Unloading Systems. See MIL-HDBK-1003/6, Section 5, for railroad and truck
delivery and coal handling. Unloading systems are required at the plant site for
removing or discharging coal from the primary carrier. The unloading system is an
integral part of the overall coal handling system for a power plant.
Barge. Barge unloaders are required for river barges (195 feet long x 35 feet
wide x 12 feet side, 1,500 ton capacity each) and for ocean barges (462 feet long x 82
feet wide x 28 feet side and combing, 13,000 to 18,000-ton capacity each depending upon
River Barge Unloader. Unloaders for river barges are usually of the
continuous bucket ladder type. Unloaders of this type range in capacity from 1,500 to
5,000 tons per hour. The unloader supports one or two bucket elevator type digging
elements as shown in Figure 36. The unloader may be arranged to either lower and raise
the elements relative to the barge or swing into and out of the barge from a pivot at
the upper end. Barges are moved under the unloader by use of a barge haul system
especially designed for this type of machine. The haul system consists of a hauling
winch, return winch, sheaves, and wire rope cable. The hauling winch is designed to
move the barge at an adjustable speed from approximately 5 to 50 feet per minute. The
return winch is designed to move the barge at faster rates of speed. Each winch must be
designed to exert a retarding torque during barge unloading in order to keep the cables
tight and prevent barge drift. With the continuous ladder type of unloader, the barge
being unloaded must be kept in alignment with the dock face.
The barge unloading procedure used with the continuous ladder unloader will
vary with the size of barge being unloaded and number of digging elements. For a 195-
foot x 35-foot standard hopper barge, a "two pass" unloading procedure is generally used
with a single digging element and a "three pass" unloading procedure is generally used
with twin digging elements.
Ocean Barge Unloader. Unloaders for ocean barges are usually of the clamshell
bucket type; however, the continuous bucket ladder type has also been used. Clamshell
bucket unloaders range in size from 400 tons per hour to 2,000 tons per hour. The
capacity of a clamshell unloader is specified in tons per hour-free digging rate based
on a mean duty cycle. The mean duty cycle is based on the average travel and lift of
the bucket required for a barge of specified dimensions.
The tower structure of the unloader may be either stationary or movable.
The stationary type requires a system of hauling and breasting winches and cables to
move and position the barge along the dock relative to the unloader tower structure and
boom. Ocean barges have four or more