On Fri, 06 Dec 2013 19:47:58 -0600, Ignoramus5722
wrote:

I have a strange sounding idea. I want to have an extra compressed air
tank in my system. I have a bunch of tanks at this moment to choose
from. This way, I could run air hungry tools like breakers, etc,
intermittently but longer.

So far, so good, and nothing complicated. I have a whole bunch of
tanks right now, and thought to do something nice for my shop and have
an extra tank.

Examples of tanks I have a

http://igor.chudov.com/misc/ebay/tmp/Marengo/186.jpg
http://goo.gl/PdkzbM (Tank with "QT-15" lettering on left)

However, I also realize that with a big extra tank, it will take so
much longer for the air system to come up to pressure. This will be my
wasted money, as people will be waiting longer to use compressed air.

So, I thought, can I have a valve, that would not fill the tank, until
system pressure reached 120 PSI. And then it would shut off again if
the pressure ever falls to below 90 PSI.

To use the accumulator tank to supply air, another pipe would be used
with a check valve.

This setup makes sense to me, as

1) I would get the system to reach pressure quickly and
2) I would eventually get a supply of air in the accumulator,
without ever dropping the system pressure below 90 PSI.

I do realize that I can accomplish what I want, with a electric
pressure switch and a normally open pneumatic valve. But I thought,
perhaps, there is a purely pneumatic valve that does this? What would
it be called?

Here is what I would use. The 60cfm model.

http://www.airtekltd.com/valves.htm#LOAD_GENIE

It will open and allow flow to the secondary tank at 120psi and block
the flow when the primary tank drops to 90psi. A parallel line with a
check valve to allow flow from the secondary to the primary would
allow both tanks to supply the system below 90psi.