Page 121 - Plant design and economics for chemical engineers

GENERAL DESIGN CONSIDERATIONS 97

and setting the labor requirements. It should be remembered that maintenance
work will be necessary to keep the installed equipment and facilities in good
operating condition. The engineer must recognize the importance of such
factors which are directly related to plant operation and control and must take
them into proper account during the development of a design project.

Instrumentation

Instruments are used in the chemical industry to measure process variables,
such as temperature, pressure, density, viscosity, specific heat, conductivity, pH,
humidity, dew point, liquid level, flow rate, chemical composition, and moisture
content. By use of instruments having varying degrees of complexity, the values
of these variables can be recorded continuously and controlled within narrow
limits.
Automatic control is the norm throughout the chemical industry, and the
resultant savings in labor combined with improved ease and efficiency of
operations has more than offset the added expense for instrumentation. (In
most cases, control is achieved through the use of high-speed computers. In this
capacity, the computer serves as a vital tool in the operation of the plant.)
Effective utilization of the many instruments employed in a chemical process
is achieved through centralized control, whereby one centrally located control
room is used for the indication, recording, and regulation of the process
variables. Panel boards have been developed which present a graphical
representation of the process and have the instrument controls and indicators
mounted at the appropriate locations in the overall process. This helps a new
operator to quickly become familiar with the significance of the instrument
readings, and rapid location of any operational variance is possible. An example
of a graphic panel in a modern industrial plant is shown in Fig. 3-4.
Because of the many variables found in processing and the wide range
over which these variables must be determined and controlled, the assistance of
a skilled instrumentation engineer is essential in setting up a control system.
Instrumentation problems caused by transmission lags, cycling due to slow or
uncompensated response, radiation errors, or similar factors are commonly
encountered in plant operation, but most of these problems can be eliminated if
the control system is correctly designed.

Maintenance

Many of the problems involved in maintenance are often caused by a lack of
thoroughly evaluating the original design and layout of plant and equipment.
Sufficient space for maintenance work on equipment and facilities must be
provided in the plant layout, and the engineer needs to consider maintenance
and its safety requirements when making decisions on equipment.
Too often, the design engineer is conscious only of first costs and fails to
recognize that maintenance costs can easily nullify the advantages of a cheap