practical info source??

WH wrote:

I have managed dual, parallel valves by putting the small one on the
PID temperature controller, and using that controller's output as the
process variable for a slower tuned PI controller, whose setpoint is a
constant somewhere around 50%.  This forces the large valve to restore
the small one to the middle of its range after a load change, so the
small one is in an optimal position to make smooth and quick (but
modest) corrections from that starting point.

If you put the small valve in manual and close it, the large one (if
left in automatic) smoothly closes.  During startup, open the small
valve to anything less than 50% and it is the total control.  But be
careful leaving it in manual above 50%.  This forces the large valve
to smoothly go to 100% at a speed proportional the the amount of
output on the small one, above 50%.

Thanks John.  That's great info!  I'll give it a go.
-Warren

Not Temperature specific but have a look at these:

http://www.controlloop.co.za/
http://www.tcnj.edu/~rgraham/PID/popelish.html

[snip]

I'm probably missing something here (as I often do). Is the vessel
pressurized? Because if it ain't you're gonna have a hell of a time heating
water to 123C.