2 questions~Over/Under and PSI

O

omnicron

Bronze Member
Jun 14, 2012
1,017
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Caldwell, Idaho
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Prospecting
Ok guys here are two questions I need answered.

1. When running a over/under sluice how should it be ran, end in the water or out of the water?

2. What kind of PSI should I see with a P180 and proper sized jet?

I ran my 3 stage last year out of the water but I had "water dams" adding some resistance to the flow. This last weekend, my trial run I had the end in the water (no dams) and it wasn't clearing but it wasn't backing up. I have seen them ran both ways

Keep in mind I'm running a 6" with twin 6.5hp engines/P180's Currently I see 22psi at about half throttle and 35psi at full throttle. I've seen the number 50psi thrown around.
 

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Omni keep the tail end out of the water. when submerged it kills the waters action for a good portion of that sluice. John "Hoser" Oates has stated that many times before.i try to do what ive been told by people that know so much more than i ever will!:)
 

Thanks Bill and Rus! I was going to raise my tail end up a inch (thank god I made this all adjustable) and give it a run tomorrow.

I was considering blocking off one of my tubes to see if it increased my pressure.
 

I found that to be a very interesting question Omnicron. If we look at all the adds for the new Keene "triple layer/over under" we see the end IN THE WATER.

I changed my triple over under frame set up by "flipping" the frame over. This raised the sluice by 5 inches. I did so because in shallow water the end of the sluice would damn up with tailings in just a matter of minutes.....(I dredge alone). It did increase the lift factor; which I overcame by increasing throttle speed. I had to make/fabricate a box frame to raise the engine and allow the power jet to clear the same. Many of my friends have also done this and I have used it successfully for quite a number of years:....."7" I believe.


Bejay
 

Thanks Bejay. Being this is on my homebuilt 6" dredge (assuming you seen my thread) and I built a double screen over/under like Dave Mac runs for fine gold recovery. I did like my 3 stage (homebuilt) that I ran last year it did have it's quirks I figured it was time to do something different with more water.
 

Hi Omni,

I will try and add my 2 cents on what I have experimented with and hope it helps.
As far as the jet goes 35psi is a great working number, but I would want to see the PSI increase to 45-50 when at full throttle so you may want to either close off some jets or insert a short constrictor into each tube close to the exit after you know your new sizes. If you had a method to create on hourglass shape in the tubes close to the exit, this could work also.

The undersluice can be done either way. If building for submerged the backpressure of having the undersluice lower into the water has to be overcome. A separation of pressure using flat plate like a Keene 3 stage is what is required and the plate must go high enough up the sluice to provide a few inches of water pressure to the outlet end to overcome the backpressure. This is why this cannot be done well with a simple punchplate as water pressure is always trying to equalize across the punchplate and flow is killed. A combination of width, height,(of the outlet area) and pressure (height up the sluice above water including water column height) will set the flow rate in the undersluice. If you try this, I recommend some clear plexi so you can monitor operation at different settings.
 

every pump has a sweet spot, mine is at 2800 rpms, don’t know about yours. a sweet spot for a jet log is at 35 lbs. when you go over 35 psi it takes a lot of hp for a little improvement. normally it is not worth the gas or the wear and tear on your motor. I gauge my jet size by when I get to 35 psi. too soon and your jets are 2 small wot and not being at 35 psi your too big. seems like you hit it first time, now your sluice should work at wot and 35 psi with the right water flow. this is the way I build my stuff and it has worked well for over 30 years and still does. timberdoodle how many big dredges have you built ? or do you just read a lot ?

bill/cr
 

every pump has a sweet spot, mine is at 2800 rpms, don’t know about yours. a sweet spot for a jet log is at 35 lbs. when you go over 35 psi it takes a lot of hp for a little improvement. normally it is not worth the gas or the wear and tear on your motor. I gauge my jet size by when I get to 35 psi. too soon and your jets are 2 small wot and not being at 35 psi your too big. seems like you hit it first time, now your sluice should work at wot and 35 psi with the right water flow. this is the way I build my stuff and it has worked well for over 30 years and still does. timberdoodle how many big dredges have you built ? or do you just read a lot ?

bill/cr
 

Bill, The biggest I have built is a 4" but there is not much difference in designing a 2" or a 6" than factoring increased flow necessary to move the bigger material. The difference between the PSI required for a log jet and a multiple jet design is what is at play here. I think the point of Omni's question is that his jet only reaches 35psi at FULL throttle which is probably not his pumps BEP so he is not getting the maximum benefit from his multiple jets and he could reduce the jet orifice size to better match his desired run speeds.
 

Why I'm asking these question is I want to make sure I am getting the most from everything. With running a double screen classification over/under sluice I need the max flow I can squeeze from my pumps. I have 3.5' of 1/4" wire mesh up top, 3.5' of 1/8" wire mesh sitting about 1 1/8" below the 1/4" mesh and it is 1" from the bottom. My underflow section is 3/4 swd raised expanded over deep v mat. My riffle section takes up the last 1.5' and sits at the level of my 1/8" screen.

As originally built, I was at 3/4 and 3/4 for my screens but my underflow section was not cleaning like it should. I added adjusters to the bottom screen and my upper screen already had. My sluice was about 1" in the water so I believe that was the issue but to be on the safe side I raised both screens. I want to make sure all bases are covered. Also I really don't want to run my engines at wot , up to 3/4 is the most I will run.

I think I will take 2 dowls and sand them down and beat in one tube and see if there is any improvement, if there is then see what a second one does. Honestly I don't think I'll see anything except a degrees in flow. I'm sure its working as good as it can but who knows...
 

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Omni, I would try pushing the limits on this to check results. If you can configure a few compression fitting using small screws and rubber grommets/rubber hose I would close off 4 of the jets and test results, then remove one at a time. It should be easier to do than pounding in a dowel and would allow easier removal with a screwdriver as your testing. Remember that PSI is king in a jet, but it's the balance of PSI to volume induced by the jet which really makes the difference in suction level. Every extra gallon of induced water from the jet takes up volume in the outlet of the jet and this results in having to increase total velocity of all the water exiting the jet to increase suction volume into the jet which is one of the major limiting factors/losses in a jet, especially a log jet.
 

Timberdoodle said:
Bill, The biggest I have built is a 4" but there is not much difference in designing a 2" or a 6" than factoring increased flow necessary to move the bigger material. The difference between the PSI required for a log jet and a multiple jet design is what is at play here. I think the point of Omni's question is that his jet only reaches 35psi at FULL throttle which is probably not his pumps BEP so he is not getting the maximum benefit from his multiple jets and he could reduce the jet orifice size to better match his desired run speeds.
Click to expand...

I might be on the same track with mines; ima have to try to get a guage on mines also..

It Should'nt matter where i adapt it ? Between close to the pump or jet.
 

I haven't found that placement of the gauge matters much as long as flow rate is not restricted by a small hose, long hose, or something else that would create losses in delivery to the jet chamber. I usually just locate the pressure gauge on the pump.
 

Blocked off 4...just worry I'm going to blow something apart....heading out!
I have my gage installed in the compression chamber, welded a piece of 3/8 pipe (2" long) and a coupler and mounted my gage.
 

Omni Hope you haven't left yet. I don't recommend pushing the pump to it's limits, just the test procedure to see how 4 jets, 6jets etc. work at 35-50 psi. I probably would not go much over 50psi.
 

Wish I was there. Still fighting too much snow here although today it's above 40 and melting fast but we have another 8" of snow expected tomorrow.
 

For that same reason tim i think i would rather put it at the jet. I understand how omni installed it; it should be pretty easy if i get a pressure guage with that small of a hole.. The ones i have seen comes with a female thread for you can attach it on a falcet. I think the diameter of those guages are about 1" wide .
 

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