April 1, 2013
Follow these operating and maintenance recommendations to keep your fine material screw washer operating efficiently.
by Dave Schellberg and Rick Madara
Many aggregate plants throughout North America use fine material screw washers for dewatering slurries of minus 1/4-inch x 0 fines in the production of varying specifications of sand. The feed to a fine material screw washer could be dry screened minus 1/4-inch material or it could be a slurry directly from a wet vibrating screen, a sand classifying tank, a hydrosizer, or other wet processing system that is able to control the top size of the material and classify the sand to the desired grading.
Making sure that a fine material screw washer is operating efficiently and retaining the product gradation required can be simply evaluated by checking the following items:
• Make sure the weirs that allow the overflow of water and undesirable fines are level and are positioned to retain the fine sand desired while removing the unwanted excess fine sand. As an example, often times, if excess fines are in the product, raising the side weirs and dropping the back weir allows for a maximum removal of undesirable fines.
• It is important to review the amount of material passing 200-mesh in the feed. In most applications, this material is considered to be unwanted fines and needs to be removed. With too little water, the operator will not be able to remove the desired amount of minus 200-mesh material. As a general rule of thumb, the operator needs 50 gallons per minute for every 1 short ton per hour of minus 200-mesh material in the feed.
• Water that comes in with the feed must also be taken into account. A washer box can only process so much water while retaining material to a certain size. Most manufactures will provide a water volume chart specific to their equipment. This will determine the size of a washer box that must be used in order to retain-product sized material while overflowing (removing) unwanted fines.
• Rising current classification water beneath the pool area of the screw washer may also need to be adjusted. If product-sized fines are overflowing — the screw washer and any rising current water is being injected beneath the pool area — reduce the flow or shut it off completely.
• When desired product-sized fines are being lost and are overflowing the screw washer, sometimes correcting improper sand slurry feed entry into the screw washer helps. The pool area in back of the baffle plate near the overflow needs to be as calm as possible so turbulence is reduced to a minimum, allowing for the settling of fine, product-sized sand. Directing the sand slurry feed to the discharge side of the baffle plate will allow for minimum turbulence at the rear of a fine material screw washer for optimum fine-sand retention.
• Screw shaft speed, which in many instances is set by the manufacturer for the original application, may need to be adjusted, particularly if the screw is turning too fast for finer sand. Generally, for dewatering an average concrete sand having up to 20 percent of the product passing 50 mesh, the peripheral speed for a fine material screw washer should not exceed 150 feet per minute or 75 percent of the full speed. Keep in mind that the reduction in the speed that is often needed for fine mason sand will result in a capacity reduction.
• To ensure that the sand product discharging from a fine material screw washer is as dry as possible, the proper amount of wash back, or flushing water, needs to be injected in the dry deck dewatering area. The addition of water in two places in the dry deck area ensures the least amount of moisture in the sand discharging from a fine material screw washer. The addition of a dewatering screen following a screw washer can reduce the moisture content of sand by up to half.
While the 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 lock out-tag out of the motor starters.
Proper lubrication of the bearings that support each end of the screw shaft should provide long bearing life since the speed of a shaft is very slow in comparison to other types of process equipment. The lubrication needs of seals on submerged rear bearings, if a part of the manufacture’s design, are less than the needs of the bearing itself, but assure proper protection of the rear bearing. A non-lubricated seal almost always 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 that 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 a motor/V-belt drive and shaft-mounted reducer. Changing of the reducer gear oil in accordance with the manufacturer’s recommendation usually provides long life for this component. If the reducer on the machine has been in service for several years, periodic evaluation for contaminants in the gear oil is warranted 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 failure.
The need to replace wear shoes can easily be made 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 may be dependent upon the chemical composition of the sand being dewatered and its physical structure. The most abrasive sand may be manufactured (granite-like) sand that can abrade or gouge synthetic wear shoes, which means these shoes do not often provide suitable life when compared to their costs.
Doing your own check-up or getting the assistance of the manufacturer’s local distributor/representative can often provide more product-sized sand retention and less downtime, providing the highest possible availability for the operation of a fine material screw washer.
Dave Schellburg is McLanahan Corp.’s application specialist – Processing Engineering and Rick Madara is its sales manager – Aggregate Processing. They may be contacted at firstname.lastname@example.org and email@example.com.