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1 people online in the last 1 minutes - 0 members, 0 anon and 1 guests. (Most ever was 29 at 13:36:32 Sat Aug 3 2002) |
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kevyluvu 13:14:52 Sat May 7 2011 Offline 261 posts Reply |
Hello,
I have a hydraulics question and I am looking for a quick answer so my welder/fabricator can keep working today. We started on this 10" dredge so late that we cannot get the Eductor (keene calls it a jet log) and Diffuser (keene calls it a jet flare) from any suppliers, so we are making our own. I have an extreemly talented welder/fabricator here in the shop and we have hit a thin wall. We have an 8x6" Berkeley pump pushing 6" of water into a chamber that is basically an 18" wide by 6" long donut that is welded on top of our 10" ID Steel pipe that our jets are going to be welded to. The donut has (4) 1.5" ID pipes going from it to the main 10" steel pipe at a steep angle taking the 6" of incoming water and changing dirrection 180 degrees to pull water up into the sluice box. The whole eductor will be underwater and the purpose of the eductor is to increase the water velocity by taking a larger volume of water and trying to force it through a smaller area in a different dirrection which creates the venturi (suction) affect. PSI has very little to do with this process. If the jets were coming from the diffuser into the main tube @ 90 degrees they would be creating no venturi because the water would just be hitting a wall. The venturi affect takes place when the jets are at an extreeme angle in one dirrection or another. Here is the question: What is the correct angle? I know 45 degrees is not steep enough, but is ONE degree (if it were possible) too steep? If the 1.5" jets were coming into the 10" pipe @ 45 degrees the oval hole that is created is only 3" long. At 22.5 degrees the oval hole is 6" long (which seems to me very close to what we want). Does it even matter if the oval hole gets really long? I would think the steeper the angle the better but what do you think? As you go steeper the hole opens up bigger and bigger. Thanks -Kevin I'll upload pictures of our jet this morning when I get to the shop. |
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LipCa 13:57:21 Sat May 7 2011 Offline 649 posts Reply |
once upon a time, I read 11 degrees. but I don't know where....
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Muley 15:10:19 Sat May 7 2011 Offline 377 posts Reply |
Keene doen't show the jet logs in thier catalog anymore, but on thier price list they sell just the logs by themselves. I beleive the one you are looking for is the PJL3 ($130)ea. This is only the jet log and if you get one you can make the other 3. this is a 3" inlet and is recommended for the larger powerjets. But if you are using 4 you may be able to get by with using smaller inlet sizes and still maintain high flow rates or at least be able to run the pump at lower rpms and save some fuel.
On the other hand if you find or maybe have another PJ you can measure the angle. There all the same angle no matter what the PJ size is, I think. I'm thinking they are 22 deg, but not sure ( 22 deg x 4 = 88 deg which put the apex of flow in the center of the PJ) Good luck and keep us posted |
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kaveman 15:55:14 Sat May 7 2011 Offline 314 posts Reply |
I'm with LipCa on 11 degrees, but also don't know from where. That's been my number for at least twenty years. Here's the thing tho,.............that 11 degree angle is supposed to be the best compromise angle for a single eductor jet where the jet is working on the water column alone. In a double jet system; or in your case a quad set-up; each jet gets to work in tandem with the opposed jet, so I think it's not so critical. Hydraulics is only part of it, I'm sure. Other factors are the difficulty of welding them in at sharper angles, the length of the resulting oval(which reduces the exit pressure,.....which DOES impact the power), and the impact on the jet tube both across from and downstream of the eductor. All that constant pressure hammers rocks into the side of the tube at these two spots.
In a twin system with a flair, it's my understanding that the left eductor should fire in line with the right side of the flair and the right eductor should fire up the left side, but I've never run that system myself. |
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kevyluvu 15:57:43 Sat May 7 2011 Offline 261 posts Reply |
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kevyluvu 16:01:39 Sat May 7 2011 Offline 261 posts Reply |
   Right now we are sitting @ 22.5 degrees... Think I'm going to go with it. My fabricator found Reggie Goulds article and he says anywhere from 22-45 degrees.. Time to weld, and we'll see how well she work in Nome. edit.... I read the article my fabricator found and Reggie was talking about nozzle angles not jet angles. |
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AK_Au_diver 17:26:11 Sat May 7 2011 Offline 249 posts Reply |
I concur with 11 degrees, actually I think it's 11.5 degrees. 22.5 should work well, especially since you have 4 jets spaced at equally around the pipe. I know one operation that has the 4 jets shooting directly into the tube (at 0-degrees), and the material enters at about 20-degrees. So it's more like a classic eductor jet pump. They have a vibrator mounted to the top of it to shake loose the rocks that occasionally get stuck. The main problem with 22.5 degrees is that the low pressure zones on that inside angle will be stronger, which will cause greater wear, shorter life. But it should still last a season or two, only a little shorter life than the 11.5-degree recommended compromise. |
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micropedes1 18:03:35 Sat May 7 2011 Offline 65 posts Reply |
There is an optimum range; anythere between 8 and 13 degrees. But it is absolutely imperative that all 4 jets converge at a single point, preferably at the center of your jet log. Other angles will work, but efficiency falls off rapidly as the angle increases beyond optimum. Turbulence is a killer.
I am more concerned with the pressure differential between pump and eductor. I recall Zooka telling me of a 4" dredge built with inadequate restriction. Almost no pressure differential and almost no suction until a .75" restriction was added at the end of the pressure flow. Are you absolutely certain of that 1.5" pipe? |
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colo_nuggets 20:03:11 Sat May 7 2011 Offline 176 posts Reply |
I have new Jet Logs made by Jim Harrand of Precision Dredges. 11 degree's
Blaster nozzle's when reversed, 11 degrees. Im having some machined up today. |
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kurt_Blumberg 03:48:13 Sun May 8 2011 Offline 473 posts Reply |
I fabricated my twin jet 5" at 15 degrees and it works just fine. I'm using 1" pipe. They are powered by two 2" P180 pumps on 6.5 hp engines.
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kevyluvu 14:27:43 Sun May 8 2011 Offline 261 posts Reply |
I did some measurements on the picture of the completed jet system above. By using the known quantity of the 6" camlock which is 4" across the back I figure the completed jets are about 16" long at the longest point, which is 22.5 degrees. Gonna give this one a shot. Maybe next year we'll fab one up with a sharper angle.
Thanks for all the quick replys. -Kevin |
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kevyluvu 03:55:24 Mon May 9 2011 Offline 261 posts Reply |
Bolt her up and go to work...
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growler 15:33:11 Mon May 9 2011 Offline 36 posts Reply |
Kevin, powerjet is too short. 5' min. from jet to flare connection. Bring xtra tubing with you if time is short. jimmy
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micropedes1 21:37:22 Mon May 9 2011 Offline 65 posts Reply |
The general rule of thumb is that the length from jet opening to flare (in feet) is the same number as the diameter as your jet tube (in inches). That means that a 10" dredge needs about 10 feet of tube. It has to do with the efficient transfer of pump energy to the column of water you are trying to move.
Growler is right; carry more tubing. |
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dickb 22:45:48 Mon May 9 2011 Offline 102 posts Reply |
Kevin:
I would agree with the others. Since your building a prototype, you can expect that some tweaking will be necessary. I'd take some extra 10", at least 4 more 1.5" jets and maybe some 2" as well. Until you get it in the water to test it and check the suction, you won't know how it will work. By taking extra you can fab in Nome to correct any problems. That short season won't leave you a lot of time to be off the water, except for high wind days and then you can make corrections to use your time well. Good fortune to you. :smile: Dickb |
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kevyluvu 15:22:34 Fri May 13 2011 Offline 261 posts Reply |
Yikes, I finally got around to reading the last 3 posts. I based my design for the length of the eductor and diffuser off a current system we use on my boses two 10" dredges. The total length of the jetting section of the eductor is 5 feet not counting the 1' 12" ID for the hose to slide into. The diffuser is only 8' long. The total length from begining to end is 14'. The total length after the jets enter to the sluice is 11 feet. What you do not see in the picture is a 2 foot extension I added with two flanges on either side to be bolted between the jets and diffuser for a little extra length.
I guess I was under the impression that the turbulance we experience on my bosses two dredges came from the sharp angle (45 degrees) at the end of the diffuser not giving the material enough time to settle down before entering the sluice box. I didn't think the length of the pipe after the jets had much to do with it. Let me make sure I am hearing you guys right... When you say length of the pipe after the jets should be 5' minimum do you mean the total length including the diffuser, or the length until the 10" pipe reaches the diffuser and starts spreading out? I do know that (4) 1.5" jets is the right combo for the Berkeley 8x6 since we have the same pump and jets on the boat I run in Nome and it has a massive amount of suction power. The pump is rated at 1700 rps, but when working rock we never go above 1350. We do boost it up 100rpms when working non-paying sand and it sure is nice to throttle up to full capacity when breaking a nasty jam. Last summer using Bob Dahlke's Nozzle and the Berkeley pump we very rarley had to use the sledge hammer to break jams, and our jam rate dropped dramatically with the well designed nozzle. With a good diver 1 easy jam in a 6 hour dive is normal. -Kevin |
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kevyluvu 22:03:50 Fri May 13 2011 Offline 261 posts Reply |
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colo_nuggets 22:57:41 Fri May 13 2011 Offline 176 posts Reply |
Any chance of some underwater video of this working?
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growler 12:50:47 Sat May 14 2011 Offline 36 posts Reply |
Kevin, that looks good. We know this is trial and error engineering with 3 factors in play, jet tubes (i.d.), jet tube angle and total length of power jet. When everything is sweet you will have to HOLD BACK the nozzle as it will pull material from almoat 2' away (@ full throttle). Nice work. jimmy
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AK_Au_diver 19:56:36 Sat May 14 2011 Offline 249 posts Reply |
The configuration you have now should work, it's not an optimal design for efficiency, but it might be optimal for space considerations. A typical optimal "throat" length is approx 7 times the diameter, so just under 6'. Less than that and there might be some loss of head (which requires more engine power to provide the same amount of suction) and other turbulence issues (hindered gold settling). A typical optimal diffuser has a 5.5-degree divergence for conical, 7-degree for square, and 11-degree for rectangular, although different angles have different magnitudes of effect depending on factors such as flow rate. The key here is that you want to maintain a laminar flow, too steep of a divergence and turbulence will result (again loss of head, power efficiency, poor gold settling). That flare is not made with the typical shape that looks like a round tube, split and wedge inserted on the top and bottom, see the Keene flares for an example. Thus, it has a sudden enlargement at the transition from throat to diffuser, which is probably ok. Except that it will be a bit of a gold trap in the bottom two corners, again not a problem as long as they keep the hose connected. If you pull out your copy of "Hydraulics and its Applications" Second Edition, 1919, and start at page 84, you can read up on the physics of flare, page 86 is where it gets real exciting, rectangular pipes. The trade off is turbulence for steep divergence vs. pipe wall friction for long and slow divergence. Since that flare design already accepts the abrupt transition from round to rectangular, then a trumpet flare would have been the best way to go. Rectangular trumpet flares can be much shorter than straight flares for the same head loss, take more calculations to get the right curve, but are only slightly harder to cut and weld (would help to use a square stock or quarter round to weld to the top and bottom panels first, to act as a backing for the sides). Rectangular trumpet flare has curved side walls, think hourglass shape, as viewed from top, or see link below. Outdoor Loudspeaker Really we only care about loss of head on the suction side, so I'm not sure how relevant these numbers are, that is to say, I'm not sure how much the loss of head in the diffuser effects the suction head. The diffuser dumps into open-to-air-above sluice, where only flow rate, gravity, and down angle are factors in moving material. Rectangular straight wall taper: 11-degree angle between divergent sides of pipes is 18% loss of head 15-degree angle between divergent sides of pipes is 30% loss of head 20-degree angle between divergent sides of pipes is 50% loss of head At any rate, looking at the pictures, it is easy to experiment with throat length, by simply adding another section, since it's so easy to do (relatively). The only problems with that are making the flanged pipe, adding another "jet riffle" that traps gold, and the mounting, weight, and balance issues. |
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