Keeping the ‘Class’ in Classifying Tanks
| March 21, 2012 |
Long-term operation and maintenance of your classifying tanks can be achieved by following five simple tips.
By Tom Moore and Dave Schellberg
Sand classifying tanks have been used for more than 50 years for refining glacial/alluvial ‘natural’ and ‘manufactured’ sands and sand slurries. They have been used primarily in North America for three typical reasons:
Concentrating or ‘scalping’ water in a low-percent solids slurry for more efficient dewatering by downstream equipment;
Removal of excess intermediate sand grain sizes to produce a uniformly graded product for many common construction sand specifications; and
The production of two controlled and one uncontrolled sand product from a single feed.
As with any process equipment, periodic operational and maintenance checks are recommended to optimize a machine’s operation. Your manufacturer’s installation and maintenance manual can be used as a guide to allow the most efficient performance and yield of desired sand products.
This type of sand tank is not a ‘set it once and walk away’ device, as sand feeds can vary due to changing pit feeds or the performance of crushers and other equipment that may precede it.
Periodic sampling of the sand discharging from the sand tank valves, particularly at the first two or three stations, is recommended to make sure that the product is mostly coarse sand particles. If a specification concrete sand gradation is found in the product valve sampling, adjustments to the rising current water injection and tank overflow weirs should be made to move finer sand particles further down the sand tank. This provides a more controlled blending of coarse, intermediate, and fine sands being made by the PLC control system.
During a non-operational period, an empty sand tank’s operating system should be inspected to assure that the valves at each station are opening and closing as programmed by the controller. This would include stalling the torque motor paddle at each valve station.
To assure fine product-sized sand retention, make sure that all weirs are level, allowing uniform depth of silt slurry overflow.
To minimize downtime during peak production, periodic inspections of key components should be made.
All sands cause wear on components to varying degrees. Valves and valve seats are among the most commonly replaced items. While some original components are hard cast-iron alloys intended for abrasion resistance, these parts are often replaced by polyurethane components, which offer extended wear life in most instances.
Valve rods, torque motor paddles, and paddle rods should be periodically inspected for wear and replaced when needed.
Sand discharging from the valves exits the sand tank via down pipes, often equipped with elbows, to a sectionalized collecting flume. These elbows handling coarse sand experience extensive wear and require more frequent replacement than valves discharging finer sand. While many sand tanks have used steel pipes and hard iron elbows, these metal components have been replaced with thick walled PVC pipes and polyurethane elbows that are easier to handle, quicker to replace, and provide equal or extended performance.
Flume liners in the sloping two- or three-cell compartment trough need to be visually inspected and replaced as needed.
Types of liners include AR steels, rubber, and polyurethane and should cover not only the bottom of the flume, but also the sides to provide protection to these cell compartments of classified sands.
The opening of the sand tank valves is typically made possible by a hydraulic power unit with dedicated lines connecting hydraulic cylinders attached to each valve rod. Periodic replacement of the hydraulic fluid and filter, as specified by the manufacturer’s manual, should be followed.
For sand tanks that are not operational during a seasonal shut-down period, even if not changing the fluid, the hydraulic reservoir should be checked for water that may have accumulated from temperature-caused condensation. The water should be drained from the reservoir and the proper hydraulic fluid level maintained as recommended by the manufacturer.
All electric systems, wiring, and hydraulic lines should be checked periodically to make sure all are functioning properly and do not cause an improper grounding or leakage causing a hazard to maintenance personnel. The control enclosure on top of the sand tank housing the hydraulic cylinders, torque motors, solenoid valves, and associated electrical wiring and hydraulic lines should be cleared of any accumulated sand to assure no interference that can cause component failure.
If a sand tank is equipped with a rising-current recirculating-pump system, the impeller should be checked for wear and replaced when worn. V-belts driving the pump and bearing lubricant should be inspected and maintained as specified by the manufacturer’s manual.
Sand classifying tanks, while from an initial overview may seem complex, can be easily maintained with periodic inspections and repairs providing higher value sand products.
The Sand Tank Top Five Check List
1. Check critical wear components and replace if needed.
2. Periodically sample sand discharging from the first three valve stations.
3. Change hydraulic fluid and filter as recommended by manufacturer.
4. Inspect and maintain all electrical and hydraulic components as recommended by manufacturer.
5. Keep service platforms cleared and properly guarded for the safety of maintenance personnel.
Tom Moore is a graduate of Penn State University with an associate degree in mechanical engineering. He is manager of Customer Service of the Aggregate Processing Division for McLanahan Corp. in Hollidaysburg, Pa.
Dave Schellberg is a graduate of Ashland University (Ohio) with a bachelor’s degree in broadcast communications. He is manager of Application Services for McLanahan Corp. in Hollidaysburg, Pa.