What Is an Open-Top Hopper Rail Car?

Take an in-depth look at the rail equipment used in the transportation of aggregate, rock, coal, coke, sand, gravel, wood chips and other bulk commodities

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Bulk commodities like aggregate, rock, coal, coke, sand, gravel and shale help build America. Learn how open-top hoppers allows them all to ship by rail.

Open-top hoppers are perhaps the most &#;no fuss, no muss&#; of all the rail car types. Why? Hoppers were developed specifically to be loaded and unloaded quickly and easily, with wide open tops for loading and gates at the bottom that allows bulk commodities to be unloaded simply by using gravity. The products open-top hoppers haul aren&#;t fussy either, as they aren&#;t affected when exposed to the elements. That&#;s why, unlike their cousin the covered hopper, open-top hoppers don&#;t have to have roofs. But even though these rail cars and their contents are low maintenance, they&#;re important, as together they ship bulk commodities like aggregate, rock, coal, coke, sand, gravel, shale, wood chips and other that help build America.

What Are Open-Top Hoppers Used For?

Open-top hoppers are used to ship bulk commodities that aren&#;t affected by the weather. These include:

• Coal
• Sand
• Rock
• Aggregate
• Cullet (recycled glass)
• Wood chips
• Ballast
• Petroleum coke
• Copper concentrate
• Shale

What Does an Open-Top Hopper Look Like?

Open-top hoppers look just like covered hopper cars, with fixed sides and ends, but they don&#;t have a roof on top.

The inside of an open-top hopper has compartments that can be loaded and unloaded individually. Open-top hoppers have anywhere from two to four compartments. Small open-top hoppers have two to three compartments and large open-top hoppers have three or four. Heavier products ship in small open-top hoppers.

The bottom of an open-top hopper has chutes (also known as &#;gates&#; or &#;outlet gates&#;) that can be opened to unload the product. Each compartment has its own chute, and these chutes are angled so product can flow through quickly and easily.

What Are the Different Types of Open-Top Hoppers?

• Large Open-Top Hopper &#; Large open-top hoppers carry commodities like coal, petroleum coke, sand and rock. They range from 51 to 57 feet long, have three or four compartments and can carry loads from 185,000 to 243,800 lbs.
• Small Open-Top Hopper &#; Small open-top hoppers carry commodities like sand, rock and copper concentrate. They range from 37 to 44 feet long, have two to three compartments and can carry loads from 166,000 to 230,300 lbs.

How Are Open-Top Hoppers Loaded and Unloaded?

Bulk commodities are loaded into open-top hoppers from the top and unloaded through gates at the bottom of the rail car.

Open-top hoppers have sloped floors and doors that open at the bottom that let products discharge quickly using only gravity. This is referred to as the &#;sliding gate&#; method.

Open-Top Hopper Fun Facts

• The word &#;hopper&#; dates back to the 13th century and means a "container with a narrow opening at the bottom.&#;
• An early predecessor of the hopper was a small, two-axle car called a &#;jimmie.&#; These were similar to gondolas with chutes at the bottom.
• In , a coal hopper could only haul about four tons. Over time, these hoppers were designed to be larger and stronger, hauling increasingly heavier loads. By the late s they could carry about 40 tons. Today, a single hopper can haul well over 100 tons of freight.

Learn More

To learn more about open-top hoppers or shipping coal, sand, shale, aggregate, rock, gravel and other bulk commodities by rail, get in touch.

Want to learn more about other types of rail car equipment, including auto racks, covered hoppers, flatcars, tank cars boxcars and gondolas? Check out our rail car type guide.

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"What Is an Open-Top Hopper Rail Car?" Video

1. Definition of funnel flow and mass flow in silos

Hoppers / Silos can be classified in 2 types : mass flow silos and funnel flow silos. This classification is done according to the flow pattern of bulk solids in the silos.

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In funnel flow, also called core flow, the product located at the center of the silo will move much faster than the product located at the sides.

In mass flow silos, in contrary, all particles will move at the same speed, which means that the distribution of residence time in the silo will be reduced, whereas such a residence time can be very large with funnel flow hoppers.

Related pages : powder properties, design of silos for good flow

Figure 1 : Bin discharge patterns

2. Pros and cons of each design

Table 1 : Pros and Cons of mass and funnel flow design [1]

Hopper type Advantages Disadvantages Funnel flow Can offer larger capacity with a reduced height (cone walls angles are flatter than with mass flow hoppers)
Reduced abrasion of hopper walls since particles at wall do not move Hoppers are sensitive to arching and ratholing making discharge unpredictable (issues can be mitigated with disharging aids)
Risks of formation of archs or ratholes, then collapse of archs that can cause structural issues to large silos
Collapse of arches can fluidize the whole silo and make the product suddenly flow very quickly
Silos sensitive to segregation
Mass flow First in - First out, particles have more or less the same residence time in the silo
No arching, stable flow output
Less segregation (density is stable at outlet, important for feeder design), such silos even give a certain degree of re-mixing right at the outlet of the silo [2]
Flooding happen rarely since arches are not stable to form Can require tall geometries that may not fit in the installation
May not be possible for all materials - use of discharging aids is then to consider

When having designed a mass flow hopper, it is also very important to have a proper design of the equipment taking out the product from the hopper in order to keep the mass flow effect : gates (butterfly valves, gate valves) should be 100% opened, and feeders for instance screw conveyors) should be taking product on the whole length of the outlet opening of the hopper (use of a variable screw diameter).

3. How to chose in between mass flow and core flow ?

What do you need ? [2]

What you want or have... What do you need ? The powder process can easily segregate, demix Mass flow
As mass flow implies at the outlet a mixing of the different streams within the silo, risks of segregation are reduced The powder spoils easily Mass flow
Mass flow will ensure that the residence time of the whole mass of powder is more or less constant avoiding dead area where the product can degrade
Have a constant density at the outlet of the silo Mass flow
As for segregation, mass flow will ensure a more constant product at the outlet of the silo over time The powder can fluidize very easily Mass flow
Using mass flow will avoid that the powder fluidize too much at the center of the silo and flush out The powder is cohesive Mass flow
Having a mass flow silo properly designed will help to avoid arching and bridging within the silo No particular concerns... You may consider core flow silos as their construction is usually cheaper and design more compact. But beware of all what is written above ! In any case, the silo must be designed carefully by 1st assessing the flow properties of the bulk solid to store

Note that V shaped hoppers (wedge hoppers) can present an interest vs cone shaped hoppers : they can provide a mass flow while being more compact.

4. Silo inspection and cleaning

Be it mass flow or funnel flow, a large silo of several dozens of m3 must be inspected and maintained on a regular basis.

In order to avoid that some material stay too long in the silo (in case of compaction for mass flow silos, or due to the design for funnel flow silos) it is advised to empty completely a silo on a regular basis (it is then practical to have 2 silos as 1 can take over while the other one is empty).

External inspection (especially for outside silos) and internal inspection should also be carried out yearly. It helps to detect structural problems, or issues that could lead to damages. For example, it is important to check if no material is hanging on the inside of the roof as some material buildup could dramatically increase the weight and lead the roof to collapse. If required, a cleaning of the inside can be done by specialized companies (this requires a specific training and very strict safety).

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