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Lateral Pipe Sizes

Discussion in 'Irrigation' started by BrasiliaFlyer, Aug 11, 2010.

  1. BrasiliaFlyer

    BrasiliaFlyer LawnSite Member
    Messages: 3

    Hello, just a quick question for the pros on this forum...
    I am a homeowner installing my own system on an acre. I want to make the system as robust as possible. The pressurized secondary irrigation system comes through a 2" valve, and averages 55psi, but can drop into the low 40's on occasion. Because of this, I have added more zones with a lower gph requirement, and plan on running two at a time when the pressure is good, but maintain the ability to run one at a time if the pressure and volume ever drop significantly. I have read the irrigationtutorials.com site and the Rainbird design handbook completely.

    Originally, I planned on using 1-1/4 inch sch 40 laterals going down to 1" for each head. I know its probably overkill for a 13 gpm zone, but I want to minimize the pressure loss as much as possible, and allow me the flexibility to add larger nozzles if I need to. Both reference sources say that there is virtually no downside to using a slightly larger than required pipe.

    Well, then I was at the local irrigation supply house, and the guy said that you must always maintain at least 2 fps inside the pipe to scour away gunky buildup inside the pipe. Irrigationtutorials.com never mentioned this, but if true I am looking at 1" and 3/4" (even 1/2"!) laterals.

    So, the question is: has anyone ever heard of this 2 fps requirement, or is this guy drinking the same kool-aid as the guys who say that you'll lose pressure if you go to a larger pipe?

    Thank you for your time, and for contributing to this great resource!

  2. DanaMac

    DanaMac LawnSite Fanatic
    Messages: 13,184

    Never heard of that before. But we don't get a lot of gunky water unless it is from a well. I doubt drip systems are flowing at that rate, and I don't see problems with what they are describing.
  3. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Messages: 50,238

    If you got gunk, then use Maxipaw heads, which function well at lower pressures.

    You can avoid the loss of a psi or two by not using any swing pipe connections, but that's a pain unless you run poly pipe.
  4. BrasiliaFlyer

    BrasiliaFlyer LawnSite Member
    Messages: 3

    Good point!
  5. txgrassguy

    txgrassguy LawnSite Gold Member
    Messages: 3,083

    I have never heard of a minimum fps require to scour gunk from a pipe.

    Now, gunk from my ex's trunk - that requires a whole hell of a lot more than 2 fps.

    Personally, if all you are working with is 13 gpm, I'd stay with SDR 21 1" pipe for everything, lateral and main and design/install to the lowest sustained volume and pressure. The safest bet is to assume the worst and install for it.
  6. Stuttering Stan

    Stuttering Stan LawnSite Bronze Member
    Messages: 1,504

    I've never heard of 2 fps requirement. Also, larger lateral pipe not will make a noticeable difference compared to 3/4. Go for the bigger pipe.
  7. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Messages: 50,238

    put in 5-inch pipe, just to be safe :p
  8. irritation

    irritation LawnSite Gold Member
    Messages: 3,540

    5" ?
    We use 6". A lot less friction loss but it does take longer to blow out.
  9. BrasiliaFlyer

    BrasiliaFlyer LawnSite Member
    Messages: 3

    My pressure loss would be 0 with 5 inch pipe... Unfortunately the budget won't allow for it (fittings are just too much for my cheap arse).

    Anyway, thanks for the tips here. I've decided to go with 1-1/4" laterals, with 3/4" branches (to each individual head). Figure with only 2-4 gpm per head, the 3/4 will be fine, and actually save quite a bit of money.

    Seriously though, I appreciate getting the insight from the pros on this forum. Thanks for your valuable time.
  10. AI Inc

    AI Inc LawnSite Fanatic
    Messages: 26,607

    Go 1" everywhere. The guy who ends up servicing it 10 yrs from now will not hate you for it.

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