Pneumatic ship unloader vs. mechanical ship unloader

"We are planning the construction of a cement terminal and would like a comparison between a pneumatic and a mechanical screw unloader, which is the better unloading system?"

This question is put to me very often and my answer actually always starts with: "That depends..."

 

As you have already guessed, there is no clear answer to this question. It is necessary to take a closer look at the framework conditions. An unloading system cannot cover all the possible requirements of unloading a ship and transporting the material to the silo facility. Therefore, both systems have their place in the market.

It is generally true that pneumatic ship unloaders are better suited for smaller ships while screw unloaders are better suited for larger ships but the situation is more complex.

Ship sizes up to approx. 15.000 dwt

The first thing to clarify is whether only one ship unloader is needed or a whole transport system to existing or newly built storage silos. If the material is only unloaded from the ship and filled directly into trucks, the first choice is a screw type unloader. It only brings the equipment needed to unload the material, i.e. the conveyor system and a loading equipment for closed cement tankers, ideally with integrated filter system (Fig. 1).

 

Fig. 1: Road mobile mechanical screw type unloader with two loading spouts for truck loading

This is because in pneumatic unloaders the transport system to the storage silos is already integrated. A vacuum system sucks the material out of the cargo hold and a pressure system blows it through pipes into the storage silos. This configuration makes it difficult to load directly into cement tankers. In contrast to mechanical screw unloaders, pneumatic ship unloaders (Fig. 2) have advantages when loading silos, because only pipelines are needed between the quay and the silo. Screw unloaders additionally require horizontal and vertical conveying systems for silo feeding.

 

Fig. 2: Road mobile pneumatic ship unloader for direct silo feeding

Ship sizes up to approx. 50,000 dwt

For larger ships, up to 50,000 dwt (Dead weight tons), for example, it no longer makes sense to load directly into cement tankers. The annual handling rates here are 1 million tons or more.  The demands on the unloading system are becoming more complex. The difference in functionality between pneumatic and mechanical ship unloading systems leads to a different ratio between investment costs and operating costs. Pneumatic ship unloaders (Fig. 3) usually have lower investment costs in this segment, but significantly higher operating costs. This is mainly reflected in energy costs, in addition to spare parts costs. An economic operation of pneumatic ship unloaders is therefore only reasonable up to an unloading capacity of approx. 600 t/h.

 

 Fig. 3: Port mobile pneumatic ship unloader

The competition between the two systems is greatest in this market segment. The general conditions, such as ship size, unloading capacity, conveying distance and the annual throughput of the terminal must be considered carefully and the energy costs must be compared in order to make the right decision. 

If higher unloading capacities of e.g. 800 to 1500 t/h are required to keep ship lay times as short as possible and ship sizes up to 100,000 dwt are to be unloaded, pneumatic ship unloaders are no longer an issue. This task is exclusively covered by mechanical ship unloaders and mechanical transport systems to the silo plants.

 

Fig. 4: Mechanical screw type unloader for 100.000 dwt ship size

 

In summary one can say:

The choice of a ship unloader system is not so much a choice between mechanical or pneumatic, but rather depends on which requirements are to be covered. These requirements determine which unloading system is the better choice.