Pond Pressure Question

[jimh]

I have about a 1/4 acre pond built on a slight sloping hill.  On the deep end, opposite of where the water enters, there is a drain pipe for the pond.  This is a diagonal pipe through the dam side.  During the winter, the overflow goes over the top through a culvert.  I was wondering if pressure is increased if you can grab the water at the bottom of the pond and route it through the side of the dam, possibly how my drain pipe is placed.  I put a sketch of my pond in my files.  If someone could explain the physics of this, I would appreciate it.  I have read a lot on this website about "head", referring mostly to the vertical feet of water travel through a pipe.  I have not read whether water "compression" multiplies the PSI, only that depth multiplies velocity.  Same thing?  I have between 300 and 400 gallons per minute.  Does anyone know what my pressure would be and what kind of watts or kw I can expect?

Thanx

Jim

[Capt Slog]

As you go down into the water the pressure increases, I can't remember the exact number but I think it is around 1 atmosphere (14.7 psi) for about 32 feet of depth. Hence if you were next to a big tank of water that is 32 feet deep, the pressure of the water coming out of a hole at the base would be 14.7 psi

The water isn't 'compressed' of course, although it is under pressure it doesn't significantly change it's volume untill much greater pressures than that.

I've never looked into the mechanics of using water power, so I don't really know how this all relates to your question or what you will get from your pond. Theoretically you can calculate what you will get by

potential energy = mgh   (that is mass x acceleration due to gravity x height)

if used fully, this PE will convert to Kinetic energy which equals 1/2 m V^2

The units are really easy if you use metric.  1 cubic metre of water has a mass of 1000 kg (one metric tonne), acceleration due to gravity is 9.8 ms^2, height in metres, and the velocity comes out in metres per second.

I doubt this will answer your question, but it might help you to understand with what you've already read.

[Nando]

Jim:

I examined your sketch, and 300 gallon/min = 18.9 liter/sec to 400 gallon/min = 25.2 liter/sec .

What is the diameter of the drain pipe ?.

Can you prolong the pipe down hill, if such can be done?

And, what increment in HEAD you can attain ?.

For each meter of head ( height difference from the top of the water to the Turbine ) with this minimum 18.9 you may obtain 18.9 l/s * 1 meter-head * 6 = 113.4 watts.

6 is the product of 9.81 * efficiency of the system.

So for each meter of head with the stated water volume you may get around 100 to 113 watts, and if the pond is at 10 feet height = 10 * 0.3048 = 3 meter head, the you can get around 300 watts -- the pond height has to stay at 10 feet.

For a stable system one needs to use a water volume that does not "empty or lowers" the pond, also by increasing the head, which means going down hill You may obtain a lot of more energy with less water usage.

For every 3 meter of head increment, You harvest 100 additional watts

I see that you are may be interested producing hydro energy and if so, then

Can you have additional head by going down hill ?. If so how much ( accurate measurements are needed)

Also accurate water volume flowing

Does Your area freezes in winter ?.

I can assist You in such effort.

Nando

[Kwazai]

don't know much about the subject , but ran across this page a few years ago. hope it helps.

Mike

http://sleekfreak.ath.cx:81/3wdev/VITAHTML/SUBLEV/EN1/WTRWHLOS.HTM

[jimh]

To all,

Thanks for the help.  Nando, I got some answers to your questions.

"What is the diameter of the drain pipe?"

It is a 12 inch galvanized right now. However, it is rusting out at the bottom of the pond and I will be replacing it, maybe with PVC.  I am thinking of an 8-10 inch or maybe two or three 6" pipes.

"Can you prolong the pipe down hill, if such can be done?

And, what increment in HEAD you can attain ?."

The pond sits on the lower edge of the property.  Unless I dredge another pond higher up, that is all the head I can get.

"Does Your area freezes in winter?"

Short periods of freeze are common.  That is also why I was thinking of taking it from the bottom.  However, if my pressure does not increase much from doing it that way, I might prefer to take the water off the top so the pond level does not fluctuate.

I will work to get a more precise water flow test.  However, by the initial tests, I am sure I have not exagerated the calculation.  Thanks again.

Jim

[Countryboy]

If your pond is X feet deep, then the water at the bottom of the pond has X feet of water pressure on it.

If you cap off the bottom of your diagonal drain pipe, and allow it to fill with water, you will once again have X feet of water pressure.

X feet of head is X feet of head.  It doesn't matter if you begin piping water that has X feet of head, or if you fill a pipe from the surface of the water - you will have the exact same feet of head, and the exact same water pressure either way.

The ONLY difference you will see is a slightly (probably not measurable) difference in flow rates as long as pipe sizes are equal in both scenarios.  This is due to the friction of water in the pipe.  A vertical pipe draining water from the top of a pond will be longer than a horizontal pipe directly to the bottom of the pond - you will have more friction inefficiencies in a long pipe than a short pipe.

[Gary D]

Just a thought, if your spring/water supply to the pond has a fair ammount of drop, you could potentially harvest it before the pond. A pipe catching a portion going to a water wheel or such. No need to dredge a separate higher pond, or chance draining down the pond in dry weather...

[alancorey]

One advantage of taking off the bottom is using the pond as storage.  If demand ever exceeds supply for a short time the pond can make up the difference.  This could happen if you're using near the full flow and you've got a centrifugal governor or something keeping an induction motor running as a generator at a constant speed.  As the electrical load increases the RPMs (and output frequency) would tend to drop, but the governor should counteract this.  The pond would increase the chance of enough flow so the governor could have more water to work with.  If you aren't planning to run out of flow the above doesn't apply.

Other than that, feet of head is feet of head, as has already been mentioned.

  Alan

[Vtbsr]

hello jimh, You need to get more than the 10 ft head of the pond if possible.  Can you go upstream to get more vertical (head).The pipe or penstock contains the water and increases the presure with increased head. The penstock can not have any air pockets or you won't get the full pressure. The pipe can be quite long if it is the right dia. The pipe has to be sized for the flow. 300 to 400 gpm is a nice flow. One way to think of it is the formula W= a x v . Watts = power, Amps = flow, and volts = the head. You want to increase your head.Is the overflow of the pond running over the side of the edge of the pond? Most ponds would have a vertical pipe running into a exit pipe. This sets the height of the water level of the pond. If the over flow is running over the surface, you could  cut the edge of the pond and the pond could drain in flood conditions. It would be nice if you could by pass the whole pond, Set up a few 100 feet of 6 in pipe and get the head up to 75 ft. How far would you have from the turbine to the house? Can you give a few more details?

[jimh]

Thanks all for setting me straight on the head vs. depth thing.  As for the possibilities of increasing the head, that is possible and was Plan B.  There is a location higher up I was thinking of dredging and putting in another smaller pond.  How can you tell the elevation/head increase?  Take a guess, I guess.  Vtbsr, the turbine would be about 150-200 feet from the house.  The water flows into a 12" diameter vertical pipe in the pond.  Kinda like you described "vertical pipe running into a exit pipe".  Then flows out through the floor of the pond, through the berm, then out 50' to the road (no further drop after it exits the pond.  Just imagine a 12" pipe extended vertically from your bathtub drain.  Once water hits 12" depth, it starts to flow into the pipe.  I won't confuse you explaining it further.  I could flow over the side, I suppose.  I will post soon on what kind of head I could increase by taking it from higher up.  Thanks again.

Jim

[scottsAI]

Hello jimh,

You do not need a pond except to buffer the water flow.

Buffering for seasonal changes is common, can become a large pond.

Many use no pond.

Beware of using several smaller pipes vs one large.

Flowing Water has head loss (frictional losses) in the pipes, the faster the flow, more friction.

Assuming one large 12" pipe compared to 4 smaller having the same cross flow area.

12" has 37.7" circumference, cross area 113.1 sqin (area of a circle)

Using 4 pipes, each needs 113.1 / 4 = 28.275" cross area, circumference = 18.85", Diameter = 6"

Yes, 4 pipes half the diameter to have the same area as the larger!

Surface area is the circumference times the length. Let's use 1 foot to compare the two.

37. 7 / 12 * 1 = 3.14 sqft.
    
4. x 18.85 / 12 x 1 = 6.28 sqft or twice the surface area therefore twice the friction.
   
   

Low flows the friction does not play a major role, best to check a pipe flow chart to see if the flow will be an issue.

As others pointed out with the same flow, the higher the head the more power your going to get.

Power increase is linear, if twice as high then you have twice the power potential. How you use it is up to you!

Have fun,

Scott.

[nick1234]

Hi jimh

Your pond has only the pressure on is sides.

If you make a hole somewhere on its side water will leak out to the level of the hole (your culvert/pipe)

Then if you want to make use of the water leaking out

 You have only the power it can give from the top of your pond to the ground there the water drops to,

If the flow is greater (bigger pipe) more flow more power/volume

Now if you punch a hole in the bottom of the pond and exit to the same spot as before

 You will have the same power as from the top

 Same applies for the size of the pipe larger pipe more power/volume

What gives you more power is a greater height from the top of your pond to the point where you use it