Page 52 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

what to observe in order to monitor the effects of the change. Plant simulators (similar to flight
simulators) are sometimes involved in operator training. These programs are sophisticated, real-time
process simulators that show a trainee operator how quickly changes in controlled variables propagate
through the process. It is also possible for such programs to display scenarios of process upsets so that
operators can get training in recognizing and correcting such situations. These types of programs are very
useful and cost-effective in initial operator training. However, the use of P&IDs is still very important in
this regard.

The P&ID is particularly important for the development of start-up procedures where the plant is not
under the influence of the installed process control systems. An example of a start-up procedure is given
in Example 1.10.

Example 1.10

Consider the start-up of the distillation column shown in Figure 1.7. What sequence would be followed?
The procedure is beyond the scope of this text, but it would be developed from a series of questions such
as
a. What valve should be opened first?
b. What should be done when the temperature of... reaches...?
c. To what value should the controller be set?
d. When can the system be put on automatic control?

These last three sections have followed the development of a process from a simple BFD through the PFD
and finally to the P&ID. Each step showed additional information. This can be seen by following the
progress of the distillation unit as it moves through the three diagrams described.
1. Block Flow Diagram (BFD) (see Figure 1.1): The column was shown as a part of one of the
three process blocks.
2. Process Flow Diagram (PFD) (see Figure 1.5): The column was shown as the following set of

individual equipment: a tower, condenser, reflux drum, reboiler, reflux pumps, and associated
process controls.
3. Piping and Instrumentation Diagram (P&ID) (see Figure 1.7): The column was shown as a
comprehensive diagram that includes additional details such as pipe sizes, utility streams,
sample taps, numerous indicators, and so on. It is the only unit operation on the diagram.

The value of these diagrams does not end with the start-up of the plant. The design values on the diagram
are changed to represent the actual values determined under normal operating conditions. These
conditions form a “base case” and are used to compare operations throughout the life of the plant.

1.4 Additional Diagrams

During the planning and construction phases of a new project, many additional diagrams are needed.
Although these diagrams do not possess additional process information, they are essential to the
successful completion of the project. Computers are being used more and more to do the tedious work
associated with all of these drawing details. The creative work comes in the development of the concepts
provided in the BFD and the process development required to produce the PFD. The computer can help