Page 66 - Design of Simple and Robust Process Plants

50 Chapter 3 Design Philosophies
. Process sequences for start-up and shut down with detailed step descriptions.
. Alarming strategy including interlocking and trip actions.
. Protection levels ± who has access to what.
. Program testing.
. Operator training on sequences and interrupts.

3.3.7.2 Robust hands-off control
In the present situation a panel operator is still part of the control loop, but this
situation needs to be resolved. It is not an easy task, and will require additional
investment in the design of robust control. The control of a process is carried out
in three hierarchical layers, see Figure 8.2:
1. Basic control: this must be designed in such a way that it functions indepen-
dently of the model-based control, as well as the operation optimization layer.
2. Model-based control: this will be dependent on the basic control layer, but
can operate independently of the operation optimization layer.
3. Operation optimization: this layer depends on the basic control layer, as well
as the model-based control layer.
Basic control design is currently mainly based on heuristic rules (Luyben et al., 1998),
the governing philosophy of which is: ªIt is always best to utilize the simplest control
system that will achieve the desired objectiveº. In respect of the hierarchical control
layers (of which the model-based and optimization layers cannot be considered sim-
ple), a philosophy for the basic control layer is introduced in line with Luyben's
statement:

Basic control design has conceptual to be simple

This concept means that we should avoid closing loops which will cause much inter-
action, but will also avoid inverse responses. These situations would require intense
operator attention and thus would not comply with a simple and robust design. For
example, if an operating unit has two controlled variables and two manipulated vari-
ables that cause a high level of interaction (e.g., the two qualities of a distillation
column), it is preferable to close only one quality loop. In practice, the stream used
for internal recycling will not be directly quality controlled, and this stream will
absorb the disturbances. In the case where both streams are final products, one
stream will run at a higher purity if the model-based controller is not functioning.
In the case where the interaction or disturbances can be de-coupled at the basic con-
trol level by simple algebraic equations, it should be implemented at the basic level.
The design of basic control configuration will move from heuristic to fundamen-
tal design procedures, whereupon the following aspects must be considered:
. Control design will have to be carried out in interaction with the process
design. Controllability analysis techniques would be used to select the final
flowsheet to balance optimal design and controllability.