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How to maintain sand classifying tanks, fine material screw washers

By |  July 27, 2021
 Photo: EIW

Although sand classifying tanks may seem complex, they can easily be maintained with periodic inspections and repairs. Photo: EIW

Sand classifying tanks are used in sand and gravel plants, as well as manufactured sand plants.

These units provide a simple way to remove excess water and slimes or undesirable grain sizes from natural, alluvial sand or manufactured crushed sand feeds. Sand classifying tanks are typically used to classify sand for most construction specifications by removing an excess of certain intermediate mesh sizes while retaining finer mesh sizes and making multiple products from a single-feed material.

Sand classifying tank are an effective, low-maintenance unit producing one or more specification products. With either slurry or dry feed, sand classifying tanks handle sand gradation swings in the average plant while minimizing non-spec sand production.

Sand classifying tanks are important for producers who:
1. Remove excess water, such as in a suction dredge feed plant
2. Produce multiple products from a single feed
3. Classify material by removing and/or adding certain mesh sizes
4. Retain finer mesh sizes

Sand classifying tank maintenance

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 that 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 valves exits the sand tank via down pipes that are often equipped with elbows to a sectionalized collecting flume. Elbows handling coarse sand experience wear and require more frequent replacement than valves discharging finer sand.

While many sand tanks utilize steel pipes and hard-iron elbows, 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.

Liners in the sloping double- or triple-cell collecting flume should be visually inspected and replaced as needed. The flumes are used to direct the classified sands to dewatering equipment. Types of liners include AR steel, rubber and polyurethane. Liners should cover not only the bottom of the flume but also the sides to protect the fabricated steel flume.

The opening of sand tank valves is made via hydraulic cylinders connected to a valve rod. A self-contained hydraulic power unit is supplied to operate the cylinders via hydraulic hose or tubing. Periodic replacement of hydraulic fluid and filters, as specified by a manufacturer’s manual, should be followed.

For sand tanks that are not operational during a seasonal shutdown period – even if not changing the fluid – the hydraulic reservoir should be checked for water that may have accumulated from condensation. The water should be drained from the reservoir, and the proper hydraulic fluid level maintained as recommended by a manufacturer.

All electrical systems, wiring and hydraulic lines should be checked periodically to make sure they are functioning properly and do not cause an improper grounding or leakage that can be a hazard to maintenance personnel. Also, 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 ensure no interference 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 a manufacturer’s manual.

Although sand classifying tanks may seem complex, they can easily be maintained with periodic inspections and repairs, providing high operational availability.

Fine material screw washers

Photo:

Fine material screw washers are used primarily to dewater, classify and wash sand. Photo: EIW

Many aggregate producers need to wash sand, and fine material screw washers are an option.

Fine material screw washers are used primarily to dewater, classify and wash minus 3/8-in. or 10-mm sand – or other fine material with a specific gravity of 2.5 to 2.7.

These machines are used following sand classifying tanks, belt conveyors, other fine material screw washers or hydrocyclones, but most often appear after a wash screen when the sand is in a dilute slurry.

Producers often choose fine material screw washers because of their simple operation. They are generally inexpensive and deliver an immediately saleable product.

Compared to other dewatering, classifying and washing machinery, the fine material screw washer requires minimal process knowledge and can usually handle variability throughout the feed material in a pit. Not all machines can say this.

Another advantage is that, most often, a fine material screw washer can be placed into an operation without a lot of rework or special hookups.

Fine material screw washer maintenance

While a fine material screw washer may be among the simplest machines to maintain in an aggregate plant, appropriate safety precautions must always be taken – including a lockout-tagout out of the motor starters.

Proper lubrication of the bearings that support each end of the screw shaft should provide long bearing life. If a part of the manufacturer’s design, the lubrication needs of seals on submerged rear bearings are less than the needs of the bearing itself, but ensure proper protection of the rear bearing. A non-lubricated seal leads to more frequent rear bearing failures and subsequent downtime with a loss of production.

If the discharge end of the screw shaft is supported by a standard pillow block bearing, it is often recommended that a lube line be run down the support to near ground level so maintenance personnel can regularly supply grease if a service platform is not at the level of this bearing.

Typically, the screw shaft is turned by an electric motor with a V-belt drive into a shaft-mounted reducer. Changing the reducer gear oil in accordance with the manufacturer’s recommendations usually provides long gear box life. If the reducer on the machine has been in service for several years, periodic evaluation for contaminants in the gear oil is recommended to avoid an untimely failure in the middle of a critical production period.

Periodic inspection of V-belts for both wear and tensioning is also recommended to reduce downtime and unexpected failures.

The need to replace wear shoes can easily be detected by visual inspection. Replacement shoes are typically available in the manufacturer’s standard iron, urethane or rubber types.

The best abrasion-resistant material for wear shoes depends on the chemical composition and particle shape of the sand being dewatered. The most abrasive sand may be crushed granite sand or sandstone that can abrade or gouge synthetic wear shoes. This means these shoes often do not provide suitable life when compared to their costs.

Remember that performing your own checkup or getting the assistance of a manufacturer’s local dealer or representative can often result in less downtime, providing the highest availability for the operation of a fine material screw washer.

Information for this article courtesy of EIW (Eagle Iron Works).


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