Preventing Ignition Sources

For most processes where the minimum electrical spark ignition energy of the working dust is considerably greater than 10mJ, elimination of ignition sources is a satisfactory protection against dust explosions.

Ignition sources can be broken down into two groups. The first group consists of those which are caused by general work in the plant:

Smoking
Open flames
Open light (bulbs)
Welding
Cutting
Grinding
These sources can be eliminated by sufficient staff training and enforcing adequate working routines. The second group of ignition sources is made up of ignition sources presented by the process itself:

Open flames
Hot surfaces
Self heating, Smouldering Nests and Exothermic decomposition
Heat from mechanical impacts
Exothermic decomposition of dust via mechanical impacts
Electric sparks and electrostatic discharges
These ignition conditions are presented in the actual process and hence need to be solved in a multitude of ways.

Open Flames
Open flames are most easily avoided by rigorously enforcing a no smoking policy within the process area. Hot work must not be carried out in an area unless it, and any area that could be directly or indirectly affected, is entirely free from dust. Gas cutting torches that use excess oxygen should be avoided as they can cause ignition to be easier than it would be in air.

Hot Surfaces
Hot surfaces occur on items of process equipment that use heat, such as heaters, dryers, steam pipes, electrical equipment, etc. They also occur in less obvious process equipment too, such as engines, blowers, mechanical conveyors, mills, mixers, bearings and unprotected light bulbs. Further hot surfaces can be created during hot work which can provide enough heat for ignition.

Dust lying on hot surfaces can be ignited, as can dust clouds, thus leading to dust explosions. Lying dust on hot surfaces also acts as a layer of insulation which can cause equipment failure.

Following these measures can help minimise the chance of ignition from hot surfaces:

Removal of all dust before performing hot work
Prevent/Remove dust on hot surfaces
Shield or isolate hot surfaces
Only use electrical equipment approved for use in the presence of combustible dust
Use equipment with low chance of overheating
Regular and thorough inspection and maintenance procedures
Self Heating and Smouldering of Dusts
Preponderance to self heating is usually determined by the chemical characteristics of the dust, but there are several ways to minimise the chance of it occurring:

Control dust temperature, moisture content etc. before putting into storage
Make sure no heated bodies are in contact with the stored dust
Monitor temperature of stored dust carefully
Monitor dust storage for combustion gases
Inerting of bulk material with a suitable inert gas (e.g. nitrogen)
Addition of water is not recommended as this can cause mechanical problems with dust transport (it sticks together). Nitrogen inerting is the best way to avoid smouldering fires, but does add the hazard of making the atmosphere unbreathable. If a smouldering mass does occur the first thing to do is to stop any exothermic reactions, then reduce the temperature of the rest of the dust, it should also be checked for any toxic combustion products. Process equipment must be checked thoroughly if a smouldering fire is generated as this can weaken vessels significantly.

Mechanical Impacts
Mechanical impacts can produce either glowing sparks or hot spots where the impact occurs. Usually these sparks are not sufficiently energetic to provide ignition, but in the case of repeated impact or when exotic metals are used there is a risk of igniting a dust cloud. To best avoid these sparks occurring it is recommended that you:

Remove foreign objects from process streams as soon a possible
Avoid any construction materials that could give incendiary sparks or thermite flashes
Inspect odd noises to see if they are caused by impacts, and if so remedy the situation in a safe manner
Electrical/electrostatic sparks
Electrical sparks are best minimised by observing the regulations for electrical equipment in dusty areas and ensuring that any installed electrical items comply. Making sure that workers are aware of the electrical items and how to use them safely is also wise.

Electrostatic sparks are less well understood and there are many ways to try to minimise their formation. It has been recommended that:

Use conducting materials for plant items to avoid charge buildup
Earth any equipment that may become charged
Earth workers is the dust is easily ignitable
Earth coarse non-conducting powders via an earth rod through their storage vessel
If in doubt, earth it.