Page 337 - Design of Simple and Robust Process Plants

8.4 Control Design 323

Fig. 8.15. Control design activities for a robust basic control
layer based on first principle models.

The control objective for the basic control layer is restricted in the decentralized con-
trol concept. The opportunities for process optimization and constraint control are
reserved for the higher control layers. Therefore, the control objective for the basic
control layer is defined as: Robust control of a process plant under defined process
condition of operation, by setting master set points for capacity, feed, product mix
and specifications, for a selected level of disturbances while respecting safety and
environmental requirements and operational constraints. Respecting safety and
environmental requirements must be interpreted as: initiate action in terms of alert-
ing, (pre)alarming and interlocking actions to comply with these requirements, in
case normal control actions are insufficient. Operational constraints are process con-
ditions where the process would not be able to function optimal by exceeding. These
conditions include capacity limits and temperature limits. The definition does not
exclude self-optimizing unit control (to be described later) as practiced at the basic
control level.

8.4.3
Evaluate Open Loop Stability

A controllability study is started with the determination of the open loop stability±
through perturbations of static process simulations around the operating point(s).
Normally, we like to avoid open loop unstable designs. Process modification, mostly