Prevent Dust Explosions

Aug. 1, 2011
You don’t have to work in the food industry to appreciate the dangers of combustible dust. By learning the nature of the problem you can take practical measures to control it.

A catastrophic dust explosion occurred at a sugar refinery in 2008, resulting in 14 fatalities and major facility destruction. While dust explosions in the food industry are nothing new, as facilities have increased in size, the consequences of incidents have also grown.

Many foodstuffs and food ingredients, including grain, sugar, artificial sweeteners, starch, flavors and cellulosic additives, will burn slowly or with difficulty as a layer on a surface, but can explode if dispersed as a cloud. In fact the vast majority of powders in the food industry can form explosible dust clouds if the particle size is small and moisture content is low. The concentration of dust in the cloud is important.

The concentration of an explosible dust cloud resembles a dense fog. 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., when bins are being filled, powders are being transferred, or dust is being collected in a dust collector.

When Conditions are Right

The particle size of the dust is a property which influences the explosibility of the dust cloud. Powders include pellets (sizes greater than about 2 mm), granules (approximate sizes between 2 and 0.4 mm), and dust particles (sizes less than about 0.4 mm). 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. The presence of dusts should therefore be anticipated in the process stream, regardless of the starting particle size of the material. For example, friable materials such as sugar will create dust in transfer operations.

The moisture content of a product will also affect the explosion risk. A dry dust contains less than 5% moisture. Dry dusts of small particle size will be more easily ignited and produce more violent explosions. Moisture within or on the particle surface reduces both the flame propagation ease and rate. 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.

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

• The dust must be combustible (as far as dust clouds are concerned, the terms “combustible,” “flammable” and “explosible” all have the same meaning and could be used interchangeably);

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

• The dust concentration must be within the explosible range (above the minimum explosible concentration);

• 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;

• The available ignition source must have sufficient energy to initiate combustion.

The ignition sources causing most explosions in dust handling/processing plants include welding and cutting, heating and sparks generated by mechanical equipment failure, sparks generated by mechanical impacts, hot surfaces, open flames and burning materials, self heating, electrostatic discharges, and electrical sparks.

The sensitivity of a dust cloud to ignition by different ignition sources should be determined through appropriate laboratory tests.

Take Appropriate Steps

A systematic approach to identifying dust cloud explosion hazards and taking measures to ensure safety involves:

• Determining the dust cloud’s ignition sensitivity and explosion severity characteristics through appropriate laboratory tests on representative dust samples;

• Identifying areas of the facility where combustible dust cloud atmospheres could exist under normal and/or abnormal conditions;

• Identifying potential ignition sources that could exist under normal and/or abnormal conditions;

• Preventing the formation of explosible dust clouds in the plant and reducing the extent and duration of any clouds that may be formed;

• Taking measures to eliminate/control ignition sources; and

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

Explosion protection measures include explosion relief venting, explosion suppression, explosion containment and explosion isolation. Where practical, one could consider the application of inert gas purging and/or padding to prevent the combustion process.

Elements of an Explosion

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 and on its 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 hoppers;

• Aerated powder test: Simulates fluidized bed processing;

• Powder layer test: Simulates powder deposits on dryer walls/surfaces and tray drying; and

• Basket test: Simulates large-scale storage or transport conditions.

The maximum explosion pressure and maximum rate of pressure rise are measured and the latter is used to calculate the Deflagration Index (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, and to classify a material’s explosion severity. This test answers the question, “How bad is it if it happens?”

Explosion Prevention and Protection

A technically 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 a pressure wave to propagate into the workplace, and dust deposits around the workroom are dispersed into a cloud, which subsequently ignites. This can happen in a series of connected rooms and result in horrific building collapses.

Secondary dust explosions are especially common in industries where traditionally little concern existed for the presence of dust outside the process equipment as the material was not toxic and not particularly expensive. The food industry is one of those.

Housekeeping activities must ensure that secondary fuel sources are not available. 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 aspiration systems, etc., than to spend the time cleaning up the dust that has escaped.

Spread Awareness

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 noises indicating mechanical malfunction or misalignment need prompt attention before a small smoldering clump of dust leads to a serious explosion. Likewise protective safeguards such as explosion relief vents or abort gates must be appropriately designed and maintained with clear, safe discharges. A routine inspection and testing program should be created for safety critical explosion protections.

Vahid Ebadat, Ph.D., is the CEO of Chilworth North America, a firm specializing in process safety management.

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