Analysis and design
Dynamics and control
A steady state says where a plant settles. Dynamics says how it gets there, and whether it stays when something upsets it.
The steps, in order
- Solve the steady state. A dynamic run starts from a converged flowsheet — the only defensible starting point.
- Vessels — give each holdup a volume. In steady state a drum needs none; in dynamics the volume is the time constant, so a flowsheet that was never sized has no dynamics to speak of.
- Lines & machines — pipe resistance and pump or compressor curves, where flow should follow pressure rather than be fixed.
- Controllers — what each loop measures, what it moves, its setpoint, and its gain, integral and derivative times. Direct or reverse acting.
- Disturbances — scheduled upsets: a step or a ramp in a feed flow, a feed temperature or composition, a valve or a duty, at a time you choose.
- Results — run, and read the trends and each controller's faceplate.
Making a run useful
- Run once with no disturbances: the plant should sit still. If it drifts, something in the sizing or the controllers is not at its steady state yet.
- Tune one loop at a time — level first, then pressure, then temperature and composition.
- A reacting CSTR keeps reacting in Dynamics, with its cooling as something a controller can move — take the cooling away and watch where it settles.
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