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kevyluvu
20:06:55 Fri
Jan 21 2011

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Water and Gravity

If you have a pipe that is infinitly long and has the same inside diameter the whole way, and you have the top of the pipe in a water source that will not run out, how fast will the water move before it reaches terminal velocity?

What I'm trying to find out is, at what point would the length of your hose for a gravity dredge become obsolete due to forces of friction that would slow the water down .
If you extended your hose past that length would the friction start slowing the water down more and more as the hose became longer and longer or would the water reach a certain speed and just not go any faster?

Besides the hydraulicing method are there any other ways to increase the speed of the water without a pump?
[1 edits; Last edit by kevyluvu at 20:08:25 Fri Jan 21 2011]

  
kevyluvu
21:39:37 Fri
Jan 21 2011

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Re: Water and Gravity

When water freezes it stops moving. I wonder if there is any fluxuations in the fluid state between ice and steam. Would hot water move faster than cold water? Heat rises, so maybe the hotter water rising would counteract any gain in speed. If not maybe a person could get a small boost in speed by using a black pipe in the sun rather than a white one?

  
overtheedge
22:05:52 Fri
Jan 21 2011

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Re: Water and Gravity

Friction loss is cumulative and affects pressure loss. The loss of pressure reduces the flow. As the flow is reduced, the friction loss is reduced. Double the flow (GPM), increase friction loss (PSI) by about 3.6X for a given pipe size.

Infinite length is a non-issue. Pipe length is finite. The question is not when does the friction loss prevent the movement of water, but rather when does pressure and volume drop below what is required for operation. Water will flow through a 2" pipe a mile long. It just won't provide any meaningful volume or pressure. Dependent upon your requirements, it might still be enough.

Terminal velocity. This isn't so cut and dried that a formula exists for all installations. Here are the rules of thumb: Larger pipe=less pressure loss/volume. Greater gradient= less pressure loss due to less pipe length. Restrictions increase pressure loss. Lower volume=less pressure loss. Maximum velocity is gravity/friction loss dependent. A waterfall reaches maximum unrestricted water velocity dependent upon heigth of fall and air resistence.

Barring any increase in head (pressure), the easiest way to increase water velocity is reducing the diameter. Reducing diameter increases velocity and decreases pressure. The trick is using the largest diameter pipe for the longest run and only use smaller pipe just before the hose appliance on the end. Larger pipe=less friction loss and therefore higher volume and pressure.

Think of pressure as push. Volume as leakage. You get maximum push with minimum leakage and the absolute max pressure when there is no leakage at all. This is the reason for looking at the pressure/volume curves for pumps. What pressure can you get for a given volume delivered?

Hopefully I haven't over-simplified this and still got the point across.
--------------------
Might I suggest the "Pocket Ref" by Thomas Glover, Sequoia Publishing as a handy reference tool. Has all sorts of data on water, minerals and just about everything you might need.

  
AceHand
06:38:23 Sat
Jan 22 2011

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Re: Water and Gravity

OTE was in a rather lengthy discussion on another forum regarding gravity dredges and one thing he posted that I didn't forget was, if you don't mind.....

"There would be a bit of friction loss from the length of pipe, but not that much if you use larger pipe than 2" to get 120' of head. At 50psi, you lose 4.6psi per 100' of 2" plastic. My book doesn't have tables of head loss for plastic larger than 2.5", but for steel pipe @ 50psi, 2" is 8.5 psi/100 feet, 3" is 2.3psi/100 and 4" is 0.3psi per 100'."

From this I assume that friction loss from 4" pipe is almost nonexistant and from 3" is minimal. These figures are taken from a line that is pump pressured. Wouldn't gravity pressured be similar? If you keep most of your pipe this size or larger there should be no problem.

Good Luck,
Tom

  
overtheedge
18:19:47 Sat
Jan 22 2011

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Re: Water and Gravity

If your desire is a gravity fed washing/sluicing operation. There is a product that is cheap enough to get you started at your local big-box hardware store known as unperforated corrugated drainage pipe made from polyethylene. As I recall, I bought a few 10' sections for just about $1.50/ft. You will need couplers and they were just a couple three bucks each.

If you have a way to haul it, the pipe also comes in 100' lengths. It coils pretty good when warm, but 100' is serious bulky.

Oh, I bought 4", but 6" is supposedly available. I would avoid using the pipe for a siphon dredge due to ... Imagine trying to clean a poop-tube concentrator 100' long. Well that is just what you'd have. For that, use plastic pipe such as abs or pvc.

See the postings by kiwijw for siphon dredges. He is the undisputed master so far.

  
tenderfootminer
17:13:37 Sun
Jan 23 2011

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Re: Water and Gravity

for what its worth I saw this "In a truly theoretical sense "the relevant equation is
v=vf*(1-e^(-t/tc)) where tc=m/b. m is the mass of the droplet and b is the drag coefficient deteremined by the conditions.
:confused:

  
kevyluvu
18:02:52 Sun
Jan 23 2011

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Re: Water and Gravity

My brain is having growing pains. Gonna have to look at this a few times to let the info sink in. I googled my questions and I was rerouted to a physics forum with all kinds of crazy formulas. I think my general conclusion is real world testing needs to be done to achieve the required results due to the amount of variables. I did do a little testing with 90' of 2" a couple years ago and threw the video up on youtube. Another youtuber asked me a question I didn't have the answer to, now I do.

Thanks tender I'll just plug that into my TI calc and get right back to you.

  
AceHand
23:42:00 Sun
Jan 23 2011

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Re: Water and Gravity

lol, yup, the only way to really tell if it'll work is to try it. I still don't know just what you want to accomplish though, gravity dredge, siphon dredge, or highbank. Just how far do you need to go?
2" should work for several hundred feet. 3" or 4" will work better. Bends in the line will prolly be your real killer. Very many and you'll have more friction loss from them than the line itself.
You need to get the air out to achive a true siphon effect. You'll get much more volume that way. I was getting 60 gpm with 100' x 3" drain pipe and only had 5' or 6' fall. It was open ended for a siphon dredge.

Good Luck,
Tom

  

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