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Mud Cat Case Studies

Dredge

New Equipment Boosts Sand, Gravel Production
A U.S. Bureau of Mines survey reports that 300 million tons of sand and gravel were removed from underwater deposits alone in a single year. This volume of material along with its corresponding dollar value has spurred both aggregate producers and dredge manufacturers to action.

Today's plant operators are seeking a higher level of sophistication. To accommodate their desires, dredging equipment designers and manufacturers are performing research aimed towards technical advancements which would primarily benefit the sand and gravel industry.

Among the issues investigated are dredge-feed monitorization and control, design and testing of equipment for deep water digging applications, and excavating devices especially suitable to non-cohesive material.

To design a system to monitor and control dredge flow, the key variables had to be identified. These were found to be specific gravity of slurry or mixture and velocity of mixture in the pipeline. Today, there are metering systems available which not only provide instantaneous readout of these two variables, but also integrate them to provide simultaneously an instantaneous reading of the production rate in tons of material per hour. Included with a metering system of this type is a time chart strip recorder which graphs the changes in the three variables mentioned for the time period in question, and also provided is a readout of accumulated production in tons, for any desired time period.

Meter Benefits

The benefits of such a system to the sand and gravel industry are several. It provides a means of optimizing production. The operator can obtain immediate feedback regarding the effect of changes in dredging technique on production and can operate accordingly. A production metering system affords the operator a means of controlling the GPM of slurry or mixture to the plant because velocity is a monitored variable, and can now be maintained relatively constant.

Also a production meter is a managerial tool in the form of an information system which aids in the analysis of dredge capacity, operator efficiency and deposit characteristics.

Although the metering system is extremely beneficial to the plant operator, it still doesn't allow him to overcome the limitations of nature which permit an operator to dig deeper only if he is willing to accept a lower percentage of solids in the slurry recovered. Since this article is meant to discuss the technical equipment advances, and not the theoretical research required to accomplish the design of the particular equipment, let it suffice to say that for many years the barometric pressure limitation placed a restriction on deep digging especially when cost was prime consideration.

Appreciation of land values and the trend toward prohibitive zoning codes are making site expansion a costly and sometimes impossible alternative. As a result, more and more producers are seeking to recover the full depth of their deposits. One of the most recent solutions to the plant operators' dilemma is the fully submersible pump and drive unit. By submerging the pump and locating it closer to the mouth of the suction pipe, the barometric pressure limitation is virtually eliminated; and by submerging the drive unit and close coupling it to the pump, the alignment, maintenance and durability problems associated with long drive shafts are eliminated.

Totally submersible pumps and drive units are available with electric or hydraulic drives, with motor configurations ranging from 250 to 1000 horsepower, and in pump sizes from 10-inch to 34-inch diameter.

There are also various types of pumps available. The operator who wishes to modify his dredge and add a submerged pump as a booster does not have the same requirements as the operator who wishes the submerged pump to be the prime mover for his operation. As a result, wide-bodied pumps which produce the low head and high volume desired in a booster operation are presently available. Because of their production capacities and corresponding head curves, these wide-bodied pumps are ideal when utilized as a booster to the main pump or on projects requiring short discharge distances. On those projects with varying pumping distances, the conventional dredge pump can also be mounted underwater and powered by a submersible drive unit without any loss in production capacity. (Mud Cat can supply you with the latest in flow meters or underwater pump systems.)

Excavating Unit

The latest facet to become the subject of manufacturer research was the deposit itself. Since producers are concerned for the most part with material that is granular and non-cohesive, why should they utilize an excavating unit that was designed for general dredging purposes?

With this reasoning in mind, the criteria set up for an excavating unit with primary application in the mining industry was the following: The excavating unit should provide a constant rate of feed and a maximization of percentage solids in the slurry. These objectives led to the development of the underwater wheel excavator which can best be described as eight cast steel bottomless buckets welded onto two steel rims. As the wheel is driven by a hydraulic motor, the buckets feed material to a hopper located inside the wheel. This hopper then leads to a conventional suction pipe.

There are several advantages of a wheel excavator in a mining operation. The wheel is capable of cutting while swinging in either the port or starboard direction. This causes less variance in the flow of material to the plant and overcomes the inefficiency associated with a conventional cutter which has a preferential cutting direction. The wheel provides a positive mechanical feed to the suction pipe and permits complete recovery of all material cut. It allows the dredge ladder to be operated at greater angles, and possibly most important, through speed control of wheel rotation, the amount of material fed to the plant can be regulated. (Note: Mud Cat has supplied more bucketwheel excavators than all other manufacturers combined.)

To combat the effects of inflation, environmental regulation and zoning restrictions, it is imperative for the would-be profitable producer to turn to technology for the operating efficiencies and cost benefits required. It is essential that constant lines of communication be maintained between equipment manufacturers and the sand and gravel industry. In addition to being aware of new developments and being able to apply the technology available to them, the producers who maintain contact with the equipment manufacturers can influence the direction of future research. This type of coordination benefits the manufacturer who can develop a marketable product, the producer who can utilize the new equipment, and the final consumer who can purchase quality aggregate at a reasonable cost.

Reprinted from World Dredging & Marine Construction