BRILON – Most production processes, e.g. in mills, mixed feed plants, breweries and power stations, start with the unloading of the “raw material”. After its delivery by road or rail, the material is often poured into hoppers. Organisational precautions can be taken, e.g. leaving the truck to cool down for at least 15 minutes before starting the unloading process.
This substantially reduces the ignition hazard from hot brakes, hot exhaust pipes or catalytic converters. Earthing, too, can offer prevention against spark discharge. If movable objects are involved, such as trucks and railway wagons, it is vital to proceed with great caution, and regular staff training should therefore be provided. In addition, agreements can be concluded with suppliers, specifying that the material must be supplied without hot embers, as this helps to ensure effective prevention against explosions within the intake facilities.
In our example the supplied material is taken to a downstream elevator by a screw conveyor. Conveyors can differ from one another and have their specific designs, so that they require different protection methods. All these methods are primarily designed to reduce or even prevent ignition hazards that might arise from the conveyors themselves – by limiting the speed, ensuring appropriate combinations of materials and using a safety-compliant setup. Open, uncased conveyor belts are considered to be the least hazard-prone, as the conveyed material is not usually stirred up and is not in direct contact with hot surfaces – unlike on chain conveyors and screw conveyors, which operate differently. Depending on the fineness, moisture level and dusting propensity of the material, the conveyance principle and speed as well as the connected plant sections, the zoning and the ignition risk assessment, it may be necessary to provide protection through explosion venting devices. As a minimum, any explosions in interconnected plant sections must not be permitted to spread along the conveyors, and decoupling systems are therefore required. The general standard is to install ATEX-approved rotary valves, chemical barriers, quench valves or quick-closing valves.
If expert instruction is provided, the actual screw conveyor themselves can be converted into a protective system with a decoupling effect. One or two flights of the screw conveyor need to be removed. Whether or not this is feasible should depend, in particular, on the flow quality of the material and the installation position of the screw. Ideally, the conveyed product will fill the entire cross section of the pipe in this area, thus plugging it.
The situation is different with trough auger where the upper cross section is not filled with product and where the design strength is not sufficient, so that chemical barriers are necessary. Those are controlled by pressure detectors or infrared sensors which identify flames and explosions and cause an extinguishing agent cylinder to open. The extinguishing agent, which escapes within a few milliseconds, stops and cools down the flame. Outdoors, on the other hand, it may often be wiser and more cost-effective to use approved pressure venting devices, and we would recommend consulting experts for this purpose.