The majority of grain exported from the United
States is carried by barge down the Mississippi
and Columbia Rivers to export elevators, where it
is unloaded and stored until it can be loaded onto
ships. In addition, some grain is transferred
directly from barge to ship, without being stored
in an elevator. The following diagram shows
commonly used equipment arranged in a typical
layout, although each elevator has its own unique
layout.
Loading Ships
An export elevator can load a ship with 60,000 tons
of grain, worth over $30 million, in less than two days. The
weight certificate must be very accurate, due to the
large sums of money involved. GIPSA typically has
teams of 3-5 people to inspect and weigh that grain:
a shift supervisor, a weigher, a sampler, a grader,
and protein analyst or mycotoxin tester.
An order of grain to be loaded aboard a ship,
referred to as a "lot", is divided into smaller
units called "sublots" for inspection purposes. A
sublot which does not pass inspection, referred to
as a "material portion," is not allowed to be loaded
onto the ship. Grain is Inspected by sublots rather
than waiting for the entire lot to be loaded so the
lot will be of uniform quality. A lot is often
divided among several customers at its destination,
so it is important for no customer to be stuck with
a pocket of bad grain. Also, a pocket of bad grain
could spoil during the voyage and contaminate the
rest of the shipment.
Every elevator has a different layout. The grain is weighed, sampled,
and held in a shipping bin while it is being graded.
Grain which has passed
inspection is loaded onto a ship.
Terms and Definitions
BARGES: Barges carry most
of the grain bound for export elevators in the
United States. About 60 percent of this grain is
carried on the Mississippi River alone.
BULK-WEIGHING SCALE: A
bulk weighing scale weighs a batch of grain in a
WEIGH HOPPER which sits on load cells or levers.
Grain cannot flow into or out of the Weigh Hopper
while the scale is recording a
weight. During this time, grain flowing to the scale
accumulates in another hopper, the UPPER GARNER,
directly above the Weigh Hopper. After the weight is
recorded, the Weigh Hopper dumps its grain into a
LOWER GARNER, which releases it at a rate which will
not overload the conveyor belts underneath. Grain
enters the Weigh Hopper through SLIDE GATES at the
bottom of the Upper Garner and leaves through slide
gates at the bottom of the Weigh Hopper. The
operation of the gates is controlled by a computer
(SCALE CONTROLLER), which also records weights and
generates an alarm if it detects a malfunction.
DIVERTER-TYPE (D/T) SAMPLER:
The D/T, located either before or after the scale,
sweeps a sampling container (called a pelican)
through the grain stream once every sampling period,
which is
usually set between 12 and 25 seconds. The grain
sample flows through a pipe to the inspection
lab, where it is graded.The law requires grain
arriving by barge at an export elevator to be
officially weighed. However, official inspection of
this grain is optional.
GALLERY : The gallery is
the structure above a ship berth containing conveyor
belts and spouts for loading grain into the holds of
a ship. An operator sits in the gallery and controls
the loading operation. Often an elevator can load
grain into several holds simultaneously. Ship
loading is a precision operation.
The ship must not be under-loaded or over-loaded,
and the load must be balanced so the ship will
be stable on the long ocean voyage.
GATES: Gates control the
flow of grain in an elevator. Every storage bin has
a proportional slide gate
underneath it to regulate the flow onto a conveyor
belt. By simultaneously opening the gates on
several bins by precise amounts, the elevator can
blend grain to meet the grade specified in the
contract.
INCLINED BELTS: Inclined belts
are the preferred way to elevate grain when it must
also be moved horizontally. A belt causes less
damage to the grain than a leg and generates less
dust. Dust must be controlled in grain elevators to
prevent fires and explosions and maintain the
quality of the air near the facility.
LEG: A LEG (also called a
BUCKET ELEVATOR) is a vertically-oriented conveyor
belt with scoops bolted to it, used to elevate
grain.
MARINE LEG: A marine leg is a
steeply-inclined conveyor belt with scoops bolted to
it, used to scoop grain out of a barge and elevate
it. An operator can move the leg around while it is
running to reach most of the grain in the barge. A
front-end loader can be placed in the barge to push
grain which is out of reach over to the leg. Some
facilities finish unloading barges
with a vacuum system.
SHIPPING BIN: A shipping
bin holds grain which has been weighed and is being
graded. Once the grain passes, it can be sent
directly to the ship. If the grain does not pass, it
must be returned to the
elevator. Some elevators load grain directly to the
ship, without using shipping bins. At these
elevators, a sublot of grain is graded after it has
been loaded. If it does not make grade, the elevator
has to
get a clamshell rig and unload that sublot from the
ship. The expense of this operation explains
the popularity of shipping bins. Grain returned to
the elevator is not wasted. Usually it is cleaned
and mixed with better grain so it will pass
inspection, then submitted as another sublot.
SHIP HOLD: A ship hold is
a space for carrying cargo. Ship holds must be
examined before loading by GIPSA or the delegated
state agency to ensure they are free of moisture,
rust, loose paint, rodents, insects, odors, or any
other contamination.
STORAGE BINS: Storage bins
hold large quantities of grain, sorted by type of
grain and grade, until it is needed to fill an
order.
TRIPPER: A tripper is a
structure that removes grain from a conveyor belt.
It can be moved along the belt to remove grain at
different positions.
TURNHEAD: A turnhead is
used in some facilities to direct grain from a
single inlet duct to any one of several outlet
ducts. The entrances to the outlet ducts are
arranged in a circle, and the inlet duct is mounted
on a shaft so its discharge opening can be rotated
over the desired outlet.