Unstable variables: The levels in

the reactor (L1) and

the flash drum (L2)

Stable variables: Examples include

Pressure in the flash drum (P1), which is stable, but the steady-state may be unacceptably high.  In this case, the pressure is a key safety-related variable even though is it stable.

Temperature after the feed heat exchanger (T3) which will achieve a steady-state after a perturbation in the feed flow or heating medium flow.