extreme in the requirement for surge capacity
As an example of one extreme in the requirement for surge capacity we have those plants whose primary (and, possibly, secondary) units operate only one shift of eight or ten hours per day, whereas the re-crush and screening units run full time, or nearly so. The problem of calculating surge storage for such an operation is a simple one if operating conditions, such as character of feed and product size, remain unchanged, and if the feed rate to the primary units is fairly constant from shift to shift. Where all of these conditions are subject to significant variation the calculation is a more complicated one: we must calculate the requirement for each combination of conditions, and provide a surge storage that will take care of the maximum.
Regardless of how extreme the requirement for surge capacity may be it is usually unnecessary to provide an extensive storage ahead of more than one crushing stage. If the plant units are properly balanced the flow rate can be smoothed out by regulating the feed from this one point.
This does not necessarily mean that mechanical feeders need be used to regulate the flow; the crushers themselves will do this for the succeeding stages of the plant if the stage at which the regulating is done is the "bottle-neck" in the flow line, i.e., the stage of least capacity. Naturally, this regulating stage should be as near the head end of the flow line as possible, and, if it were possible, or practicable, to achieve an "ideal" plant layout, each successive stage would have a capacity slightly in excess of "the preceding stage.
Any multi-stage crushing plant will have a "bottle-neck" stage, and it is this stage which establishes the production rate of the entire plant; therefore, it should be kept busy, and the only way this can be done, if the feed to the plant is a fluctuating one, is to provide sufficient surge storage ahead of it to absorb such fluctuations-at least, such as may be expected under normal operating conditions.
We have mentioned elsewhere that some operators of gravel plants have realized the desirability of incorporating in their flow line surge storage of sufficient proportions 10 compensate for delays of considerable duration, such as might be caused by a major breakdown of the loading equipment. There is no reason why this same expedient cannot be applied in a quarry operation-following the primary, or secondary, crushing stage. Aside from the production insurance it affords, such a stockpile is an excellent point from which to regulate the feed to the reduction crushing stages and screens, by means of one or more mechanical feeders. With such regulation the need for surge storage at other points in the flow line is minimized.
Thus far we have discussed only that type of surge storage which lies directly in the flow line, a "series" storage, as it were. There is another type which can be utilized to advantage in certain operations, particularly in those gravel-pit operations where the gradation of the pit-run material is subject to considerable variation. This type of storage might be called the "bleeder" system, inasmuch as it consists of drawing off an intermediate size, or sizes, of product the amount of which, at any time, runs in excess of the capacity of the reduction crushers. Then, during periods when the pit-run material is running to fines, this storage is drawn upon to keep the reduction crushers busy.
This is Jan excellent system for smoothing out plant production, especially in those plants which intermittently run heavily to the production of fine sizes of crushed rock. It is difficult to forecast the requirements for such a system; the application can best be made in a going plant where the operator knows his pit and his requirements for certain sizes of Product.
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