Microhydro planning, 1200 ft transmission cable

[sailawayrb]

Sure, sounds pretty simple.   Or is it?  

I'd need to grease up a cap (make it watertight and able to slide off easily) and apply the appropriate weight.   For a 2" pipe, the area is 3.1415 sq inches.   (pi * r^2)   So, I'd have to balance 314 lbs on top of a vertically oriented capped 2" pipe to counteract 100 psi.    

Unless you tee off to 1" standpipe...then you would only need 78.5 lbs...  

Seems like something that would just fail open (and that could be cheaply replaced) might be the simplest solution.  Like tee off to a closed section of threaded plastic pipe that would rupture at the desired pressure (perhaps use lathe to cut a groove to the required wall thickness).  You could also drill a very small hole in the cap to bleed the air (so the rupture occurs very fast without the delay of having to first compress a pocket of air) and this very small hole would not waste any significant energy/water.

Unless something unusual occurs, you should never encounter this failure...similar situation for having circuit breakers and fuses...

If you try something like this, be sure to use pressurized water (and NOT air) to determine/test the actual rupture pressure point to avoid injury!

Bob B.

[mike90045]

re water hammer.

I'm looking at a 4" siphon pipe to use in my pond, with a 50' drop, to a turbine.  Since it would run from pond storage, and could drain it down between rainstorms (a 4" leak is a big deal on a 2 acre foot pond), I was thinking of an electric butterfly valve, takes about 30 seconds to close, would slow the water down, and enable me to easily conserve water on sunny days between storms.

[sailawayrb]

Hi Mike,

The empirical water hammer pressure spike equation is P = 0.07VL/T where:

P is pressure in PSI
V is maximum velocity of water in pipe in FPS
L is length of pipe in feet
T is valve closure time in seconds

It is left to the astute student to calculate V knowing the pipe diameter and flow rate...  With a 30 second closure time, you will not induce any significant water hammer pressure spike.

The theoretical water hammer pressure spike equation is more interesting in that the wave frequency is also a function of the pipe length since the pressure wave travels at the water speed of sound.  Both these equations can be found in the hydro ram pump Excel spreadsheet I posted.

Bob B.

[mbryner]

For extensive calcs and description of water hammer as it relates to penstock design see:

http://www.microhydropower.net/download/layman2.pdf

Specifically page 41-42.

(Don't know why the link has "layman" in it, because this book is anything but a layman's book unless it means you are going to fall asleep reading it.)

[mike90045]

For extensive calcs and description of water hammer as it relates to penstock design see:

http://www.microhydropower.net/download/layman2.pdf

Specifically page 41-42.

(Don't know why the link has "layman" in it, because this book is anything but a layman's book unless it means you are going to fall asleep reading it.)

Sweet link, thanks

[sailawayrb]

That's a good resource for info and I have used it in the past too.  Looking forward to hearing what your consultant recommends.

Bob B.

[mbryner]

Well, after meeting w/ Jerry O (from apmhydro.com) I'm reassured that my plans will work well.   Max flow of 45 gpm through 2" poly pipe.   He's going to wait until I install the penstock so we know the exact head height before he completes the turbine (custom stator).   We're planning for about 170 V DC (rectified 3 phase), using the first wiring schematic I posted.   I've ordered 1500' of 2' polyethylene in 300' rolls.   It's utility grade -- $0.84/ft, not cheap, but only rated to 80 psi.   The higher grade is rated to 100 psi.   That psi rating is marginal for my use, so I'll probably use PVC for the lower 50' of head.   He did say that some installs do include a pressure release valve.   I'll give an update when I get the pipe and can snake it down the hill and check the pressure.

Marcus

[sailawayrb]

Cool Marcus...this is getting very exciting!

Agreed, rectified 3-phase is good approach and assume you get it via some PMG.

Hey, if you have 250 feet of head and are planning on using 80 PSI rated PE tube for the upper penstock, you can only handle the first 186 feet of head with this tube and you will need something else for the last 64 feet of head...and you may want to have some design margin like not letting the PE tube see more than 60 PSI (which means the PE tube only handles the first 140 feet of head and something else handles the last 110 feet of head).  Or maybe go with 100 PSI tube and design so as to not let PE tube see more than 80 PSI and use 187/64 approach?

Did you check the PE tube price at Grover in Medford?  Seems like I got my 2" PE, which was used for my hydro ram pump, for much less.

Bob B.

[mbryner]

Bob, 

Those prices are from Grover (Grants Pass & Medford would have the same price).   I'm going to decrease the head to about 200', and I'll transition to 100' psi pipe for the lower section, and probably PVC for the bottom.   Yes, it is from a custom PMG head (APM hydro builds pretty efficient, solid PMG gen heads from what I've seen in their shop).   It's still raining here, so I can't even drive the tractor up the hill to start building a diversion point, yet.

Marcus

[Tom Reed]

Sounds like an awesome project Marcus. I had similar plans here, but since living off-grid for 4 years now we really don't need the extra power. Also my water source looked good in '05 when we had loads of rain. This year there has been almost no runoff.

One source for cheap pent-stock pipe are well drillers. I've got a friend who is a registered water system operator that has given me 700' of 3" iron pipe and another 700' of 2 1/2" pvc. Seems if a pump has issues and needs to be pulled, they don't always want to reuse the pipe. The pvc pipe is interesting in that it has threaded couplings. Never seen that before.

Another option if you want to stay all poly is to use water service line which is a heavier wall rated for around 160 psi.

[sailawayrb]

Good deal Marcus, Grover had best prices on local PE when I purchased last summer.  I also really like the stuff they have on-hand in general and the knowledgeable service they always provide.  If I ever get my hydro electric power plant approved, I will need to check with APM about PMG.  I already designed what I want for PMG, but it might be better to buy all things considered if they do things right and are local too.  My penstock will be 18" diameter but only 40 feet long...

It was snowing here last night and just the normal Seattle rain now.

Bob B.

[mbryner]

Thanks for the tip about well-drillers.  Hadn't thought of that! 

[XYZER]

The pvc pipe is interesting in that it has threaded couplings. Never seen that before.

Schedule 80 for hanging all that weight I suppose......plus you don't have to cut it as you pull it.

[BruceM]

The well drillers have switched to schedule 100 PVC for new installs here with the deeper wells.  They continue to use standard pipe wrenches on the PVC, and they were getting a lot of perpendicular hairline cracks in the schedule 80 pipe, near the fittings...du oh.  These micro cracks don't show and don't leak until the pressure is high enough.  They have been very annoying for customers (me).

The schedule 100 has an even heavier wall- perhaps that might suit, somewhere.

[Tom Reed]

The pipe I have says Sched 120 on it, but the wall thickness looks less that sched 40. I got a bunch of nice brass check valves with both sets of pipe too. Don't know what I'll do with them though.