POWER April 2012 - 26

5. Microwave switch. A microwave blocked chute switch is best used in applications
where the flowing material is dry. Courtesy: Hawk Measurement Systems
guide extension to direct the signal to a
remote amplifier.
Sensors using microwave technology
for reliable dry solids point detection to
eliminate the chances of a plugged chute
have become the standard in many mining
facilities around the world. There is
a 75% chance that one transfer chute
will become blocked over the course of
one month in every mining facility, and
that means a reliable plugged chute detector
that will not fail under duress is
required.
Acoustic Wave Technology. Acoustic
wave technology relies on a very
low frequency (15 kHz), high-powered
transducer pair that creates a pressure
wave on the sensor face of each transducer.
The technology requires a pair
of transducers to be located apart but
aligned with each other. The transducers
both pulse and receive signals from
each other, and as soon as the signal is
blocked by wet or dry solids material,
the attenuated acoustic signal is amplified
and sent to the plant-monitoring
system (Figure 6).
The low-frequency, high-power signal
6. Acoustic switch. An acoustic blocked
chute switch can be used in moist or dry material
flow applications. Courtesy: Hawk Measurement
Systems
plications, such as plugged chutes. Either
of these technologies can be used
for point level detection or flow/no flow
in a process chute.
Microwave Technology. Microwave
technology has been in existence for
many years and as a plugged chute
detector for the past 15 years. If the
conditions or an area of the process is
free of moisture from water sprays, then
microwave technology is very reliable,
maintenance free, and should provide a
lifetime of operation. It is not the right
solution if there is a possibility of moisture
from water sprayers in the process
due to the potential of coating the instrument
with a dust film (Figure 5).
This technology makes use of a
tory, rotating paddle, or tilt switches,
all are exposed to heavy, abrasive raw
materials that cause excessive wear and
tear to contacting switches. The coating
from the wet, moist coal dust can also
adhere to the contact probes, causing a
false trip. Even though these switches
are fairly inexpensive, their reliability
for plug chute detection application is
rather low, and the cost of downtime is
too high to rely on an exposed switch
(Figure 4).
In our experience, microwave and
acoustic wave point detection provide
the most reliable operation in solids ap26
sender
and receiver transducer throughbeam
design that is mounted outside of
the material and transmitted through
a transparent, wear-resistant window.
Once the material fills the chute, the
microwave beam is broken and the product
sends a signal to the control system
to quickly provide an indication of the
blockage.
Since this technology does not make
any intrusion into the chute wall, but
rather, through a high-grade ceramic
window, the material cannot wear down
the sensor face. Even under high temperatures
in excess of 160F, the sensors
can be remotely mounted with a wavewww.powermag.com
applied
to the sensors also has a selfcleaning
feature when the pressure wave
is created. This pulsing pressure wave
keeps material from adhering to the
face and provides for maintenance-free
operation in critical applications. By
installing the transducer system in the
optimal location, plant personnel will
get an early detection of chute plugging
in order to take corrective action. Prevention
is the best method for ensuring
that flow continues unhindered through
the power generation process.
Pick Your Switch
If it's a reliable point level detection device
that you're looking for in a transfer
chute that is going to be immersed in a
moist, dusty coal environment and reliability
is very important, then acoustic
wave technology may be your best option.
If the process material is dry, then
the microwave detection switch may be
your best choice.
Regardless of the application, it's best
to thoroughly understand your process
conditions before picking a switch technology.
Be cautious, explore all the available
technologies and their track record
in harsh coal-fired plant applications, and
then make an educated decision. ■
-Contributed by Jerry Boisvert
(jerry.boisvert@hawkmeasure.com),
a mining specialist with Hawk
Measurement Systems.
POWER | April 2012
http://www.powermag.com
POWER April 2012

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