Type of unloading systems
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Enclosed have prepared a short introduction to the various unloading system which you could consider.
Vessel's based equipment
In basis there are two different selfdischarging systems for bulk cement vessels, i.e. mechanical and pneumatic systems, and both installed in specialized vessels (named Cement-carriers)
Mechanical unloading
The cement-carriers having pure mechanical systems mainly equipped with an chain- or screwconveyor system in holds, with vertical screws to decklevel, or with airslide in holds and with vertical screws, bucket-elevators or a sucktion and cyclone system to decklevel. From decklevel the bulk cement being either carried overboard by use of screw conveyors or by an airslide beltconveyor to hoppers or direct to trucks. The mechanical system fits well into shore terminals where silos located close to the berth, or where the distance from the berth to the silos are fairly short. The mechanical systems primarily used in Japan, in Middle East,Med Sea and for direct screwconveying into silos also in Scandinavia, and on many cement-carriers the owners have combined the mechanical discharge with a pneumatic blow system, enabling vessel to be more flexible.
The mechanical system normally permits loading rates of 600-1000 Mts/hr and discharging rates of 300-700 Mts/hr, and the KW per transported Mts of bulk cement corresponds to 0.5 Kwh/mts for the unloading system based on a normal unloading rate of 3-400 Mts/hr, but if raised to 600 mts/hr the effective energy consumption is reduced to 0,4 Kwh/ton, which for larger quantities, may be of interest.
The advantage is that the system is totally enclosed free of all environmental problems and both loading and discharging are normally fully automatic and controlled by one man only. The equipment is due to the simplicity, very reliable and requires a minimum of service and maintenance. The costs of operation for loading and discharging are very low compared to any pneumatic alternative, and relates to 0,4-0.5 Kwh/mts for the vessel based equipment and about 0.5 Kwh/mts for the ongoing belt or screwconveyor to the silos ashore. Normally we see an effective discharge rate at close to 95% of the design criteria. It should be noted that mechanical transport only requires minimum filtercapacities on the receiving silos.
The drawback of this system is that the installation of the equipment is relatively more expensive than the open hatch system or even than the closed pure pneumatic system. Further the use of a mechanical discharge system requires that the receiving facilities are fully adapted for such system, as unless there is a mechanical receiving and convey system ashore, to carry the cement further (open or closed) then unloading is impossible.
Pneumatic unloading
The cement-carriers having their own pneumatic systems are mainly equipped with chain/screw conveyors in holds or with airslide in holds, both systems to bring the bulk cement further into pneumatic pump(s). This can be either a screwpump, in which the bulk cement is continously being feeded, or into a pressurized pumptank where the cement is dumped down into the tank and when same filled, being pressurized and discharged ashore. The system could also be a sucktion-pump equipment, where the cement is sucked from the airslide in the holds and into the sucktion tank, and when filled, then put under pressure and discharged ashore. It should be noted that the filters on the receiving silos shall be dimensioned for receiving 250-500 cbm air per minute. The pneumatic systems, onboard vessels, are most common in Scandinavia, USA, Mexico and most of Europe.
Some smaller cement-carriers (between 200-2000 Dwt) are equipped with pressurized bulktanks i.e. 8-10 tanks on each vessel, which to be loaded by gravity or pneumatic and discharged pneumatic by adding compressed air to each of the tanks. These vessels are mainly trading on rivers and large inland waterways.
The advantage of the pneumatic system on cement-carriers is that the system is total closed, free for environmental problems, and fairly easy to install. The pneumatic system normally allow you to pump the cement, at a speed of 150-300 Mts/hr, via steelpipes to a distance of upto 300 m, thus larger distances can be obtained by use of larger compressors, or boosters. Normally we calculate an effecticely rate to 90% of the design rate. Most of the heavy equipment can be purchased locally, which reduces cost and enables quick service for spares. The system is more flexible than a mechanical system and vessels are able to call other ports, than the dedicated, in order to pump the cargo into silos located close to the berth, or bulk trucks on the quay (by use of flexible hoses), or to transfer the cargo into other vessels or floating silos.
The drawback of this system is the rather high energy consumption for pneumatic conveying, both onboard the vessel and ashore for further transport to the silos. The onboard consumption normally 0,9-1,2 Kwh/mts bulk cement, and ashore same naturally relates to the distance of transportation and normally varies from 1,5-2,0 Kwh/mts, however for high rates and long distances up to 600 m we shall calculate 3,5-4,0 Kwh/mts. It should also be noted that the longer the distance from vessel to the silos the more air needed for the pneumatic conveying and thus the extra power requirement for cleaning filters on top of silos increases. Normally we would have to install filters capable to receive 250-500 cbm air/min.
Shore based equipment
The use of shorebased equipment can either be used with standard open hatch vessels without any equipment installed onboard or with semi-specialized vessels equipped with airslide in holds. Surely grab discharge together with beltconveyors can also be used, but hardly recommended and not presumed to be accepted by any local authorities.
Standard vessels
Using standard open hatch vessels, we can either use a stationary or mobile pneumatic sucktion unloader or a mechanical screwunloader both combined with a pneumatic or mechanical transfersystem, enabling the vessel to be discharged (based on one or two units and with open hatches only) at a rate of 100-400 Mts per hour.
Pneumatic unloading:
We can use a mobile or stationary pneumatic shipunloader, which system, by use of a long sucking arm, sucks the bulk cement (with open hatches) and discharges it pneumatically at a distance of 2-300 m at rates between 100-400 Mts per hour. The requirement for a large unloading capacity can often be secured by using two smaller unloaders, thus securing a faster clean up of the holds and a more flexible operation. Alternatively we can use booster arrangements to secure the necessary pressure. Utilizing such pneumatic shipunloader normally requests a consumption of 3-4 Kwh/tons unloaded plus 1-2 Kwh/ton for further conveying in pipes to silos. Such unloaders either stationary, or mostly placed on a truck-trailer, or placed on rubber wheels for moving alongside the quay. The weight of such equipment ranges from 25-95 mts. The use of mobile ship-unloaders for bulk cement on standard vessels, mainly being used where the imported quantities are fairly small, where the contract period is short or where the receivers for any reason want to be able to reposition the unloader to another port. The unloaders are mostly equipped with own diesel and compressors thus totally independent of any shore requirements. It should also be noted that the longer the distance from vessel to the silos, the more air needed for the pneumatic conveying and thus the extra power requirement for cleaning filters on top of silos increases.
Normally we would have to install filters capable to receive 250-500 cbm air/min (on ship and onshore). The mobile system mainly being used in smaller ports in Europe, Indonesia, Taiwan, Korea, USA and in Mediterranean Sea, as well as in ports world-wide where the port authorities do not allow any stationary equipment to be positioned on the quay.
The advantage of this system is that the units are independently powered and mostly mobile (subject to road conditions), enabling the units to be placed alongside the vessel and restored into parking position, or moved to another location for another discharge.
The drawback of this system is that whatever we call the system mobile or fixed, the equipment due to the weight will hardly be removed from the position once it is placed, unless strong demands require same. Further the shifting of the arm(s) from one hold to another, as well as moving of the arm inside each hold to secure full sucktion depth and continous supply, normally reduces the effective discharging rate till 65-75% of the design capacity. By using the system for larger vessels (5000 - 20,000 Dwt) we will have to use larger units with very long sucktion knuckle-arms,to reach over the hatchcoaming, in which case the manoeuring of the knuckle-arms will be limited, and sucktion will require more vacuum.
The units are all very weather dependent, as all discharging will have to stop in case of rain or winds, and with open hatches, one can never avoid some of the bulk cement, in form of cement dust, to be spread around the berth facilities.
Mechanical unloading
Using a mobile or stationary mechanical shipunloader, does in many respects work like a pneumatic shipunloader, however as the system works totally mechanically, by use of screws and further transport ashore by screws or beltconveyors, our demand for energy is drastically lower than for the pneumatic systems. Normally we can calculate a discharge capacity of 0,4-0,5 Kwh/ton for the overboard discharging and further 0,4-0,5 Kwh/ton for the beltconveyor (or screw conveyor) transportation ashore basis 300-600 m and 300 mts/hr, but if raised to 600 mts/hr the effective energy consumption for the unloader is reduced to 0,4 Kwh/ton, which for larger quantities, may be of interest.
The advantage of this system is that the units are also independently powered and mostly mobile, enabling the units to be placed alongside the vessel and restored into parking position, or moved to another location for another discharge.
The drawback of this system is that whatever we call the system mobile or fixed, the equipment due to the weight will hardly be removed from the position once it is placed, unless strong demands require same. Further the shifting of the arm(s) from one hold to another, as well as moving of the arm inside each hold to secure full sucktion depth and continous supply, normally reduces the effective discharging rate till 70-80% of the design capacity. The units are all very weather dependent, as all discharging will have to stop in case of rain, and even the mechanical system does not create too much dust, one can never avoid some of the bulk cement, in form of cement dust, to be spread around the berth facilities.
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