by hausen » September 4, 2010, 3:12 pm
It depends entirely on the facility and loading equipment used, and less on the size of the vessel being loaded (i.e. whether or not a thousand-footer should be used in an example.)
Most ore and coal docks load from stockpiles or from storage pockets. A direct rail-to-vessel transfer as the sole means of loading is rare. I think the coal dock at Sandusky does it this way - someone else might know the details of that operation.
The Midwest Energy coal dock in Superior, WI lists the train-unloading speed of their car indexer/rotary dumper at 5,000 tons per hour. That's about 40 coal cars per hour at a standard capacity of about 125 tons per car. That speed is well within the capacity of their shiploader and its attending conveyor belts, so if the dock did a direct transfer that would be about the speed.
Don't know if direct rail-to-vessel transfers are possible at newer style ore docks like CN Escanaba or BNSF Superior, but again the restricting factor there would be the speed at which the cars can be emptied onto the conveyor belts that feed the loaders. Docks like these can often dump 2-3 carloads at a time onto the belts, so a rough estimate would be about 40 - 60 railcars per hour, and at least at BNSF they're using big ore cars with capacities about 100 tons each.
Direct transfer is simple on the old gravity style docks, but if that was the only method used things would go slowly. Each gravity dock-compatible ore car (or 'jenny' as they're called in the region) holds 70 or 80 tons. Although they can be emptied straight through the dock, down the chute, and into a ship in mere seconds, it takes hundreds of ore cars to fill a ship and they cannot all fit on the dock above the loading chutes at once. The constraint here would be the time it takes to pull each string of empties off the dock and replace it with a fresh set of loaded cars. I'd imagine that it would be fairly time consuming, and even an average ship would require many such switch-outs. A wild guess at an estimate for a gravity dock with today's crewing levels would be about 80 cars an hour for a 700' freighter.
The storage pocket behind each chute can typically hold about 300 tons or about 4 carloads of ore. Typically at a gravity dock, before a ship shows up to load the dock crew has been busy running strings of cars out onto the dock and dumping them until enough pockets are filled to match the cargo capacity of the ship. When the ship loads it shifts back and forth along the length of the dock until it's been under enough different sets of pockets to become fully loaded.
The CN/Missabe conveyor shiploader in Duluth was built into an existing section of gravity dock. It can put 30,000 tons of iron ore onto a vessel in one hour, though due to vessel stability concerns, crewing levels, etc. they don't usually run the system that fast. That speed can only be achieved if they're loading from the storage pockets above the conveyors; there's no way for them to keep up with 30,000 tph either by loading directly from railcars or by recharging by conveyor from their stockpile.
It depends entirely on the facility and loading equipment used, and less on the size of the vessel being loaded (i.e. whether or not a thousand-footer should be used in an example.)
Most ore and coal docks load from stockpiles or from storage pockets. A direct rail-to-vessel transfer as the sole means of loading is rare. I think the coal dock at Sandusky does it this way - someone else might know the details of that operation.
The Midwest Energy coal dock in Superior, WI lists the train-unloading speed of their car indexer/rotary dumper at 5,000 tons per hour. That's about 40 coal cars per hour at a standard capacity of about 125 tons per car. That speed is well within the capacity of their shiploader and its attending conveyor belts, so if the dock did a direct transfer that would be about the speed.
Don't know if direct rail-to-vessel transfers are possible at newer style ore docks like CN Escanaba or BNSF Superior, but again the restricting factor there would be the speed at which the cars can be emptied onto the conveyor belts that feed the loaders. Docks like these can often dump 2-3 carloads at a time onto the belts, so a rough estimate would be about 40 - 60 railcars per hour, and at least at BNSF they're using big ore cars with capacities about 100 tons each.
Direct transfer is simple on the old gravity style docks, but if that was the only method used things would go slowly. Each gravity dock-compatible ore car (or 'jenny' as they're called in the region) holds 70 or 80 tons. Although they can be emptied straight through the dock, down the chute, and into a ship in mere seconds, it takes hundreds of ore cars to fill a ship and they cannot all fit on the dock above the loading chutes at once. The constraint here would be the time it takes to pull each string of empties off the dock and replace it with a fresh set of loaded cars. I'd imagine that it would be fairly time consuming, and even an average ship would require many such switch-outs. A wild guess at an estimate for a gravity dock with today's crewing levels would be about 80 cars an hour for a 700' freighter.
The storage pocket behind each chute can typically hold about 300 tons or about 4 carloads of ore. Typically at a gravity dock, before a ship shows up to load the dock crew has been busy running strings of cars out onto the dock and dumping them until enough pockets are filled to match the cargo capacity of the ship. When the ship loads it shifts back and forth along the length of the dock until it's been under enough different sets of pockets to become fully loaded.
The CN/Missabe conveyor shiploader in Duluth was built into an existing section of gravity dock. It can put 30,000 tons of iron ore onto a vessel in one hour, though due to vessel stability concerns, crewing levels, etc. they don't usually run the system that fast. That speed can only be achieved if they're loading from the storage pockets above the conveyors; there's no way for them to keep up with 30,000 tph either by loading directly from railcars or by recharging by conveyor from their stockpile.