First system / Design a challenge

Discussion in 'Irrigation' started by Ground Master, Mar 11, 2003.

  1. Ground Master

    Ground Master LawnSite Senior Member
    Messages: 505

    I design most systems at 10 to 11 gpm in my area (3/4" main and 5/8" meter). Today I'm looking at my first design of the season. It too has a 3/4" main (type k copper) and a 5/8" meter.

    The only diference is this house sits way back off the street (flag lot). The 3/4" copper main from the street to the house measures 200'. The pressure is way too high, 120 psi (will have to be regulated down to 70).

    I figure I can only pull 7 to 8 gpm out of this to keep pressure loss low and velocity safe. The entire yard will be rotors with a max of 3 per zone.

    Have any of you had to design a system with such a low design gpm?
  2. DanaMac

    DanaMac LawnSite Fanatic
    Messages: 13,226

    Oh yeah. GM I've done quite a few systems in the older parts of town that still have the old galvanized service line to the house. Inside of the pipe has gotten corroded and GPM are limited. But I still designed them at 7-8 and they work fine.

    I've been away from the site for awhile. I'll try to post to more questions now.
  3. MikeK

    MikeK LawnSite Member
    Messages: 145

    Although I do not install Hunter, here is a real handy tool to caculate the amount of pressure you will have.

    Or, hook up a flowmeter to the outside spigot and measure.

    If you could regulate the pressure to 90 PSI Static, you will have around 10 gpm with around 40 PSI at the head. Not too bad.

    Depending on the situation, you could put 4 or 5 rotors on a zone.
    5 Ful circles with 2.0 GPM nozzles ( just run them longer) or 3 rotors with 2.0 nozzles ( running 180 degrees and 2 with 1.0 nozzles ( for the 90 degree corner.

    However, you would probably want to design for about 8 GPM max since pressure and flow will most certinally drop over the years
  4. greenworldh20

    greenworldh20 LawnSite Senior Member
    Messages: 659

    i agree with should try to get your cic status. they teach you how to calculate for pressure loss, friction loss, how to size pipe....and other fun stuff like that. be sure to size all nozzles for matched precipation rates. also, since pressure is high, making sure you size nozzles right will prevent excessive misting of heads while operating.

    you should be fine putting 4 rotors on a zone. most homes in my immediate area have a maxium of 9 gpm @ 40-50psi.

    have fun:p
  5. HBFOXJr

    HBFOXJr LawnSite Bronze Member
    Messages: 1,712

    Don't sweat it. My handy Palm program says 200 ft K copper @ 10 gpm costs 28.67 psi. Meaning you will still have to regulate.
  6. Ground Master

    Ground Master LawnSite Senior Member
    Messages: 505

    Thanks for all your replies

    Dana- have done a few of those older galvy service lines. Hows that pond retention project of yours going?

    Mike- I do have the hunter program your talking about. 80 psi is all we are allowed on a PVB.

    Greenworld- what is cic status?

    Harold, I came up with your figure of 28 psi loss and another figure of 33 psi loss (depending on which chart i was looking at). I'm shooting for 8 gpm so I can reduce the mainline loss to 19 psi.

    The design came in at 26 rotor heads on 7 zones, 7.8 gpm being the largest zone. I'll set the pressure regulator at 70 to 75 psi so that i can get at least 30 at the heads.
  7. MikeK

    MikeK LawnSite Member
    Messages: 145

    CIC= Certified Irrigation Contractor. It's a Certification administered by the Irrigation Association.

    Harold, Where can I get a copy of the palm program you use?

    The Febco 765 is rated for 150 PSI. Why do you have to stay under 80? Is it a local code thing?
  8. Ground Master

    Ground Master LawnSite Senior Member
    Messages: 505

    Mike- yes 80 psi is a local code thing.

    There are many components rated at 150 psi and up. You would have some serious velocity and water hammer issues if you used that kind of pressure.

    How many of you guys are CIC's? Perhaps I'll take the test for grins.
  9. SamIV

    SamIV LawnSite Senior Member
    Messages: 324

    Why not just run a larger mainline from the front of the property. Seems like this would cure your problem. Are there reasons why you can't?

  10. HBFOXJr

    HBFOXJr LawnSite Bronze Member
    Messages: 1,712

    These can be controllable by design without regulation. Velocity is determined by flow (gpm) in a given diameter pipe. If velocity is kept to less than 5 to 5.5 ft per second, with slow closing irrigation valves, hammer is not an issue.

    But you must make your calculations carefully and completely. If you use flow of a nozzle based on 30psi and the actual working pressure at the nozzle is 60 psi then your flow and velocity will be greater and water hammer can occur.

    You must first determine the desired operating pressure at the nozzle to know what the flow will be. Once flow is known, loss calulations can be made for all components from the source to the regulator and the regulator through to the heads. Any pressure difference left over msut be taken care of by the regulator and you will know what outbound pressure to use.

    Example if you only need 90 psi to operate the system, meaning all losses in components plus desired pressure at the nozzle and the static pressure is 120psi, then by gauge the the out bound pressure must be 30 psi than the inbound pressure at the regulator.

Share This Page