December 1, 2008
Selecting the right size and type of screen improves the overall success of an aggregates operation.
by Leon Luadzers
The proper sizing and selection of a vibrating scalping screen is critical to the success of most aggregate and mining processing plants. In general, vibrating scalpers improve the overall performance of the entire processing plant, including crushers and other vibrating screens.
Since conducting research on this subject 21 years ago, my vibrating scalper sizing formula has been incorporated into popular aggregate-flow computer programs, making it easier to size vibrating scalper screens. However, with new vibrating screen technology and screen media it’s important for operators to take advantage of new options and choices.
The original scalper screen sizing formula was required because formulas that existed at that time only worked for small openings and finishing screen applications. The large openings used by vibrating scalpers and the uniqueness of the application required something new. Throughout the years, the formula has proven to work well and has been incorporated into modern sizing programs.
For the purpose of this article, the vibrating scalper is described as an extra heavy-duty vibrating screen designed to accept larger feed size (typically larger than 8 inches). The scalper typically screens or scalps at openings from 4 to 8 inches and larger. The scalper is normally designed with extra heavy-duty design features and is equipped with heavy-duty media such as grizzly bars, perforated plate, rubber panels, etc.
The function of the scalping screen is usually described as the separation of oversize material from a feed of finer feed; however, this is not the situation in every application.
Typical scalper applications
(There are many other scalper applications.)
The following information is required to size a vibrating scalping screen: Feed rates – tons per hour, feed gradation, and deck openings. It’s also important to know and understand other application criteria including specific material specifications, screen media specifications, general application requirements, and scalper service duty.
The vibrating scalper sizing formula determines the square footage required based on the specific application factors. A screen width is chosen which will result in acceptable material bed depth and a length is chosen which will provide the required screen efficiency rate. Today, scalpers are typically available in 4- by 8-foot to 8- by 20-foot size units.
Scalping screens can be single-deck units but many applications require them to size other products, therefore it is common to see double- and triple-deck scalpers. A flow program will individually size each deck. In some cases, the middle or bottom deck requires more screen area than the top deck.
A speed and stroke combination is selected based on the deck openings of the scalper screen. Generally speaking, a maximum stroke is required for the large deck openings to prevent material from plugging or sticking in the opening. It’s common for an inclined scalper to have a 1/2-inch stroke and to operate with a speed of 650 to 750 rpm, which typically results in a 3.2 to 3.5 G force.
Vibrating scalpers are available as inclined and horizontal-type units. Today, inclined scalpers are typically two-bearing, circle-motion units. Horizontal units are either linear motion (two-shaft) or triple-shaft (elliptical motion) units.
The inclined scalper is typically used in stationary plant applications and in some portable plants. The inclined scalper relies on gravity to move the material down the deck. This is a useful benefit when handling high tonnage rates and large feed sizes. It’s typically easier to prevent deck openings from plugging on an inclined scalper screen.
The horizontal scalper is often used on portable plants because it helps reduce travel heights. The linear or elliptical vibrator motion of the horizontal scalper performs the dual purpose of shaking the feed material and also conveys the feed down the deck. The material travel rate is lower on a horizontal screen, but one of the benefits is that this allows the operator to control the velocity of the material coming off the deck. The disadvantage of the horizontal screen is that the top deck opening must typically be limited to 4 inches or less to prevent plugging problems.
One of the new developments in vibrating screens is the modular vibrator, which can create the necessary stroke and speed for scalper screens. The modular vibrator is durable and provides the highest bearing life at the lowest operating cost. The versatile unit may be used on both the inclined scalper screens and horizontal scalper screens.
The modular vibrator is a true cartridge-type vibrator that eliminates bearing replacements and dramatically reduces plant downtime. The modular design versus the traditional design eliminates oil leaks and shaft deflection. It also can be easily and safely maintained and replaced.
There are many media options that can be customized for any application. In the past, perforated steel panels or grizzly bar decks were the only choices. They are still viable choices, depending on the application, but there is new media that will also perform well.
Perforated plate decks are available in abrasion resistant (AR) steel or mild steel and are available with a wide range of openings typically from 1-1/2 to 5 inches. The deck openings are usually available in square or round holes and can be placed in a staggered layout to improve screening. Perforated plates are available with skid bars to help prevent plugging and to help reduce wear on the plate.
Grizzly bar decks are often used to increase screening capacity when exact sizing is not required. The grizzly bars are typically available in 4- to 6-foot-long sections and, depending on the length of the scalper, there may be multiple grizzly sections with a step between each one. The grizzly bars can be set up with a wide range of spacing from 2-1/2 to 8 inches with standard bars. Special taller grizzly bars are required for larger openings.
Today, a good alternative media are rubber panels. These systems offer excellent wear characteristics and are available in a wide range of openings with molded skid bars. These panels can make accurate separations and reduce noise levels.
There are a number of media options available for the other decks. Urethane panels, non-blinding rubber, or wire cloth may be used on the lower decks to size other products on the scalper screen.
The scalper typically handles large feed sizes or higher tonnage rates – or both. Therefore, the scalper body has heavier duty features. In most cases, the side plates will be thicker and include more reinforcements. It has been found that a huck-bolted design is superior to a heavily welded design or a design using traditional bolts. The liberal use of welding increases the opportunity for cracking, and traditional bolts tend to loosen on vibrating units.
The scalper screen deck frame needs to be a heavier duty design in order to carry higher tonnage rates and to accept impact from larger feed material. The media used on scalper screens also weighs more and this requires larger deck cross members. It’s a good idea to use cross member wear liners to prevent premature frame failure.
A scalper typically has extra side-plate height above the top deck to contain the higher tonnage rate and large feed size. The top-deck side plate is typically lined with AR steel or rubber. The space between the lower decks may need to be increased as well, depending on the tonnage rate.
Vibrating scalper screens can improve the overall performance of almost any plant. It’s important to correctly size the unit, choose a modern heavy-duty screen and vibrator design, and select the optimum media for the given application.
Leon Luadzers has 32 years of application, sales, and manufacturing experience with crushing and screening products. He is currently the capital equipment manager – vibrating equipment for Metso Minerals in Columbia, S.C. He holds both bachelor’s and master’s degrees and has published and presented many papers on the subject of vibrating and crushing equipment.