Thinking back

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.

dust collector bag
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  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.