Screening For Maximum Efficiency
- Published: Wednesday, 01 March 2006 07:00
Screening equipment is one of the largest determining factors in plant productivity and efficiency. But with the many variables and other machinery affecting the screening process, it takes rigorous attention and effort to maintain maximum production. It also takes quite a bit of knowledge.
Joe Schlabach is director of marketing and sales for Deister Machine Company with over 28 years experience. In January, he led a seminar on screen performance at the National Stone, Sand & Gravel Association's Plant Operators Conference in Long Beach, Calif.
Schlabach explains that screening material is based on the art and science of stratification, which is the phenomenon that occurs in a bag of chips or a box of cereal. By the time the product makes it from the packaging line, to the grocer and finally to the kitchen; the larger sized product remains at the top while all of the fines have worked through to the bottom.
With aggregates, the material is added to the top deck, and the machine's shaking action causes the coarse material to rise as the fine material falls. Towards the discharge end of the machine, the undersize material becomes closer in size to the deck opening.
It's a simple concept that quickly grows more complex as manufacturers and engineers synchronize and incorporate this process with the rest of production, especially the crusher. And many questions need to be answered. For instance, is a horizontal or incline screen more suitable? The choice may be obvious if there is no headroom for an incline. And portable plants and dewatering screens typically are horizontal, Schlabach says.
The initial cost of a horizontal screen is greater than an incline screen because of the gears. Horsepower and energy needs are greater because there is no gravity working with the screen, and they are more prone to plugging with the linear stroke. And, with a rate of travel at 45 to 50 feet per minute, at a specific tonnage a horizontal screen results in a greater depth of bed that diminishes capacity. Because the stroke on a twin-shafted horizontal screen is a straight line, it must stop and change direction at 180 degrees, which causes stress on the machine frame.
In comparison, an incline screen has a lower initial cost, is less susceptible to plugging and uses gravity to reduce energy needs. On a 20-degree incline, rate of travel is about 70 to 75 feet per minute. Schlabach says an inclined screen has 20% to 25% more capacity than a linear-stroke horizontal machine. And since the machine has a circular motion, there is less stress on the vibrating frame.
Once the proper type of screen is installed, proper adjustments need to be considered to maximize efficiency. The adjustable operating parameters on a vibrating screen are speed, stroke, direction of rotation and angle of inclination.
The higher the angle, the faster the material will travel, which is good for dry screening material that tends to stick. However, there may be a point where incline would hinder production because fines may roll right over the media rather than fall through.
Schlabach says that both linear and triple-shaft horizontal screens can be adjusted for inclination as well. ìYou can gain some capacity, rate of travel, some better screening results at times, by pitching up the horizontal screen a little bit.î
Direction of rotation on a linear-type horizontal screen has little effect. The stroke action is still following the same straight line at the same angle and amplitude. He says it has very little impact on operation. But it would have a tremendous impact on a triple-shaft horizontal screen because it would change the rate of travel. Running counter flow could lead to plugging problems, which might be resolved by reversing the rotation.
Rotation also can have a dramatic impact on incline screens. The screen retains material longer when running counter flow or uphill. This slows the rate of material travel and increases the depth of bed. Schlabach says this may sound like the wrong thing to do if the material bed is heavy, but he has seen it defy logic. It is worth a try because there is no cost associated with doing so.
It also may be advantageous to increase speed. Doing so can decrease the depth of bed but also increase the G-force, which decreases bearing life. Also, as material travels over the media faster, the chances of particles falling through the hole are diminished. Schlabach says it may be beneficial to have a slightly larger opening than the desired particle size. However, this also increases the risk of oversize. Using the proper mesh size with increased speed, will leave a small percentage of the desired material size in the oversize. Speed can be measured using a photo tachometer and a reflective sticker.
Stroke can be measured with an address label and a pencil. First, attach the label onto the side of the screen and draw a horizontal line on the label using a level. Then touch a pencil to it long enough to generate the stroke action of the machine. Stroke readings also can be taken electronically with vibration analysis equipment that measures both vertical and horizontal directions.
To change the stroke of a machine, the eccentric mass of the machine needs to be altered by the shaft or with counter weights. Removing weight decreases stroke; adding weight increases the stroke.
The advantages of increasing stroke are more carrying capacity and higher acceleration and travel rate. Increasing the travel rate by increasing stroke is not as dramatic as increasing travel rate by increasing speed, but it can reduce plugging and blinding, and it does enhance stratification.
Increasing stroke can increase the stresses on the machine. However, Schlabach says stroke is a linear type function rather than an exponential function, so changing stroke has a less dramatic effect on G-force and bearing life. It also can create some inefficiency due to bouncing caused by the decreased depth of bed. Generally, coarse separation requires larger strokes and less speed (rpm). Fines separation generally needs less strokes and higher speed.
These four operating parameters need to be explored. ì(Screening) is art and science. There are no books or formulas,î Schlabach says. ìA lot of times it is about trial and error by experiment.î
Another thing to experiment with is the various media available for screening. Changing it could eliminate plugging or blinding. And the options are seemingly limitless. Media is produced in various types of rubber, stainless steel, high-tensile wire, perforated plate and polyurethane. And it can be shaped in squares, rectangles, zigzags and in varying grades of wire with different diameters.
Choosing the right media size and material, like operating parameters, have a strong impact on screening efficiency. ìYou don't want to be sending material back to the crusher that doesn't need to be crushed,î Schlabach says. ìScreens and crushers need to work together.
Material feed has a dramatic impact on screening efficiency, and it is important to use as much of the media surface as possible. Rock ledges can help consistently distribute material from conveyors. A half-round plate with a ledge around it is one option. Material collects on top of it, creating a half-cone of material that cascades at 180 degrees onto the screen.
An inconsistent method of distribution is the pant-leg feed chute. Schlabach says they are susceptible to changing feed conditions to screens because of buildup under the head pulley. Volume often shifts from one side to the other, and material segregation can occur between the two chutes. Also, material tends to fall at a slight angle relative to the machine, and coarser material tends to fall farther than the fines.
The pant-leg feed chute can be avoided by splitting material feed at the tail pulley and using two conveyors. Smaller conveyors typically would be needed, and eliminating the pant-leg feed chute reduces height needs, so less horsepower would be required.
Other mechanical alterations or additions could be costly in the long run. Schlabach says mechanical means call for additional maintenance and he recommends first looking at media changes. For instance, moving to a smaller diameter wire could increase vibrations and eliminate blinding.
If changing the media fails, consider ball trays and heated decks. Ball trays incorporate rubber balls into pockets beneath the screen cloth. As the machine vibrates the balls strike the media to free collected material. The brick industry and those trying to screen clay, often rely on heated decks. These have an electric current in the wire that heats and dries material so it is knocked loose of the screen.
Despite all of the additions and alterations that can enhance screening efficiency, ìyou do reach a point where it is doing all that it will do,î Schlabach says. But regardless of capacity, a machine can always last longer with better maintenance. The bearings are especially important to watch. The elements most harmful to bearings are heat and dirt.
Schlabach says to avoid getting dirt in the lubrication by keeping it in clean containers and watching for dirt that could fall into an oil-fill hole when making changes. Also it is important to use the equipment manufacturers' recommended lubricant.
Consistent oil sampling is another smart maintenance practice, Schlabach says. It can give telling signs about the fate of the machine. ìIt is better for us to predict or to schedule when it is time to change a bearing, then for the machine to dictate to us when it is time,î Schlabach says. Oil samples often have traces of iron and copper, which are common wear metals. Dirt and water should be nearly nonexistent in a sample.
Spray systems also need to be kept clean. Material can clog a nozzle and decrease the efficiency of the spray. It also is good to maintain the right volume and pressure. Schlabach recommends 40 psi or slightly higher depending on the application. Spray headers should maintain 3 to 5 gallons per minute for every ton per hour of material. And for rinsing, spray headers should maintain 1.5 to 3 gallons per minute for every ton per hour.
It also is smart to ensure spray pipes do not spray at right angles. Schlabach recommends 30 degrees from the vertical. Water typically is the first thing turned on and the last thing turned off on a plant. That means there will be periods when water sprays directly onto the media. This can start to cut wire and synthetic media in half.
Belt tension is another common problem. And because the machine is sitting on springs, belt-tension gauges are inadequate. One way to tell that a belt is too tight is if the machine is physically pulled cockeyed on the screen mounts when tightening a belt. Schlabach says overtightening a belt is usually a gradual process. Perhaps there was a humid day that contributed to some slippage, and the belt was tightened and never loosened. Perhaps that same belt was tightened repeatedly.
Schlabach also says that it is important to avoid material buildup around moving parts and to prevent the machine from impacting on anything solid. ìEventually something is going to break,î he says. And as screens continue to get larger, they are more susceptible to breakage due to greater leverages and centrifugal forces. Impact stresses the entire frame.
The same is true with material buildup. When a machine is trying to go through the motions and one corner is interrupted, that energy has to go somewhere and often twists the entire frame. This can lead to cracked side plates, premature bearing failures, cracked mechanism tubes, broken deck frames and bucker bars that break off. Even without impact or material buildup, side motion may still be twisting the frame. It could be a structural issue, one that adding braces could relieve. A speed adjustment away from harmonic frequencies could be another solution.
When looking to maximize screening, the right knowledge, equipment and setup can keep operators focused on the ìart and scienceî of making money.