Page 31 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Since ABC is only a moderate-sized company, it does not have sufficient staff to prepare
the 120 P&IDs (Piping and Instrumentation Diagrams) needed for the new ABS plant.
ABC hires a well-known engineering and construction firm (E&C Company), DEFCo,
to do this work for them. The company assigns two of the ABC process teams to work at
DEFCo to coordinate the job. DEFCo’s process engineers, specialists, and drafting
department prepare the P&IDs. They do much of the detailed engineering (pipe sizes,
valve specifications, etc.) as well as the actual drawing. The job may take two to six
months. Every drawing is reviewed by DEFCo’s project team and by ABC’s team. If
there are disagreements, the engineers and specialists from the companies must resolve
them.

Finally, all the PFDs and the P&IDs are completed and approved. ABC can now go
ahead with the construction. They may extend their contract with DEFCo to include this
phase, or they may go out for construction bids from a number of sources.

This narrative describes a typical sequence of events taking a project from its initial stages through plant
construction. If DEFCo had carried out the construction, ABC could go ahead and take over the plant or
DEFCo could be contracted to carry out the start-up and to commission the plant. Once satisfactory
performance specifications have been met, ABC would take over the operation of the plant and
commercial production would begin.

From conception of the process to the time the plant starts up, two or more years will have elapsed and
millions of dollars will have been spent with no revenue from the plant. The plant must operate
successfully for many years to produce sufficient income to pay for all plant operations and to repay the
costs associated with designing and building the plant. During this operating period, many unforeseen
changes are likely to take place. The quality of the raw materials used by the plant may change, product
specifications may be raised, production rates may need to be increased, the equipment performance will

decrease because of wear, the development of new and better catalysts will occur, the costs of utilities
will change, new environmental regulations may be introduced, or improved equipment may appear on the
market.

As a result of these unplanned changes, plant operations must be modified. Although the operating
information on the original process diagrams remains informative, the actual performance taken from the
operating plant will be different. The current operating conditions will appear on updated versions of the
various process diagrams, which will act as a primary basis for understanding the changes taking place in
the plant. These process diagrams are essential to an engineer who has been asked to diagnose operating
problems, solve problems in operations, debottleneck systems for increased capacity, and predict the
effects of making changes in operating conditions. All these activities are essential in order to maintain
profitable plant operation.

In this chapter, we concentrate on three diagrams that are important to chemical engineers: block flow,
process flow, and piping and instrumentation diagrams. Of these three diagrams, we will find that the
most useful to chemical engineers is the PFD. The understanding of the PFD represents a central goal of
this textbook.

1.1 Block Flow Diagrams (BFDs)