minimum psi/gpm and remedies?

Discussion in 'Irrigation' started by terracare, Jul 20, 2006.

  1. terracare

    terracare LawnSite Senior Member
    Posts: 353

    Just wondering what the minimum psi and gpm you work with and what can be done to boost them up to workable levels. Thanks :)
  2. bumper

    bumper LawnSite Member
    from So. Cal
    Posts: 187

    The minimum thus far....12 GPM and 25 PSI...however that is rare..generally its expected to have 15-18 GPM and 60-70 PSI for residential.
  3. hosejockey2002

    hosejockey2002 LawnSite Bronze Member
    Posts: 1,195

    I suppose, in theory, that the minimum GPM is what one head requires. The lower your GPM, the more zones you need because fewer heads can run at one time. I'm definitely not the expert on this forum, but I beleive most heads need around 20 PSI at the head to perform properly. This is not static pressure, but the residual pressure at the head while it is running. You will lose pressure through the backflow preventer and friction loss in the pipe and fittings you use.
  4. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 47,720

    Make that "practical" minimum 35 psi, for gear drive heads, anyway. Sprinkler heads don't often spray further, in feet, than their (at the base of the head) pressure, in psi.
  5. PurpHaze

    PurpHaze LawnSite Fanatic
    Posts: 5,496

    Your GPM is a given known at your POC. PSI can be boosted or restricted to your heart's content depending on what you are attempting to accomplish and there are different ways to do this.

    We run between 35-100 PSI and 10-200 GPM. It all depends on the system and particular zone. :)
  6. terracare

    terracare LawnSite Senior Member
    Posts: 353

    Thanks for the responses guys. What about correcting low psi/gpm problems on both a well system and a city water system?

  7. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 47,720

    It's all a matter of money. Once you know what usable flow you have, you run the numbers to calculate how many hours are needed to cover the lawn. If the numbers work out like one example I had, of 37 hours of watering a day, it's fairly obvious an upgrade is needed. If you have the flow, but the pressure is less than optimal, then look for low-pressure-capable heads, and see what it will cost you to design with them.

    How you deal will vary, depending on the job site. Some locales with city water may allow a new oversided water-main tap and meter. That gets you the flow you need.

    Sometimes, a booster pump can be used to raise operating pressure. You might have it just for the sprinkler system, or you might boost pressure for the entire home.
  8. JimLewis

    JimLewis LawnSite Fanatic
    Posts: 6,841

    It depends on whether you are working with a new install or trying to fix an existing system.

    When we find an existing system that has deficiencies pressure or flow and 1 or more zones have spray heads that aren't popping up all the way, there are a few things we'll consider doing, in this order;

    * Pressure Compensating Screens. Rainbird makes these and they lower the pressure right at the head. Right at the screen inside the nozzle, to be exact. Often times, just retrofitting a zone with these screens will make a zone work right.

    * Switch the heads to Rainbird PRS heads. And switch nozzles to U-Series nozzles. This lowers both the water usage and pressure, right at the head.

    * Switch all heads in the zone to MP Rotators. This will almost always fix the problem. But is more expensive than the first two options. And now MPRs come in so many different radius sizes. They even come in side strips and end strips now. You can retrofit almost any zone with MPRs and solve all of your pressure / flow problems.

    * Split the zone into two separate zones. This is the most expensive option and usually our last resort.

    As for minimums, we have designed several systems for customers who had about 40-45 psi and only 4-5 gpm flow. There is one neighborhood in particular with these exact numbers and I've installed 3 systems in that neighborhood. We used MP Rotators. I can't remember how many we got per zone but I seem to recall we had about 6-8 heads per zone.
  9. PurpHaze

    PurpHaze LawnSite Fanatic
    Posts: 5,496

    One of the biggest challenges we're facing is dwindling PSI-GPM from either wells that haven't been kept up or domestic supplies where tremendous residential building in the area is lowering everything. Seems to affect the older systems first and the suggestions that Jim and Boots have listed are pretty much some of the same things we will do.

    Each area is different so it will depend on what we actually run into that determines our course of action. Last resort is to abandon and start all over with new calculations and then installing zones that are well under those parameters so we get more serviceable time from the new system zones.

    I'm facing that exact problem right now with an "island strip" in front of one of our high schools. We've done all we can over the years to try and salvage this 45-year-old manual system (4 zones) on galvanized pipe but we're running out of options. It will eventually be easier, faster and more cost effective to just go back to the POC, set new valves and a WVC and install MP-Rotators which match this area very nicely.

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