Rotary Railcar Dumpers, also known as Wagon Tipplers in some parts of the world, are the primary mechanisms used for unloading open-top railroad cars. A dumper holds a railcar to a section of track and rotates the track and car together in order to spill the car's contents. Together with bottom discharge railcars, the rotary dumper is one of only two accepted methods for unloading these types of railcars.
Selecting a railcar unloading system means that a number of factors must be considered.
Capital costs: Unlike operating costs, which are the daily or monthly expenses related to the routine operation of a power plant, a capital cost is a one-time expense incurred in the purchase of new equipment, which increases production and often lasts for years. Capital costs are fixed, and are therefore independent of the level of output. A rotary railcar dumper represents just such a capital cost, whereas there is no corresponding cost for a bottom dump system. Both methods involve a deep receiving pit to be dug in the ground and a system of conveyers to carry newly-dumped material to its destination, but the rotary dumper is the only one of the two that requires such a large piece of equipment. The rotary dump method may also require auxiliary equipment such as a train positioning system to carry out the unloading procedure.
Maintenance: Technical maintenance involves fixing any sort of equipment should it become out of order or broken, and also includes performing routine actions to keep the device in working order or prevent trouble from arising. For a Rotary Railcar Dumper, there is only one centralized system to maintain, rather than an entire train line. Bottom dump railcars feature sloped chutes covered by door hatches for unloading, using the force of gravity to allow the material to fall out of the car. Any imperfection in the seals of the doors can allow material to spill onto the track. With a rotary dump system, there are also no door seals to replace, no railcar gate failures, no railcar linkages and no hot shoe maintenance.
Climate: As the temperature drops, it becomes more of a factor in the transfer of bulk materials than some may realize. Materials such as coal, coke, lignite, iron ore and wood chips all tend to clump together as the water moisture they contain turns to ice. In fact, there is a greater opportunity for a load of coal to freeze if the haul is more than one day away from the receiving station. Clumped or frozen materials are easily discharged with the Rotary Railcar Dumper through the single opening in the top of a coal car, but not so easily through a bottom dump car's smaller chutes. These types of cars often must be kept in thaw sheds overnight to allow the unloading process to be more free-flowing. The railcars can also face demurrage charges while they are detained, payable either to the shipper for holding the car whether laden or not, or to the connecting railroads while the car is empty and returning to the home road. Multiply all that by the number of cars in the train, and bottom discharge has significant disadvantages while a rotary dumper continues its more speedy unloading.
Versatility: There is a greater opportunity for a plant to receive coal if the facility handles both standard and bottom dump railcars. A rotary dumper requires a deeper receiving pit, about 10-15 feet, so that it can include a hammermill to break up clumped or frozen 
materials. In addition, the platen can include various integral devices to aid in safe car handling. These include a weigh scale that allows railcars to be weighed before and after dumping, an retarder to stop or hold cars on the platen, an ejector to remove empty cars and facilitate throughput, and a rerailer to eliminate truck associated misalignment.
The best of both worlds the Bottom Dump Rotary hybrid made by Heyl & Patterson, shown in the image to the left. This machine is a rotary dumper with an open platen for bottom dumping capability. The platen can be covered with planking for worker safety. This provides facilities with a greater opportunity to receive coal, since both standard and bottom dump railcars can be handled. Employing this type of dumper means that a rotary dumper facility has the capability of handling bottom dump railcars, but not vice versa.
Economics: A bottom discharge railcar, such as the Superflow Coal Car from American Railcar Industries, holds a capacity of 4,603 cubic feet of material in a 10-foot average heap, with a load limit of 235,000 pounds. In contrast, a standard railcar that can be rotary dumped, such at the ARI RotoFlow Coal Car, holds a capacity of 4,911 cubic feet of material in a 10-foot average heap, with a load limit of 243,100 pounds. A standard coal car carries 8,100 pounds more material, for an additional 308 cubic feet. In a 110-car train of standard railcars, that equates to an average of 7.63 more railcars than a train with the same number of bottom discharge cars. That means that if a receiving station accepts one train every other day, 180 days per year, then a Rotary Railcar Dumper handles 1,325 more railcars per year than a bottom dump station. When all other things are equal, a rotary dumper can handle more material.
Bottom dump railcars may have a lower capital cost and less maintenance of plant infrastructure, but these features are offset by less efficient train operations, the increased maintenance of rolling stock and the inability to handle as much material as a rotary dumper. The dumper may be a larger up-front cost, but its versatility, centralized maintenance and more efficient and economical train operations give it a definite advantage. In colder climates especially, there can be no argument.
For a copy of Heyl & Patterson's presentation on this topic, which was recently given at the 2012 Coal Handling & Storage Conference in St. Louis, MO, click here:
