General Chemical Industrial Products

Shipping Weights

It is vital to maintain strict control over shipping weights, especially if the invoiced weight of soda ash received is a primary indicator of raw material consumption in controlling production and evaluating process yields. Many sites formulate mixes or reactions using package count or depend on a secondary variable, such as pH, to control use rate or quantity.

The alkali industry uses automatic machinery to bag soda ash in multi-wall paper bags. These machines are rugged, dependable and have a high degree of reliability. Trucks that carry soda ash are weighed on truck scales before and after loading. If they are reweighed at a destination, fuel consumption must be subtracted. The tractors hauling self-unloading bulk trailers may be changed between loading and delivery, so gross shipping weight at the origination and delivery points may not be comparable.

Weighing

Measurement of dry soda ash should be based on weight, not volume. Even measurements made by package count are based on weight, because packages are preweighed. Most industrial scales use gravity in a counter-balancing system of levers and linkages. Inaccuracies due to friction are generally on the order of 0.1% of scale capacity for new and well-maintained scales.

For maximum accuracy, scale capacity should match as closely as possible the weight to be determined. One hundred pounds can be weighed on a 100-lb. capacity scale to accuracy of 1/10 lb. or better, but only to 1 lb. on a 1000-lb. scale. Accuracy is not related to the precision with which the scale may be read or set. A 250,000-lb. capacity railroad scale read to the nearest 10 lb. may still have an inherent accuracy no better than 0.1% of its capacity, or 250 lb. (Railroad rules permit a difference of up to 1% of the lading between any two railroad scales, with a maximum of 500 lb.)

The levers in mechanical scales can actuate trips, alarms and control devices. They also can be adapted to automatic weighing (see below). Attachments available for most mechanical scales allow them to print, record or transmit a signal. Photoelectric cells or magnets can permit this without mechanical contact.

Non-mechanical scales use hydraulic, pneumatic or electronic load cells rather than mechanical linkages to generate signals proportional to the forces involved. They are useful for large quantities, such as bulk storage bins or tanks. Load cells are sturdy and compact so they are often mounted on crane hooks and lift trucks to read the load handled. They give continuous, full-range signals that can be amplified and converted. This is more useful than the simple over-under signal from many mechanical scales. Load cells require calibration before use and may have a comparable accuracy to mechanical scales at a lower installed cost. They do not have the same kind of progressive wear as mechanical scales.

Batch Weighing

Manual batch weighing allows the scale operator to put or take so as to trim the scale balance to the desired weight. The accuracy of manual batch weighing is affected by the mechanical interferences described above and by the feed system to the scale. Batch-type scales operate on the basis that a flowing stream of material has constant density. The stream of material flowing to the scale must be cut off far enough in advance so the weight on the scale places it in proper balance. This requires a uniform flow to the scale. One possibility is to keep the flow rate relatively low to minimize the effect of flow variations. Since this slows down the operation, two rates of feed are often used to charge automatic batch weighers: most of the charge is fed rapidly and then the flow is reduced to a dribble for finishing.

Many types of manual and automatic batching scales are available. Most of them have a hopper to contain the soda ash. The hopper can be movable or resting on a dormant or built-in scale with the platform flush with the floor level. It also can be suspended from a trolley on an overhead track scale. Such arrangements are frequently used when soda ash is combined with other dry ingredients at a central mixing station. In most cases, the hopper is an integral part of the scale, and the dry soda ash is fed to the hopper and removed by gravity or mechanical conveyors.

Continuous scales used for batching should have an integrating device to register the flow of a predetermined amount, but this type is more properly classed as a feeder.

Continuous Weighing

Continuous weighing involves a device that is sensitive to the amount of material flowing and to changes in the flow. Continuous weighing scales use a section of a belt conveyor over which the material to be weighed passes. The belt is mounted on a weight-sensitive platform, typically equipped with load cells, that can detect minute changes in the weight of material passing over the belt. The load cell output is integrated over short time intervals to give a rate of flow and total flow.¹

All scales require continuous monitoring to assure that the desired set weight is maintained and does not drift off because of changes in bulk-density or flowability. They must be clean, carefully maintained and checked regularly with test weights. Dust or other foreign matter should not accumulate on levers or pivots.

Feeding Dry Soda Ash

Dry soda ash can be continuously fed based on volume or weight. Weigh feeders are not subject to error when material density changes as are volumetric feeders. Continuous feeding is different from simple conveying or batching because the flow of material must be maintained at a controlled rate. Almost any type of conveyor can be a feeder if it has a reasonably uniform discharge rate that can be varied by changing conveyor speed or amplitude or by throttling the flow to the feeder. Some of the better known feeders are: variable speed screw feeders; rotary pocket, star and vane feeders; belt or apron feeders; oscillating tray, pan or undercut gate feeders with adjustable throws; vibratory feeders; and revolving table or disk feeders with adjustable scraper blades. These volumetric feeders can be calibrated by measuring discharge over a set time and are subject to error when the density of the material varies.

Gravimetic feeders measure the material being continuously fed to the process and have an endless conveyor partially or entirely supported on a scale. Scale position is transmitted to a primary feeder controlled to keep the weight of the material on the belt constant. Belt speed controls the discharge rate. The amount discharged is measured by integrating the travel of the belt.

The loss-in-weight feeder is a special case. This hopper scale has a discharge mechanism controlled by the scale beam that always keeps it in balance. A controllable drive moves the poise down the weigh-beam at a predetermined rate, which continuously unbalances the scale. Balance is restored by controlling the hopper's discharge rate. When the hopper is nearly empty, discharge ceases while the hopper is refilled. This system requires dual units for continuous flow. Feeders should be checked dynamically by calibrating their delivery against a scale of known accuracy.

Solution Feeding

Soda ash solutions have uniform physical characteristics and are much simpler to meter than dry ash. When concentration, which is readily determined, is known, measurement can be made by volume as easily as weight. As a result, most liquid metering systems are volumetric.

The metering equipment chosen depends on the accuracy required. The concentration of batch quantities can be determined by its level in a tank, although the design should consider possible scaling effects and crystallization tendencies. A manual or automatic valve in the pipeline followed by a pressure gauge can be used to detect variations in flow (a rough correlation exists between pressure and flow rate).

Volumetric or proportioning pumps are commonly used when flow rate is low and measurement is secondary to flow control. This can involve any positive-displacement pump, such as a gear pump. Precision feeding usually calls for a special diaphragm pump (proportioning pump) that has an adjustable stroke and is protected by a strainer.

Various volumetric flow instruments, such as rotameters, orifices, magnetic flow meters and turbine meters, indicate flow directly and continuously. They also can integrate quantity to provide direct or auxiliary control. Displacement meters, similar to those used for water and similar liquids, integrate flow quantity but may not give flow rate directly. Consult the meter manufacturer when procuring such equipment. Mass flow meters can measure flow rate, mass (weight), specific gravity and temperature. These meters can be alarmed to indicate changes in flow rate, mass flow (two phase flow caused by crystallization) and changing liquid concentration.

References

1. Chemical Engineers' Handbook, Fifth Edition, 1973.

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