A new type of pneumatic unloading system for self-unloading ships is being introduced and compared with existing systems. The new unloading system uses a large-volume pressure vessel in each cargo hold which is filled via a screw arranged in the vessel itself and which conveys out of the cargo hold directly to the storage silo located ashore, without transfer points. Several of these senders are connected with each other in such manner that a quasi-continuous unloading takes place: One sender is filled while another conveys. The new system is suitable for fine-grained bulk solids, such as cement. Compared with other conventional systems, it is operated with two-stage compressors and is characterised by a clearly lower specific energy consumption at simultaneously higher unloading mass flows and considerably lower numbers of switching cycles of the discharge vessels. The structure of the new system will be described and discussed in detail.
The main requirements of unloading systems for self-unloading ships transporting bulk solids are:
- High discharge flow rate
- Low specific power consumption
- High availability, i.e. long lifetime of valves, wear parts, etc.
- Matching different port setups
- Easy to maintain
- Simple design, easy to operate
- Low space requirement
In the case of unloading systems for fine-grained/dustlike solids, these requirements have to be supplemented by appropriate measures in order to prevent dust nuisance. Here, closed systems are required with a minimum of transfer points ashore as well as on the ship. In particular, the latter requirement has led to an increased use of pneumatic conveying systems for unloading of fine-grained bulk solids, though their specific energy consumption is generally higher than that of mechanical systems.
This article will provide a description and discussion of three exemplarily chosen unloading systems for fine-grained solids available on the market, followed by a comparative analysis of the newly developed system. Cement is assumed as the unloaded material in all cases.
Existing unloading systems
The systems used for comparison are shown schematically and strongly simplified in Figures 1-3. Transport to the storage silos ashore is realised pneumatically. The connection with the systems on board the ship is realised by means of flexible hoses. Usually two parallel conveying lines on shore are used. The cargo holds of the ships are equipped with a gas-permeable aeration bottom, which is inclined by approx. 10° to a central discharge point and is aerated section-wise in such manner that at all times a continuous flow channel forms to the discharge point. The aeration dissolves the friction lock inside the bulk solid close to the bottom and on the bottom itself so that the material flows to the outlet due to the load of the material column. Only during residual discharge is the bulk solid/cement fully fluidized .