Balancing Pressure Loss

Discussion in 'Irrigation' started by burlap, Apr 19, 2006.

  1. burlap

    burlap LawnSite Member
    from Indiana
    Posts: 66

    I'm designing a four zone system using a well source and a new 3HP pump whose chart says I should have 56 PSI @ 50 GPM and a 10 foot drawdown. My zone stats are as follows:
    Zone 1 49.5 gpm with a psi loss of -3.45
    Zone 2 48 gpm with a psi loss of -3.83
    Zone 3 45 gpm with a psi loss of -6.096
    Zone 4 45 gpm with a psi loss of -7.532.

    Zone 4 can be lowered to 4.43 if I use all 2" pvc. zone 1,2,&3 can be adjusted somewhat by using different size pvc in a few places. My supplier says that loss from the PVB and the swing pipe will be of little consequence. How close should my operating pressures be and what options would I have in making those adjustments. Thanks, Burlap
     
  2. PurpHaze

    PurpHaze LawnSite Fanatic
    Posts: 5,496

    I always leave a buffer. Kind of a built-in safety valve as far as GPM and/or pressure. That way there are no surprises when I design using a higher GPM per zone. But in my particular situation I may be going back into an area within a year and adding/moving sprinklers. I must be able to add additional sprinklers to zones.
     
  3. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 47,312

    If you allow for a 10 psi loss from supply to head, you should give yourself enough wiggle room. A PVB and zone valve will cost you over 5 psi, regardless of flow.
     
  4. burlap

    burlap LawnSite Member
    from Indiana
    Posts: 66

    Thanks for your comments. I don't think I explained my situation clear enough. My sprinklers are placed at 40' and 40 psi. At that combination the heads will be producing 6 gpm. If every time a valve opens on a different zone I have a different resistance, the heads will a have different pressures and therefore, different gpms. I realize I can't have all four zones identical-but how close should they be? Would I be in the ball park if my pressures were within 10% or some other percentage? Am I missing something obvious here? Thanks, Burlap
     
  5. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 47,312

    Okay, now the warning bells begin to sound. 40 psi is not the pressure you'd want to have on 6 gpm heads. Hunter PGP nozzles #8 or #9 or higher should really have at least 50 psi at the heads, should you be looking for full 'textbook' performance. Like if you expect the heads to give you a 40 foot radius, in the event you need head-to-head performance.

    You'd be better off looking at the pump curve for the flow you'd get at a 65 psi supply pressure, and then you could plan for having at least 50 psi at the heads. Reducing the nozzle sizes one notch would probably match up the system to the pump.
     
  6. burlap

    burlap LawnSite Member
    from Indiana
    Posts: 66

    Thanks for the advise, but I've got another question. You're recommendation concerning the Hunter PGP 8 or 9's needing 50 psi doesn't correspond with the literature. I had planned to use 10's and the literature says 40 psi, 45 feet and 6 gpm. Am I reading the chart wrong or is your recommendation based on your experience with those heads or do designs vary that much that you should plan on 10 more pounds than the charts call for. I'm a little confused as how to use these sprinkler charts if you can't take them at face value. Thanks, but I still haven't found an answer to my original question. If I have two zones with the same heads but different pressure losses would I have the same coverage? Burlap
     
  7. skurkp

    skurkp LawnSite Member
    Posts: 248

    Is his pressure difference do to the fact that the zones area at a different distance from the supply and he should expect this? Please don't slam me I am just trying to learn.
     
  8. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 47,312

    The literature, ahem, exaggerates. Big nozzles get big pressure. Sprinkler companies are notorious for publishing performance tables that suggest good performance at lower pressures. It doesn't work that way. Give those Hunters at least 50 psi to get that 40 foot radius. Just use #9 nozzles.

    The original question? Pointless. Who cares what the precise pressures are, in various sections of various zones, as long as you get the coverage. I'm still kind of dumbfounded that you would present a 'pressure loss' that doesn't take into account the PVB, the zone valves, and the one or two psi the swing pipe/fittings lose.

    Just get the 65 psi flow performance from the pump curve, and match it up with the 50 psi gallonages consumed by the heads. That allows you to lose as much as 15 psi in the system, and still get coverage. If your pressure losses are less, then the heads will see higher pressures than 50 psi.

    That is not a problem. If the heads see higher pressure, then they will consume higher gpm. But higher gpm means the pump puts out less pressure. You achieve a balance,
     
  9. PurpHaze

    PurpHaze LawnSite Fanatic
    Posts: 5,496

    No two zones will be exactly the same even if the ultimate PSI at identical heads/nozzles were the same. Don't worry about trying to match up zone for zone as this can be overcome through prudent controller programming. It would be the same thing if say you had a small rotor zone next to a spray pop-up zone next to an MP-Rotator zone. You will balance things out through the program. Concentrate on coverage within each specific zone. Although sprinklers are dependent upon the head next to them and zones are dependent on the other zones next to them the balancing act comes about through the watering program.
     

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