“Depending on the product, we unload 12 cars per hour,” Yelvington says. “If it’s base or screenings or a fine product, it might be 10 cars per hour. If it’s a coarser material that flows and spreads freely, we might do as many as 14 per hour.”

Bob Dominick, PE

With big money on the line, many producers moving material by rail are becoming increasingly savvy about how little changes in the process can improve overall metrics. “In the past, I think there just had never been a study of process flow for rail unloading,” says Bob Domnick, vice president of sales, marketing, and engineering for Morris, Minn.-based Superior Industries, LLC. Factors such as the size of the hopper and the mix of rail cars sometimes vary widely, which can lead to inefficiencies in the process.

“It seems to me that, in the past, operators would build pits with as big of a hopper as they could,” he says, noting that size would be the maximum available, based on the site’s water level to ensure that pumping would not be necessary. “By and large, there would have been several hoppers and then a conveyor under those hoppers.

“Today, simplicity is chosen, so if a single hopper with one discharge point under that conveyor can be done, that’s what is chosen because you don’t need as much depth to accommodate the conveyors,” Domnick says. “That way, the operator doesn’t have another drive to maintain, which can be a headache.”

The key, Domnick says, is to watch material flow. How long does it take to move from one car to another? “Ideally, the hopper would just be emptying as you get to the next car and start dumping. This ensures continuous flow, and it’s nice for the drives on all the conveyors going away from the rail because they can be sized efficiently,” he says. “Otherwise, you will have surges, which means wider belts and higher capital costs.”

Another step toward more efficient unloading of cars is for each train to be comprised of the same type of rail car. “Sometimes, there are aggregate cars that have two hopper bottoms, some have three, some have four. They just vary in size; they aren’t all 100-ton cars,” Domnick explains. “If the rail cars themselves are identical, you can get a much nicer flow.”

Often, the cars are owned by the railroad, which can make the composition of the unit train a little more difficult to control, but Domnick says that, in the situations he’s seen, approximately two-thirds of the cars have been the same. “More and more, people are trying to get them identical, so the trend is toward better flow,” he adds.