Covered hoppers can be used for a great variety of loads. Some of these loads can be a little stubborn when it’s time to be unloaded. Pressure differential, or more commonly referred to as simply “pressurized” covered hoppers are one of the more common modifications to facilitate unloading these commodities.
Pressurized covered hoppers come in all shapes and sizes. Like other conventional cars, their size and capacity is tailored to the density of the load. This maximizes payload while staying within the weight limits of the trucks. The size and number of roof hatches, angle of the slope sheets and sides of the car and size of the discharge outlets can also vary according to the specific needs of the shipper. And of course there are the different looks offered by competing car builders as well.
The one easy spotting feature for a pressurized covered hopper is the extra piping around the hopper bays used to pressurize the car and remove the contents.
These cars are not pressurized for the entire trip. At the receiver’s siding, the cars are attached to a stationary air supply. Air flows into the car, often through outlets beneath the load, pressurizing to about 15 psi.
As the air passes through the lading, it loosens and dries the load, like the Airslide cars discussed in an earlier blog. The added pressure inside the carbody then helps push the contents out through the discharge pipes at the bottom of the hoppers and into storage silos or even waiting trucks.
Because the load can be pumped out through outlets on the sides of the hopper bays, no pit or conveyer system is needed. Unloading facilities can be minimal. Even an empty siding or team track can be used if the load is transferred into a truck for delivery to an offline customer. (These are easy “industries” to add to your model railroad when you’re short on space.)
Besides these offline customers, pressurized covered hoppers can be seen serving a lot of industries. Smaller cars (around 3000 cu ft. capacity) are most commonly associated with cement but there are other similar mineral loads as well. Mid range cars (4600-5100 cu ft.) haul flour, corn starch, kaolin and fly ash. The largest pressurized cars are around 5600 cu ft capacity and typically haul certain types of plastics – most commonly PVC and polystyrene – as well as cars lined for food-grade service or unlined for mineral service.