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sprinkler rookie

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Saturday, July 5th 2014, 2:11am

lateral pipe design

Hello Everyone

I am installing a sprinkler system at my house. I had a sprinkler design done by a professional. I am running off of a 1 horse well pump which feeds a pressure tank in basement. There is a 1" line running outside for the sprinkler connection. Pressure tank is set at 65 psi. The design is made to run each zone at 18 gpm max. After my one inch valves, I am told to increase my lateral pipe to 1.5 inches for "x" amount of feet then back to one inch lateral pipe. I was told that the increase in pipe size was for more volume of water. Does this seem right?

Central Irrigation

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Location: Central Minnesota

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Saturday, July 5th 2014, 11:27am

For 18 GPM (depending on length of pipe runs) 1.25" mainline and laterals are sufficient. I'm assuming they also designed your mainline at 1.5"?

1.25" poly pipe will lose approx. 2psi per 100' of pipe at 18gpm. 1.5" poly will lose roughly 1psi per 100' of pipe at 18gpm. If nothing else, I would run 1.5" mainline and 1.25" laterals.

You need to change your thought process on pipe sizing. Larger diameter pipe isn't for more volume, it's for reducing the amt. of pressure lost as the water travels through the pipe.

I would measure the length of pipe it would take to feed the zone farthest from the water source. Using the values above, you can determine the amt. of pressure lost for your "weakest" zone.

sprinkler rookie

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Saturday, July 5th 2014, 5:06pm

lateral pipe

Yes my mainline is 1.5". On one zone it shows installing 45 ft of 1.5" the rest is 1" on that zone. Does this have to be an exact measurement in the field? Due to trenching, I might have 10 more feet of 1.5" on that zone. Will having more or less of the 1.5" pipe on lateral affect sprinkler operation?

sprinkler rookie

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Saturday, July 5th 2014, 5:12pm

There was a mistake in the plan. Plan shows pressure tank set at 65 psi and my pressure tank shows 45 psi

Central Irrigation

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Posts: 362

Location: Central Minnesota

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Saturday, July 5th 2014, 11:36pm

Not a mistake. More than likely you'll need to adjust the pressure switch to a 65psi cutout once the system is installed
As for the 1.5" pipe, if the pro thought shaving a .5psi loss was worth the added expense of larger pipe, then so be it. Not knowing the exact specs of your well, its hard to say one way or another if 1.5" is necessary. If 18gpm is max, I would think 1.25" would be more than enough.

A rule of thumb I like to use:
<15gpm= 1" pipe
16-24gpm= 1.25" pipe
25-35gpm= 1.5" pipe

There are exceptions like long runs, and low operating pressures where upsizing pipes might be recquired, but usually not until you reach the upper limits of the above flow rates. 18 gpm falls rather low on the 1.25" scale.

Wet_Boots

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Location: Metro NYC

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Sunday, July 6th 2014, 8:46am

The dollar cost of 1-1/2 pipe is not so much more than that of 1-1/4 pipe, so it can be an easy choice for a mainline. As for laterals, they might be all 1-inch on an 18 gpm system, provided the mainline is located so as to pass through the middle of zones, so that two diverging pipes comes from a zone valve, each one carrying 9 gpm.

On well-water systems, more attention can be paid to minimize pressure losses in the plumbing, because the supply pressure will max out at the cut-off pressure setting. (actually a few psi below that cutoff setting, so as to avoid cycling)

Central Irrigation

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Posts: 362

Location: Central Minnesota

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Sunday, July 6th 2014, 11:46am

$30 difference between 1.25" and 1.5" per 100'. Not to mention the added expense of fittings, which on some can be as much as $2 more per fitting. I just can't see where it justifies saving 1psi. But, again, I haven't seen the property nor have I seen the drawings.

Better that he overdesigned than underdesigned.

Wet_Boots

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Location: Metro NYC

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Sunday, July 6th 2014, 4:45pm

I think it's the all-PVC systems that some pros don't bother with 1-1/4 pipe, probably like some poly-pipe guys don't ever use 3/4-inch poly, because they don't want to carry the extra inventory in pipe and fittings.

Central Irrigation

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Posts: 362

Location: Central Minnesota

9

Monday, July 7th 2014, 7:52pm

Understandable.

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