Combining 2 different pressure penstock lines

[Mountain Clan]

I have 2 springs I would like to combine the flow of into one penstock pipe.

A lower and a higher (on the hillside) spring. The lower spring has 3-4 times the flow of the upper spring.
The higher spring is 40' above (vertical head) the lower spring, and the 2 springs are more than 350' apart horizontally.
If I combine the flows below the lower spring, then will the pressure in the penstock below that point balance to the pressure of the lower spring where it is combined?
Will the flow of the lower spring reduce at all because of mixing with the higher pressure of the upper spring ?

If I think about the interaction of the different pressure flows simply, then I guess that the pressure of the upper spring will go up the lower spring pipe from the combination point, until it reduces and balances with the lower spring pressure.

If you know about combining penstock pipes, please do let me know what you think.

[richhagen]

I am no expert on pen stocks, but water will only flow from higher pressure to lower pressure.  While it is possible to use the energy in some of the water to power a pump to pump a small portion of the water uphill (or to higher pressure), it costs in efficiency if you are only planning to capture energy from that higher pressure water relative to just getting what you can out of it at lower pressure.   In your case, it would seem to me that likely the highest yielding energy capture would involve separate generators on each flow.  Hopefully someone with a bit more experience with a similar problem will chime in here.  Rich

[SparWeb]

Penstocks are not in my personal experience, but what you're describing sounds possible.  It reminds of me of the "ejector effect".  A high pressure flow is directed through a nozzle that drives a low-pressure high-volume flow.  IIRC you can do it in a way that allows them to mutually reinforce the flow, but if you don't take care of some details they will interfere with each other instead.  See what you can find on the ejector effect and how to make it work for you... 

[bigrockcandymountain]

Or just have a combination wheel hooked to the same generator.  The high pressure will be larger diameter smaller cups.  Low pressire will be a smaller diameter wheel with larger cups. 

[mab]

If I combine the flows below the lower spring, then will the pressure in the penstock below that point balance to the pressure of the lower spring where it is combined?

yes

Will the flow of the lower spring reduce at all because of mixing with the higher pressure of the upper spring ?

probably - unless the draw at the bottom of the penstock is matched to the combined flows from both springs

If I think about the interaction of the different pressure flows simply, then I guess that the pressure of the upper spring will go up the lower spring pipe from the combination point, until it reduces and balances with the lower spring pressure.

yes

If you know about combining penstock pipes, please do let me know what you think.

I've sort of done this unintentionally: my penstock had a single collection point where the two streams came together - just a simple strainer. I then installed a coanda screen about 2m (vertically) above the original strainer on one leg - but have not yet installed the coanda screen on the other leg (to be installed at the same height).

What happens in practice ATM is that the water from the coanda screen 'chutes' down the pipe to the lower strainer and comes out through the strainer (actually carries air down under the water with it and blows bubbles out through the strainer). The pelton wheel 'sees' the same pressure as it would see without the coanda screen collection point and will do so until I've installed the 2nd screen and removed the original strainer.

best answers are (as bigrockcandymountain said) two penstocks and two wheels, or (as richhagen said) two penstocks and two generators IMHO

[Mountain Clan]

Thanks for the answers and info everyone.

In regards to the Ejector effect, I had thought that something like this would be needed to combine the two flows/pressures with the least interference, and am likely to experiment with this and an adjustable nozzle/jet to achieve the best mix. Possibly an 'injector' style mixer would work best for my needs.

I would rather not run two separate penstocks down the hill (all the way down) as the current flow from the upper spring is only 2-3 gal/min and likely not worth the cost of all the extra pipe.

I do have two turbines to work with here and could setup two separate systems. I think that the 2 gal/min flow is just too little to generate enough rpm on the turbine wheel, even though it would have 240' of head.

[bigrockcandymountain]

The nose off of a convertible jet pump would maybe have exactly what you need.  Similar pressures and flows to what you are looking at.  They have an ejector nozzle and the proper venturi to make it work. 

What is the head and flow of the lower spring?

[Mountain Clan]

The nose off of a convertible jet pump would maybe have exactly what you need.  Similar pressures and flows to what you are looking at.  They have an ejector nozzle and the proper venturi to make it work. 

What is the head and flow of the lower spring?

The lower spring is 8 gal/min (might be 10gal/min as I am adding a few small springs together here) and the head will be 195' once I have the new pipe in place.

[SparWeb]

OK.  I'm still optimistic that the ejector would help, and would save a second run of pipe.
Best guess is that the 190' source at 8 GPM will offer 250 watts, and the 240' source at 2 GPM offers less than 100W.
Bearing in mind that no matter how we combine the flows, they won't make more than 350, there are things we can do to get as much of it to your turbine as possible. 

My suggestion is to combine the flows by making (or buying, if you can find such a thing!) a section in the large flow tube that has a smaller tube entering it at a very shallow angle like a "y".  The small tube would enter not just like a simple T at right angles, but give it an angle so the flows are almost going the same way before the combine.  Also, the smaller tube would continue inside the larger tube for some distance, straightening out and running through the center of the larger tube.  The mouth of the smaller tube then opens in the middle of the flow of the larger tube, finally mixing the flow.  Make the end of the tube have sharp edges, or maybe cut at an angle, so that there's no blunt end to slow the flow down.

I probably need a drawing.  My explanations never seem clear when I re-read them.  I'm trying to google up a good photo but they all have venturis and nozzles and that's not what you need.  All this takes is a gentle introduction of the 2 flow together with the least turbulence added.

[SparWeb]

These wouldn't be bad, but not exactly what I was describing:

[Mountain Clan]

My suggestion is to combine the flows by making (or buying, if you can find such a thing!) a section in the large flow tube that has a smaller tube entering it at a very shallow angle like a "y".  The small tube would enter not just like a simple T at right angles, but give it an angle so the flows are almost going the same way before the combine.  Also, the smaller tube would continue inside the larger tube for some distance, straightening out and running through the center of the larger tube.  The mouth of the smaller tube then opens in the middle of the flow of the larger tube, finally mixing the flow.  Make the end of the tube have sharp edges, or maybe cut at an angle, so that there's no blunt end to slow the flow down.

I probably need a drawing.  My explanations never seem clear when I re-read them.  I'm trying to google up a good photo but they all have venturis and nozzles and that's not what you need.  All this takes is a gentle introduction of the 2 flow together with the least turbulence added.

It is likely that the power generation will be right around what you guess.
We are a low-medium power using family so we will make due with whatever we can get from the hydro system and be happy with it.
 
I understand well what you are describing. It is along the lines of what I imagined I would need if combining the flows would work out. I was considering the simple idea of a venturi-like jet to mix them. After considering it further and reviewing the Ejector diagrams, I expect that a larger and longer mixing chamber will be needed.

If I try your suggestion, then I would make a spacer for the middle of the larger flow section to hold the smaller pipe in the middle of the flow. A spacer with lots of room for the larger flow to get by and around it.

The plumbing piece you referenced is a long street ell or bend. Although they do make them with a longer ell, I think that I might use a sealed compression fitting (maybe a saddle) for the smaller pipe where it will enter the the larger pipe.

[SparWeb]

If I try your suggestion, then I would make a spacer for the middle of the larger flow section to hold the smaller pipe in the middle of the flow. A spacer with lots of room for the larger flow to get by and around it.

Perfect

Head loss through sharp elbows is also an issue, so gentle curves make a difference.  They also affect the pocketbook, so keeping the total number of bends down is really important.
I'm not sure if you can find saddle fittings with angles other than 90 or 45 degrees, but if you can, it will help a lot.

Do you have to deal with freeze/thaw?
A saddle fitting could be split off the pipe by ice expansion, depending on how it's attached and sealed.

[Mountain Clan]

If I try your suggestion, then I would make a spacer for the middle of the larger flow section to hold the smaller pipe in the middle of the flow. A spacer with lots of room for the larger flow to get by and around it.

Perfect


Do you have to deal with freeze/thaw?
A saddle fitting could be split off the pipe by ice expansion, depending on how it's attached and sealed.

Hardly ever here. We may get to 28-32 degrees on a clear night when the inland weather is blowing in, but the Ocean usually keeps us in the 40-50 degree range throughout Winter.
As this system is new, I have not buried the penstock yet as I wanted to test it and be satisfied before making the effort of burying 540' of pipe...by hand.

I am glad I waited, as the way I am handling the water from the springs, and the output I expected from the turbine, are changing from my original guesses about the flow and output information from this turbine manufacturer.

[SparWeb]

Colorado's pretty far from the ocean, but I'll take you at your word.

[Mountain Clan]

You are right, and I had not looked at my profile since a long time ago.
Thanks for the nudge, I have updated it so its less confusing.

[skid]

The best way would be to dump the flow from the higher spring into the waters of the downhill stream. An energy dissipater may be required depending on the elevation change. I've done this professionally on a 30MW Pelton.

[Mountain Clan]

The best way would be to dump the flow from the higher spring into the waters of the downhill stream. An energy dissipater may be required depending on the elevation change. I've done this professionally on a 30MW Pelton.

When it comes to simplicity, I agree. Dumping the higher spring into the lower one is the easiest and least costly method.

I need to come up with a creative and fun way to use up the energy in the water coming from the upper spring, so I dont feel bad about losing the extra psi.

...Hot tub frother, ....Neptune and Nymphs water sculpture, ...mini vertical waterwheel

[SparWeb]

 

[casrwm]

Hi, I don't know if you are still interested...it's a while since the last post.

Basically if you don't want two separate systems you have two options:
(a) if it is feasible to run water from the top spring to the lower one, do that.  Have a small turbine at that point to extract energy from the difference in head between the two springs.  The water coming out of that turbine would then mix with the lower spring water and continue down to your main turbine at the bottom.   If the pipe lengths are reasonable this could be your most efficient option.

(b) Alternatively, mix the flows using something like an ejector pump (as mentioned in a previous post).  At the mixing point, the flows will have the same static pressure but different total (stagnation) pressures (look at Bernoulli's equation) and therefore different velocities.  There will be some energy loss when the two flows moving at different speeds mix together.   You have some control over this static pressure because you can pick the pipe diameter at the mixing point.

[Mountain Clan]

Thanks for that information.
Right now, I am combining the 2 flows at the source of the lower spring and losing the energy from about 45' head from the upper spring. It is the method that I know I can count on the results of, for the effort I put into it.

In the upcoming Winter, when our heavy rains return, I will experiment with a directional flow mixing unit further down from the lower spring source and see how that goes. This seems a somewhat simple process and I am hoping not to stress about it, and overthink the dynamics and engineering.

Again, thanks for the response.