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Odell, Oregon 97044
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Phone: 541-352-7942
Fax: 541-352-7943
cmconsulting@hotmail.com
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C. M. CONSULTING
A Division of Cliff Mansfield Incorporated
Consistent Baghouse Dust Control
by
Cliff Mansfield
Thinking back on my
travels around our great country working on various asphalt plants this last
summer one recurrent theme stands out in my mind. Inconsistent sample results
relating to material passing the 80 through 200 screens. I’ve encountered this
at more than a few facilities in the North, the South, the East and the West.
Some of the plants were easy to fix. But after eliminating the obvious problems
like poor quality control of the aggregates, operational protocols and
calibration issues I was left with a core of plants that still exhibited
inconsistencies in their sample results. This problem was predominantly evident
on plants with pulse-jet baghouses that used an automatic pulsing card. What I
discovered is that when the baghouse is being pulsed a set rate of fines were
removed from the thing, but when it was not being pulsed none was being removed.
A brief explanation of baghouse operations should help to understand the
problem. As air is pulled through the baghouse the main fan generates a certain
amount of suction. As that same air is pulled through the bags there is a
certain amount of restriction. The fan suction is measured by a gauge such as a
photohelic meter in the control room. The suction is also measured at the inlet
to the baghouse and the two readings are compared. The resultant difference is
called the “pressure differential”. When the plant is running this air is drawn
through the drum and carries with it a certain amount of fines. These fines
collect on the bags and increase the “pressure differential”. The higher the
“pressure differential” the less air that can be drawn through the baghouse. As
the air volume falls, so does our production rate since the burner requires air
to make heat. To correct this problem we pulse a jet of air through our bags to
clean them off. This operation is handled by a pulse card located on the side of
the baghouse or in the control room. By using two adjustable pointers on the
photohelic the plant operator can adjust the operational “pressure differential”
as the plant runs. The low “pressure differential” is typically set at around 4”
while the high “pressure differential” is set around 6”. As the plant is running
the photohelic will automatically begin pulsing the bags when the pressure
reaches 6” and stop when it falls to 4”.
What happens is that while the baghouse is being pulsed the dust that
collects in the baghouse is put back into the mixing drum by augers or a blower.
When the baghouse is not being pulsed, no fines are being added to the mixing
process. It doesn’t take an expert to see that this process causes the gradation
of your hot mix to be continually changing. In essence, when pulse card is not
calling for cleaning action the baghouse is storing the fines drawn off of a
large amount of aggregate and when the card again calls for pulsing action the
baghouse dumps an elevated amount of dust in a relatively small tonnage. This,
of course, would lead to inconsistent sample results and, more importantly,
inconsistent mix.
To correct this problem at these plants I added a “manual” pulsing mode
so that we could set the baghouse up to be pulsed any time the plant is running,
regardless of the “pressure differential”. This way a constant amount of
baghouse dust is being added to the mixing process. I used the manual settings
on the pulse card to set the “pressure differential” to 4”. The downside of this
is that the plant operator must adjust the pulse card himself to assure the
“pressure differential” stays in the range he wants. This “inconvenience” is far
outweighed by the improvement in dust control and improved mix consistency. This
change can be implemented at your plant for perhaps $500, maybe less. It simply
requires some wire, a switch and someone to do it.
I’ve changed many plants, both drum and batch, to this configuration and
have seen a significant improvement in mix quality and more consistent sample
results.
A secondary issue that contributes to dust related sample problems is
start-up and shut-down protocols. When we start our plants the majority of them
will exhibit an elevated level of dust for a short period of time. This material
is added to our mixing process regardless of the amount of aggregates coming in.
On a batch plant this can contribute to surges of fines into #1 bin, especially
if the plant doesn’t use a dust run around system. On a drum plant it means we
have to waste material onto the ground through our reject chute. During daily
operations a plant can be started and stopped many times which leads to a
considerable amount of rejected material. It also leads to poor quality mix for
a short period of time, due to an elevated level fines. To combat these problems
I advocate the use of hot-stop or mid-stream stops during normal daily
operations at all plants, including batch plants.
A hot-stop is performed as follows: When the time comes to shut down you
would first switch your burner control and asphalt control (on a drum plant) to
manual, note the burner position, then kill the feeders. When the aggregates
stop falling into the dryer, kill the dryer, the burner, the fans, divert the
oil on a drum plant, turn off the baghouse pulse control and any associated
blowers/augers, then close the damper.
To restart the plant: Start the main fan and the burner fan. Once the
burner controls go through their ‘purge’ cycle you can light the burner, open
the damper and start the aggregates. When the rock spills into the dryer you
would start the dryer, turn on the baghouse pulse control and any associated
blowers/augers, then either restart the oil injection on a drum plant or restart
the batching process on a batch plant. Next, ramp your burner up to the last run
position. You are now back to full production in less than two minutes.
If you do these operations correctly you should have no wasted materials
and a very smooth transition from running to stopped and vice versa. One note:
The temperature of the aggregates in your drum is what controls how long you can
remain hot-stopped. On hot summer days I’ve set for as long as three hours with
little adverse affect. On a cold, windy day in Alaska the down time will be
considerably shorter. Given the improvement in mix quality due to even fines
distribution and considering the abbreviated start-up time, hot-stops have
become a matter of course in my training regimens.
For additional information on this subject
or help with any problems encountered
contact Cliff Mansfield,
541-352-7942,
7:30am to 9:00pm Pacific Standard Time.
Email me-
cmconsulting@hotmail.com
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