tests on representative dust samples;

* Identifying potential ignition sources that could exist under normal and abnormal conditions; * Taking measures to eliminate/control ignition sources; and

* Taking measures to protect against the consequences of dust cloud explosions.  

Laboratory Testing to Assess Explosion Characteristics of Dust Clouds

In order to assess the potential for an explosion and to select the most appropriate Basis of Safety for any operation, the explosion characteristics of the dust(s), handled in the processes, should be determined.  

The explosion characteristics normally fall within one of two groups, “likelihood of an explosion (Ignition Sensitivity)” and “consequences of an explosion (Explosion Violence)”.  Taken together these two groups define the dust explosion risk of a material. 

Laboratory Testing to Determine “Ignition Sensitivity”

Group A/B Classification Test

This test determines whether a dust cloud will explode when exposed to a sufficiently energetic ignition source.  The test results in classification of either Group B (non-flammable), whereby no more has to be done from the respect of explosions or Group A (flammable) whereby further work has to be performed. These powders would make the processes fall under the ATEX1 or DSEAR2 regulations.  This test answers the question “Can this dust explode?”

Minimum Ignition Energy - MIE (EN 13821:2002) 

The MIE test determines the lowest electric energy that is capable of igniting a dust cloud at its optimum ignitable concentration.  The test is used primarily to assess the susceptibility of dust clouds to ignition by electrostatic discharges (sparks).

Table 1 lists Explosion Characteristics of some dusts that are commonly used in the food industry
Table 1 lists Explosion Characteristics of some dusts that are commonly used in the food industry

Minimum Ignition Temperature of a Dust Cloud – MIT (EN 50281-2-1, 1999

This test determines the lowest temperature capable of igniting a dust dispersed in the form of a cloud.  The MIT is an important factor in evaluating the ignition sensitivity of dusts to such ignition sources as heated environments, hot surfaces (electric motors), and friction sparks.

Layer Ignition Temperature – LIT (EN 50281-2-2)

This test determines the lowest temperature capable of igniting a dust layer of standard thickness (5 mm). The LIT is used in evaluating the ignition sensitivity of powders to ignition by hot surfaces (electric motors). 

Self-Heating (JA Abbott (ed.) “Prevention of Fires and Explosions in Dryers”, Institute of Chemical Engineers, 1990)

Ignition of bulk powders can occur by a process of self-heating when the temperature of the powder is raised to a level at which the heat liberated by the exothermic oxidation or decomposition reaction is sufficient to exceed the heat losses and to produce a “runaway” increase in temperature.  

The minimum onset temperature for self-ignition of a powder depends mainly on the nature of the powder, its volume and vessel dimensions.  If these variables are predictable, a reliable assessment of the onset temperature for self-ignition and also the induction time to self-ignition can be made by appropriate small-scale laboratory tests:

* Bulk Powder Test: Simulates bulk powder in IBCs, bags, bottom of dryers, hoppers

* Aerated Powder Test:  Simulates fluid bed drying 

* Powder Layer Test:  Simulates powder deposits on dryer walls/surfaces and tray drying

* Basket Test:  Simulates large-scale storage or transport conditions

Minimum Explosive Concentration - MEC (EN 14034-3)

MEC test determines the lowest concentration of a dust cloud in air that can give rise to flame propagation upon ignition.  This test answers the question “What is the minimum concentration of a dust cloud to sustain combustion”.

Limiting Oxygen Concentration - LOC (EN 14034-4)

LOC test determines the minimum concentration of oxygen (displaced by an inert gas such as nitrogen or carbon dioxide) capable of supporting combustion.  An atmosphere having an oxygen concentration below the LOC is not capable of supporting a dust cloud explosion.  

Laboratory Tests to Determine the “Explosion Violence”

Maximum Explosion Pressure (Pmax), Maximum Rate of Pressure Rise, Dust Constant (Kst Value) (EN 14034 (part 1 : 2004 and part 2 : 2006))

Table 2 specifies the type of data that might be required to assess dust explosion hazards associated with some common unit operations in the food industry and to define a Basis of Safety
Table 2 specifies the type of data that might be required to assess dust explosion hazards associated with some common unit operations in the food industry and to define a Basis of Safety

The Maximum Explosion Pressure and Maximum Rate of Pressure Rise values are determined by using a 1m3 or more commonly a 20-Litre Sphere test apparatus.

The maximum explosion pressure and maximum rate of pressure rise are measured and the latter is used to calculate the Dust Constant (Kst) value of the dust cloud.  These data can be used for the purpose of designing dust explosion protection measures such as explosion relief venting, suppression, and containment. 

Table 2 Notes:

1. Explosibility Screening test is only conducted if the combustibility of the powder/dust is not yet established. If the powder is found to be non-combustible, other tests in the table may not be required. 

2. LOC is determined if the Basis of Safety is inert gas blanketing.

The basis of safety for dust cloud explosion hazards

Safety from dust cloud explosions includes taking measures to avoid an explosion (explosion prevention) or designing facilities and equipment so that in the event of an explosion people and processes are protected (explosion protection).  The selection of explosion prevention and/or protection measures is based on:

* Define where flammable atmospheres occur

* The available flammability data 

* The nature of the processes and operations

* The level of personnel knowledge regarding the consequences of a dust explosion and adherence to the preventive measures (employee training)

* Environmental effects of a dust explosion

* Business interruption resulting from a dust explosion

Explosion Prevention and Protection Measures

The risk of an explosion is minimised when one of the following measures is ensured:

* An explosible dust cloud is never allowed to form

* The atmosphere is sufficiently depleted of oxidant that it cannot support combustion

* All ignition sources capable of igniting the dust cloud are removed, or

* People and facilities are protected against the consequences of an explosion by suitable “protection measures”. 

Secondary Explosions and Good Housekeeping

A well-designed plant is no guarantee of safety, if those who operate it do not understand the hazards involved, and the precautions designed to control them.

A majority of the most serious dust explosions over the years have not been caused by the primary explosion inside the plant, but from a secondary explosion within the building.  A small initial event causes pressure and shock waves to propagate into the workplace, and dust deposits around the workroom are shaken into a cloud, which subsequently ignites.  When this happens in a series of connected rooms, the result can be fatalities and horrific building collapse. 

Secondary dust explosions are especially common in industries where traditionally little concern exists for the presence of dust outside the process equipment due to the material not being toxic and to not being particularly expensive. Unfortunately, the food industry falls squarely into this category.

Housekeeping activities must strive to minimise the presence of dust outside equipment.  Of key importance is evaluation of dust release points and exhaust ventilation needs.  It is much easier to replace a gasket, refit a manway, or install local dust extract systems, etc., than to spend time cleaning up the dust that has escaped.  Other common practices are venting, inside a building. The flame and pressure waves from such a release can cause unimaginable damage to people and plant let alone the increased risk of a secondary explosion occurring.

Safeguard Reliability

In the EU, legislation is now in place to reduce the likelihood of dust explosions occurring. The ATEX1 directives and DSEAR2, in the UK, are designed to show awareness of dust explosion hazards and to minimize the potential risk to employees. All safeguards intended to prevent dust explosions must be recognized, understood and maintained by operating forces. Operators should be aware that signs of overheating, excessive vibration, or noise indicating mechanical malfunction or misalignment need prompt attention before a small smouldering clump of dust leads to a serious explosion. Likewise protective safeguards such as explosion relief vents must be appropriately designed and maintained with clear, safe passages of discharge. Maintenance programmes for equipment should be in place and a routine inspection and testing programme should be created for safety critical explosion protections. 

References:

1 – ATEX, EU Directive 1999/92/EC

2 – DSEAR, Dangerous Substances and Explosive Atmospheres Regulations

Author:

Mike Weaver is a Consultant Engineer and business development executive at Chilworth Global specialising in the field of process safety testing and CAD, ATEX and DSEAR compliance.

After joining Chilworth Technology in 1991, he managed Chilworth’s Industrial Explosion Hazards (IEH) laboratory for seven years and was a major force in its expansion. He was also responsible for establishing the Physico-chemical laboratory and is a qualified Study Director under the Chilworth GLP scheme. 

Mike’s specialities include dust, gas and vapour explosions and electrostatic hazards, especially in the field of data interpretation, and is also the main contact for regulatory compliance in the field of NONS, GHS, PPORD, REACH and transportation issues.


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