Gravity dredge variation.

[impfected]

I had planned on using a good length of lay-flat hose as the supply with a short section of rigid suction hose to keep the end top opening from collapsing, but sounds like it would suck itself closed with the vortex at the top.
Carrying lengths of pipe would be as or more difficult than as my 2-1/2 dredge pump in the area I'm thinking of. I probably would be better off finding a lighter engine than the Brig-strat 3 hp lump and just take the pump up.
I can't wait till the snow melts!

Thanks for the info guys!
Al

[Geo-George]

I'm using 3" fire hose followed by 1 1/2" fire hose.
Basicly "Lay flat", works just fine.
When I first layed it out, it wouldn't fill with water.
I squeezed the edges of the 3" to get the water started in and
that's all it took. About 5 min later there was a good strong flow
comming out the lower end.

[rob2dog]

It is the hydrostatic pressure that counts, not volume. Think of diving into deep water. deeper you go, more pressure on the ears. The pressure is the same in a tank of water or a lake 10 miles accross.
Robbie

[ROKONRANDY1]

with a gravity dredge how fast the water is moving threw the pipe has a lot to do with the suction and being able to keep the suction going. Right now the conditions are good for some larger gravity dredging. A little cold maybe. rokonrandy

[dickb]

The problem with a gravity dredge is grade.

If you are at the bottom of a dam or water fall the has a 30' drop you can use about 50' of hose to get the dredge working at the bottom of the falls.

If you are on a creek that drops 2' for each 100' section of creek, then to get the same 30' drop would take 15 times 100' of creek or about 1500' of hose plus what you need to connect to the dredge or highbanker. You also have friction loss thru the pipe that you need additional head to add the the length of hose to get the same pressure. There's 30 times more friction in a 1500' hose than a 50' hose.

Just some food for thought.

Dickb

[Sean C]

Obviously a gravity dredge won't be practical for every site a miner could want to dredge, which is one of the drawbacks. Fortunately many higher elevation rivers have the necessary drop available especially feeder creeks. The biggest interest for me is in the design. In particular I would like a design which uses a jet tube as opposed to the style in which your material has to run the full length of your hose, which might be 100+ feet in length.

When thinking about design considerations I have to think about available elevation drop, diameter of hose, water collection apparatus, possibly using multiple diameter hoses in a reducing sequence, etc. Basically I'm trying to determine which configuration will be the most efficient and produce the most power using a 3-4" suction nozzle.

[Hoser John]

The 100" hose carries water only to a powerjet,suction nozzle,hydroblaster,long tom,highbanker or sluice. When trying to use a gravity dredge you must REALLY get into physics,intertia and depth pressures to initiate the highest psi at the source as the feed in collection chamber must be as deep underwater as possible. Some folks here would have you believe that hose size does not matter. Well physics is physics as that is akin to saying a semi will produce the same effect as a motorcycle smashing into a wall. How do do you think the fellas of old old got them 10-12" streams of water to shoot 100's and hundreds of feet and wash down whole mountains. The bigger the hose the more weight is carried(multiplied expotentially) and hence more weight =more inertia ,hence more inertia more psi and more volume and force. When properly necked down in a conical fashion you loose the impedence formed by any and all sharp restrictions that create friction and cause a disruption of the flow. Smooth reduction is how the resistance is foiled and your good to go. The deeper the water source(think putting your head and/or diving underwater) as you have to equalize the increase of pressure in your ears and when going down deeper you must again and again neutralize this immense pressur increases or your brains will squirt out your nose,eyes and ears at about 25' or so, THIS pressure increase can be utilized to start your project off to a HUGE advantage pressure wise and increases are all expotential and you then rock,rip,blast and tear with no noise,no gas and nuttn' but sweet oro puro as your reward-party on and tons a u 2 u 2-John

[chief613]

[/quote]

Take one sq inch, now 1 sq ft. The overall wt. is more for the sq ft, but it's still the same per sq inch.
Now if you take the 144 sq inches from the sq ft and stack them, you have increased the "PSI" by 144.
Take that sq ft and stack 144 more on top, you have increased the "PSI" by 144.
In both cases the PSI is the same. Something to remember, "Gravity" pulls straight down.
By using a larger pipe you are only increasing surface area, not pressure.

Another name for the system you are talkin' about is a "Simple Simon" dredge.
If you have enough drop available, this is a very effective system.

You are right where I was, but I got my answers right here.
Again, I thank members for the solutions to my quandries.

If you go here, you'll find the sumation of my inquiries on the subject.
viewtopic.php?f=5&t=428&hilit=gravity+sluice&start=10[/quote]



would the larger pipe give you more gmp or will that be controlled by the opening of the venturi

[dickb]

Let's simplify here for easy understanding and assume our pipe is 2" square and 4" square.
area = base times height in inches.

2" x 2" = 4 Square inches area for the small pipe.

4" x 4" = 16 square inches area for the large pipe.

So we see that the area of the larger pipe is 4 times larger than the small pipe.

So we get 4 times more volume from the larger pipe.

Pressure is figured on feet of head. so if there is 40' of head on both pipes, then the larger will give 4 times more water than the smaller pipe at the same psi in both pipes.

Same is true for round pipes only about 80% less area.

Hope this helps.

Dickb

[MadJack]

I have always wondered how this would work until we found one in the back country, out of the water with no one around. This is in a steep ravine [canyon] which would limit your working/dredging time even on a two day trip. Just walking in with no equipment takes a half hour and it's nearly straight down.
We studied it and examined the dredge hole under the 12 foot waterfall. (late 2008)
July 2009 while getting some lunch, one of the guys who was running that grav-dredge pulled up to say hi and ask how we were doing. It gave us a chance to interview him on his design and results...

They had two 3" pressure [heavy-wall] coming from the pool above the falls. Both of these were fed into a homemade dual-offset 4 inch jet tube about 10 feet from the sluicebox. Another 20+ feet of 4 inch suction hose allowed them to dredge under the falls, which in my opinion was too close.
He said he thought it worked OK but that he thought the two 3 inch pressure hoses over-powered the jet and the suction at the nozzle wasn't what he hoped for.

John and I discussed this later and figured there was no way to slow or limit the high pressure. Now, if the power jet had two 3 inch to one inch cones like the Keene or Proline, (or other professionally designed eductor) it would have had better pressure at the intake nozzle without over watering the sluicebox. (Pretty much what Hoser said above)