October 1, 2008
Use these dewatering tips to keep your sand operation flood free.
by John Bennington
When fine material screws fail to remove enough moisture from their material, many sand operations decide to introduce a dewatering screen into their material flow.
The producers who own them already know that dewatering screens typically require little maintenance. While some dewatering screens resemble traditional sizing screens that use eccentric shafts to induce their shaking motion, the majority of dewatering screens use high-frequency vibrators and have few moving parts. The screens are designed for an operator to simply flip a switch and run them, day in and day out. However, even low-maintenance machines still require some care, and the necessary maintenance is essential to their successful operation. In addition, operator error can affect both capacity and drying capability on these simple screens.
Opportunity for error
With either dewatering screen type, the theory behind water removal is the same; as the wet sand feed creates a deep material bed on the screen deck, the combined material weight and motion of the screen’s throw work to squeeze water through tiny apertures in the screen media. At the same time, the throw moves the material down the screen’s length. The desired result is that sand coming off the end of the screen should have a moisture content of 10 percent or less.
The most common complaints noted among dewatering screen operators – those of wet material and decreased capacity – are usually the result of not feeding enough material. If the screen cannot form a deep bed of sand, it will not be able to retain the product on the screen to remove the moisture and will also have trouble effectively moving the sand down the length of the screen. It is the vertical/diagonal vibrating motion of the screen, along with the inertia of the sand, that work to squeeze the water through openings in the screen media. The thinner the bed depth, the lower the inertia, resulting in lost and wet material – ultimately defeating the purpose of a dewatering screen.
Another common mistake among producers who are not familiar with dewatering screens is purchasing a unit based on desired plant capacity, rather than current production capability. In order for the screen to handle the feed correctly and create the ideal bed depth, the producer must look realistically at the plant’s current production numbers. A high-capacity dewatering screen that is too large for the feed will not effectively dewater or move the material.
Most dewatering screens allow for adjustment of the throw and the screen’s angle of incline – from horizontal to about 5 degrees uphill or downhill. Both adjustments can affect the capacity of the screen and its drying capability. If an operator adjusts the screen for a steeper uphill angle with less throw, it will require that less tonnage be introduced to the screen. The opposite is also true, with a horizontal screen or downhill angle with a harder throw allowing higher feed tonnage. Producers should initially work with the manufacturer to adjust the screen for the best angle and throw to meet their desired results. Later, if material properties change or production increases or decreases, the manufacturer and/or the equipment dealer can help determine the best settings and recalibrate the unit to operate under new parameters.
With sizing screens, the purpose of the screen media is for the material to go through the openings. With a dewatering screen, the goal is to retain the material on the deck, allowing only the water to filter through. Dewatering screen media does not wear as fast as media used on sizing screens, but when it does wear, it should be replaced immediately. Although dewatering urethane typically tends to wear at the surface of the screen, instead of the openings, any wear that does appear at the openings will allow salable material to fall through the screen along with the water. In addition, because the openings are very fine, they can punch through or tear easily if stepped on or if tools are dropped on the screen panels. In order to ensure retained product, the media should be examined regularly for signs of wear or holes, and worn or torn panels should be replaced.
Vibrating motors: For dewatering screens that use high-frequency vibrating motors, operators need to know that these motors run fast, from 900 to 1,800 rotations per minute, depending on the screen and motor. Additionally, their linear throw can equal up to 6 g (six times the force of gravity, which means that one ton of sand is equal to six tons – or 12,000 pounds – of force), so they generate a lot of force driving up and down. If these motors and their mounting apparatus are not kept properly maintained, they can destroy themselves rapidly and create safety hazards.
As a standard step in safety and operating the machine efficiently, producers need to maintain the bearings on the motors and should follow manufacturer-recommended intervals. The motor’s manufacturer provides greasing schedules for the bearings, as well as startup procedures, which should be included in the screen manufacturer’s operation and maintenance manual.
Ensure that bolts are tightened to the correct specifications. If the bolts or the frame are cracked, repair them immediately. Most manufacturers will recommend checking the torque on the bolts prior to initial startup when the unit is first installed, followed by regular inspections at least twice a year. Inspect the motor mount regularly; if cracks are found, repair them immediately.
Eccentric drives: For eccentric shaft dewatering screens, the maintenance of the motors and bearings is similar to that of traditional sizing screens. Manufacturers should provide schedules for maintenance, but most recommend checking the belt torque on the wheel case approximately four times a year. It is a good idea to check the drive belt and sheaves daily, as well as for leakage from the drive shaft seal. The pillow block bearings should be greased every couple of weeks or according to recommended intervals. Producers should also check the wheel case oil for contamination after 250 hours of operation and change it according to manufacturer specifications.
While dewatering screens may not be maintenance intensive, a basic understanding of how they perform and what maintenance they require will guarantee long-term success.
Bio: John Bennington is vice president and general manager for Columbus, Neb.-based GreyStone Inc., a manufacturer of sand washing and classifying systems. He also is a past chairman of the National Stone, Sand & Gravel Association’s Manufacturers and Services division. Bennington has more than 15 years of experience in the aggregate industry. He may be reached at 888-346-9274 or firstname.lastname@example.org.