Mobile systems
The systems illustrated above have all been fixed in a given location and the only mobility has been in terms of vacuum nozzles where, with flexible hose, limited movement is possible such as that required for ship off-loading and the clearing of materials
from stockpiles and spillages. Many bulk particulate materials are transported from one location to another by road, rail and sea.
Many materials, of course, are transported in a pre-packaged form, or in bulk containers, and can be transported by road, rail, sea or air, in a similar manner to any other commodity. Many transport systems, however, are specifically designed for bulk particulate materials and have a capability of self-loading, self-off-loading or both. These are generally mobile versions of the above static conveying systems, depending upon the application and duty.
Where materials are transported by road, rail and sea they will be subject to con- siderable vibration, and hence compaction and de-aeration, and so this must be taken into consideration when designing the off-loading facilities.
Road vehicles
Many road sweeping vehicles employ vacuum conveying for their operation. These are generally single stage in operation with an on-board exhauster providing the power for material pick-up. The reception hopper on-board the vehicle is generally hinged so that it can be off-loaded by gravity. Vehicles used for clearing materials from stock- piles are generally designed on the basis of Figure 2.5 so that they have the capability of delivering the collected material into a reception vessel.
Road vehicles are widely used for the transport of a multitude of bulk particulate materials, such as cement; sugar, flour and milk powder in the food industry; sand and soda ash in the glass industry; and nylon, PVC and polyethylene in the chemical indus- try. Road vehicles often have their own positive displacement blower mounted on-board and so can off-load their materials independently of delivery depot facilities. The material-containing vessel on-board doubles as a reception hopper for the collection of material and its ultimate discharge. This may be tipped to facilitate discharge, which can be via a rotary valve, or the vessel may be capable of being pressurized so that it discharges as a blow tank.
Rail vehicles
Railway wagons generally rely on delivery depot facilities for off-loading, because of their length tilting is not an option and multiple point off-loading is often employed. They may be off-loaded by rotary valve, or the wagon may be capable of being pres- surized so that it can be off-loaded as a blow tank.
Whereas road vehicles are typically designed to operate with air at 1 bar gauge for this purpose, railway wagons are generally designed to 2 bar gauge and a full length wagon can usually be off-loaded in about 1 h. The base of the wagon is generally sloped at about 5 degrees in herringbone fashion around each discharge point and fluidized to facilitate removal of as much of the material as possible.
Ships
Large bulk carriers usually rely on port facilities for off-loading and these are generally similar to that depicted in Figure 2.6. Intermediate bulk carriers, however, often have on-board facilities for self-off-loading. Such vessels are often used for the transfer of materials, such as cement, to storage depots at ports for local supply, or to off-shore drilling rigs.
Materials are typically transferred from storage holds in the ship by a combination of air-assisted gravity conveyors and vacuum conveying systems, into twin blow tanks located in the centre of the vessel. High pressure air is supplied by on-board diesel driven compressors and materials are conveyed to dock-side storage facilities through flexible rubber hose, which solves the problems of both location and tidal movements.