Hazards associated with explosives manufacture
Fire from combustible insulation on process tank
In 1993, a fire occurred in the combustible insulation material of a stainless steel heated tank containing ammonium nitrate. Over a period of years, ammonium nitrate solution spilled over the tank and absorbed into the insulation material. At the time of the fire, the tank was operating at a temperature higher than normal. The fire was believed to be due to spontaneous combustion.
Suggested control measures include:
- avoiding the use of combustible materials in explosives manufacturing plants (including ammonium nitrate)
- cleaning up any spills immediately
- protecting insulation on tanks from spills from the top of the tank
- providing adequate fire-fighting equipment and water supply at the point of manufacture, particularly material transfer locations.
Compression ignition of explosives pumps
There is a risk of compression ignition events during the pumping of emulsion or slurry explosives where there may be enough energy to initiate the explosives. Possible causes of compression ignition include:
- entrapment of air in the product
- highly volatile components, such as fuel vapour
- high pressure
- rapid increase in pressure
- pumping of products above their minimum burn pressure.
Suggested control measures include:
- following manufacturer and supplier requirements for pumping explosives
- minimising pumping pressures and providing pressure sensor trip switches in equipment
- reducing the rate of pressurisation (i.e. lower the stroke rate or pump speed)
- avoiding single-stroke piston pumps
- avoiding deadheading and starting the pump running slowly
- using low volatile fuel ingredients in the pumpable product
- allowing volatiles to disperse from a tank or hopper before pumping.
Sources of ignition on mixing or transfer augers
Augers are used in the transfer or manufacture of ammonium nitrate-based explosives, including ANFO.There have been several issues, including tramp material, metal grindings, rust flakes, loose bolts or fibrous material form bags being caught between the auger spiral and the auger casing.
This has led to the generation of heat, sparks and other means of ignition. Ammonium nitrate prill has also been ground to a fine powder within the auger under such circumstances.
Suggested control measures include:
- ensuring that at least 1 end plate of the auger assembly is designed to be removed so you can inspect the auger spiral
- avoiding or reviewing the location of bolts, pins, nuts, screws or other protrusions within the auger tube to ensure they can't become loose and cause rubbing and heating, or other means of ignition
- ensuring that end bearings external to the end plates and seals or bearings are made from non-combustible material
- providing an air gap between the bearing mounts and the adjacent seals to allow any leakage to fall free and not allow it to enter the bearing
- washing down and checking all parts for corrosion and wear at regular intervals, and repairing or replacing any damaged parts
- checking the tolerance between the auger spiral and the tube to ensure they haven't differed from the design specifications
- ensuring that electrically powered augers have wiring and electrics to at least IP65 standard
- ensuring equipment is suitable for its intended use and maintaining it in a safe condition (also considering section 3 of AS2187 Part 2 - 2006).