mix it up
Keep Baghouse Surges from Disrupting Mix Quality Elam Construction managers go ghost busting to solve fluctuating problems by Sandy Lender
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onsider for a moment that your quality control (QC) team has perfected a mix design for a state project. Everything is seamless from the aggregate to the binder to the percentage of RAP. The test strip your paving crew placed on the back lot gave beautiful compaction numbers. Then the department of transportation quality assurance inspector ran a test and your air voids were all wrong. Your crew paved an extra test strip and couldn’t get density to save their lives. What on earth went wrong? Jeremy Dade is the plants manager for Elam Construction in Grand Junction, Colo., and told a similar story. “We’d either have too much 200s in the mix or not enough.” It took more than a year of trial and error to realize the problem came from the baghouse fines. “We even turned the baghouse return off,” Dade said. Nothing seemed to give them a consistent result. Instead, they saw a big swing of 1.2 to 1.7 tons per hour (TPH) in the 200s, Dade said. By installing a device from Clarence Richard Co. that weighs and meters the dust going into the drum, the plant personnel slowed the rate to a steady 4.2 TPH, according to Dade. Elam Construction personnel had found a problem with the sags and surges in the baghouse’s normal operation. A typical pulse-jet baghouse has about 64 rows of 10 to 15 bags that pulse three rows at a time on a timer. The dust that falls to the bottom of the baghouse gets augered out in surges, thus the fines returning to the drum move in surges. This gives varying amounts of fines in your mix design. A rotary air baghouse could be considered “worse” as it can stop sending material for up to a minute at a time. But a plant operator can’t blame the baghouse. It’s operating as it was intended. “The primary purpose of the baghouse is for emission or pollution control from the plant process,” Rick Rees said. He’s the CEO of Stansteel/Hotmix Parts, Lexington, Ky. “Some contractors over the years have used the baghouse hopper as a temporary holding device to feed the fines to the plant. Most producers today have much more sophisticated feeding and metering devices to properly proportion the dust/fines back to the material feeding and mixing operation.” David Fife, the QC manager for Elam Construction, spelled out his experience in detail. “There are many aspects as to how dust effects mix designs,” Fife began. “I can remember standing next to a hot plant several years ago with a Colorado Department of Transportation (CDOT) inspector trying to 16 january 2012
Here’s How It Works: Material slides down the EZ-Flo flow scale’s feed throat and impacts a suspended plate (plate suspension is patented). The material impact force on the plate is transferred to a load cell, which converts the gravimetric force to digits on the readout. It displays both the flow rate in tons per hour (pounds per minute, etc.) and the tons of material that has passed through the scale since the totalizer was last zeroed. Photo courtesy of Clarence Richard Co.
figure out why the properties of the mixture where fluctuating. I hadn’t been making changes in the percentages of materials at the hot plant, and we religiously had run crusher control on the aggregates as they had been manufactured. I told the inspector that I was chasing ghosts. Every time I had my fingers on the problem it would disappear and show up again on the other side of the specification. “At that time I had a pretty good idea this problem was based on dust but I couldn’t put my finger on exactly why it