BULKtalk, Engineering

BULKtalk: Hazards associated with conveying and storing bulk materials

Some of the bulk materials carried on conveyors bring inherent hazards to the design. Steve Davis explains some of the risks involved and how they can be mitigated appropriately.

Current safety regulations require a safety in design review to take place that shows a design provides the lowest practical risks. It isn’t possible to meet this requirement unless the risks are clearly defined.  

We have risks with flammable bulk materials such as ammonium nitrate, coal, coke and sulphur, potentially explosive atmospheres from dust that is formed by flammable materials, chemical issues where noxious gasses are emitted or the bulk is corrosive, health issues from silica, coal, and other respirable dusts, and the risk of public and environmental nuisance from dust emission. 

What data is required and what can we do to manage these risks? The Material Safety Data Sheet (MSDS) will have specific data on risks associated with each material.

To have a fire, three things are usually needed: a flammable material (fuel), oxygen and an ignition source with sufficient energy to ignite. If the material doesn’t burn or there is no heat/ignition source a fire is generally not possible. Some materials carry their own oxygen and others may ignite in the presence of chlorine or other chemical, even water. Some spontaneously heat and then burn, such as coal. A detailed list of the different categories can be found in the International Maritime Dangerous Goods (IMDG) code for shipping.

For an explosion five things are needed: a combustible dust, dust dispersion, dust confinement (chute, bin, building) oxygen and an ignition source. If one of these five components is missing, then there can’t be an explosion. Some dusts hold and discharge static, which can be an ignition source. Dust from most flammable solids can explode if the circumstances are right. 

A small fire in a dusty environment may cause an explosion, a small explosion may ignite a larger secondary explosion. The primary explosion shock wave lifts dust from the secondary area forming the dispersion in a confined are and acts as the ignition source. The secondary explosion is usually bigger and more dangerous.

Our first action is to assess the risk associated with the material. There are many tests available to confirm risks from potential flammable or explosive materials, including: Burning tests to determine the ignitability and combustibility of powders, Combustible Dust Screening Test: OSHA SLC Go and No-Go, Minimum Ignition Energy (MIE), ASTM ASTM E-2019-99, Minimum Auto-ignition temperature (MAIT) of dust clouds in air on hot surfaces, Minimum Ignition Temperature, dust cloud (MITdc), Dust Explosivity Lower Explosivity Limit (LEL), Maximum Explosion Over-pressure (Pmax), Explosion Pressure Development (dp/dt)max, Dust Specific Characteristic (Kst), Limiting Oxygen Concentration (LOC). All are used to determine the possible conditions under which a fire or explosion is possible and how bad the resulting fire or explosion could be. Long lead time for testing can be expected and few laboratories have the suite of equipment necessary for all tests. Consider Simtars in QLD, Gexcon in Norway, and CMC Technologies in USA. These laboratories offer guidance on the test that are best for the circumstances.

There are other circumstances where fire or explosion can occur. Transport of Dangerous Goods Model Regulation (TDG) has several classes that identify risks and include solids: Class 1 Explosives, Class 4 Flammable Solids etc., includes seven classes of self-reactive materials, pyrophoric and self-heating spontaneously combustible materials and a class of materials that are dangerous when wet, and readily combustible solids. Again, the MSDS will highlight these risks.

Other TDG Classes include Class 5 Oxidizing Substances and Organic Peroxides, Class 6 Toxic Substances and Infectious Substances, Class 7 Radioactive Material, Class 8 Corrosive Substances, Class 9 Miscellaneous Dangerous Goods. 

Data may not be available from websites and data from other sources may not relate to specific bulk materials. Past experience included consideration of explosion relating to AS 60079 (IEC60079), where 35-year-old data for a similarly named, but different solid, was used as the basis for assessment. The evaluation assumed there would be an explosion hazard and applied AS 60079 zoning when there was sufficient justification to question whether all five explosion components were present to create an explosion. The recommended action was to extract dust into a baghouse, which then created a larger risk envelope. Of note, the evaluation ignored the risk of fire, which was definitely present and neither supplier nor end user had previously considered the possibility of an explosion hazard with the material being handled. The AS60079 review only considers electrical ignition sources and the installation was considered safe even though there is potential for mechanically generated ignition sources through various conveyor upsets that can generate heat. Risk assessment should use correct data and look at the issue from all perspectives.

To manage fire and explosion risk we must understand the causes and take relevant action. For example, sulphur granules in storage can be ignited by a spark from a front-end loader on concrete, static or other ignition sources. Sulphur has a low ignition temperature so burns easily. The same front-end loader may be able to extinguish the fire by smothering the fire (removing the oxygen) in more sulphur as the fire burns relatively slowly. If the fire grows, water or foam can be used to extinguish it, so typically there will be deluge systems on conveyors and high-volume water sprays in storages. Burning sulphur produces sulphur dioxide, which is poisonous, and sulphur may contain pockets of hydrogen sulphide, which is also poisonous. Typically, we will install sensors for both gases linked to alarms for evacuation. Fire resistant anti-static (FRAS) belts are the norm for conveying sulphur.

Sulphur explosions are common in chutes, bins and stockpiles and are ignited by static discharge that is built up on the sulphur during transport. Sulphur dust has a low lower explosivity limit and ignition temperature, so it doesn’t take much to start the explosion. These primary explosions can start a secondary explosion if there is sufficient dust nearby. We manage these explosions by including various methods of static discharge to earth and other detailed design methods. Good housekeeping prevents the secondary explosions. Dust is extracted into wet scrubbers because bag filters have a high explosion risk. Clean up via wash down, not vacuum systems, for the same reason. 

Sulphur is also classified as an H315 skin irritant, so PPE is required. Sulphur may react to form acids if wet, and when certain bacteria / microbes are involved, so measures are taken to avoid sulphur build up and facilitate cleaning. Sulphur can react with several metals, so we avoid their use in contact with sulphur.

So, for sulphur we have managed: fire, explosion, corrosion, health risk, environmental dust risk and chemical risk. Each hazardous bulk material has its own associated risks and mitigation should be assessed to suit.

Many materials have known or perceived risks, but the details may not have been defined fully. Some materials are commonly handled without any consideration of potential risks. If designing a new facility, don’t assume that previous assumptions are correct without some evaluation. If relevant data is not available to define the risk, it could be prudent to assume a worst-case situation and risk assess this and the mitigation cost, then use the hierarchy of controls to assess how far down it is possible to go within the capital constraints of the project. Current legislation allows as low as reasonably practicable (ALARP) to be defined by financial constraint as well as technical limitations. The legislation does not make exceptions for incidents resulting from a design that does not consider all potential risks.

Other risks and considerations that can occur from handling of bulk materials, and that should be considered in the risk analysis include: chemical attack on belts and other components (e.g., alumina and natural rubber), radiation (uranium and other), spillage where there is an environmental risk, structural collapse from build-up of spillage (encrustation) or of dust on roof structure, consideration of impact from adjacent operations, environment and the like that could initiate a risk with a new installation. Is there a chemical that reacts with the new bulk material or an ignition source that intrudes into a high explosion risk zone? Is the material subject to slumping (some coals) or liquefaction under certain conditions (oil sands), is there a transportable moisture (TML) risk when loading ships? 

If your risk assessment is based solely on a typical ‘high flow, no flow, reverse flow, no flow’ style of analysis, it may miss many of the potential hazards which do not readily fall into this method of interrogation. When hazards are not identified by the risk assessment, it is likely that they won’t be mitigated, and if an incident occurs that could reasonably have been foreseen, defending ALARP after the fact becomes more difficult. 

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