All companies that process bulk solids have their own specific features, yet their plants all share certain components, though they may vary in arrangement. These include unloading stations, conveyors, elevators, screens, dryers, mills and filters. This article will look at each element of a plant on its own and describe the explosion safety measures it requires. Using a specific case, it will illustrate why a comprehensive protection setup is always more cost-effective for the plant operator than protecting each part separately. Figure. 1 shows an existing installation and the subsequent discussion is based on this particular arrangement of elements.
Figure 1. Example of standard elements in a production plant of bulk solids.
Unloading and mechanical conveyors
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 usually poured into hoppers. Organisational precautions can be taken, such as leaving the truck to cool 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 the given example, the supplied material is taken to a downstream elevator by a screw conveyor. Conveyors can differ from one another and each specific design requires 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, the connected plant sections and 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 screw conveyors 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, making chemical barriers necessary. Those are controlled by pressure detectors or infrared sensors that 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 consulting experts are recommended for this purpose.
Elevators are some of the most widespread conveyance devices in the bulk solids industry. At the same time they are a special source of danger in the event of an explosion, as both their operation and design are conducive towards explosions (‘explosive mixtures’ and ‘ignition sources’). Moreover, unless an elevator is protected, pressure waves and flames can spread across several floors. Due to this increased risk, the Association of German Engineers (VDI) has set up an expert committee consisting of plant operators, elevator manufacturers and explosion safety specialists who published a dedicated VDI standard. VDI Standard No. 2263, page 8 (sections 8.1 and 8.2) specifies explosion protection precautions for elevators, with advice on the dimensions of the pressure venting devices and the design of the explosion suppression systems.
In addition to explosion vents, which are normally used as standard pressure venting devices, the VDI standard also deals with flameless venting devices. This is because if the elevator is situated within a building, pressure must not be vented via bursting discs alone.
Based on the specifications of the VDI standard, three companies – REMBE, IEP Technologies and GreCon – have formed an expert committee for the development of a comprehensive protection system for elevators.
ElevatorEX, the result of this working group, offers a suitable solution for all types of elevators. Hazard-prone parts of the system, such as the foot and head of the elevator, are equipped with maintenance-free pressure discharge devices, such as the Q-Box or bursting disks. The elevator legs are isolated by extinguishing systems, which are activated by spark detectors. The system is suitable both for the first-time fitting and for the retrofitting of existing elevators.
Again and again expert committees have discussed the special explosion properties of elevators with round shafts. They see such a model as hazard-prone, as it has a round cross-section that offers more expansion potential for an explosion than a rectangular cross-section. This increases the severity of the explosion – i.e. the reduced explosion pressure, by a factor of about 2.
It is, of course, advisable to take a range of further precautions, such as using aspiration systems to limit the dust concentration. However, this applies to all the parts of a plant, not just the elevators.
This is part one of a three-part article written for World Cement’s February issue and abridged for the website. Subscribers can read the full issue by signing in, and can also catch up on-the-go via our new app for Apple and Android. Non-subscribers can access a preview of the December 2015 issue here.
Read the article online at: https://www.worldcement.com/special-reports/04022016/explosion-protection-part-one/