PVC vs. galvanized pipe for irrigation valve risers

[Drew]

Howdy folks,

I'm planning an irrigation system for our small farm that involves a 1" PVC main with about 7 valves coming off of it.  It's all going to be in one straight line (Except for the hill in the beginning), so it looks like this:

(Hill)\____I_____I_____I_____I_____I_____I_____I

I plan to use 3/4" risers and brass bent nose valves, but would like to know if I should use galvanized pipe or if I could stay with schedule 40 PVC.  The PVC would be easier for me to work with (I am no plumber and have no pipe threading tools), but I am worried about it deteriorating in the sun.  Each valve/riser assembly will be clamped to a short PT post, so I am not worried about driving or tripping over them.

Do I want to go through the trouble and expense of galvanized pipe for the valves or would PVC be sufficient?

Thanks!

Drew

[MikeC]

Drew -
We use sch 80 pvc attached to T-posts for risers, and the black coil pvc aboveground for mainlines.  This withstands horses trampling goats gnawing,  and freezing without a problem. 
I'm gonna guess though that the 7 risers will flow much more than your mainline resulting in mere dribbles at the end risers or very limited output throughout.  If you intend to use all valves at the same time a larger mainline might be best, and/or a loop arrangement so there is no dead end riser.  Also, prevailing wind patterns can cause very uneven water distribution upwind/downwind.  Sprinkler mfrs usually have a lot of sizing information available, usable once you know your water pressure & flow - rainbird .com is one.  Oh, & don't use cheap sprinklers - brass impact is very durable.

[Drew]

Thanks Mike.  This system is going to be for drip irrigation with gravity feeding it.  The tank is ~80' above the field.  I can get > 25 psi before the pressure regulator, and I plan to run one line at a time.

This is for a family operation with some scalability in the design should we decide to do more than limited market gardening.  The whole farm is 20 acres and the growing field this will serve is about 5 acres (Though it will take us some time to get out that far).

"...the 7 risers will flow much more than your mainline resulting in mere dribbles at the end risers or very limited output throughout." - I admit I am hoping that the friction in the line won't be too bad at the end of the 750' main line (Again, 80' down from the tank), even if I have only one valve open.  Is there a table or rule of thumb I can follow?  Also, am I going to lose significant pressure by adding riser/valve units to the line, even if they're closed?  Just so I'm making sense, I'm calling the riser/valve unit the piece of pipe and valve that comes off the main line from the tee connection.

Black coil PVC above ground, huh?  Don't tell my family!  We've been trenching to bury rigid pipe.  We're having a good time, though. 

[glenn kangiser]

I had exposed PVC risers and manifold at our other place.  It lasted around 25 years before UV made it brittle and it broke.  To stop UV  damage, paint it your favorite decorator color.  Should be no problem with the post -- I do it all the time.  Easy to fix if you destroy one,

[MikeC]

Oh, drip irrigation - I was assuming sprinklers for some reason.  Here is a pressure drop calculator:

http://irrigationtutorials.com/faq/psi-loss-calculator.htm


I think you'll need to know the total flow rate for the drip heads though. 
Guessing that your family has volunteered to hand trench for the lines?