Jul 052012
 
Henningson Contro lHouse
Henningson Control House

Henningson Control House

If you wish to trim asphalt cement (AC) costs while keeping waste material out of landfills, you’ve probably looked into the recycled asphalt shingle (RAS) availability in your area. With the quality control (QC) issues of bringing in clean RAS and having it ground to the finest size you can get, you may have run into the challenge of weighing that material accurately throughout the mix design and mix production phases. Not to worry.

Producers in the field have met with some challenges when it comes to handling RAS and they have tips and advice to help their peers ensure a quality mix for customers, owners and agencies. Let’s begin looking at this topic by addressing some of the QC aspects to watch for when working with a finely ground recycle material such as RAS.

First, remember that the amount of recycled AC you get out of RAP is a different percentage than what you get out of RAS. Even the amount of AC in RAS can vary from 15 to 35+ percent. That means your plant operator has much to track when it’s time to produce mix with recycle in it. Of course plant manufacturers and controls OEMs are part of the solution.

WeighTronix Screen

During the demonstration, Henningsen Plant Manager Duane Updike observed the bin flow rate change from plus or minus 15 percent while the feeder speed ran at a constant speed. The same 15 percent deviation was observed at flow rates as low at 6.75 TPH to 25 TPH. Photos courtesy of Clarence Richard Co.

Rick Tapia of Stansteel Asphalt Plant Products, Louisville, Ky., brings up an interesting point for the discussion of handling RAS separately from RAP. “Initially, it’s very important to know the difference between excess manufacturing tabs on the one hand and recycled shingles from a roof tear-off on the other hand,” Tapia said. “These two types of shingles must be handled, fed, metered, proportioned and blended in different ways. For example, your RAP bin is separate from your RAS bin. If that’s the case, shouldn’t you also separate never-used shingles from worn-out shingles?”

The big question producers in the Midwest have asked of Clarence Richard, proprietor of Clarence Richard Co., Hopkins, Minn., concerns measuring what comes out of the separate bins. What he discovered in working to resolve their measuring issue was an accuracy issue. Quality control of RAS needs a delicate touch.

“Most producers are trying to weigh RAS using the existing RAP feeder system that they have without making any improvements,” Richard said. “There are a few reasons that producers will want to update existing equipment for RAS. First, RAS carries a lot of AC in it. When accuracy is off, the AC content is affected by the error more than when RAP’s accuracy is off.”

Richard’s second observance should encourage producers to look not only at multiple bin use for multiple products, as proponents of fractionated RAP have said for years, but also to look at multiple scales that the control house can read. His third should encourage producers to look at equipment shape.
“Second, sometimes producers use multiple bins in series with a RAP scale with RAP in one of the bins and RAS in another,” Richard continued. “They can’t be sure how much is coming out of either bin at any given time. The RAP conveyor was built for heavier TPH flow rates (25 to 125 TPH) of RAP productions. Current RAS rates require weighing product from 5 to 25 TPH. That’s a big difference. The RAP scale system (conveyor included) was not built to accurately weigh material at such a low flow rate.

“Third, when you have multiple bins in series with a RAP scale system, there’s no way for the blending control to adjust for bin speed when one of the RAP bins or RAS bin clogs up, or density is changed. And density can change plus or minus 15 percent. Some belt scale manufacturers advise against using their scale to weigh light-loaded products for such a critical application.”

To address weighing RAS separately, Richard brought up the EZ-Flo continuous weigh belt that his company manufactures. To address flow and controls, a variety of sources discussed controls and equipment use.

The flow scale from Clarence Richard Co. works much like a belt scale except it doesn’t need a tachometer. It already “knows” the speed of the material passing through it is constant—gravity. The flow scale reads the rate of material flow as it impacts the front face of the impact plate. The rate reading, in TPH, indicates how fast material is passing through the flow scale and produces an output signal useful for blending materials. The totalizer reading, in tons, is used for calibrating the scale, cross-checking accuracy, inventory control and producing a signal capable of batching product. The flow scale impact plate is mounted 6 inches away from the end of the belt feeder head pulley to measure the RAS flow rate.

Weigh Scales

Keep in mind that continuous weigh scales and metering instruments should be checked for accuracy. The accuracy of the check is only as good as the resolution of the instruments being used, the size of the sample and the sampling method. In this photo, an 8-ton sample is gathered, which is better than a 4-ton sample. The procedure was to collect material in a truck after the material had traveled more than 100 feet of conveyor and through a scalping screen. Photo courtesy of Clarence Richard Co.

A.J. Dennis Campbell, plants superintendent for Henningsen Construction, Atlantic, Iowa, shared that he had concerns when his company won a 60,000-ton job that required about 4 percent shingles. The job was in Iowa, but on the Illinois border, so two groups took an interest when Campbell gave the go-ahead for an EZ-Flo to be designed, built and delivered in four weeks. He wanted an accurate means of weighing RAS material as it discharged from the RAS feeder belt to the collector belt.

The distance between the RAS feeder head pulley and the RAP tail pulley—and both of their support structures—is 20 inches. The distance between the bottom of the feeder belt and the center of the collector belt was 12 inches. The troughing rollers used up part of that 12-inch space. The RAP bins also have bin support structure and bin feeder conveyor drive pulley-sprocket arrangement on one side of the bin and the loader ramp barricading access to the confined space. That meant the team assembled the scale inside the confined space as much as possible.

With the careful operation done, the EZ-Flo can be used at the Henningsen operation for fine RAP and RAS monitoring. An operator can slide the scale out of the flow when using the bin for RAP.

With visitors looking on, Henningsen Plant Manager Duane Updike observed flow rate changes during production with the EZ-Flo in place. The impact scale located in the control house reads in real time and would normally be monitored by the plant computer.

“Plant computers are programmed to either compensate for the AC when the RAP scale deviates from a set point or to increase or decrease both feeder bin speeds,” Richard reminded us. “Either way, one scale is not enough. Now, the mix producer has the option of weighing the RAS and controlling that bin or the AC being added to the mix. The same RAS rate signal being used to control for the RAS can be subtracted from the RAP belt scale—which is weighing both RAS and RAP—to indicate the actual RAP TPH flow rate.”

This means the plant computer can use RAP deviation information to adjust the oil being added to the mix or the RAP feeder speed.

“Contractors should give a heads up to their blending control manufacturers about the changing requirements they’re asking of their blending controls,” Richard said. “Today’s blending controls aren’t usually set up with the software to accept another scale input or two to monitor the flow with a continuous weigh RAS scale and, consequently, control the flow from that scale information.”

That means it may be time to contact your controls OEM to get all those unused switches on the control panel connected to a new scale. Jim Richwine, director of engineering for Libra Systems, pointed out the importance of connecting new technology with new technology.

“Most plant operators open the gate on RAP bins much wider than would normally be done for volumetric control,” Richwine said. “They do this to prevent bridging, but in the process, they hinder traditional volumetric control because the material is then not cut by the gate. Instead, the material pours from the bin and the amount produced by the bin for each revolution of its belt actually varies as the belt speed changes. So, the old-style control systems that used a single ‘pounds per tach pulse’ calibration for the bin became inaccurate at different speeds. However, newer systems allow multi-point volumetric calibration to get the mix as accurate as possible at all speeds even when the gate is not set in an ideal position.”

Dual Recycle Bins

Dual recycle bins often mean the producer fractionates his RAP and treats it with the same care he treats virgin aggregate. That’s an excellent practice. When he decides to use one of the recycle bins for a much finer product like RAS, quality control management needs to take a step up. Photo courtesy of T.J. Young of TASCO2.

Getting the gate, and indeed the entire bin, in position for the new material, is T.J. Young’s primary concern. He’s the proprietor of TASCO2 and recommends producers ask about cold feed bin enhancements before purchasing more bins for recycle storage. We’ve already established that a fine material with high moisture and angular shape such as RAS will have a tendency to bridge in its bin.

“Over the last 30 years, all main-line hot mix equipment manufacturers have embraced the following design elements, often borrowing from each other, to help improve feed characteristics and reduce bridging problems,” Young said. “They build with steep side walls of typically 60 degrees or above. They keep a long opening at the bottom of the bin and a trapezoidal shape to the opening. And they use vibrators or air cannons that can be activated at loss of flow.”

Whatever bin “style” a producer has in place, the control house has to get good information from it.

“Because of the importance of maintaining a quality product through the proper blending of materials, it is essential to have electronic controls and interlocks to properly monitor the RAS feed rate,” Tapia said. “The Stansteel Accu-Track with Closed Loop Control System is an example of such a control system. With many control systems, a signal is sent from the processing unit to feed at a certain rate. However, with the Accu-Track System, the rate of flow is verified and deviation alarms alert the operator to any problems. Another important aspect is factoring in the asphalt percentage—that’s true for either RAP or RAS percentages. The percentage of liquid asphalt in recycled shingles can vary from 15 to 38 percent. In addition to being able to input varying amounts of asphalt percentage in the RAP and RAS ingredients, it is also important to know the rate of flow and have sequential timing in place to track the travel time of each RAS and RAP ingredient, as well as other aggregate ingredients, so that they all hit the mixing, proportioning and blending phases in a planned and quality-controlled manner, resulting in a quality product.”

Libra’s Richwine pointed out that these worries for a drum plant are somewhat lessened with a batch plant. “Since batch plants, with a separate RAP scale, weigh each material one at a time, bridging doesn’t affect the mix content. It merely slows the weighing of the batch.”

That seems to make Richard’s point perfectly. Weigh and measure each element and be sure the controls system has a way to incorporate that data. Above all, make sure the team “at the wheel” is your best.

“Nothing beats an alert operator and control system,” Richwine concluded. “But modern controls ameliorate the situation with automatic attempts to break the bridging and control techniques that still make the best mix possible.” The best mix possible is the hallmark of the asphalt industry.

Sorry, the comment form is closed at this time.

Slider by webdesign