We have been using CSV’s to solve these kinds of problems for 22 years now. If the supply houses haven’t heard about them, they are not very well educated on pump systems. More than likely they have heard about CSV’s, and their companies have chosen to act like they don’t know what it is, or to claim CSV’s don’t work for some reason.
Can’t say as I blame them. A CSV125-50 will only cost you $111.30, and you would only need about a 20 gallon size pressure tank, which I see selling for $140 or so. So for roughly $250, you will have every thing you need to run from as little as 3 GPM up to 35 GPM, as that is the amount that pump can produce. The CSV will make your pump, tank, switches, and everything else in your pump system last many decades.
The CSV was designed as a simple, inexpensive, and long lasting replacement for VFD’s, and we have replaced hundreds of thousands of them in the last 22 years. The supply houses would rather you purchase a couple of really large pressure tanks for about $2,000.00. Or they would be fine selling you a VFD for a couple thousand as well. Either of these will make the supply houses a lot of money. Neither of these will last very long or make your pump last very long, so they will get to sell you a new set up on average of about every 7 years.
It has been proven that a CSV will at least triple if not quadruple the life of pumps and associated equipment. So if you want the supply houses and pump manufacturers to make a lot of money off of you, go with the VFD or a couple of large pressure tanks. If you want something inexpensive that will last a long time, you have come to the right place.
I looked up the pump curve you posted and the 35 GPM, 1.5 HP pump will work fine with our plastic, domestic grade CSV125-50 for $111.30. Or you can use one of our heavy-duty, long lasting, municipal grade CSV3B2T valves, which sell for $647.50. You won’t see any difference in performance between these two valves. Just that one is adjustable and longer lasting than the other. However, for the difference in price, you could replace the CSV125-50 about 6 times for the same money as the CSV3B2T if needed.
I have had one of the little plastic valves in my well for 22 years now. But water quality has a big effect on how long these valves will last. If you have dirty or slimy water, the more expensive CSV3B2T would be best.
There are lots of good applications for VFD’s. But pumping water like this is not one of them. VFD’s are just used for these kinds of water pumps because the company that sells the pumps and VFD’s will make a lot more money. If you don’t mind paying a lot more and having a lot more problems, then a VFD is what you want.
Oh, BTW, Franklin is one of the biggest instigators of pushing those VFD's on people. So I would prefer to see you use a different pump company and not encourage Franklin. F&W is a good US made pump, and now they have their own motor. I had just rather promote companies who are not trying to shove a VFD down peoples throats for the extra profit.