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Tuesday, July 29th 2008, 12:34pm

System design help

Looking for some advice ...

Need to design and install a residential irrigation system. The water source will be via a tank and pump, beings that the "well system" (2 party) is inadequate to supply any kind of irrigation system. I know it’s recommended that you first calculate your available water supply in order to design the irr. System. So, am I better off selecting a pump and then designing the zones or design the zones and selecting a pump to match? For optimum pump performance, it should not cycle on and off, it should run constant during each zone. (correct?) So, if I design each zone to require the same GPM, I should be able to select a pump that would function correctly for this system ... yes/no?

And ...

If a pump has an outlet size of say 1" ... is there any benefit to running a main supply line larger than 1", say 1 ¼" ( to decrease friction loss due to long distance or grade/head)? I know there are calcs to figure out friction loss in main supply lines and laterals, but is there a "rule of thumb" that typically applies? Do you calc. lateral pipe size per system or run a particular size pipe for all jobs (one size fits all)? Also, is one better than the other, PVC vs. Poly, for main supply line?

And.... (one more)

Any good sources for info online pertaining to the above?



Supreme Member


Wednesday, July 30th 2008, 3:03pm

You can learn a lot by reading over . There's tons of information there, so you'll likely have to read it a couple of times.

You are on the right track with not wanting the pump to cycle and designing the zones to roughly match. Larger pipes will only reduce water pressure losses due to friction, it will do nothing for the change is grade (every 10' in elevation changes the pressure by 4psi regardless of pipe size). Since friction losses are a function of the length of the pipe, long runs from say the pump to the general area of the irrigation might need larger pipes to avoid too much friction loss. But before you can begin to design your circuits, you have to know what sort of GPM you can produce. And before you can specify a pump size, you have to know that the water source can produce as fast as the pump can pump.

So you really need to know your water supply 1st. About the only way you would want to design the irrigation system 1st and select a pump 2nd would be to perhaps 1st design the layout of all the sprinkler heads regardless of which circuit they are on. In other words, design the layout of your sprinkler heads to get your head-to-head coverage. The assume a working pressure and determine the TOTAL GPM for all of the sprinkler heads. Then divide them up into a reasonable number of balanced circuits and see how you have to group the heads to achieve the desired circuit size.

As an example, lets say that you layout the virtual heads and find you need 6 heads spraying full circles, 20 heads spraying half circles, and 8 heads spraying at quarter circles. That's equivalent to 18 heads spraying full circles (assuming quarter circles spray at 1/4 the rate of full circles). You next assume a working pressure of say 40 psi. You then look at the specs for the spray heads you plan to use and see that at 40psi, a full circle uses 4 GPM ( half circle does 2 GPM, and 1/4 circle 1 GP<). That's a total of 18 * 4 = 72 GPM. 72GPM divided 4 times is 18 GPM (likely too much), divided 5 times is 14.4 GPM, divided 6 times is 12 GPM, divided by 8 times is 9 GPM, and divided by 12 is 6 GPM. From there you investigate the water source and determine, with the right size pump, can it produce 6, 9, 12, or 18 GPM. Say you determine you can handle 12 GPM. Then you go back and group the heads into circuits, each circuit producing about 12 GPM. From this point, what you discover might guide where you go next. The distance you have to travel might help dictate the size pipe needed. But the cost of the pipe might influence the GPM you decide to design for. You might start by picking 12GPM, but once you've got the final system designed, you find that you need very expensive pipe to maintain a 40psi working pressure at the heads at 12 GPM and decide on a smaller GPM. Perhaps you find the exact opposite and discover your water source and mainline pipe are still pretty cheap at 18 GPM so you design for fewer zones. It will likely take sever iterations of calculating GPM, checking expected pressure losses, sizing pipe, pricing equipment, redesigning to take advantage of the cost of a particular pipe size etc.

As an example of how pipe size/ cost can influence your design decision, if you were working with shc 40 PVC, you'd find that 3/4" PVC pipe and fittings cost about as much as 1" PVC pipe and fitting, but 1-1/4" price jumps significantly. You would then want to try to design as much of the system as possible using 1" pipe. But then again, smaller pipe usually results in limiting your GPM, which could increase your circuit count, which could increase the linear foot of pipe needed, so maybe a larger more expensive pipe IS cheaper after all.

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