PSI rating of pipe for microhydro

[mojisimo]

I am installing a microhydro project in the mountains of Western North Carolina. The site has 140 ft of head and is a pipe run of 1300 ft. I expect 80-150 GPM of flow. I am using 4" polyethylene pipe. I am trying to decide whether to use 100 psi pipe or 160 psi pipe. I understand that the 100 psi pipe could handle 231 ft of head and I only have about 60% of that with 140 ft. What I am concerned about is the strength of the pipe in cold weather conditions when a limb or tree falls on it and also just about its general wear over time. Does anyone have any experience or insight about this?

Thank you very much

Mike Dooraghi

[pyrocasto]

Western north carolina eh? What part? I live in Hendersonville(asheville) so probably not far from here. ;-)

I'm not sure how strong each would be to a cold tree fall, but you can always cut a small piece out and slice a new one in if something does happen. Price may be a big factor as well, but dont know.

[Ungrounded Lightning Rod]

The margin seems too low with the 100 PSI plumbing when you take into account water hammer when you turn off a valve to stop the flow.

I'd use the 140 stuff AND put a pressure relief valve upstream of the (second of two) cutoff valve(s) at the low end.  Then I'd only shut off a live flow with the downstream valve - and be VERY CAREFUL to shut the upstream valve last - and VERY SLOWLY - if I ever have to cut off water to do maintainence on a leaky relief valve.

[strider3700]

I'd worry about the water hammer as well.  my 120 feet of head gives me 50 psi through a 1" line.  turn off the flow and it spikes up to almost 70 psi.  I can only imagine a 4" as hitting it way harder.

[hydrosun]

I put in a 2000 foot length of non pressure rated pvc sewer pipe over 12 years ago for my hydro. It has 50 psi at the hydro. I buried it shallow to protect it. I  made changes at the end over the years and once had the reducer fitting pop off the end after a change before the glue set up. I've never had any problem with the pipe itself from water hammer. I always try to close the valve slowly but have never seen the pressure guage go over 70 psi with a fast closing of a valve. This pipe is thin wall and probably would only have a  60 psi rating. I'd never use it for a customer but it's held up well for me. I now use heavier gasketed  4 inch sewer pipe for  other people. One system has 109 psi. I just make sure the pipe has thrust blocks at any radical turns.  One system a friend put in has 90 psi with cheap 2 inch pvc pipe. It didn't have any thrust  block and moved several feet over the last 15 years  because of water hammer. I think any pipe is vulnerable to smashing if a tree falls on it. If possible I bury everything . That protects against freezing and trees. Even just a shallow hand dug trench will make a difference.  Also if buried the weight of the ground will help alittle to hold the pipe from expanding. If you are woried about pressure use the heavier more expensive  pipe for the bottom section where the pressure is greater.  Of course the pressure at the top is zilch. Be sure to have a tee and standpipe after the intake to let in air. Nothing destroys a pipe like a vacuum caused by the water going out the bottom with the intake blocked. It implodes, I know by hard experience, having to replace 20 feet at the top of mine. Good luck.

Chris

[Vtbsr]

Hi Chris, I had a question about the sewer pipe you use. We have a green color pipe called sr 35 that has a rubber gasket. You grease the smaller end and install up to the mark. Is that the pipe you are using. I didn't think it was rated for pressure, Maybe sch 40, 4 inch with the glued couplings would be good for the high pressure end of the penstock.

[GaryGary]

Hi,

You might find some useful info here:

http://www.endot.com/support/installation.asp

The 100 psi is a working pressure rating, and there is quite a bit of margin over that for water hammer etc.  I belive they say at the link that they test each batch to 2.5 times working pressure.

I'm not sure how you would evaluate resistance to tree falls and the like -- maybe get a short section of each and see what it takes to bend/break them?

If the pipe is going to be exposed on the ground, then I would be bit concerned that where it passes through areas in direct sun that it would heat up some.  The pressure rating they provide is at 73F, and it drops off as you go up.  This won't be a problem when the water is flowing, since the water will cool the pipe, but if the flow stops and the pipe stays full, things could warm up, and it would still be under pressure.   There is a temperature vs pressure chart in one of the docs at the link.  Allowing a little bit of water to run all the time would keep it cool.  The sun will also shorten the life of the pipe even if it has the UV stabalizers and carbon black pigment.

Gary