two streams for hydro!

Hey, I'm in a pretty unique position where I actually have two streams on my property which are suitable for microhydro. Who would have thought this could pose a problem!!?

On the North stream we have about 70m head and currently have a small turbine generating 400w. We'd like to add the West stream (about 60m head) to the turbine (which can handle it) but there is only one jet input on the turbine. Adding a second jet is not an option. So we though we would combine the two pipes to essentially double the flow from 2 to 4 litres/second.

The question is, if the pressures are different (about 10psi - that is 90psi North stream and 80psi West stream) will this mean the pressures will try to equalise with flow from from one pipe pushing against the other and causing efficiency loss? To what extent?

I hope a hydro expert chimes in but in lieu of that, I'll give you my somewhat informed thoughts.

I'd plumb both water sources together before the turbine inlet in a Y with valves on each.  Then you can see how it works with one, than the other, then both.  Hopefully it would become apparent which is the best option.  I think adding the 80 psi flow would help.  If the turbine was shut down the water would flow from the 90 psi pipe up the 80 psi pipe.  But if the turbine is spinning, I am assuming that the path of least resistance for both flows of water is to go through the turbine.  I would guess that the pressures would equalize at the Y and you'd have 4 l/m of 85 psi water to run the turbine.  I would put a straight section between the Y and the turbine so that any turbulence from the confluence of the streams is reduced.  I'd make that straight section 10-15 times as long as the diameter of the pipe.

But this is all conjecture on my part based on a tiny bit of engineering experience and nothing "hands on".  Good luck!

I think your answer depends on whether the inlet to the turbine can take additional flow. If it can take additional flow, you will increase your power. If the diameter of the inlet is already saturated, you will not. As far as pressure, connecting those two pipes will result in 80psi (the lower of the two). I like to think about it this way: if you were to block the entrance to the turbine, you'd be redirecting the 80m head stream into the 70m head stream at a pressure of 10m head. Water is always going to flow out at the lowest point.

Thank for the quick replies. I'm upgrading the turbine so it will be capable of extra water input. Ideally I would upgrade to two jets but the turbines I'm interested in are only available in single jet below 3kw, and I'm looking at only 1-2kw. Which is why I'll be wanting to combine the two streams into one input.

So summing up Mike, you think 85 psi is the likely outcome.

Kyle, I'm not sure I completely grasp the ramifications of your answer; I understand the thinking (I think) but the 80psi - will that flow nicely at the output of the Y junction or will it be a mess of inefficient turbulent water hitting the jet as some tries to back up the lower pressure pipe? I guess I'm asking if efficiency will take a major hit? Can I expect almost a double of potential power by doubling the pipes or will the pressure difference (backflow etc) cause a major hit for efficiency?

Thanks guys

I would agree with Kyle. If the new stream pushes at 80 psi, the downstream combined pipe has to have less than 80 psi or the new stream will stall or flow backward. you will have greater flow but lower pressure (90 -> 80 psi) when combined; hopefully this will net to more output.

Make the combined pipe large enough that it flows at the same speed, to reduce turbulence, and give it enough length for the flow to smooth out.

Yes Muzzer, that's what I thought.  But now that these other wise gentlemen have spoken I agree that the final pressure will be 80 or lower.  

Muzzer: I think the key is that water systems are all one system, regardless of how they're connected. The instant the valve is opened, the water will attempt to find level however it can. If the turbine wasn't connected, the 70m head creek would definitely flow back up into the 60m head creek.

But the turbine is connected, which is why I'm not sure how to answer your efficiency question entirely. The turbine as it stands now is not really pushing back with 90psi of backpressure because it's allowing water through. Think about adding sprinklers on a hose… each one you add reduces the pressure of the others. The turbine is reducing the pressure by letting water through (that reduction in pressure is where your energy comes from). 70m is your static head (inlet closed), but your dynamic head (friction, joints, turbine) is probably a lot lower. So long as that dynamic head is 60m or less, the real pressure in the turbine should remain roughly the same, but with more flow.

Since the total head is so much higher than the difference (10/70), my gut says that the real pressure would remain the same at the turbine. But I'm not an expert! Just a guy who had to take an awful lot of fluid dynamics in school I think the far more important part is designing that junction and increase in pipe size without introducing turbulence.

Muzzer Ward wrote:
The question is, if the pressures are different (about 10psi - that is 90psi North stream and 80psi West stream) will this mean the pressures will try to equalise with flow from from one pipe pushing against the other and causing efficiency loss? To what extent?

Is that static or dynamic pressure?  If it's static, then the one with the longer pipe will likely lose some head.  At any rate, it seems to me with pressure that close they will balance out with the lower pressure feed producing less flow than the higher pressure, but still more flow combined than either of them alone.

Thanks for all the awesome insights guys. I've got the second pipe installed and put pressure gauges on both so now know with more accuracy the pressures/heads.

North stream static pressure is 113psi, so the head calculates out at 79m. Dynamic pressure with a 1.5l/s flow generating 400w is 100psi.

West stream static pressure is 99psi, so a head of 69m.

The next job is to get a y-junction and combine the two pipes, add another pressure gauge and test the flow.

I'm thinking for the two 50mm pipes an 80mm diameter junction should be good. Also, 80mm diameter seems to be a common input for a lot of hydros on the market. It'll be interesting if I can even get such a junction locally, probably not something often used.

I'll post the results here for those interested.