Chemical Engineering April 2020 - 38

Martin Engineering
Martin Engineering
FIGURE 4. Track-mounted components allow fast replacement by a single worker
consider track-mounted impact
cradles and belt support cradles
(Figure 2). Located under the skirtboard
and mounted with rugged
steel assemblies, the cradles feature
large impact-absorbing UHMW (ultrahigh
molecular weight) polymer
" box bars " engineered with smooth
surfaces across which the belt can
slide with little friction or belt wear.
These assemblies can be pulled out
by a single worker, working safely
from outside the conveyor. In some
cases, box bars can be simply removed
and flipped over to double
the service life.
Along the cargo path in the settling
zone and beyond, slide-out/slide-in
idlers support the belt and maintain
the trough angle. Exposed to the
punishing environment, gritty dust
and extreme weather, rollers can
easily seize. Often set close together
in the loading zone to avoid belt sag,
easy-to-service roller frames permit
workers to perform idler service
outside of the belt plane without the
need to raise the belt or remove adjacent
idlers (Figure 3).
Discharge zone maintenance
Parts that wear, such as belt cleaner
blades, need to be monitored, serviced
or changed regularly to prevent
carryback from causing dust
and spillage along the belt path.
However, blade maintenance and
changes with existing designs can
require several hours of downtime.
To address the issue, newly developed
automated tensioners have
been designed that continuously
38
FIGURE 5. Duct tape can have drawbacks when used to seal access doors
monitor belt cleaner pressure and
wear, automatically adjusting for
optimum cleaning performance and
service life. An automated tensioner
can be run off the existing electrical
service or paired with a local power
generator, which uses the kinetic energy
of the moving belt to generate
electricity. Powered auto-tensioners
are considered " smart " devices that
can deliver
realtime
performance
data from a remote location.
On most existing belt-cleaner designs,
primary cleaners - located
just under the material flow path on
the head pulley - are mounted on
rotating assemblies that maintain the
proper tension between the blade
and the belt. Secondary cleaners
are typically located under the belt
and behind the head pulley. Both
need regular inspection and adjustment
to obtain maximum cleaning
performance and service life.
One way to improve maintenance
access and safety while reducing
service time is with track-mounted
blades that can slide in and out by
simply pulling a lever and releasing a
pin. This allows maintenance to be
performed outside of the system by
a single worker - typically in less
than one hour (Figure 4).
Inspection doors
A tight seal is the key to preventing
fugitive dust from escaping the
transfer chute. Many current setups
require workers to crouch or crawl
under the system or even enter a
confined space to inspect the chute
or perform maintenance, which can
increase the risk of serious injuries.
Inspection of the system needs to
be fast, easy and safe. Inspection
doors - either solid or grated -
can allow several observation points.
Larger doors can offer access points
with ample space for maintenance of
specific parts that experience wear.
Real-world examples
The following examples of actual
solids-handling operations show the
difficulties that can arise with the
maintenance of mechanical conveyors,
and subsequent solutions to
the problems.
Example 1: Dust issues. A west
coast coal-handling operation was
experiencing dust issues at one of
its transfer points, so operators requested
an inspection by outside experts.
Starting at the top of the transfer
zone, technicians initially saw no
fugitive material. In fact, they found
several access doors that had been
cut into the chute by the company's
maintenance staff to allow inspection
at various points. Then they noticed
that the doors were all sealed with
duct tape in an apparent attempt to
avoid the cost of installing properlyengineered
doors with tight-fitting
seals (Figure 5).
Duct tape is a versatile tool, but it
has some drawbacks when used to
seal access points, a fact that was
about to become readily apparent.
The technicians had to see the material
stream
to evaluate the dust
issue below, and while they were impressed
by the ingenuity with which
the access points were cut and
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