Another option...
I like the belt-and-suspenders approach much better: First line of defense is a solenoid that opens on the end of the header when all tanks are off. Easy logic on most controllers that have two outputs. Or add relays to your control panel if they don't. Easy option to keep glycol flowing without undue heating from pumping against a pressure relief valve. And then a pressure relief bypass as close to the chiller as possible. Internal if possible. That should only actuate when the solenoid, controller, or relay doesn't work properly. VFDs are also another way to keep header pressure constant regardless of tank loading. Bit more work and a bit more complex, but they work great. Remember that all energy you put into the system costs you money once for the energy cost of input, but also (and more expensively) again when remove that energy in the form of heat. If you limit how much goes in, you save money twice. This is a cheap and simple approach.
Phillip Kelm--Palau Brewing Company Manager--