Gravity Dredge

The Gravity Powered Dredge
Requiring no gasoline, pump, and very little maintenance, the gravity powered dredge functions not by magic, but by basic physics. The gravity powered dredge uses the power of water and gravity to do the same job as that of the gasoline powered dredge. With sufficient head, a gravity flow dredge can have as much or even greater lifting power as the gasoline powered version. The gravity powered dredge works on the same principle as a syphon, that is, that the weight of water in the pipe down hill (at a height below the suction end) creates a negative pressure in the pipe that pulls the water and the gold bearing sand a gravel through the nozzle, down the tube, and delivers it to the sluice. Although the available head determines the pressure available, other factors limit the dredges efficiency at any given head. These factors include the length of pipe, the occurrence and severity of bends, the relative roughness of the pipe and fittings, the type of couplings employed, and the design of the watergate and sluice box. Although the accompanying velocity charts have been generated from both calculated and experimental data, they are based on the ideal configuration consisting of a smooth pipe without couplings or bends. To better familiarize my reader with the significant criteria involved in the efficiency of the gravity dredge, I have enumerated just a few with a brief explanation of each.


Available Head
Head is a unit of measure used by the mining engineer to quantify the pressure of water under static conditions. Even though head is pressure, its units of measure are 'feet', i.e. the pressure of one foot of water is equal to one foot of head. A pipe filled with water to a given height is said to have that amount of pressure, or head, equal to the waters height in feet. The gravity dredge takes advantage of the natural gradient of a stream or river to develop the pressure necessary to move the water through the pipe and thereby deliver the gold bearing materials to the sluice. Although the feet of head will determine the pressure at the exit level of the pipe when it is full of still water, the other factors listed above should also be considered. To decide whether or not a gravity powered dredge would be practical for your particular application, a brief survey of the terrain and gradient (steepness) of the stream or river you intend to dredge must be made. You might think that your potential stream site has plenty of available head, or perhaps you merely hope that it does, but it is essential to sketch or draw your site plan so as to determine its practicality for gravity dredging before you begin. This site plan will also help you to decide which dredge size and configuration would be best, and, the plan will better help you to decide where and how to locate your sluice relative to your dredging pool so as to economically optimize the gravity dredge's performance. One such method for generated a site plan is discussed latter in this text.


Pipe Diameter and Type
Pipe diameter is, as you might guess, the most important consideration for determining the overall performance of the gravity powered dredge. Most gold dredgers select the size of their gasoline powered dredge based on investment (cost), performance, and site practicality. These same considerations similarly effect the selection process for determining the size of the gravity powered dredge. In fact, any of the common sizes of gasoline powered dredges may be converted to gravity power by using pipe of the same diameter as the dredge size. For example, a 5 inch gasoline dredge can be converted to gravity power by using 5 inch pipe.
Another important consideration is the type of pipe to be used. The choices are varied in both material and cost. I have found, from my experience, that aluminum irrigation pipe is both durable and light weight, and it is also the most rigid, but, unfortunately, it is not the least expensive, and it is susceptible to denting. If you can locate a source for inexpensive used or scrap aluminum irrigation pipe, I highly recommend it, especially for diameters of 6 inches or larger. Aluminum irrigation pipe is available in several grades or wall thicknesses. If you plan on straddling (suspending the pipe over) long distances (more than a few feet), or walking on your pipe, I recommend the heavier grades of aluminum irrigation pipe that generally run between 0.075 to 0.938 inch wall thickness for the 6 inch and between 0.0938 and 0.125 inch wall thickness for the 8 inch aluminum pipe (not quoted).
The least expensive and the most readily available is the PVC (polyvinylchloride) pipe available from your local plumbing or irrigation supply. The durability of schedule 80 is great, but its probably to expensive and heavy for most applications. Schedule 40 would be my selection where durability is necessary, and where the cost would not be a significant burden. The class 100 and 125 PVC though, when handled and used carefully, offer the most economical options. I would not recommend any of the lighter classes or grades of PVC or ABS pipes or DWV tubing, such as the class 63 sewer pipe, because of their inability to handle the rigors and loads of the typical gravity dredging application without breakage. A look through your local phone book and the yellow page directory should list several suppliers in your area that stock almost all the possible grades, classes, and materials you might need for your application.