May 1, 2012
When it comes to screening efficiency, common culprits of lost production are blinding and pegging, says Gary Pederson, vice president of sales for Major Wire Industries, Ltd., based in Candiac, Quebec. “Usually, the bottleneck of the plant is the screen,” he explains. “It’s not the crusher. It’s not the conveyor. It’s often the screens, because you can’t get enough material through them.”
Al Grove, quarry superintendent, with Doylestown, Pa.-based Plumstead Materials, agrees. Several years ago, one of his sites was putting blast feed into the primary crusher, then scalping modified material and selling it right away. “The quickest return on investment is to produce a saleable product with the least refinement,” Grove says. “The less process the material has to go through, the less cost it creates.” In this case, the site was skipping the back end of the production process to get a quicker return.
The challenge Grove faced, however, was blinding. “If there’s moisture, it creates a blinding issue, no matter what kind of screen you are using,” he says. “If it rained for a day, we’d have to wait four or five days for the material to dry out enough to screen it effectively.” One trick he learned along the way was to decrease the volume of material that was being screened based on the moisture levels.
A splitter system was installed at the site that allowed the operator to control the feed flow in 20-percent increments. “The splitter system allows me to control the volume of modified material going to the screens,” he says. “That worked great.” The plant operator running the scalper screen was trained to eyeball it. If the operator saw the material becoming drier, he could adjust the splitter door. “It’s trial and error,” Grove notes. “Controlling the volume is the best way to control blinding.”
The selection of screen media can also reduce blinding, Pederson notes: “The operator may decide to run a stainless-steel wire mesh instead of a tempered wire mesh because the stainless steel is a little smoother and slicker and can pass some of that sticky material that would stick to a tempered wire.”
Many manufacturers suggest a self-cleaning screen cloth for applications prone to blinding, Pederson adds. For example, polyurethane strips in the place of cross wires can promote vibration in the wire that allows it to throw material off.
“In addition to the eccentric motion of the screen, you’re getting a vibration through the wire of the screen cloth,” he says. “The eccentric motion, coupled with the vibration on the screen cloth, helps prevent blinding and pegging. That makes a significant difference.”
1. Configure conveyor setup
Evaluate how the feed is hitting the screen. If rock coming off the conveyor hits only a portion of the screen, the entire screen cloth is not being used, and it will wear prematurely. Consider either repositioning the conveyor or installing a pant-leg chute or deflector that will spread the material evenly across the screen.
2. Beware blinding and pegging
Blinding and pegging are the two most common challenges operators face while screening. Control the flow of feed to ensure the screen is not overloaded, then work with various combinations of screen media to determine which will best meet the need for production volume, cost, and ease of maintenance.
If the material needs to be rinsed, set the spray bars to disperse water up the deck, opposite of the material flow. Nozzles should shoot a wide fan of water across the rock to ensure that it doesn’t create pegging problems or “burn” holes through the screen media. Avoid setting nozzles so they spray straight down or in narrow arcs onto the material and the screen media.
4. Positive tension
For longer screen media life, make sure it has proper tension. Examine items such as the clamp rails and the crown rubber to make sure the tension is evenly distributed across the screen media and that no gaps appear between the media and its supports. Such gaps will allow wire cloth to move and, eventually, break.
5. A screen for every deck
Some operators run a multi-deck screen without media on each deck, usually with the thought that more material will pass through the screen. This can cause irregular wear on the crown bars as well as the structural integrity of the screen box. Instead, consider the use of a heavy gauge screen cloth with large openings.
6. Look for the common denominator
Operators experiencing frequent problems with screen media should evaluate the “bone pile” or discards off the box. If a common pattern of wear exists, it can provide clues as to why the screen media is failing, such as feed patterns, wear on crown bars, or improper tightening of clamp rail bolts.
Gary Pederson is the vice president of sales with Major Wire Industries, Ltd., based in Candiac, Quebec. He moved from the construction industry into the aggregates industry in 1988 and joined Major Wire in 1999. He is a member of American Equipment Distributors and the National Stone, Sand & Gravel Association.
Quarry Superintendent Al Grove joined Doylestown, Pa.-based Plumstead Materials 12 years ago. A friend told Grove about career opportunities in the aggregate industry and recruited him. He spent four years at the Plumstead site and seven at Sellersville, Pa.-based Naceville Materials before returning to Plumstead last year. During his tenure with the company, Grove moved his way up from a laborer to plant operator to maintenance worker to superintendent.
VOICES OF EXPERIENCE
Insights can be gained from failed screen media, says Gary Pederson, vice president of sales, Major Wire Industries, Ltd. He describes visiting a producer who had problems with screen cloth and examining the discard pile. “Every piece of screen cloth was broken in the exact same place,” he says. After examining the screen box, he determined that they had run the deck without screen cloth. “It wore that crown bar down so there was a 3/8-inch gap between the crown bar and the screen cloth. It had the space to keep flexing up and down, and the wire broke every time.” The solution was a fairly simple one: the operator welded some key stock to raise the crown bar up so it would support the screen cloth.
Ensuring proper support of the screen media and box is important to extending the life of each. “Some people run the top deck empty because they think they’re saving money by not having the screen cloth up there,” he says. The most common result, however, is that the crown bar experiences irregular wear and the screen cloth fails; but, there can be even bigger consequences. Many screen boxes, Pederson says, are designed so the tension of the screen cloth aids in the structural integrity of the screen box. Not using screen cloth on the top deck will cause structural damage to the screen deck itself or crack the pan sides. “What they should do instead is put a piece on top with a heavy wire diameter, but maybe a 3-inch or 4-inch opening so there is a lot of open area. The screen cloth will protect the deck, but the large opening won’t restrict what goes to the second deck,” he advises.
Another common problem is using clamp rails that have worn so thin that the clamp rail bolts will bend the rail, causing the screen cloth to be tight near the bolts, but loose in between. Size of clamp rails is also an issue. Some operators will use clamp rails that are a different brand or length than the screen, which prevents proper tensioning.
Finally, some operators use different styles of crown bar on the same deck or don’t run it to the corner of the screen box. If there are various thicknesses of crown bar, say some 1/2 inch and some 3/8 inch, gaps are created. Just like with crown rubber that doesn’t run the length of the box, gaps allow the screen cloth to move and break.
“Clamp rails, crown bar rubber, and damaged crown bars are probably the three most common things we see causing problems with screens in terms of breakage and wear,” Pederson says.
“As a manager, I tend to spend a little more to buy more wear life so we’re not changing things as frequently,” says Al Grove, quarry superintendent for Plumstead Materials, Doylestown, Pa. “I’ll do a cost comparison for a side tension rail and compare one that is all steel versus a rubber lined one. If the cost comparison for the life of it more than makes up for the price of it, it’s worth it. It’s a matter of doing the math and determining which is most cost effective.”
When the operation recently replaced a crusher, Grove says they found that it needed more throughput on its 8- x 20-foot screen. “It didn’t have the throughput we needed, so when we upgraded our crusher, we knew we were going to increase our production, so we changed the bottom screen to work with it,” he adds. “It gave us more throughput. We’ve actually increased our 1/2-inch production by 20 tons per hour.” By tweaking some of the crusher settings to adjust the feed size, Grove says he hopes to boost that number even higher.
Plumstead’s deposit is largely argillite, and a key market is for stone that meets state specs. “If you’re cleaning for a state spec, you’re under stricter regulations because your stone has to meet certain cleanliness standards,” he explains. The stone passes through wash screws and an on-site lab tech performs the requisite wash test to guarantee it meets the state specs.
“As long as you can control blinding, you’re not going to get the carryover on the deck and won’t get fines in the 1/2-inch or 3/4-inch product,” Grove says. “If you can control blinding, you generally have a quality product coming out to sell.”
During the winter months, the wash screws are taken out of line for repair and rebuild. He says that, because the site is still adapting to the impact of the new crusher, the lab tech is running the wash test every day, and sometimes twice a day. “We’re actually under our wash test requirements, without even running the wash screws,” Grove says. He attributes those results to quality material, cooperative weather, and a wire screen on the bottom deck that keeps grit and fines out of the rock.
“On this site, our uptime is sometimes 100 hours a week to produce for demand,” Grove explains.
“We run two shifts, so we’re going to wear things out twice as fast. We do change things more often. If we can find things that don’t wear out as quickly, I’ll spend a little more to get it.”