- Published: Thursday, 01 March 2007 07:00
In a commodity-based business, the more time needed to convert resources into revenue generally results in reduced profits due to increased overhead. This is where machine efficiency enters the picture, and there's no more important aspect of this than screening efficiency.
From one point of view, the business of quarrying is really a process of conversion ó converting large rocks into a range of smaller, standard-sized fragments; and converting sand and gravel deposits into saleable construction materials. The paradigm extends one step further, converting these products into revenue.
In a commodity-based business, the more time needed to convert resources into revenue generally results in reduced profits due to increased overhead. This is where machine efficiency enters the picture, and there's no more important aspect of this than screening efficiency. In addition to the potential problems of decreased throughput and unplanned downtime, a poorly performing screen circuit may result in products that are non-spec or of poorer quality than expected and therefore less valuable to the customer. Plus, the overhead costs of rescreening out-of-spec products detract from profits.
At a recent technical conference Jean Leblond, president of Major Wire Industries, explained how two of the most common screening problems ó blinding and pegging, in which particles clog the screen or near-size fragments wedge into the openings ó can reduce screen capacity by 70% or more. This essentially shrinks the effective size of an 8- ◊ 20-foot screen down to 4 ◊ 12 foot.
Leblond used a hypothetical 500-ton-per-hour stationary plant that worked nine hours per day to produce 4,500 tons per day. At five days per week over 48 weeks, the plant could produce 1.08 million tons per year. If the plant's average selling price is $8 per ton, the potential annual revenue is $8.6 million.
If the plant incurred a one-hour production delay to clean a blinded or pegged screen, the resulting loss of 500 tons would cut into potential revenue by $4,000. Two such interruptions per week would result in a loss of 1,000 tons of production per week or 4,000 tons per month, eroding potential revenue by $32,000 per month. Over 48 weeks of production, the potential loss would be a potential revenue drop of $384,000.
But, it gets worse. According to Leblond, a fully loaded screen deck that is partially blinded will incur a production loss equal to the percentage of area blocked on each deck. A screen that is only 5% blinded will decrease production by 5%. At 1.08 million tons per year, a 5% reduction represents 54,000 tons. At $8 per ton, that translates into a potential revenue loss of $432,000. Ultimately, the hypothetical plant would, after incurring just two one-hour interruptions each week to clean one screen, lose as much as $816,000 per year in revenue while running a fully loaded screen deck that is 5% blinded.
In many cases, blinding of woven-wire media can be eliminated or reduced by changing screening media specifications or switching to synthetic or flexible media. For producers who don't want to switch to synthetic media, Major Wire offers Flex-Mat 3, which combines polyurethane strips bonded with individual wires to eliminate the need for conventional woven wires. The polyurethane strips allow the individual wires to vibrate independently and at different frequencies to prevent material from accumulating between the wires. Flex-Mat type screens, according to the company, have 15% to 30% more open area than equivalent square-opening woven-wire screening media.
Major Wire's most recent product is HyperSlot screen media, a slotted screen cloth with alternating crimped and straight wires. The crimped wires on the cloth slow the material, creating more time for it to pass over the openings. The uneven surface of the cloth creates a tumbling effect for material passing over the screen, resulting in better fines removal. And as the crimped wire slows the material, it tumbles onto the straight wires, which vibrate to reduce blinding.
Metso Minerals' new Lokotrack ST458 mobile screening plant has been designed for improved reliability and productivity, according to the company. The 69,000-pound ST458 is constructed with heavy-gauge materials and is mounted on twin 15æ-inch-wide, hydraulically driven tracks. The feed conveyor assembly has been mounted as a cassette and the entire unit can be removed through the rear hopper door, providing full access to the conveyor belt and all conveyor components. In standard configuration, the unit is capable of producing three sized fractions, and an optional two-deck vibrating screen can be fitted to provide four fractions. Metso says the loading width of 15 feet 3 inches and hopper capacity of 9.6 cubic yards is the biggest feed hopper available in its size class. The unit's two-bearing screens feature 85 square feet of screen area per deck. Power is provided by a Deutz BF4M1013EC four-cylinder, liquid-cooled diesel engine rated at 148 horsepower.
The ST458's IC300 controller uses SmartScreen technology to offer fully automated control of all machine functions during the screening process. Screen speed and amplitude in conjunction with conveyor speeds are monitored and adjusted for optimum productivity and classification accuracy. Remote- and radio-control options provide for safe movement around the job site. Process adjustments and tipping of the grid also are possible from the radio remote.
Extec's range of screening systems includes single-conveyor machines, such as the 6000, and three-conveyor systems such as the S series, both double screen and single screen. Extec's latest offering in this line is the S-5 series double screen, a three-conveyor unit that can provide more than 20 different screening angles, according to the company. The adjustable-angle setup provides a steep angle for the primary screen box to ensure that the majority of the material is processed in the initial impact area. Consequently, the majority of undersize is removed during initial impact, and only near-size material passes to the secondary screen box. This may be set flat enough to ensure any remaining material is screened efficiently. Clean gradations, at high output, are produced as the material is exposed to a large screening area.
Several other suppliers offer similar screening media. Durex Products says its new Dur-X-LiveWire poly-wire slotted-wire screen media also eliminates blinding and plugging problems while delivering longer wear life. Available in Durex's Xtreme high-tensile-tempered or stainless-steel wire, Dur-X-LiveWire is suited for dry screening of materials that have high moisture and fines content. The wires are held in place by molded rubber strips that allow the flexing action required to keep screen openings clear. Durex says that, while rubber molded strips provide superior wear life in all applications, they are particularly ideal in hot and humid environments where thermoplastic polyurethane is not as effective.
Dur-X-LiveWire is available in four different slot arrangements: diamond-shaped; accuslot (alternating Z and straight wires that form triangular-shaped openings); herringbone; and straight-wire slot openings (long slot screens that have parallel wires). An optional lap design is available that permits screen replacements without having to adjust adjacent screens. Screens are available with openings up to 2 inches and wire sizes up to º inch and with overall screen sizes up to 72 inches long by virtually any clamping width.
A more elementary but possibly equally effective means of avoiding screen mechanical breakdown was recently introduced to the market. The HydraNut system, which allows precise tightening of critical bolts in often hard-to-reach machine locations, allows users to secure nuts using hydraulic force axially and evenly without torsion, precisely stretching bolts with the degree of force needed for fastening in different applications and ensuring that components won't shake apart from improper fastener tension. Loosening the nuts requires only repressurizing the HydraNuts, releasing their lock rings and then releasing their hydraulic tension.
The system is described as requiring only minutes to apply correct tension to any size bolt. In the field, HydraNuts are screwed into position on the bolt, a quick-connect coupling is made and a hydraulic pump applies pressure. The bolt is stretched and the lock ring is wound down to seat on the top face of the piston. The hydraulic pressure is released and the hose is disconnected. The system is said to reduce tensioning cycle time by 80% and minimize bolt failures.
Also at Bauma, Sandvik will introduce a new feature for its CM4800i mobile secondary crushing plant. The unit on display will have a hanging screen module in closed circuit. The newly designed, high-performance screen is fitted with modular media and wear plates from the Sandvik line. The CM4800i also has been given a more powerful diesel engine that meets Tier-3 emissions requirements.
For customers interested in modular screening systems with synthetic media, W.S. Tyler now offers a Ty-Deck polyurethane screen panel in a 1- ◊ 4-foot pin-and-sleeve modular design. The company says the panels have as much as 10% increased open area over traditional pin-and-sleeve 1- ◊ 1-foot panels. It does this while continuing to offer the advantages of flat, modular screening. Ty-Deck panels are available in square and slotted openings ranging from as small as ? inch to as large as 3 inches and in a variety of compounds. The screen media thicknesses range from 30 to 80 millimeters, depending upon the application. These screens are available as a direct replacement for most popular original equipment manufacturer's designs.
Schenck's Vibromac 100 and 200 vibration monitoring and controls systems also are designed to protect screens and similar gears. The system uses a series of sensors that are mounted at critical locations on the equipment. These monitors sense motion, energy, and temperature. The sensors are wired into the Vibromac system. Here, information then can be transferred from the controls via a wireless or hard-wire connection to the Schenck Process CP-3 hardware platform. From there the collected data are sent to a Web page for viewing. Tables, alarms and e-mails are displayed on the Web page to provide current condition reports and historical data. When an alarm is triggered, Vibromac system sends the data to the Web page where an e-mail alert is generated and sent to designated personnel.
Performance and reliability concerns extend beyond screening media. In the harsh operating environments in which vibrating screens are generally found, mechanical components often break, loosen or corrode. SKF says its CMPT Copperhead Transmitter Unit functions as a brain for fault detection systems to provide early-warning indications of failure in vibrating machinery. Users can select from among several types of vibration signal process analyses and can further configure units to monitor machines operating either at normal speeds or low speeds.
SKF says the CTU ó when paired with SKF sensors or other industrial accelerometers ó can alert users to equipment problems before they can escalate. The CTU digitally evaluates overall machinery vibration and provides a processed analog signal to detect such problems as loose parts, imbalance, gear and bearing damage, lack of lubrication, and others that can adversely affect machine performance and plant productivity. The CTU can be connected directly into existing automation systems or can stand alone for monitoring and alarming (with an optional digital display module). Multiple monitoring points can be easily interconnected to a PC.
The Western Group's lineup includes its Poly-Z and Z-slot screening media, in addition to more conventional FlatTop woven wire. For buyers focused on cutting media change-out costs and downtime, Buffalo Wire Works says its GatorWire's designed-in abrasion resistance provides up to two times more screen life than standard oil-tempered wire cloth.
To learn more about these products and companies, complete the reader service card on page 55 and mail it to Rock Products or visit www.freeproductinfo.net/RP
Name RSN Buffalo Wire Works 176 Extec 177 HydraNut 178 Major Wire Industries 179 Metso Minerals 180 Sandvik 181 Schenck 182 SKF 183 Terex Powerscreen 184 Western Group 185 W.S. Tyler 186