Grain storage scenario in India is changing fast. Industry is fast converting from bag storage to bulk storage to become more competitive in the market. Bag Storages are now fast converting to Silo Storages and forthwith the whole industry is switching from low speed grain handling to high speed grain handling.
Just a few years back, it was difficult to find conveyors and elevators having capacity more then 50 TPH in the Indian grain industry but now in industry is using material handling system having capacities as high as 700TPH and even more at some port areas. Although, high speed handling of grains in the industry is a welcome move but we must not ignore the fact that it increases the chances of Dust Explosion in the grain industry.
WHAT IS DUST EXPLOSION AND WHAT CAUSES IT?
As per definition a dust explosion is the rapid combustion of fine particles suspended in the air within an enclosed location.
The grains have inherent characteristic to generate dust whenever it is conveyed or transported from one point to another and this grain dust is highly combustible in nature. It is even more combustible then Coal Dust. As per available data, dust explosions can occur where any grain dust cloud is available in high-enough concentrations in the atmosphere i.e. within lowest concentration limits of 0.050-0.100 kg/m3 and highest concentration limits of 2.0-3.0 kg/m3. Once this concentration condition is met with grain dust cloud can ignite at a temperature as low as 300 Deg C.
Hence we must take appropriate Safety Precautions to mitigate this risk and prior to this we must understand what causes dust explosions in Grain Terminals? This awareness is very important because ignorance is not a bliss and Grain Explosions can result in human fatalities, severe injuries, loss of plant and machinery and operational downtime too.
WHAT CAUSES A DUST EXPLOSION IN A GRAIN TERMINAL
There are four necessary conditions to cause a dust explosion
1. Grain dust cloud in high enough concentration
2. Confined Space
3. Ignition source
4. Atmospheric Oxygen
Wherever these four conditions are available in a grain terminal that area is prone to dust explosion.
Grain dust cloud in high enough concentration
As said above grains have inherent tendency to generate dust whenever it is conveyed or transported from one place to another.
This dust moves along with grain and wherever this dust laden grain fills an empty space, air is displaced from that empty space and this displaced air takes along fine grain dust along with it to form a dust cloud.
This process of air displacement takes place at many places in a grain terminal.
Confined Spaces in a Grain Terminal
In a grain terminal, there are many confined spaces that are prone to fire and dust explosion. Some of the locations where possibility of grain dust cloud is in high concentration are Dumping Pits, Bucket Elevators, Enclosed Drag Chain Conveyors, Basement Tunnels, Dust Bins, Aeration Ducts, Dust Bag Filter, Silos, Hoppers etc.
Ignition Sources
Among the various ignition sources the following are more prone to cause a dust explosion in a grain terminal
Direct Flames and direct heat : Cigarettes / Bidis, Lighting Matches, Lighting Gas Lighters, Sparks from Grinding Wheels, Welding Works or Gas Cutting Works are all examples of Direct Flames
Hot surfaces : Friction sparks or Metal to Metal Contact : Overheated Broken or Damaged Bearings, Slipping Elevator Belt, Hammering Spouts or Hoppers, Poking with Iron Rods, Shoes with Iron Studs, Damaged / Deformed Chain flights, Loose Conveyor Centre Guide, Elevator’s Buckets Touching Casing, Mis-aligned Belts, Damaged hanger bearings of sweep auger, Screw Conveyor scroll touching casing/floor or anything similar to above contributes to friction sparks
Grain Infestation: Water leakage inside silo results in infestation, mould growth, fermentation etc. of grain. This grain spoilage result in making a lump of grain mass. This lump size keeps on increasing gradually and inside at the centre of this mass, temperature keeps on increasing and may become very hot resulting in smouldering effect. When we start grain discharge from silo, this grain mass breaks and sometimes result in generation of spark. While grain is being discharged from silo, silo is full of inflammable dust, a minor spark may immediately result into a large fire and sometimes an explosion
Impact Sparks: All lithium Batteries like Cell phone batteries, Camera Batteries, Video Recorder Batteries, Laptop Batteries if dropped from a certain height can produce spark . Dropped tools or fasteners or any metallic component if falls from a particular height may generate spark.
Incandescent Light Fixtures : Incandescent Light Fixtures that are not flame proof may generate spark.
Electrical Sparks : Electrical short circuit or loose electrical connections may also generate sparks if it does not have proper protection. Electrical panels must not be located in dust prone area
Electrostatic sparks : The static charge is charge that builds on the surface of a material or object when it comes in contact with or rubs against another object. Bag Filters may develop static charge and result into generation of electrostatic sparks.
Phosphine Ignition : Phosphine pellets or tablets in a wet aeration duct releases phosphine gas at an uncontrolled rate which may self-ignite or explode inside ducts, if the concentration goes beyond 17900 ppm
PREVENTION OF DUST EXPLOSION IN GRAIN TERMINALS
Dust Explosion can never happen if proper care is taken while designing, operating and maintaining the grain storage facility. General Awareness of Dust Explosion among operating and maintaining staff can avoid Dust Explosion for ever.
Awareness : Awareness about the problem can mitigate the risk well in time hence we must focus on making people aware of the causes of dust explosion and dangers associated with it.
Housekeeping : Implement an effective housekeeping and sanitation program. Grain dust depth should never be allowed to exceed 1/8” inch.
Condition Monitoring & Maintenance : Prepare and practice an effective condition monitoring and maintenance schedule to check for lubrication, misalignment, bearing temperature and leakage. Keep all the equipment in good working conditions with a proper preventive maintenance schedule. Any abnormal noise coming out of any equipment must be immediately rectified.
Dust Extraction System : Keep low pressure dust extraction systems fully operational during any grain transport to collect the dust emitted during operations in various sections of the facility. Dust extraction system must essentially be installed at all grain loading and unloading points and all grain transfer points. Install dump pit baffles to reduce airborne dust during truck receiving. Clean filter bags and dust collector bins at regular intervals. All basement tunnels must have effective dust suppression system.
Usage of High Pressure Water / compressed air: High Pressure Water / compressed air results in generation of lot of dust thus increasing the dust concentration to a point where it may require a minor spark to explode. This happen mainly in enclosed spaces hence High Pressure water usage must be restricted and carefully used in confined spaces.
Instrumentation : All conveyors and bucket elevators must essentially be fitted with Zero Speed Switch and Mis-Alignment sensor to reduce chances of any metal to metal contact. Bearing temperature sensors may also be installed at critical location to get a timely notification of bearing damage / failure. Install explosion vents wherever necessary. Avoid anything that increases the concentration of airborne dust including high pressure water spray
General Maintenance Instructions : No Maintenance works should be attempted while the equipment are in operation. No cutting, no welding, no hammering and no poking with iron bars that may result in generation of sparks or flame.
Design Considerations:
1. Dust extraction system must be installed at all grain transfer, loading & unloading points so as to avoid dust concentration at confined places. Bag Filters must be installed with anti-static bags.
2. Proper maintenance platforms, catwalks and approach staircases must be installed to facilitate condition monitoring inspections and maintenance of plant and machinery.
3. Electric Panels must not be located in dust prone areas if they do not have proper protection.
4. Only explosion proof lighting and light fixtures must be used at all dust prone areas
5. All Conveyors and Elevators must be fitted with Zero Speed Switch, Mis-alignment switch and overflow switch to take care of belt/chain slips and friction heating.
6. All bucket elevators must have quick-opening cleanout doors on the elevator boot side panels for easy grain and dust cleanout. Explosion vents must be properly installed at the location specified by manufacturers. Only PVC / Poly Propylene buckets should be used in bucket elevators and these buckets must be fixed with nylok nuts and bolts. Steel Buckets must not be used. Only anti-static belting material should be used in bucket elevators.
7. Basement tunnels should be properly ventilated and must have safety exit on both ends. Maximum travel distance between two exits ideally should not be more than 60 mtrs. In case there is a single exit the maximum travel distance from end of the tunnel must not be more than 15 mtrs. However if a tunnel is having exits on the both ends and fitted with sprinkler systems maximum allowable safe travel distance can go up to 120 mtrs.
8. All silos must be fitted with temperature monitoring system, aeration system, ventilation system and fumigation system to avoid grain infestation inside silos as infestation could lead to hot spots and smouldering.