Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/908212
14 AGGREGATES MANAGER / December 2017 A ll too often, total plant through- put is used as a measure of efficient operation, says Craig Burke, P.E., director of engineering and sales support at Polydeck Screen Corp. Efficiency is more accurately thought of as how easily you are getting product into the pile the first time you try. "That's when you have to look at the whole picture," he says. "Screen operating parameters, mechancial conditions of the screen, and feed distribution are just a few critical factors that can have a big effect on efficiency." Missing the first, or subsequent, opportunities to get product into the pile can lead to recirculating loads that bog your plant down. "One key is under- standing the physical characteristics of the equipment and setting up operating parameters correctly," Burke says. Without understanding the effect of speed and stroke on the screen media, producers may have a screen set at a speed or stroke better suited to fine material, but have coarse material feed. "In a case like that, you can't get coarse material to stratify as it should so that the right sizes find the apertures," he explains. "Alternatively, some people may have a screen set at a high-energy level for coarse material, and when fine material is fed to it, the material bounces too high and can't find its way through." Inspecting the feed arrangement and physical condition of the screen is also important. "Is the feed stream segregated? Is the feed all to one side of the screen? Are there broken cross members or springs? Is the chute clear under the screen? Sometimes the screen is smacking up and down on packed material in the chute and shaking itself apart," he says. When troubleshooting feed arrangements, check to see if you are feeding to the center of the screen or to one side or the other. And remember, feeding from two different conveyors isn't a 'no-no' as long as you properly mix at the feed box. O ne of biggest issues with screen throughput is managing dust suppression and emis- sions coming from transfer points, says Michael Vacchi, quarry manager for PJ Keating's Cranston Quarry in Cranston, R.I. "We manage this in production dai- ly," he says. "No matter what material you are feeding through, you are trying to manage the right amount of water to control dust and allow the material to screen out — both of which are very important." Ensure that you're not trying to put water over the top of the screen, but wetting the material prior to hitting the screen so it has the best chance of being properly screened. Regardless of the type of screen me- dia being used, producers often see the most success when they fabricate either a feed shoot or a diverter on the feed plate of a screen to spread out the ma- terial on the top deck. "A lot of people don't use the whole top deck, so it hurts screening ability through the rest of the screening," Vacchi says. "If all of the top deck is used, you'll get the greatest efficiency out of all the decks." This also makes the finished prod- uct much more consistent. If only the middle of the top deck is utilized, it's difficult to efficiently screen material without it being spread out. It also contributes to improper wear. "If I have a box or diverter gate dumping onto the feed plate, sometimes the metal on it will begin to wear," Vacchi notes. "Instead of taking impact on the feed plate of the screen, the first row of screen panels will take the impact. It's only a matter of time before it will go through and cause contami- nation." Although it may not seem directly related, equipment maintenance plays a vital role in maximizing throughput. "As soon as you go out of spec, things can go awry very quickly," Vacchi cautions. T raditionally, wire cloth has been the predominate technology because it is relatively inexpen- sive, fairly easy to procure, and has high open area. However, it comes in large 4- by 8-foot sections, has a high bed depth, and tends to wear out quickly. Polyurethane and rubber media tends to be a much higher cost at the outset and may not have as much open area, but it may only need to be changed every six months instead of every 10 days, Sam Durnavich, regional sales manager for Polydeck Screen Corp., says of conven- tional screen media options. Now, a combination of wire and syn- thetic screen media is being used to give producers a screen with higher open area combined with the advantage of lower maintenance. A section of welded wire inside a polyurethane border in a modular format is being used to custom- ize screen system designs. "Using this type of modular approach allows producers to choose row by row to fine-tune the screen to their needs and hit their production targets," Durnavich says. However, he stress- es that even with the flexibility of a modular design, it's critical for producers to work with an applications engineer and provide accurate information on production goals, current system setup, and gradation analysis. "If one side of the screen machine is moving differently than the other side of the machine or the G Force is set too high or too low, it can affect the perfor- mance of the screen media," he says. Older panels had a tendency to plug and blind. By experimenting with the material science behind the rubber and urethane used in newer modular design screens, more robust — and lighter — designs are being developed. "Screens are being designed with lower-durometer rubbers to allow the panels to self-clean so they don't plug badly," Durnavich says. "It has a trampo- line-like action — a secondary motion for self-cleaning." Voices of Experience Craig Burke, P.E. Michael Vacchi Sam Durnavich