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Venturi Dredge: Why does this work?

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Venturi Dredge: Why does this work?

Unread postAuthor: biggsauce » Sun Jun 22, 2008 10:47 pm

Since my recent cannon-related run-in with the law, I've taken up another project which is legal and still uses my knowledge of pvc fittings.

Some of the guys from work have 2 camps off the Atchafalaya River on Sweet Bay Lake, or "The Sweet Bay Lac" During Duck season, the mouth of the canal that leads to the camps has been filling in over the years and its to the point that ist almost impassable without a go devil.

My boss asked me to do some reading on venturi dredging. I came upon this site. This is where I started my research and got many ideas from. Included are 2 pictures. The first picture is our pvc dredge. Its fed by a 4" pump that runs 750gpm @ 52 psi. The second is a concept called and Infinity dredge and is what I'm still working on bringing it to life. It is the most efficient design due to its radial water injection.

Now this is a pretty cool concept, but there is little in the ways of math that I can find on the internet that are specifically applied to this sort of setup. There are venturi equations, but adapting one to and incompressible fluid mixed with sand, rocks and god-knows-what is not easy. Any suggestions?

I also read up on how this really works. here , here, and here all offer a lot of information about a venturi effect in a convergent/divergent pipe application.

Is this dredge really venturi operated?
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Unread postAuthor: rp181 » Sun Jun 22, 2008 10:57 pm

yes, but it would be more effective if the input was sticking out of the 90, pretty much replace the 90 with a tee. Also, i think more pressure and less volume is preffrable.
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Unread postAuthor: biggsauce » Sun Jun 22, 2008 11:08 pm

Well we have awhile until we start dredging, and that is the next model I plan on testing.
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Unread postAuthor: jackssmirkingrevenge » Sun Jun 22, 2008 11:33 pm

We have a similar system to generate vacuum in the lab, attached to the water tap. It works for some applications, but not as good as a vacuum pump.
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Unread postAuthor: jimmy101 » Mon Jun 23, 2008 2:16 am

jackssmirkingrevenge wrote:We have a similar system to generate vacuum in the lab, attached to the water tap. It works for some applications, but not as good as a vacuum pump.

Yep, a water aspirator. The vacuum attainable is limited by the vapor pressure of the water, which is dependent on the temperature of the water. For typical 60F tap water the minimum vacuum obtainable is about ~20 mmHg (~0.03 ATM). Not as good as a vacuum pump but still pretty darn good.

Back to the OP, "is it really venturi operated?" Yes. Water is not really "incompressible", it is just "incompressible" compared to gases. You still get the venturi affect, Bernoulli still works, wing shapes still provide lift, all the things that make a great air propeller also work in boat propellers ...

The details of the medium are different so the details of the devices are different but the math is still pretty much the same.

You can always fall back on the "busy street" analogy. Your high flow inlet and the pump's outlet is an LA freeway, bumper to bumper at 75 MPH. Next to the freeway is a herd of deer. Every once in a while a deer wanders onto the freeway and suddenly finds itself moving at 75 MPH. Cars rarely wander of the road because the movement of the cars is coherent (they are all going in the same direction at the same speed), so the number of cars on the road stays the same but the size of the deer herd is slowly reduced.
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Unread postAuthor: jackssmirkingrevenge » Mon Jun 23, 2008 2:21 am

jimmy101 wrote:the number of cars on the road stays the same but the size of the deer herd is slowly reduced.


That raised the sort of cynical smile that keeps one going on a Monday morning, thanks :)
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Unread postAuthor: biggsauce » Mon Jun 23, 2008 8:04 am

Are you saying it works on a more molecular level? That the molecules in the high flow inlet "grab" the molecules from the suction inlet?

I wish I could find the website that told me for every gallon you pump through, 2 gallons are sucked through the suction inlet. I thought this was ridiculous. Even If I build the most efficient design, I really doubt it can be that efficient. I was thinking more along the lines of for every gallon pumped in, 1/2 gallons would be drawn in through the suction nozzle.

Regardless, we have to move what I think is rougly 40 yards of sand, so I'm predicting it may take a few weekends...
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Unread postAuthor: jrrdw » Mon Jun 23, 2008 5:18 pm

Maby a bigger exhaust pipe? Run a booster pump in front of the main pump, (turbo charge the main pump) to see if you can push more flow out your drain, thus increasing the suction.

Are you going to exhaust the sand back into the water? Above the water or under the water? I say above. No restrictions above.

EDIT: I can only see 1 picture.
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Unread postAuthor: jimmy101 » Mon Jun 23, 2008 6:01 pm

biggsauce wrote:Are you saying it works on a more molecular level? That the molecules in the high flow inlet "grab" the molecules from the suction inlet?

yep, that's one way to look at it. Doesn't help much with trying to predict performance though.

biggsauce wrote:I wish I could find the website that told me for every gallon you pump through, 2 gallons are sucked through the suction inlet. I thought this was ridiculous. Even If I build the most efficient design, I really doubt it can be that efficient. I was thinking more along the lines of for every gallon pumped in, 1/2 gallons would be drawn in through the suction nozzle.

I would think the efficiency is much less than 2 gallons flow moving 1 gallon of load. I'd wag it at more like 10:1 or worse.

The water pump (aspirator) that jacksmirk posted will pump both air and water.

A typical garden sprayer that screws onto the end of a hose and pumps weed killer from a resevoir is a water / water aspirator pump.
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Unread postAuthor: biggsauce » Mon Jun 23, 2008 7:18 pm

well lets say the ratio of flow to load is 10:1. Best efficiency, there should be 10 - 20% sediment in the load/intake water. So in a minute, ~700 gallons of water is pumped through, which gives 70 gallons of water + sediment, which yields 7 gallons of sand per minute. ( conservatively)

thats not a lot... :? this is goin to be a long process...
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Unread postAuthor: biggsauce » Mon Jun 23, 2008 7:21 pm

jrrdw wrote:Maby a bigger exhaust pipe? Run a booster pump in front of the main pump, (turbo charge the main pump) to see if you can push more flow out your drain, thus increasing the suction.

Are you going to exhaust the sand back into the water? Above the water or under the water? I say above. No restrictions above.

EDIT: I can only see 1 picture.


I'm working on the second picture sorry,

And the discharge will be above water in order to give some back pressure to make it work
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Unread postAuthor: psycix » Tue Jun 24, 2008 10:30 am

There is one question in my mind:
Can it be a problem if the pressure is too high, making the air/water also come out of the port which should suck? So would the port for blowing in have to be smaller then the outlet port if your pressure is too high?
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Unread postAuthor: jrrdw » Tue Jun 24, 2008 6:46 pm

I would think back pressure is the last thing you want. Any opposing pressure at all will slow the siphon down. Run that sucker wide open to create as much draw as possible on the intake.
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Unread postAuthor: biggsauce » Tue Jun 24, 2008 11:01 pm

jrrdw wrote:I would think back pressure is the last thing you want. Any opposing pressure at all will slow the siphon down. Run that sucker wide open to create as much draw as possible on the intake.


Well the venturi dredge is a strange animal. Ive done a lot of reading on websites dedicated to gold prospecting and gouldeng.com is a great place to start. For a production dredge, the exhaust pipe must be a certain length. Gould tried making the exhaust pipe as short as possible and there was no suction. The water from the pump was coming out the exhaust end in a hurry, but the venturi effect wasn't happening. He then added 18" to the exhaust pipe and it worked. I at first thought the same as you. On some production dredges, the Venturi jet is a minimum of 5' away from the exhaust/sluice box.

We plan on 40 -50' of discharge hose. From the dredge to the surface of the water should only be 6' or so and the rest will be floated on the surface. For some reason, the venturi jet pump must have a certain amount of back pressure. As far as I have read, this will be a fairly effective setup, although no the most efficient design.

Still working on getting that second picture up... summer school has kept me on my buddies pc... :roll:

There is one question in my mind:
Can it be a problem if the pressure is too high, making the air/water also come out of the port which should suck? So would the port for blowing in have to be smaller then the outlet port if your pressure is too high?


The picture I have posted is of the dredge we built. This is the crudest and most inneficient design. Ideally, the clean water flow should be directed from the front of suction elbow in the middle of the exhaust pipe (as has been stated before) Also, the clean flow should be necked down with a nozzle that is a diameter less than that of the exhaust. This is known as the jet and from what I understand (which may be wrong) will induce much more load water that an input the same diameter as the exhaust. I hope that makes sense...
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Unread postAuthor: jrrdw » Wed Jun 25, 2008 3:55 pm

You better get some video's of this in action.
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