By Steve Fiscor, Editor-in-Chief
More than 20 new prep plants have been constructed in the U.S. during the last five years and a number of older plants have recently completed upgrades that were long overdue. A noticeable trend among both new construction and upgrades is the increased use of large scale Banana screens. While plant operators admire the performance of these screening systems, high vibration frequency, capacities and velocities place a lot of stress on the screen deck and screen media.
Historically, coal operators have had mixed emotions about screen media. Prep plant purists prefer profile wire, while others see advantages in modular synthetic media. Similar to any new technology, when the concept of urethane screen panels was introduced in the 1970s, some coal operators had a positive experience while others did not. Simply put, synthetic media did not have enough openings to allow the heavy media and water to drain fast enough therefore steel would do the job better. During the last 40 years, the technology has improved to the point where urethane screen media has a greater level of acceptance now and it’s growing.
“As coal companies consolidate, we are seeing a change of attitude about steel vs. urethane,” said Ron Bennett, coal industry sales manager at Polydeck Screen. “That trend will continue. There are some people who still think urethane doesn’t work, but we have been very successful applying it in the coalfields. We are seeing people get more comfortable with it over the last three to five years.”
The largest manufacturer of synthetic screen media is Polydeck, located in Spartanburg, S.C. The company’s primary product lines include modular polyurethane and rubber screen panels, and the frame systems to support the media.
The differences between steel and urethane should be obvious, but they are not. One would think steel is harder and therefore would last longer. That may be a common assumption, explained Bennett, but more often he hears plant operators say they tried urethane a number of years ago and it just didn’t work. Aside from brute strength and tradition, other differences between steel and urethane are more subjective, such as effective wear life and ease of handling during maintenance.
Because the Polydeck system uses 1- x 1-ft or 1- x 2-ft screen panels, one of the biggest advantages for modular screening media is the ability to solve a wear problem in one area of the deck. For instance, Banana screens are hard on screening media. “Many of these new plants are using more water and creating higher velocities than was used in the past,” Bennett said. “We have created different panel styles that improve wear in these high velocity areas.” Maintenance personnel can replace just the worn panel instead of an entire section of steel media. “That modular approach also gives us the advantage on Banana screens of alternating rows of media to fine tune the overall performance,” said Dennis Zeiger, marketing manager, Polydeck. “You can put a row of panels with dams to slow material down or change the slot widths. You can adjust the material flow over the deck to achieve production objectives.”
From a maintenance standpoint, modular screening media is much simpler, easier and safer to handle than steel decking, Bennett said. “It takes less people and time to change out the decking and, from a safety standpoint, instead of handling a 4- x 10-ft piece of punch plate, the maintenance crews are handling 1- x 2-ft urethane panels, there is no comparison,” Bennett said. One man can crawl into a vibrating screen and change out a couple of panels compared to a maintenance crew using a crane to remove a whole section of steel.
Handling Tougher Conditions
Ask any coal miner and they will quickly lament that all of the good coal is gone. No one knows this better than the screening professionals like Polydeck that serve the industry. To get at lower quality seams, mines are having to work through clay partings and that clay has an adverse impact on a screen’s performance. Prep plants wash coal and that means water will be added and removed from the process, and the clay becomes a sticky nuisance plugging apertures and blinding screens. Heavy clay build-up is a major problem for many prep plants, explains Bennett. “We’re talking about clay material that you can wad into a ball, throw it against a wall and it will still be there a month later. We have a rubber formulation we use in our 1- x 2-ft screen panels that we call our Flexi design. It’s softer and allows more vibration than standard urethane panels. We have been able to solve some major clay problems in scalping applications with this product,” Bennett said. “Operators usually say there is no way to solve this, but we have proven we can go in on the deck and fix the problem.”
For challenging situations like this, Polydeck field personnel work closely with applications engineers to help operators make informed decisions regarding media selection, Bennett explained. “Over the years, our applications department has assembled an extensive database of different applications,” Bennett said. “To verify our own decisions, we can look at the database to reference successful solutions applied to a particular application in the past. We can also provide a cost analysis to show plant operators how much money they can expect to save.”
The Science of Screening
Polydeck’s 1- x 1-ft and 1- by 2-ft modular screen panels are available in square, slotted, round, zigzag and slotted zigzag designs. Openings range from 140 mesh to 9 inches and the panels are available in traditional pin-style, PolyRail or the patented PolySnap fastening systems. In addition to the standard range of panels, the company also has various design options for integrated surface features like skid bars, dams, diverters and restricted flow bars to improve performance. These features are molded into the screen panel, never glued or screwed on.
Over the years, Polydeck has made a number of performance improvements to its synthetic media, including those on the drain-and-rinse side, according to Bennett. “Years ago, we could not compete with profile wire because we did not have the open area,” Bennett said. “Today, we can provide as much open area as profile wire.” A key to achieving higher open area occurred when Polydeck introduced the PipeTop frame concept in the late 1990s. The frame’s narrow ½-inch stringer support rail allowed each screen panel to contain more holes, increasing open area by as much as 30%. The next generation, PipeTop II frame added easily removable polyurethane ferrule inserts to further increase the versatility of the system.
Through its on-going R&D work, Polydeck introduced the Maxi screen panel design that, when combined with the PipeTop II frame, provides the highest open area in the industry. The concept is relatively simple—more holes per square foot allows more product to pass. In many cases the system’s open area meets or exceeds the true effective open area of conventional wire cloth. “We have so many more screen panel options than we had in the past,” Bennett said. “There are very few applications where we have not been successful in using our synthetic media.”
The formulation of the screen panel material is not the biggest achievement, Zeiger explained, it’s the screen panel designs. “When we developed this frame system several years ago, it allowed us to put more holes on the surface of the panel, which gave more open area to drain off the heavy media and water,” Zeiger said. “From an efficiency standpoint that has really raised the profile of synthetic media,” no pun intended.
Zeiger also believes the company’s approach to customer service is a Polydeck strong point, particularly in the Midwest and Eastern U.S. coalfields. “Our sales engineers complete screening surveys of the prep plant, showing the vibrating screens and the screen panels they have, and where they’re positioned,” Zeiger said. “These surveys provide the plant operator with the information they need to make proactive instead of reactive decisions in terms of their screening operations.” The company also stocks a warehouse in West Virginia to be responsive to immediate need with quick turnarounds.
Polydeck is a family-owned business. “Another advantage is that screen media is all we do,” Zeiger said. “Our R&D efforts are focused on the whole screening operation at a prep plant. We are always looking at ways to improve different parts of the system, such as the fastening system or panel design. Our success results from the fact that we are nimble enough to make changes and get them into the field quickly.”