SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

I am not familiar with designing with poly pipe. I use schedule 40 and class 200 in Houston. The distance is enough to make a difference. You may want to find the water meter and tie into it, and that may change the entire picture. It sounds like you may need the 1.25".

Good Luck.

BTW what is the water source and what size is it? Are You tieing in at the meter or off of the spigot? What size is the pipe you will tie into? How far away is the point of connection to the water source? What Kind of heads are you using and what is their pressure requirement? This information is necessary to answer your question.

The larger pipe is being specified because of the GPM and distances involved.

Check out the PE Friction loss table here: http://www.hunterindustries.com/Resources/PDFs/Technical/Domestic/LIT091w.pdf

At 14+GPM, a 1-1/4" pipe over distance of 300' will only loose less than 5psi.

But if you change to 1" pipe, your pressure losses can jump to over 15psi AND puts you in the "danger zone" of water velocities exceeding 5 feet/sec. That's likely enough of a pressure loss to invalidate the design.

At the expense of complexity, you might be able to save some money and use the 1" pipe (perhaps even 3/4") in some parts of the distribution. It depends upon the layout of the system. As an example, if you have one main trunk to get the water to the area of the zone, but then you effectively have spider legs feeding each of the individual heads, you likely could limit the use of 1-1/4" pipe just to get the water to the area, and then drop to the smaller pipe that feeds each individual head. But if you are having someone else install the system for you, you're likely better off sticking to a single pipe size rather than running the risk of the wrong pipe size installed in the wrong place.

Quoted from "AdamT"

None of the water will have to go more than 165' past the valve.If I am reading the chart correctly, I should have @ 9 psi loss at 9 fps velocity with 1" pipe. Am I doing this right? Do I have to add up all of the pipe going to all 7 of the heads?

9 fps is WAY into the "DANGER" zone. If you'll notice, the chart shadows everything where the water speed is exceeding about 5 fps. Remember, kenetic energy quatruples as speed doubles.

Additionally, a 9psi loss in a system that was designed for less than a 5psi can still affect performance. By how much? Can't say because we don't know how concervitive the design was. Too little pressure will result in heads not throwing the water as far as it's supposed to. An extra 5psi pressure drop may or may not make or break, the design.

As for how to add things up right... here's what you have to do. You start by determining what the gpm is EXPECTED to be at each head. Then you have to account for that much gpm from the valves to the head. So if a pipe is supplying two heads, each expected to output 2gpm, then before the pipe splits to the two head, you have to account for 4gpm flowing in the pipe before the split and 2gpm in each pipe before the split. For pipes that lead to three heads, you have to account for the gpm flowing to all three heads, etc. So once you know the expected gpm of each pipe segment and the approximate length of each pipe segment, you can calculate the psi losses in each segment. You then determine if the point the fartest out will still have enough pressure (after you account for all the pressure losses through the valves and before the valves) at the head to see if it will perform within spec.

For more information, try checking things out starting at this page: http://www.irrigationtutorials.com/sprinkler01.htm