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Why PVC mainline?

Discussion in 'Irrigation' started by ddthomas2, Aug 9, 2005.

  1. ddthomas2

    ddthomas2 LawnSite Member
    Messages: 26

    Several posters were emphatic about using PVC for mainline. One popular reason was "poly is not rated for continuous pressure". Here in Louisiana, all of the utilitycompany supply pipe is poly. Possibly it is a different grade, but it certainly has continuous pressure.

    Also, I always install a master valve, so my mainline doesn't have continuous pressure anyway.

    I know many of you have been doing this longer than I have been out of school, so....... What am I missing?

  2. Planter

    Planter LawnSite Member
    from Utah
    Messages: 214

  3. DGI

    DGI LawnSite Member
    from SE Mich
    Messages: 173

    A PVC main should be more resistant to seeping and also has less friction loss after each fitting.

    Poly is fine, though.
  4. jcom

    jcom LawnSite Senior Member
    Messages: 791

    It is all a matter of preference and comfort zone, I guess. I am not comfortable having a poly line "live" but since I use a master valve, I need only have PVC where there is static pressure all the time. But that is my comfort area. Others would say I am way too conservative but I am not going to change.

    As long as I have a master valve, the dynamic pressure is in my comfort zone for the poly.

  5. turfman59

    turfman59 LawnSite Senior Member
    Messages: 801

    I have done nothing but poly mains for 5 years, no seepage anywhere, However if you get sloppy with fittings and clamps, you will be paying for it in call backs.
  6. bicmudpuppy

    bicmudpuppy LawnSite Silver Member
    Messages: 2,781

    Check the product you are installing. "regular" irrigation poly (80# or 100#) you normally see is NOT rated for continous pressure. The insert fittings w/ otiker clamps are not rated for continous pressure. There is a lot of 160# and 200# rated poly pipe out there w/ quest or similar compression fittings that is perfectly acceptable as mainline. Price that grade of pipe, and you will be back to PVC on the double quick. Price the fittings alone for use w/ 100# poly, and I still think you will want to be installing PVC mains. That's how it works out. Do guys do it the other way? Yes! Is there anything wrong with a 100# poly main just because the pipe and fittings are not rated for constant pressure....That's a call I won't make for anyone else. I fix a lot of leaks on the poly mains I'm stuck with. The justification was "the poly is after the MV". If the MV closes faster than the last valve that runs....maybe. But it doesn't work that way forever. If the zone valves close before the MV, the master valve may allow a small leak to leak indefinetly before it closes. Remember, a solenoid valve requires flow/pressure differential to close. A system that works fine for the first few years will eventually become a good repair tech's nightmare in 5-10 years, and/or the customers money pit.
  7. turfman59

    turfman59 LawnSite Senior Member
    Messages: 801

    So if you were using a pump sytem out of a lake poly would be an acceptable choice, being that the only time the poly is under pressure is when the pump is running. or are you still calling that static pressure because there are still valves closed in the system and water pushing against them?
  8. Dirty Water

    Dirty Water LawnSite Fanatic
    Messages: 6,794

    A properly set up pump would have a check valve to prevent it from draining back into the lake.

    So yes, the static pressure would be considered .
  9. bicmudpuppy

    bicmudpuppy LawnSite Silver Member
    Messages: 2,781

    BINGO, and the mainline would still drain to any leaks that presented themselves.
  10. turfman59

    turfman59 LawnSite Senior Member
    Messages: 801

    Actually it would have two. One to keep the prime in the suction side and one on the discharge side to prevent recoiling through the pump after it shuts off, ( possibly blowing the screen off )

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