Page 57 - Plant design and economics for chemical engineers

‘to PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

developing the individual components of the design. Each safety factor must be
chosen on basis of the existing conditions, and the chemical engineer should not
hesitate to use a safety factor of zero if the situation warrants it.

SPECIFICATIONS
A generalization for equipment design is that standard equipment should be
selected whenever possible. If the equipment is standard, the manufacturer may
have the desired size in stock. In any case, the manufacturer can usually quote a
lower price and give better guarantees for standard equipment than for special
equipment.
The chemical engineer cannot be an expert on all the types of equipment
used in industrial plants and, therefore, should make good use of the experience
of others. Much valuable information can be obtained from equipment manu-
facturers who specialize in particular types of equipment.
Before a manufacturer is contacted, the engineer should evaluate the
design needs and prepare a preliminary specification sheet for the equipment.
This preliminary specification sheet can be used by the engineer as a basis for
the preparation of the final specifications, or it can be sent to a manufacturer
with a request for suggestions and fabrication information. Preliminary specifi-
cations for equipment should show the following:

1. Identification
2. Function
3. Operation
4. Materials handled
5. Basic design data
6. Essential controls
7. Insulation requirements
8. Allowable tolerances
9. Special information and details pertinent to the particular equipment, such
as materials of construction including gaskets, installation, necessary delivery
date, supports, and special design details or comments

Final specifications can be prepared by the engineer; however, care must
be exercised to avoid unnecessary restrictions. The engineer should allow the
potential manufacturers or fabricators to make suggestions before preparing
detailed specifications. In this way, the final design can include small changes
that reduce the first cost with no decrease in the effectiveness of the equipment.
For example, the tubes in standard heat exchangers are usually 8, 12, 16, or 20 ft
long, and these lengths are ordinarily kept in stock by manufacturers and
maintenance departments. If a design specification called. for tubes 15 ft Ion&
the manufacturer would probably use 16-ft tubes cut off to the specified length.