Page 53 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

with the drawings but cannot create a new process. Computers are valuable in many aspects of the design
process where the size of equipment to do a specific task is to be determined. Computers may also be
used when considering performance problems that deal with the operation of existing equipment.
However, they are severely limited in dealing with diagnostic problems that are required throughout the
life of the plant.

The diagrams presented here are in both American Engineering and SI units. The most noticeable
exception is in the sizing of piping, where pipes are specified in inches and pipe schedule. This remains
the way they are produced and purchased in the United States. A process engineer today must be
comfortable with SI, conventional metric, and American (formerly British, who now use SI exclusively)
Engineering units.

We discuss these additional diagrams briefly below.

A utility flowsheet may be provided that shows all the headers for utility inputs and outputs available
along with the connections needed to the process. It provides information on the flows and characteristics
of the utilities used by the plant.

Vessel sketches, logic ladder diagrams, wiring diagrams, site plans, structural support diagrams, and
many other drawings are routinely used but add little to our understanding of the basic chemical processes
that take place.

Additional drawings are necessary to locate all of the equipment in the plant. Plot plans and elevation
diagrams are provided that locate the placement and elevation of all of the major pieces of equipment
such as towers, vessels, pumps, heat exchangers, and so on. When constructing these drawings, it is
necessary to consider and to provide for access for repairing equipment, removing tube bundles from heat
exchangers, replacement of units, and so on. What remains to be shown is the addition of the structural
support and piping.

Piping isometrics are drawn for every piece of pipe required in the plant. These drawings are 3-D
sketches of the pipe run, indicating the elevations and orientation of each section of pipe. In the past, it
was also common for comprehensive plants to build a scale model so the system could be viewed in
three dimensions and modified to remove any potential problems. Over the past twenty years, scale

models have been replaced by three-dimensional computer aided design (CAD) programs that are
capable of representing the plant as-built in three dimensions. They provide an opportunity to view the
local equipment topology from any angle at any location inside the plant. One can actually “walk through”
the plant and preview what will be seen when the plant is built. The ability to “view” the plant before
construction will be made even more realistic with the help of virtual reality software. With this new
tool, it is possible not only to walk through the plant but also to “touch” the equipment, turn valves, climb
to the top of distillation columns, and so on. In the next section, the information needed to complete a
preliminary plant layout design is reviewed, and the logic used to locate the process units in the plant and
how the elevations of different equipment are determined are briefly explained.

1.5 Three-Dimensional Representation of a Process

As mentioned earlier, the major design work products, both chemical and mechanical, are recorded on