What are the causes of fire in Local Exhaust Ventilation (LEV)?
What dusts are explosible?
The dust of many combustible materials in everyday use, such as coal, wood, grain, sugar, certain metals and synthetic organic chemicals, when dispersed in air to form a cloud, can explode if an ignition source is present.
When a mass of solid combustible material i.e a log of wood is heated it burns away slowly, layer by layer, owing to the limited surface area exposed to the oxygen of the air. The result is quite different if the same mass of material is ground to a fine powder and suspended in air as a dust cloud. The surface area of material exposed is much greater, if ignition occurs the whole of the material may burn very rapidly. If the dust cloud is contained i.e. within an extraction unit, the rapid release of heat causes the pressure to rise to levels which most industrial plant cannot withstand.
In addition, explosion may be caused in the dust of combustible and non-combustible materials due to the build-up of static electricity in the particles. Such explosions have occurred in cement and metallic dusts.
Fires Involving Combustible Dust
Small smoldering fires may develop in dust accumulations from any of the common sources of ignition.
If a fire is suspected inside a dust handling plant, it may be dangerous to open up the inspection points to look inside.
A sudden rush of air into the plant could cause a smoldering deposit to flare up, or a dust cloud to form, followed by an explosion which vents out through the inspection point. It is preferable first to try and cool the affected plant from the outside, or where practicable to apply a fine water spray into the plant through a small opening. High pressure water jets applied to a smoldering fire are dangerous as they can raise dust clouds. Water should not be used to extinguish fires involving powdered metals. Fire fighting means appropriate to the material should be employed.
All people involved in plants handling explosible dusts should be given training in general terms about the nature and hazards of dust explosions, typical sources of ignition, safeguards provided, precautions to take and any emergency procedures on their plant. The importance of good housekeeping should be stressed and the need to report any release of material or equipment malfunction that could be a source of ignition.
Common sources of ignition include:
faulty or unsuitable electrical equipment
overheating of moving mechanical plant, e.g. by friction; [i.e. blunt cutting tools]
careless use of welding or flame cutting equipment.
CONTROL SOURCES OF IGNITION
Prohibit Smoking & Naked Flames. Enforce a no smoking area.
Reduce & Control Hot Work Processes. Where possible employ cold cutting methods rather than hot work, implement a permit to work system for hot work.
Protect Electrical Equipment. Observe the requirements for protected electrical equipment for use in potentially explosive atmospheres (see DSEAR Regulations 2002)
It is preferable to site electrical equipment away from dusty areas, but if it is installed close to areas such as sack tipping points, sanding machines, sampling points or similar foreseeable dusty areas.
A dust tight enclosure is recommended to prevent the risk of dust ignition inside the apparatus.
The dust tight standard is IP6X. Where electrical equipment is only likely to be exposed to dust in the event of a plant malfunction the protection IP55 is likely to be adequate. The DSEAR Regulations 2002 introduce new zone identification rules and specific ATEX / zone suitability labelling for motors.
Control Plant Temperature. Control the surface temperature of plant and restrict dust ingress into electrical equipment enclosures.
Equipment Location. In the layout of workshops ensure that potential sources of ignition are segregated from dust sources where possible. There are strict restrictions on the location of "open bag"' type filter units within specified distances from workers/operatives, walkways and flammable stores.
Do Not Use "Lead Lights" It is especially dangerous to lower a mains-powered portable lamp or other electrical item into a vessel containing combustible dust. Battery lamps certified for use in gaseous flammable atmospheres are unlikely to cause ignition of a dust cloud but could overheat, starting a fire if they were dropped in a dust heap.
Control Friction Frictional heating of moving parts of process plant may raise the temperature locally to the point where ignition of a dust occurs without spark or a flame.
Prevent Metal Entering LEV. Impact sparks are likely to arise where tramp metal or stones enter process plant.
Earth Metalwork & Flexible Hose. Electrostatic charging of plant or process material is likely to occur whenever dusty materials are moved in quantity. Earthing of all metalwork that may be in contact with the dust is necessary.
Typical precautions required are earthing of plant items that stand on non conducting floors and avoiding the use of non- conducting fastenings to join metal components together. Earthing arrangements should be made at each maintenance or modification. Flexible hoses containing a wire helix must be earthed to comply with BS EN 1127 –1 Supervision.
Access to workshops should be controlled and limited to authorised, trained personnel. In some circumstances, it could be possible that unsupervised or irresponsible personnel could enter a workshop and start fires either maliciously or by misuse of equipment. It is therefore important that, in these environments, all users have the appropriate supervision and workshops are locked when not in use. Restricted access signs should be prominently displayed.
CONTROL FORMATION OF DUST CLOUDS
Good Housekeeping Where plant handling combustible dust is contained within a building it is necessary to maintain a high standard of housekeeping.
If dust deposits are allowed to accumulate they can provide the fuel for a secondary explosion. Dust deposits can be shaken into suspension from all the ledges within a room by a small primary explosion and then ignite. Comparatively small amounts are needed. A layer of flour 0.3mm thick on the floor can produce a 3m explosible cloud.
To prevent dust accumulations regular cleaning is likely to be needed, a vacuum cleaner should be used, not brushes. The use of compressed air lines to dislodge dust deposits will cause unnecessary danger by creating a dust cloud.
Dust deposits should never be more than 5mm thick, surface deposits greater than 5mm deep of wood dust may spontaneously ignite. Surface deposits of dust can also block ventilation holes or otherwise interfere with the cooling of electrical equipment.
The regular removal of collected dust prevents spillage from the collection receptacle and possible build-up through the extraction system. Also, care must be taken when removing collected dust to ensure that it does not become airborne at any stage of its disposal: filled collection bags must be sealed prior to moving.
Maintain plant in a leak tight condition. Dust Filters provided as part of the LEV concentrate the dust and are likely to contain an explosive atmosphere within them, even if the dust concentration in the extract ducting is well below the lower explosive limit. Appropriate routine maintenance of mechanical and electrical equipment within the workshop ensures that equipment parts do not become overheated and consequently become a source of ignition.
It may be necessary to electrically interlock the process machinery with the local exhaust ventilation so that it can only operate when the LEV is operating.
Explosion Relief Venting
The simplest means of protecting process plant against the consequences of a dust explosion inside the plant is to provide some deliberate point of weakness. This is called an explosion relief vent. If it is of suitable size and in the right place an explosion within the plant will be vented safely. To function effectively explosion vents are ideally ducted to an outside wall, with a straight duct of minimum length so it can relieve to a safe place in the open air. If the explosion vent is not ducted outside it should not relieve to an area that is regularly occupied.