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Okay, I found out that the city water supply is roughly 50 psi at 15gpm although I know that it would be user specific so I won't worry too much about that as you suggested.Unless you know your city water supply can match the well, with pressure to spare, don't worry about switching supplies.
You don't have utility-grade poly under 24/7 water pressure, so your system must be designed accordingly. I don't know from Silverline poly pipe, so no first-hand opinions from me.
If the well install has a pressure tank and a pressure switch controlling pump operation, you are probably fine. If pump operation will be controlled by a relay, then it's a different story, since the pump can probably generate higher pressure than the pipe can withstand. You will then have an absolute need for a pressure relief valve at the wellhead. For relay control, you definitely upgrade the mainline pipe, since there is no protection afforded by a pressure switch and pressure tank, and you must always allow for a zone valve not opening when energized.
You aren't restricted to just poly and PEX tubing. There is also sch 40 PVC, which will have a much higher pressure rating, at a competitive price. (I'm assuming pipe will be laid in open trenches)
Wet_Boots, you gave me lots of good information to consider and what you are talking about makes perfect sense in my mind and operation. I check out the website and understand what is going on. From what I gather the cycle stop valve does keep the mainline under a pressure however you get to set that pressure which makes sense. Instead of the pump pushing against a deadhead or close valve it pressurizes a tank till it reaches the cutoff value of the pressure switch. A pressure tank out in the yard would be an eye sore but certainly seems like the right way to irrigate from a well.You might consider skipping the relay in favor of a pressure switch and a small pressure tank, and to use a master valve to feed the system, so that the mainline is not under pressure 24/7 ~ the advantage of such an approach is that you aren't subjecting the mainline to excessive pressure
I looked at the design, and I don't see that you are matching zone flows to the pump performance. The pump doesn't just set a ceiling for zone flows. It also sets a floor. You need a certain minimum flow in order for the pump to not be short-cycling. I know this is where people think that relay control is better than pressure switch control, as it forces continuous operation, but what will be the mainline pressure if a zone is only using a fraction of the pump output? The answer to that question is that the pressure will be higher than you'd like. How much higher is an answer that will tell you what pipe your mainline must be.
By the way, your zone valves need to have flow controls if the supply pressure may be excessive.
One means to get a handle on pump operation and system pressures is to employ something like a Cycle Stop Valve upstream of a tank and pressure switch. That would let you employ a master valve and a mainline of the ordinary 100 psi poly.