BGT in deep water

[benny]

The scoop really doesn't create much additional pressure to the tubes and the main function of the scoop shape is for creating a stratified flow to the upper deck. A small amount of additional pressure is created from the scoop depending on water speed and is called stagnation pressure. A simple test of this is to take a funnel with a short hose attached and put it into a stream at different flow speed locations and raise the hose above the waterline and see what additional pressure is created causing lift (not much). The main pressure to provide flow in the tubes is from the head pressure difference in height between the scoop inlet and the tube outlets. So lets say if you only need 4 inches of head pressure to run the sluice well, you could locate the sluice in a spot that gives you 3 inches of head drop and 1 inch of stream stagnation pressure for a total of 4 inches of pressure. The speed of flow required to create 4 inches of stagnation pressure would be very fast and isn't realistic for a stream (and your sluice would go bye-bye). A 1" increase in pressure is around 3ft per second and is possible. So finding a spot with a couple inches of drop is important part to gain the majority of the total pressure needed.

I do want to thank you for defining stagnation pressure. That exactly defines what's going on in the scoop. That'd be the kinetic energy stored by moving mass, and when the mass is stopped, the energy is converted to pressure. True?
Are you saying that the difference in water pressure from a couple of inches of drop accounts for the main pressure applied at the tubes? That'd be the same as a two inch water column, wouldn't it?
At any rate, if you were to remove the funnel from your hose, the stagnation pressure would not be practically nothing. It'd be comparable to putting a finger into a running stream to stop the flow, and putting the palm of your hand in the stream to stop the flow. You'd certainly notice the difference in pressure. Would you agree that the difference in pressure between the water in the tube, and the water in the trap is what makes it all work? If so, then it'd seem that the BGT should work the same whether it were submerged, or not. Wouldn't you agree?

 

[KevinInColorado]

The upstream end is always submerged. The real question is the difference in pressure between the upstream end and the downstream end. The upstream end high pressure benefits from the velocity of the incoming water (as well as any vertical drop). The downstream end is either in air or if submerged benefits somewhat from the low pressure zone created by how the sluice blocks water flow. Clearly having open air on the downstream end is ideal (as made clear by Goldwasher) but even without that benefit, the difference in upstream and downstream pressure seems to be enough to make the fluid bed functional in the field...based on my real life experience.

 

[Timberdoodle]

I do want to thank you for defining stagnation pressure. That exactly defines what's going on in the scoop. That'd be the kinetic energy stored by moving mass, and when the mass is stopped, the energy is converted to pressure. True? yes but keep in mind the increased pressure equalizes back to the inlet so no major increase in pressure is seen inside the scoop and the water rise in front of the scoop that you will see is the same pressure increase inside the scoop. (the same as looking at a rock in a stream that you will see a water rise in front of the rock due to the increased pressure.)
Are you saying that the difference in water pressure from a couple of inches of drop accounts for the main pressure applied at the tubes? That'd be the same as a two inch water column, wouldn't it? Yes, 2 inches of drop is 2 inches of head pressure. The point is that it is easier to apply drop than finding fast enough flow that will provide the same amount of pressure increase/difference.
At any rate, if you were to remove the funnel from your hose, the stagnation pressure would not be practically nothing. It'd be comparable to putting a finger into a running stream to stop the flow, and putting the palm of your hand in the stream to stop the flow. You'd certainly notice the difference in pressure. Would you agree that the difference in pressure between the water in the tube, and the water in the trap is what makes it all work? If so, then it'd seem that the BGT should work the same whether it were submerged, or not. Wouldn't you agree?

pressure is measured in psi and therefore the amount of square inches is important in understanding the difference between the area of a finger and the area of a hand so you can't really compare a finger and hand approach to how the scoop functions except to say that the dam of water and the increase in water height in front of your hand is the result of the relative pressure increase which results in higher head.
running it fully submerged would require significant flow speed and while you can get it to function at some degree probably close to a slow flow with a small drop, my point is that it's going to be hard to get the same function in the pressure tubes as a drop of 4-6 inches can provide.

 

[benny]

pressure is measured in psi and therefore the amount of square inches is important in understanding the difference between the area of a finger and the area of a hand so you can't really compare a finger and hand approach to how the scoop functions except to say that the dam of water and the increase in water height in front of your hand is the result of the relative pressure increase which results in higher head.
running it fully submerged would require significant flow speed and while you can get it to function at some degree probably close to a slow flow with a small drop, my point is that it's going to be hard to get the same function in the pressure tubes as a drop of 4-6 inches can provide.

Hi Timberdoodle
One of the things that's causing a misconception is the fact that I have a mini BGT, and you're probably running a Prospector. A 4 inch drop on a mini is really significant. I understand the difference between putting a finger in the current, and putting a hand. I was comparing that to having a hose without a funnel, and with. Same PSI, but more square inches. I think that you're right that's going to take quite a bit of current, and to keep an eddy forming on the slick plate, you're going to have to run it completely flat, of even raise the trap end a tiny bit (Here, I'm referring to Goldwashers comment on laminar flow). Doing that will lessen the difference in pressure at the tubes though. Basically, I just think that it can be done. Just like with any sluice setup, somebody's just going to have to try it.

 

[Timberdoodle]

Hi Benny, I've never run a mini so I'm not sure on the usual setup and drop required for it to clear properly. Would be interesting if someone could attach a meter to the tubes and measure different flow speeds and psi differences.

 

[benny]

Hi Benny, I've never run a mini so I'm not sure on the usual setup and drop required for it to clear properly. Would be interesting if someone could attach a meter to the tubes and measure different flow speeds and psi differences.

It's only 16" long, so a 4" drop is significant.

I need to thank bronze for bringing this topic up. I'm learning a LOT!

 

[Goldwasher]

The scoop most definitely adds pressure. As discussed drop as a major affect. take a box with a full scoop and upend it and pick it up listen to that pressure...for those that ask if A BAZOOKA THAT IS HOW YOU MAKE THEM DO IT...WHEN YOU DO A CLAEN OUT..! A bazooka in the water has a little bit different dynamic than a funnel and a hose BAZOOKAS DO NOT HAVE HEAD PRESSURE! The pressure in a bazooka is a result of resistance an pressure. There is pressure developed by the water "slamming" into the scoop as Kevin stated.We have ran them so many ways over the years. Every model (too many by the way. I can have any model I want and only own two the smallest being a 36" prospector) I can run them in slow water pool to pool with little flow or super fast water and the KEY is always water over the grizzlies and exit out of the water as much as possible. The main thing that happens with less than optimal flow is that you lose the ability to feed fast. depending on where your parameters fall on the scale it will be because the water isn't pushing your waste off fast enough. And even though your trap is fluidized it could be at a lower than preferred amount and therefore not exchanging quickly also affecting feed rate. Having the end of the box submerged most definitely affects the exchange. your trap is going to be very fluid yet, now you have a large heavy wall of water that your out put flow has to break through creating back pressure on the trap exit. For every bit of depth you add there is pressure exerted down on the material in the trap that needs to exchange. I promise you that those conditions are not your friend and the pressure in the tubes vs. the pressure exerted against the trap outflow is not optimal. It may seem on paper that it should work or not matter....however having ran TONS of material through bazookas field experience is the prevailing illustrator of its working dynamics. This is a miner in large volume but slow water... Too deep material was building up pre trap and under the grizz. We lifted it up and problem gone. The other thing is tailings they create back pressure against out put just like deep water.View attachment 1251132 Here is a 36" pro fast water but low volume wedge it dam it and look at her run perfectly!!and at 70 degrees to the original creek flow. View attachment 1251133Heres a 36" pro fast water large volume...IN LOVE!!!!! notice up and out with flow over the grizz. fast feed, great recovery, tailings go away nothing hindering flow out of the trap IDEAL!!View attachment 1251135

 

[Goldwasher]

It's only 16" long, so a 4" drop is significant.

I need to thank bronze for bringing this topic up. I'm learning a LOT!

A mini can have too much drop pretty easily...run it relatively flat and with fast flow

 

[KevinInColorado]

A mini can have too much drop pretty easily...run it relatively flat and with fast flow

Indeed, the smaller the trap the flatter you have to run it. Otherwise you start losing fine gold.

PS thanks for the long post just a couple above this one Goldwasher, your expertise is highly valued!!

 

[Timberdoodle]

I believe all the different terms for pressure are getting misunderstood. In the example of lifting up a full scoop of water and listening to the pressure what you are explaining is head pressure (a difference in static water level) and having a smaller scoop or no flare to the scoop with the same water height would produce the same pressure and flow through the tubes because the pressure is due to the water height not the volume. When running with a setup in a dam pool with little to no stream flow hitting the scoop and the outlet water height lower than the inlet water height the result is a head pressure difference which is also why you have flow down the sluice deck, in this example the only pressure created to run the flow out the tubes is head pressure, there is no pressure increase happening in the scoop simply due to it's shape. There are a multitude of dynamics that do result in pressure differentials though and even the fast water speed off the upper deck in close proximity to the outlet also plays a role since the fast water has less pressure than the outlet water below and water flow is always from high pressure to low pressure.
I tend to get a bit to technical on my approach sometimes so sorry for any confusion and a merry Christmas to all.

 

[Goldwasher]

I was pointing out the difference between head pressure and the pressure created by the resistance of the scoop water entering smaller tubes. submerged bazooka...at a tilted angle under water would not have any head pressure

 

[Goldwasher]

Under water the pressure exerted from all directions increases significantly.

Except the pressure in to the scoop

 

[Goldwasher]

Where as at stream level flow is able to exert more force in the direction you want your waste material to go...as easily as possible while maintaining the low pressure of the trap and not hindering it's exchange...putting the end of ANY SLUICE BOX under the surface level of water is always something you should try to avoid.

 

[Timberdoodle]

I was pointing out the difference between head pressure and the pressure created by the resistance of the scoop water entering smaller tubes. submerged bazooka...at a tilted angle under water would not have any head pressure

Ok Sorry for my confusion, sounds like we are on the same basic page.

 

[Timberdoodle]

I want to go back and address the scoop shape and pressure increase question. It is thought that the flow in the scoop is somehow increasing the pressure to the tubes due to the shape of the scoop and the water speed increasing as it flows down the inside the scoop.
I will give you my ideas on how the scoop functions in fast water conditions and we can discuss.
I don't believe the water speed inside the scoop has any potential to do this and I will try and explain why. The tube outlet holes are very small and once the scoop is full of water the only water that can enter the scoop is the same volume leaving the scoop through the tubes outlet holes to agitate/fluidize the trap. The volume leaving the tubes is very minimal and probably less than 10 gallons a minute. So the speed of the water travelling down the scoop once filled is very slow especially in relation to the water hitting the scoop area in front of the scoop. The scoop is acting as a vessel simply holding water and transferring the different head height from the head of the scoop to the tubes. Why does it function better in fast water then? What is created in the condition of fast water hitting the area of slow moving water at the front of the scoop is a hydraulic jump just like putting a small dam in the water. The fast water has energy and as it reaches the slow water at the front of the scoop it must transfer this energy somewhere and you get a water height change around the scoop and water flows over the deck better due to the increase in water height from the dam effect created from the scoop below combined with the speed of the flowing water. The total water height now created at the front of the scoop has increased and this is transferred as a head pressure increase to the tubes also. So the faster the water the greater the hydraulic jump height and the greater the flow over the deck and faster water results in flatter operation while still achieving a pressure differential from the head of the scoop to the tubes to keep material fluidized.
I am a fan of the BGT sluice and it's design features which result in great operation. Having the scoop follow the same deck size and shape is one of the design features that I have always admired as a change not included in the original Schmidt patent and results in better operation as it transfers flow much better to the upper deck.
Any thoughts?

 

[Goldwasher]

Yea that is all out the window when submerged.....and the water that" backs up" out of the scoop will only go up if it can't go down or sideways having personally built several hundred bazookas I promise what you speak of is not a design feature or function. You are almost on the right track. That water coming through the tubes is being forced by laminar flow and the weight of the water in the river, stream or pool feeding it. Simply when you have the flow and water depth you need you can run flatter because you don't need as much drop to get the material to clear. Drop or head pressure just doesn't come into play. It is very difficult to over pressureize the tubes with natural flow. The chance of loss in the trap when you have too much drop is from the trap scouring out due to extreme angle and the eddy that is created. Unlike a riffle sluice that require the eddy for concentration and exchange. The bazooka relies on the fluidization and exchange based on relative density of the introduced material. The tubes are designed per trap size to deal with the pressure created by the water that is being forced through them. Different models have different size holes or number of holes...and even different size holes per tube placed specifically to deal with the pressure variance at the up stream end of the tube vs the end near the output of the trap...what we call the front plate. That is done so the pressure is even for the entire tube and therefore the trap will have even fluidization.The main reason these holes vary is the scoop size and the pressure change as the box and scoop get bigger.
THINK LESS SHOVEL MORE

 

[Goldwasher]

It runs better in fast water because it moves gravel out of the way faster. There is rarely an issue getting the trap fluidized with low flow it's getting the water over the grizzly.

 

[benny]

Goldwasher (or anyone else), I have a question for you. If the BGT were underwater, and the trap end lower then the front, then the sluice is creating an eddy current and keeping material from passing through the trap. Would the force of the current move material through the trap better if the trap end were raised higher than the front of the box? I realize this goes against normal thinking, but I was thinking that if the current were driven down against the deck, it'd force the gravel through.

 

[Timberdoodle]

Thanks GW, makes sense to have different size holes for different models depending on pressure needs. I always looked at the basic shape of the scoop as a function of simply following the upper deck shape and the shape on the upper deck designed to increase flow speed on the upper deck prior to the grizzlies and better stratify material in much the same way that a pinched sluice works to some degree.

 

[Goldwasher]

Goldwasher (or anyone else), I have a question for you. If the BGT were underwater, and the trap end lower then the front, then the sluice is creating an eddy current and keeping material from passing through the trap. Would the force of the current move material through the trap better if the trap end were raised higher than the front of the box? I realize this goes against normal thinking, but I was thinking that if the current were driven down against the deck, it'd force the gravel through.

Thanks for bringing that up I mentioned to say something about it. Yes under water if the input end is lifted you end up with low pressure on the deck and therefore less puss on material. As well as into the trap. Flat would be best but like I said you have cetain things working against you at the out put end of the box. Keep it out of the water as much as possible.