[supercat]

It opened the valve and held it open against water pressure.

That requires a 120CUFT N2 bottle? Why not use a pilot-valve arrangement like that used in electropneumatic pipe organs (in a pipe organ, a small valve allows pressurized air into a bellows which then operates the main valve; for water, I'd sugget having a small valve let water into a piston to do likewise).

[Old Professer]

I didn't design it, I just made the rounds with the fire inspector testing the units. I arranged to have all the bottles hydrostated and they all passed - even the one stamped 1937 as its first hydrostat date. This was in 1991.