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All of the companies who gave me quotes are licensed companies that have long histories (10+ years) of work in this area. None of them are "no-account-trunk-slammers." All of them claim to be following all of the new state laws (head to head coverage, permits and plans filed, freeze/rain sensor, etc.).Texas lawn sprinkling became a lot more expensive with their new rules. Anyone who does the work by the book is going to be significantly more expensive than some no-account trunk-slammer.
40 psi is extremely low for sprinklers
All I can say is 40 psi is a joke. No way will 5 zones cover your property. I highly doubt 7 zones will either. I would install some sprinklers above ground as a test. See how many sprinklers you can run. I've never seen a functioning sprinkler system with 40 psi unless it's a dripline.
Apparently they exist because your neighbors have them. Check out your neighbors sprinklers and see how many they have on one line. Compare it to what the three guys have in mind. Just remember that a full circle is 1 sprinkler. 2 half sprays is equal to 1 full. So if see 6 sprinklers at your neighbors on one line and they're half sprays. Don't expect you can run 6 full sprays in your yard.
With all that said keep in mind I'm not a landscape architect. I do however repair sprinkler systems full time. Working with systems that have very low pressure is a pain in the rear. In my opinion the more valves you have the better your odds of everything working correctly.
Even without the new Texas requirement (your local mileage may vary) about leaving trenches open for inspection, having a 40 psi supply pressure makes for an expensive sprinkler system. Every conceivable means to conserve water pressure gets utilized. No master valve, for instance (use zone valves with flow controls for operating reliability). Even then, heads won't throw as far as usual, so they get spaced closer together. Pipes may be oversized, because you want near-zero losses. Flows will be limited, according to your water supply pipe size and water meter size. This all costs money. Sometimes, it's so much extra money that boosting the pressure with a pump is considered.
There is a tremendous difference between 40 psi and 50 psi, when it comes to a supply pressure you can work with. If your pressure is subject to fluctuation, then different companies might be seeing a different picture, depending on when they measured pressure. (I myself, encountered an extreme example of this, and submitted a design and bid that was more than double anyone else's, on account of having measured a supply pressure less than half of what was usually present)
Since Texas has curbside meters, an installer can minimize pressure losses, unlike other states in the north, where meters are in basements, and pressure gets lost in the pipe supplying the home.
I have to agree with the other posts. 40 PSI static is very low for a sprinkler system. Most rotary sprinkler heads operate at their optimum
at the 45-50 psi range. You're starting out with 42 PSI. By the time you figure all your pressure losses in the service line, meter, backflow device, valves, piping, elevation, etc. etc. you could very well be operating at best in the low 30's but likley in the 20's. You might want to inquire with the three contractors what they figure the pressure is going to be at the base of the heads for the worst zone. A booster pump is looking good for this application. I doubt you will have much success any other way. Where did you come up wth 70-100 heads on seven zones with 30-50 PSI?
If all you can count on is 42 psi, then you deal with it. Pipe sizes are bumped up. If there was going to be a master valve, that could be eliminated, getting you back about 3 psi. If you already have a one-inch water meter, then you can't do anything there. Flow losses in a one-inch meter should be less than 3 psi, if your flow is less than 20 psi.
You still have the losses of a backflow preventer and a zone valve. 3 psi here, 5 psi there, and it adds up pretty quick. In your best scenario, you will be lucky to have as much as 30 psi at the sprinkler heads. The best designs will be very conservative. Better to have 10 zones that work, than to have 5 zones that don't.