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View Full Version : Gravity Venturi pump for dewatering pit

09-01-2012, 12:17 AM
Hey all, I am out of the hills for a few days and thought I would try and get some thoughts on a thought of mine.
Lets say that I have a 70' pit that I am dewatering with an 6" pump. The water table is at 20'. To dewater the pit I have to pump the water to a height of 100'. I have a huge water supply that is 200' higher than the bottom of my pit.
With a water supply that is 100' higher than my pumping outlet why would a gravity Venturi pump not work? If I was to start with a 4' foot pipe that reduced down to 6" at the pumping level it would seem that I could pump the water out of the pit. I dont need a huge volume to keep up with the inflow of water into the pit if the pump ran continually.
I am sure that Im not the only one to think of this so there must be some reason that it is not in use. Maybe a venturi pump cant lift that much head? I dont know but I was betting that someone here probally does.

09-01-2012, 06:32 AM

The question that needs answered is; Is the suction created by the venturi greater than the pressure created by the 100 ft head of water?

What we know:

One square inch of water x 1 ft = .4 lbs. So, 100 ft height = 40 psi. The question is; can you create a negative pressure (suction) of greater than the ambient 40 psi?

We know how a venturi works. A venturi creates a pressure drop because of the differential speed (v1-v2) through the venturi.


Next, do the math:

p1 - p2 = Rho/2 (v2 exp2 - v1 exp2) Note Rho is water density = 1 gm/ccm and "exp2" means the same as "squared" ( v1 x v1 and v2 x v2).

Pounds (PSI) is/are messy.

The reason is, we have to have uniformity of units. i.e. CGS, MKS or SI units. We can't mix apples with oranges or grams with pounds!

SI units are the most recently adopted units. So, one psi = 6895 Pascals. Are we having fun yet? :)

In SI units, evertything must be in Meters, Kilograms, Seconds, and Pascals. So, do the conversions. Do the math and you're there!

I have put together an Excel spreadsheet for those that want to plug in the numbers. If anyone wants a copy, send me a PM and I can email it to you.

Note also: The assumption is that we are running water - not slurry.

- Geowizard

09-01-2012, 08:08 AM
Get a copy of "Hydraulic Mining" by Capt C.C. Longridge. On page 51, he covers lift ability of hydraulic elevators. He states that lift capacity is roughly 20% of head with 16% fairly easy. Now consider that the material has a SG of roughly 2.7 versus water at 1.0 but the material is only about 5-7% of the slurry, uh, er ... maybe?

Keep in mind that reducing the diameter of the fall pipe does not increase pressure. Pressure is solely a matter of water column height. It will increase velocity but you also get a substantial increase in friction loss: this is a matter of water velocity and diameter.

It is best to keep the diameter fairly large (8-12" ABS should be enough) until you get to the injector which should be long (3-4') with a nice smooth taper. The orifice itself needs to be crowned on the outlet side just as with a rifle barrel. You want the injected water stream to maintain its integrity for the longest distance possible. Another engineering thingy that seemed common was the venturi went directly in to the elbow. This was poor engineering at best. Ideally the venturi should be necked just as the drawing posted by geowizard above. You might recognize the necking as the same as the barrel of a carburetor on an engine.

Just as with the standard log type venturi, you need a fair length of pipe past (outlet side) the actual injector to maintain a static head and increase the efficiency of the water piston effect before you have any bends. See the info posted by Reggie Gould aka joebazooka on Gold Prospectors Network and on his website, I think it is Gould Engineering.

You should be able to download the book from archive.org

All suction dredges are spin-offs of the hydraulic elevator.

Can't help much beyond that, but hope it helps.

Steve Herschbach
09-01-2012, 08:21 AM
http://books.google.com/books?id=0kIsAQAAMAAJ&pg=PR2&lpg=PR2&dq=%22Hydraulic+Mining%22+by+Capt+C.C.+Longridge&source=bl&ots=8ffW9QySWI&sig=emyRpbRZ85UdZFnz1Qbl250xqDg&hl=en#v=onepage&q=%22Hydraulic%20Mining%22%20by%20Capt%20C.C.%20Lo ngridge&f=false

09-01-2012, 09:23 AM
The question may need some clarification.

A (power) jet pump aka loosely referred to as a venturi pump uses a Y joint as on a typical "suction" dredge.


Looking at Keenes largest dredges, the lift is at the dredge and can lift as as much as the laws of physics (50 feet) above water level. They advertise something less.

Eductor pumps are an option, if that is the question presented here. The "suction" limits are stated at 20 to 30 feet.

My understanding is we are pumping out a pit containing water. We are using water from a 100 ft higher elevation to produce motive force that will lift water from the pit 100 feet. A venturi (jet tube) suction pump would be an option if the water only needed to be pumped almost horizontal. Lifting the water 100 feet is another matter.

The line pressure in the pit would represent a 200 ft - 100 ft column of water = 40 psi and would flow at a rate based on a 40 psi differential between the top head and the exit level 100 ft above the pit. Would the 100 ft elevation difference create enough flow (motive force) to create a suction and also push with sufficient force to lift 100 ft? The water line is being required to suction and blow at the same time.

- Geowizard

09-01-2012, 11:06 AM
We can probably all agree; "Water is good."

This past summer, I spent much of my time mining the "nugget patch". The nugget patch is in a large 2-3 acre cut that was made last season. Problem is that the cut has water inflow. Because I am bent toward the Bobcat - washplant operation, I needed to de-water the cut on a continuous basis.

Dewatering a cut is not trivial work.

The pump as a matter or course must be located near the lowest point in the cut. When you return the next day, the pump is submerged unless you keep moving it to high ground. My solution was to put the pump on pontoons. I have a 23 HP Vanguard pump and I put it on the pontoons from the mini-six inch suction dredge. So, the pump rises with changes in water level. The next issue is rope starting the 23 HP pump in the water. Yes, it has an electric starter, but anyone that has bought one of these probably has found a lack of sufficient clues to hook up the what normally should be simple starter wiring. Anyway, I can certainly sympathize with dewatering a pit. And, BTW, rope starting the 23 HP pump requires full attention and care to avoid getting an arm yanked! Fun stuff!

- Geowizard

09-03-2012, 02:59 PM
Thank you Geowizard and Overtheedge for all the great information. The link to Captain Longridges' information on the hydraulic elevator is great stuff. I will be trying the hydraulic elevator idea out and will let everyone know how it works. It will probally be next spring before we will really be able to use it but we will be experimenting some this fall. In the right spot this could save alot of fuel and wear on the pumps. Plus it may be a good way to get the muddy soup out of our pit. It takes 25 to 30 gallons of fuel a day to keep the pumps running, so if the math works out for the h2o elevator buying the pipe for it is affordable.

Well the caribou is in the freezer and my moose is hanging from the backhoe, so I will be going back out in a couple of days. Thank you all again for the great information.