RPZ Minimizing Pressure Loss

Discussion in 'Irrigation' started by dypsisdean, Sep 4, 2010.

  1. mitchgo

    mitchgo LawnSite Silver Member
    Messages: 2,940

    sure if you want 15fps ++
  2. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Messages: 50,402

    Not my meter :)
  3. dypsisdean

    dypsisdean LawnSite Member
    from Hawaii
    Messages: 38

    This is a lively forum. I never expected so many responses so quickly. Thanks.

    Here's some more info for those still willing to help. I wanted to avoid all the technical mumbo jumbo because my set up is truly unique and one that you all will laugh at, and you probably won't believe it works as well as it does. But it's proven remarkably efficient, reliable, easy to install and customize, and is extremely inexpensive. So for those curious minds, I'll briefly explain.

    I'm on very rocky ground - lava, on the island of Hawaii - in a forest loaded with hundred foot trees and the accompanying root systems. So in other words trenching is impossible. I have placed thousands of rare plants in and among the trees and needed overhead irrigation to assist the natural rainfall.

    So, I used 1" polypipe with the full flow compression fittings - not the barbed kind - but the insertable kind. (BTW - I learned all my hydraulic knowledge in the swimming pool business, that's why all this will sound unorthodox) I am using a Hunter rotary (not a popup) with 3gpm nozzles) I started with my first station eight years ago and just started adding sprinklers and changing nozzles by trial and error until I had a station that took 15 of these sprinklers working well, and keeping the pressure in a "sweet spot" to operate them effectively without popping the fittings from excessive pressure. (Are you laughing yet?) So in other words, I'm at the maximum "balanced load" for each station now.

    Each station covers about a 1/2 acre and although not perfectly configured for overlap, trees in the way, etc. it assists the natural rainfall (60 inched/year) very well. They are all on risers 6-10 feet tall, hidden by various plants/stumps, and the polypipe slowly disappears beneath the leaf litter with time. The older stations have worked flawlessly for eight years now.

    If there is enough interest, or disbelief :), I can take a video to show you all.

    Now, the problem is that I need to use an RPZ within 5 ft of the meter to satisfy the water dept. and make me eligible for ag rates. As mentioned, there is about a 100 ft. fall to the property, over about a 500 ft. run to the valve manifold using 1 1/4" Drisco Pipe (don't know the ID). That's what they use a lot over here.

    The most interesting thing I have discovered is the extreme pressure a hundred foot fall over 500 ft. with 1 1/4" pipe can generate. I experienced this when I shut off the meter valve, opened a valve near the meter, then opened a hose bid down on the valve manifold. The pressure had to be 50 psi initially with no meter pressure applied. So I thought this might be a factor in any equation to figure pressure/flow loss and the RPZ. It would almost appear to generate a "head gain" instead of loss. :) Maybe even helping to "suck" the spring(s) in the RPZ - is that wishful thinking?

    So the short story is that I already have the large (three acre) set up - with the stations, spacing, valves, etc. sized and working perfectly. My problem is that I'm worried that by installing the RPZ, everything will have to be rearranged - more stations or valves because the pressure will not be high enough to throw the water at the spacings I now have. Given the terrain and all, that would be very labor intensive, with 90 sprinklers all on tall risers, tied to stakes set in lava, all hidden away.

    I'm stuck with having to used a licensed plumber here, and they want $1700 to install the device. (I know, a rip off) Last time I checked, to swap the meter out to 1 inch was about $6000 (I know, another rip off). So I'm starting to wonder if the savings for the ag rate will be worth it. I'm at about $500/month for water now.

    So, since you all tell me that there is no RPZ that is better than another for flow/pressure, that leaves me with only two choices. Unless you guys have another.

    1) An oversized RPZ - will that help my situation?
    2) A larger meter - will that be worth it? Would that actually help with keeping the pressure adequate to operate the set up as configured?

    Or I guess there is always the ability to downsize the nozzles to accomodate any pressure/flow drop. This will probably leave some dead spots, but maybe I could live with it until I could afford a new meter.

    Thanks in advance for your feedback.
    Last edited: Sep 4, 2010
  4. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Messages: 50,402

    We already told you that a larger RPZ won't help. The next time you ask the same question, the same answer will still apply. If it's about the money, then you will simply have to re-nozzle all the heads, or purchase and install a booster pump.

    These are simple concepts. You will not be making any end runs around your situation.

    By the way, some more info would get you more detailed answers. Your current static pressure. Your current flow rates. What the ag rate is. (how much less for how much water for how much savings) Is there any backflow prevention already in place?
  5. dypsisdean

    dypsisdean LawnSite Member
    from Hawaii
    Messages: 38

    Not everyone is in agreement that a larger RPZ won't help. Given my situation with such a steep long downhill flow (with an "oversized" supply line), some plumbers I have talked to choose to disagree on that - suggesting that such a strong "suction/syphon" pulling downhill would create a situation not often encountered. So I assumed some of you here may also disagree with that.

    At any rate, it seemed to me that if a larger RPZ might help things, it would be better to go that route first, then if I still needed more I could upgrade the meter. Sizing the RPZ for the present meter/supply line, and then wishing it was larger if I had to upgrade the meter later, seemed like backward thinking.

    Everything here is dealing with theory, so the real answer comes only when the system is tested. That's why I didn't want to get into all the math because my question could be phrased as simple as "How do I install an RPZ on a system that is already maxed out under given pressure/flow/head and keep it functioning as designed?" And I guess the answer is "I can't." And I guess all RPZs are pretty much the same.

    So the next question would probably be, "Will the system still function adequately with a minimum pressure drop of 10 psi?" Would anyone like to speculate?

    For anyone still willing to offer suggestions:
    - There is no backflow in place now, There are anti-syphon valves on the zones, but I have already upsized those to 1 1/4". I have used every trick I know to squeeze maximum amount of water to each station, while keeping the total number to only 6 stations for the entire 3 acres.
    - There is no place to check pressure at the meter, but I'm told it's around 85psi. Because of the extra weight of the water downhill at the valves, the pressure is around 120psi. As stated, I don't know the ID of the polyhose, so flow rates would be a guess. But I'm guessing it is closer to sch40 1 1/4". But if 15 rotary sprinklers with 3gpm nozzles are running (indicating probably 15-20psi in the active line) that should give some indication.
    - As stated, the system is already close to "maxed out" for optimum coverage per station. Downsizing nozzles will leave dead zones because of the decreased throw of any replacement nozzle available.
    - The price of ag water and the cost savings etc. is something I can easily calculate. But since rainfall varies greatly (from 30 in/yr to 110 in/yr) and given that we have the highest electric rates in the nation, figuring in a booster pump and then projecting the complicated tiered billing system would be close to a guess anyway over a 5 year time frame. But I do know the savings in ag rates would certainly offset the cost of a RPZ alone.
    Last edited: Sep 4, 2010
  6. 1idejim

    1idejim LawnSite Fanatic
    Messages: 11,291

    i come from a pool building background too., shoot, install the rpz and see what happens. you can adapt to the challanges that may arise.

    best of luck
  7. mitchgo

    mitchgo LawnSite Silver Member
    Messages: 2,940

    $1700 is insane, That's like a car repair guy saying I'll replace your breaks for $1000.. Dig out the hole your self and get another plumber / quote to install the rp..

    I could have a professionally installed rp in 1-2 hours if it was dug out for me.

    I would use a 1" rp, like I said the biggest difference is a little less friction loss.. Were talking extremely minimal.
    Remember, your only working off a 3/4" pipe to begin with.

    The other difference is cost.. a 1 1/4 is much more expensive then a 1" and wouldn't justify the possible .5 psi savings in return.

    It sounds like your going to have to deal with it, you shouldn't have over extended it in the first place. You should have had more zones installed...
    For the amount of cost you spend on plants and water, you should have had your irrigation system done the right way, not jimmy rigged it.

    I live on a slightly larger then 1/3 of an acre and I have 7 zones.

    In the long run you will end up with a ton of damage, just because it's worked for 8 years doesn't mean it's going to keep on working. Think of it as if it was electrical- It's like plugging 10 things into one outlet and thinking it's okay to plug one more into that same socket. Eventually it will fail.
  8. Kiril

    Kiril LawnSite Fanatic
    Messages: 18,334

    Why don't you just look up the info you need from the various manufacturers? Every single one of them provides a flow vs pressure loss graph.
  9. dypsisdean

    dypsisdean LawnSite Member
    from Hawaii
    Messages: 38

    I understand what you are all saying.
    1) But - 7 zones for 1/3 acre would equate to 62 zones for my three acres. Even 20 zones would have so many pipes running across the rocks it would look like a junk yard. In addition, it has to be flexible pipe to conform to all the nooks and crannies, ups and downs of the terrain - and black in color to blend in with the lava. Rigid sch 40 PVC just would never work. Plus the UV here is intense, regular PVC gets brittle fast. I've already run over 2000 feet of pipe, so flex pvc would have bankrupted me, even if available. That's why I used what I did. To try and explain, check the photo below for an idea of the terrain I have beneath the few hundred years of growth and leaf litter.

    2) $1700 for the installation is a rip off. But I wasn't even allowed to hook up my own sinks and toilets for an addition I just put on the house. The unions have a death grip on the legislators and they know it. Plus, being on a small island, all the plumbers know this and price accordingly. Hawaii has never been cheap for anything.

    3) Checking the manufacturer's specs and graph's was a nightmare because the curves were very different depending on flow rates and various pipe/valve sizes. And since I may have to just install the RPZ and play with things afterward, those variables could change.

    So that is why I thought someone here could have told me that one specific RPZ was the best at reducing pressure loss, or someone would have had a suggestion I hadn't yet thought of. But it looks like I'm stuck and will just have to proceed one step at a time.

    I guess I'll use a 1 1/4" RPZ to match my supply line, and to eventually accommodate an upgrade to a 1" meter if I absolutely need it. Then just see how it goes. It still would be nice to know how different the various RPZs are in pressure loss. But I guess there is no "Cadillac" in this department.

    The feedback has been useful, and if anyone else has any thoughts I would still like to hear them. So Thanks.

    It's not quite this bad, but you can get the idea. Definitely not your typical installation.

    Last edited: Sep 5, 2010
  10. 1idejim

    1idejim LawnSite Fanatic
    Messages: 11,291

    nice and clean, you never said what you're irrigating

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