Getting The Best Performance From Your Dredge
By Reggie Gould
Getting the best performance from your dredge involves some basic physics. The most obvious problem is lifting or I should say sucking, in most cases, a dredge-hose full of sand, rock and water. The actual lift will be from the nozzle end of the dredge-hose to the exit port of the venturi device. Included in this lift load is the flair, because it has a static load composed of rock and water, on the output of the venturi jet. The lift varies from dredge to dredge and is determined by the draft (how far the pontoon sinks into the water), of the pontoons and the angle of the flair to the waters surface. This is the hardest lift in the dredge system. Once you leave the surface of the water you are lifting a column of rock and water whose weight is proportional to the diameter of the dredge- hose and the total lift in height from the top of the water. Water weighs 8.33 pounds per gallon, so if you calculate the cubic volume of the actual lift above the water and convert it to gallons, then it can be multiplied by the 8.33 value. To complicate things the water has rock and sand whose weight varies with the specific gravity of the material. What about the weight of the rock and sand being sucked from the end of the dredge-hose to the surface of the water? Good question. That is a little more complicated because it varies with the density of the rock and sand to water ratio. In short that load is added to the head pressure above it. All this sounds very technical but there is a point to all this technical jargon. Why do we lift the material anyway? Why not submerge the sluice-box in the water and eliminate this lift efficiency loss? You would probably say that sluice-boxes dont operate well under water. That is true so why dont we put a removable cover on top of it? I cannot give you a number for efficiency improvement but it is a huge factor. This would help improve the poor efficiency of the straight jet venturi (power jet). This type of jet has the worst efficiency of the all the venturi jets on the market. How do I back up this statement? Well I could say that I test the suction of each venturi jet by placing my hand over the end of the nozzle, but that is not much of a test. Or I could use my gut feeling to judge which is the best device for the job. I think pride and gut feeling seem to be the measuring devices used today by a few dredger's. Why am I attacking the power jet? Im not attacking the device, it has done a good job for many dredgers, I am just stating facts, no matter how cruel they may sound. I ran extensive tests on various venturi jets and found the power jet to be the least efficient; the suction nozzle was the 2nd. best, and the tri-jet was the 3rd. best, and the infinity jet was the best performer. How did I arrive at these conclusions? I actually tested them in a controlled environment using a water tank. The method I used was not Rocket Science; I used an old miners method for measuring the flow of water through a pipe. I modified an aluminum ruler that would slide back and forth on the output of the venturi jet. On the end of the ruler was a metal piece that stuck down exactly 4.0". I would fire up the pump and set the throttle to full, that would ensure that the pump always had the same output. I would then adjust the ruler so the piece of metal hanging down 4.0", would just touch the top of the water flowing out of the end of the venturi jet, bending downwards. I would then look at the reading on the ruler to see how far the device was extended. Once I had this measurement, I would go to a table and look up the GPM (gallons per minute) flowing out of the end of the venturi jet. The table would be different for each diameter of pipe. As simple as this may sounds it is quite accurate. You can check the accuracy by filling up a 50-gallon drum and timing it with a stopwatch. The miners method is much easier and far more accurate, taking out the human factor of when to start and stop the timer. I repeated this measurement for each venturi jet and recorded the data. If you are interested in this data you can see it on my web page at .wwwgouldeng.com, under Technical Reports. Until I ran this series of tests, I had always thought that the power jet was the best performer. The suction nozzle is my choice because it is more efficient then the power jet and it is built by many dredge manufactures. It pushes the water and material from the nozzle end of the dredge-hose to the other end of the dredge-hose to the dredges sluice-box. On the power jet, if you should ever lift the nozzle out of the water you will get an air bubble, which will immediately stop the power jet from functioning. You then have to bleed all the air from the system, including the dredge-hose. The suction nozzle does not have this shortcoming, if you get an air bubble, it will disappear as soon as the nozzle is put back into the water. The one small drawback to the suction nozzle is that the high-pressure water that drives the venturi jet has to be fed with a hose at the suction nozzle. This does not bother me because I use a sub-dredge that lays on the bottom of the river with a short piece of dredge-hose, see the Gould-Bazooka. If I were still using my float dredge, I would probably use a tri-jet venturi, it is more efficient then the power jet and the suction nozzle. What is all this talk about efficiency anyway? The object of the game is to move as much material as you can in the shortest time period with the smallest pump and motor possible. Which would you rather drag down the steep bank, two pumps or one? That will of course be your choice. If you already have your system then it wont really make any difference anyway. I talked to a group of miners from the state of Oregon, who had hung their sluice-boxes under their float system in the water with a cover over the sluice- box. I dont have a clue how they did their cleanup but they did say that the improvement in efficiency was remarkable. I like the efficiency of the infinity jet but they are difficult and expensive to build. To explain why one jet has a different efficiency then another, you have to look at the fluid dynamics going on inside the various venturi jets. The power jet directs its flow of water tangent (at an angle) to the incoming flow of water. This causes a few problems, such as cavitation (air bubbles) and side load friction (water rubbing against the opposite wall of the tube), which reduces the efficiency of the jets performance. It is however, the easiest jet to manufacture and so it is less expensive. Costs verses performance is always a tradeoff to think about. The suction nozzle has the jet inline with the column of outgoing water and material. It directs a stream of water down the center of the tube which greatly reduces cavitation and almost no side force friction. The nozzle is formed at an angle that varies from 45° to 22.5° . The jet is installed at the bend in the center of the outgoing flow. It is more difficult to manufacture and costs a little more. The tri-jet is usually located at the sluice-box but some manufactures install it 5 feet from the sluice-box to improve its performance. It has three jets that are 120° apart from each other in a tangent flow. The main difference from the power jet is that their combined flow meats at the center of the tube so there is little side friction. There is a little cavitation but is over whelmed by the improved efficiency. Although this device is a top performer, it is not widely used because of the difficulty of manufacturing and proportionate cost. This would be my top choice for a float type dredge, I wish it were more widely available. Now it comes down to the awesome infinity jet. This device has the highest efficiency of any of the jets discussed in this article. The principle is completely different from any jet on the market. The water from the 360° jet (same diameter as the dredge-hose) is applied on the outer edges of the column of water. There are many advantages to this design. The cavitation is almost zero; the outside surface tension is broken by the force of the surrounding jet. The jamming appears to be less because the force of the water is directed to the outer edges of the column of water instead of directing it at an angle towards the opposite side wall, as in the power jet, causing the rocks to tumble and jam against the side of the tube. This device does have a short fall however; the gap is very small, so it can get small rocks stuck in the gap if the input screen on the pump has too large of holes. They make the jet variable so it can be adjusted to the flow of the pump and pressure. The gap can also be opened to clear out any rocks that may have lodged in the gap. When I first tested it, I had trouble setting the proper gap. That is when I discovered the old miners flow gauge. I simply set the gap for the maximum amount of water out of the end of the device. One thing I discovered while doing all this testing, was the amount of water that was actually sucked in verses the amount of water driven into the venturi jet is poor in efficiency. In the case of the power jet, for 100 GPM applied to the venturi jet, it only sucked 50 GPM through the nozzle and dredge-hose. I repeated the test 5 or 6 times and the results were always the same. The suction nozzle drew 75 GPM and the tri-jet drew 90 GPM. I expected an efficiency of 100 GPM applied to the jet and sucking 200 GPM, with a total output of 300 GPM. Even the infinity jet only sucked 130 GPM with 100 GPM applied to the jet. I was totally amazed of the poor efficiency. As it turns out that the venturi jet system is very inefficient but that is all we have available for us to use. The reader will have to make his own conclusion as to what this all means and if it worth his money to upgrade to a more efficient venturi jet. Water measurements table, were taken from the Pocket Ref
By Thomas J. Glover ISBN number 1-885071-33-7.
Finding A Better Dredge Hose
Since I started designing and building Dredges, I have never been satisfied with the dredge-hose that is on the market place. Actually there is no such thing as a dredge-hose from the manufacture. The manufacture offers a variety of flexible tubing with various psi (pounds per square inch) ratings and circular dimensions. There is a wide variety in the catalog; the trick is to find a place that you can buy some without buying a 100-foot roll. The material that is available from most dredge shops is stiff enough to support a Sherman Tank when stretched across a creek. Well actually maybe not quite that stiff but you get the idea. The material offered has a 20-psi rating, which is totally ridiculous. The dredge is sucking very low-pressure sand and gravel mixed with water. The only requirement that I can see is that the tube does not collapse due to the vacuum present in the line. It is very hard to get much data on the actual hose itself. The catalog lists various hoses for different applications, ranging from Lawn Mower Clippings to Food Processing. When I contact the factory they are reluctant to offer real specifications. I dont really understand this industry. Some manufactures list the spacing between the reinforcement coils, which is extremely valuable when determining flexibility. Most of them will show the inside diameter (ID) and the outside diameter (OD). With this information it is possible to determine the thickness of the cording material. What is not shown is the actual thickness of the vinyl material; this material represents less than 10% of the overall thickness. The thickness of the cording material and the spacing of the coils next to each other, determine the flexibility of the hose. A lot of catalogs will show a maximum bending radius of the hose but what kind of force is required to make this bend? The dredge-hose offered today is a hose rated at 20 psi. The physical makeup of the hose that gives it that rating is a huge cording thickness exceeding ¼" and the coils are very close together, less then ½". The vinyl appears to be twice as thick as their lightest product. If you use the lighter material, will it wear out? You would probably have to dredge 12 hours per day at 365 days per year to wear out the lighter materials. Now if you have an elephant helping you dredge and he accidentally steps on the dredge- hose, the heavier hose will be more forgiving. I like the lighter weight hose when you are trying to clean out the gold in the cracks in the bedrock, because it is much easier to bend. The stiff hose is like trying to bend a tree trunk. This really irritates me, why are we supposed to use a dredge-hose that is stiff as a tree trunk? I have been trying for several years to rectify this problem. I have asked various manufactures for samples so I could evaluate them. The materials that I got from the manufactures that responded in most cases was not what I was looking for. I did get a sample from Pacific Echo but it only was manufactured in 4.0" diameter. The dredge industry is a very small industry when it comes to buying the hose. They are not going to create a hose just for our industry. I also asked Kanaflex and Tiger Flex (Kuriyama of America) for samples but as of today I have not received any. They are feeding a market that is buying thousands of feet per day and the dredge industry does not meet that criteria. I wish I could say that I was not disappointed. The other huge problem is that once I find the exact product that meets my needs, I still have to order it in 100-foot rolls. That means 100-foot rolls of every diameter size dredge-hose. I have talked to the various warehouses about stocking the material but they are reluctant unless you intend to place huge orders. Even if they would order you a roll, you would have to know which material to select. Now if they wont give you samples and their catalogs lack the detailed information, then how do you go about selecting the right material? It is a catch 22 situation. A friend of mine had bought a 20-foot length of 4.0 dredge hose from a distributor in Oregon and it was the perfect material. Unfortunately he could not remember whom he purchased it from. He did say that the hose was used for sawdust removal. This will give you a hint; wood chip removal, dust collection, and grass clipping recovery are all excellent candidates.
I would like to test each of these materials before spending the money on them. Im not sure how to proceed at this point if the manufactures will not supply me with samples.
The requirements are very simple, the coils should be at least 1.0" apart from each other, and the coil diameter should be around 3/16" in. I have a sample of this material but I dont know who manufactures it. I am really sick and tired of using what is available to us. Suppose you could only buy one kind of car without any choice of size or functionality? If you know of where I can purchase the material that I described, send me an e-mail at firstname.lastname@example.org. If all the Miners work together to help solve this problem, we can all benefit. The following is a list of manufactures that make dredge-hose.
Pacific Echo Inc. 1-800-421-5196, Kanaflex 847-634-6100 and Tigerflex 847-228-0300, 1-800-326-8940.