TM 5-810-15
b. Safety devices. Each conveyor in a conveying
i. Conveyor backstops. A backstop is a mechani-
system will incorporate electrical safety devices to
cal device which allows a conveyor or bucket
provide protection to the operating personnel as
elevator drive shaft to rotate in one direction only.
well as to prevent damage to the conveyors*
An automatic backstop will be installed on all
mechanical components.
conveyors or bucket elevators subject to reversal
(1) All electrical safety devices will be
under loaded conditions. Backstop will be sized
electrically interlocked so that when a "trip" signal
according to conveyor drive motor stall torque, and
is received from the device at the point of failure,
be provided with a removable torque arm. The
all the downstream conveyors and feed devices,
backstop will be installed on the conveyor drive
such as crushers and feeders, back to the initial
pulley shaft and not in the drive reducer.
j. Methane detectors. Install methane detectors
conveyor feed source will shut down immediately.
(2) No conveyor can be started until the
and vent system anywhere that coal is stored in an
safety device has been checked and put back into
enclosure.
proper service. Only then can the complete
conveying system be put back into operation.
5-21.
Spontaneous combustion of coal.
c. Emergency stop switches. Pull cord switches
a. General. A major problem with the bulk
and pull cords will be located along all walkways or
storage of coal is its ability to release enough heat,
areas that are accessible to conveyors to protect
through slow oxidation, to raise its temperature
personnel from falling into any rotating or moving
gradually until self-ignition or spontaneous com-
machinery. Once "tripped" these switches have to
bustion occurs. The tendency of stockpiled or
be manually reset before the conveyor can be
stored coal to self ignite increases as the coal
restarted.
ranking decreases.
d. Belt overtravel switches. Belt misalignment
b. Coal ranking. Lower rank coals tend to be
switches will be provided on both sides of the belt
very fragile, resulting in faster degradation and
at the head and tail end of each conveyor and the
particle size reduction during the handling process.
tripper, to detect conveyor belt misalignment,
Anthracite type coals, which are the highest ranking
which can result in serious damage to expensive
coals have few problems and are very easy to
belts, drive equipment and structures. Extra
handle. Lignite and subbituminous type coals tend
switches will be installed at selected intervals, no
to degrade quickly leading to spontaneous
more than 500 feet apart on long conveyors.
combustion.
e. Conveyor zero speed switch. A zero speed
(1) When these types of coal are stored,
switch will be provided for each conveyor in the
provisions must be made to monitor the conditions
system. They are installed on a nondrive pulley,
in the silo, bunker or stockpile to reduce the occur-
preferably the tail pulley to detect a decrease in
rence of spontaneous combustion.
conveyor belt speed, from a given set value.
(2) Precautions must be taken so that mate-
f. Plug chute switches. A plug chute switch will
rial in a silo, bunker or stockpile can be evacuated
be installed at each conveyor transfer point. They
in the event of material self ignition. Without
operate when a plugged chute condition occurs,
oxygen, the oxidation process cannot take place, so
and are arranged to stop the downstream equip-
it is important that the total coal surface exposure
ment from continuing to feed the plugged chute.
to air be as low as possible. Coal should be stored
Similar type switches are used in hoppers, bins,
so that air cannot infiltrate or move through the
silos and chute discharge points. Tilt type switches
storage pile. Spontaneous combustion usually only
are the most common type used.
results from careless storage procedures. Where
g. Warning horns. Horns are used prior to the
coal is properly stored, this likelihood is remote.
conveyor start sequence to warn operating or
maintenance personnel that the equipment is being
5-22.
Coal bunkers.
placed into operation. Horns will be operated for at
a. General. In the design of bunkers, careful
least 15 seconds, before starting any conveyor.
consideration will be given to the capacity, shape,
Provide enough horns to cover all conveyor areas
bunker material, and bunker location within the
in the plant.
steam plant.
h. Guards. Rotating or moving machinery which
b. Storage capacity. Bunker will be sized for a
provides a safety hazard to the operator will be
minimum of 30 hours supply for maximum boiler
provided with a guard or guards to warn the
capacity.
operator that a particular hazard does exist. ANSI
c. Shape. The shape of the bunkers are usually a
B20. 1 gives the conveyor designer guidance on
compromise between space restrictions and opti-
conveyor safety guards.
mum design for coal flow. The more common
5-14
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