TM 5-810-15
below 2000 tph and product size is below 3 inches.
for inventory purposes. It is important to weigh the
Sample system manufacturers will provide help
coal as it is delivered to the plant, and again before
with system sizing and requirements. Due to the
it is burned.
complexity and high cost of sampling system, the
b. Type. A belt scale of the weighbridge type
designer must decide if a sample system is a
which incorporate electronic precision strain gauge
justifiable piece of equipment to meet the end
load cell and microprocessor based technology with
results.
automatic calibration capabilities should be se-
c. Sweep type sampling. Sweep type (or hammer
lected. The belt scale, including weighbridge as-
type) are relatively new and have different design
sembly will be capable of withstanding at least 250
conditions than given above. This type takes less
percent material overload without damage to any
sample than a cross stream type, usually from 1/3
mechanical or electrical components.
to 1/6 less, depending on conveyor speed, material
c. Scale accuracy. If a belt scale is to be used for
size and flow rate. This allows two stage sampling
basis-of-payment contracts between the coal sup-
systems to be employed with virtually any flow rate
plier and the plant, the coal supplier may require a
using sweep samplers for the first and second
scale with 0.125 or 0.25 percent repeatability
stages. Also, for low capacity installations of ap-
accuracy. For general plant inventory purposes, a
proximately 50 tph and below, a single sweep
0.5 percent accuracy is usually acceptable. If a
sampler with sample collector can be used to meet
scale is used for billing purposes or invoicing
ASTM D 2234. At these low flow rates, a manage-
freight, approval and certification by the weighing
able amount of sample is collected for laboratory
bureau which has jurisdiction for that particular
analysis with a minimal capital investment.
geographical area must be obtained.
d. Readout. The scale can be connected to a
5-13.
Magnetic separators and detectors.
computer or a printer to provide a readout of the
a. Magnetic separators. Magnetic separators are
quantity of material delivered to the plant. The
installed to remove potentially damaging magnetic
readout will be easily readable by the operator and
tramp iron from the material on the conveyor belt.
be such that he does not have to do any manual
Tramp iron is removed from the conveyed material
calculations to find the amount of coal received.
by the separator and can be automatically or
manually discharged to a collection hopper. A
5-12.
Sampling system.
single unit mounted ahead of the crusher on the
a. General. When a given consignment of coal is
conveyor is usually all that is required to protect a
delivered to the plant, it may be advantageous to
complete conveying system. A small piece of tramp
the plant to determine by laboratory analysis some
iron can put an expensive crusher out of action
of the characteristics of the delivered coal. Sam-
very easily. The separator will also protect the
pling is used to take a representative sample from
conveyor belts from being ripped by large pieces of
the complete coal consignment lot and provide a
tramp iron. A separator is a relatively inexpensive
quality evaluation of that sample. Because of the
and necessary method for protecting crushing ma-
variability of the chemical composition of the coal,
chinery, conveyors and the plant boilers.
the analytical results from a sampling system can be
used to determine coal contract rates, reliable and
magnetic and nonmagnetic tramp iron and are
efficient quality assurance, plant operating
usually installed in conjunction with a magnetic
efficiency and compliance with environmental
separator to provide additional protection for all
standards.
downstream equipment. When tramp metal is
b. Design conditions. Each sampling system will
detected, the unit automatically die marks the
be designed for a specific location and on an
location and shuts down the conveyor before any
individual plant basis. One sampling system cannot
damage is done. The operator has to manually
necessarily be used for another similar system.
remove the foreign material before restarting the
Depending on the capacity of system, one or more
conveyor.
sampling stages may be required to obtain the
volume of the final sample required for analysis.
be used to remove tramp iron, but are usually not
ASTM standards establish the requirements of the
as effective as a magnetic separator, and are seldom
final sample for each particular system. A good rule
used in coal handling systems.
of thumb for selection is a three stage system is
used for flow rates which exceed 2000 tph and
5-14.
Coal crushing equipment.
when product size is greater than 3 inches, while a
a. General. Stoker fired or pulverized coal boiler
two stage system is used where flow rates are
plants install crushers for use when a larger and
5-9
Previous Page
