From the 90' depth of static water, 12 GPM wide open, and 130 PSI differential pressure, I can tell from the pump curve this is a 7 GPM series pump. When you open enough fixtures to use more water, the pump pressure is falling off, because the well level is dropping. If the pumping level is 350', 5 GPM is more than that pump can produce and still keep the pressure up.
But the differential pressure is what closes the CSV1A. So the higher the differential pressure the more reduced pressure fall off there is. This is usually only a pound or two difference.
The high differential pressure does two more things. One is that the 1 GPM differential figured in the CSV1A is at about 80 PSI differential. So at 130 PSI differential the minimum flow is probably closer to 2 GPM. This means using 1.7 GPM is not quite enough to set the CSV1A, which is why it continues to fill the pressure tank and shut off when water is still being used. Also 2 GPM will be your tank fill rate. So to get even get close to one minute of run time in a tank that my calculations show to hold 1.9 gallons at 50/70 pressure, you would need to set the CSV1A for 48 PSI while running about 3 GPM.
This will ensure that the CSV1A is closed down to the minimum 2 GPM by the time the pressure reaches 50 PSI. Then it would fill the entire 1.9 gallon tank at 2 GPM, and should take just under 1 minute.
However, if the amp draw of the motor reduces by as little as 20%, as it should, the motor is running cooler and does not need the old rule of thumb of 1 minute of run time. 20-30 seconds is enough. For one thing the CSV1A makes the pump run WHILE you are using water PLUS the other 30 seconds to fill the tank. So normally the pump runs for more than a minute or two anyway. Another thing is that the CSV1A doesn't let the pump shut off until you have finished using water. So it is usually off for more than a minute or two as well, and has plenty of time to cool down.
The second thing the high differential pressure will do is eat the guts out of the CSV1A much sooner than it should. It should still last several years and you can just drop in a new gut pack when the old one wears out, but splitting the differential pressure between two CSV1A's would last much longer.
However, increased differential pressure through a CSV1A increases the minimum flow for a reason. A 1HP, 7 GPM pump has 17 impellers. And the more impellers in a pump, the more flow it needs to remain cool. You probably need about 2 GPM to keep that pump cool.
So I would not add another CSV1A, because it would also reduce the minimum flow back close to 1 GPM. The only problem with having a 2 GPM minimum, is the pump will cycle during times when you are using less than 2 GPM. If any of your showers on anything you use for very long require less than 2 GPM, you should probably add a little larger tank. I am not talking about a very much larger tank, maybe 20 gallons total volume. You could add a 14 gallon to the 7 gallon you have, or replace the 7 gallon tank with a 20 gallon size.
A 20 gallon size tank would hold about 5 gallons of water. So when you are using 1 GPM, the CSV is letting 2 GPM pass. 1 GPM goes to the faucet you are using and the other gallon goes into the pressure tank until it is full and the pump shuts off. Setting the CSV1A in the middle of your pressure switch setting, you should get about 2.5 minutes of run time, and 5 minutes of off time as long as you continue to use 1 GPM at the faucet.
This is better for the pump than 1 minute on and two minutes off, which is the best you can do with the 7 gallon size tank. But really 1 minute on and 1 minute off is all the motor company says it needs. And with the low amp draw created by the CSV1A, it really doesn't need that much. So if you can set it up to get at least 30 seconds of run time, I wouldn't add another tank unless you have a 1 GPM drip system or something that uses less than 2 GPM for long periods of time.
