Page 124 - Plant design and economics for chemical engineers

100 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

is necessary, therefore, to know the characteristics of the soil at a given plant
site before the structural design can be started.
The allowable bearing pressure varies for different types of soils, and the
soil should be checked at the surface and at various depths to determine the
bearing characteristics. The allowable bearing pressure for rock is 30 or more
ton/ft2 (30 X lo4 kg/m2), while that for soft clay may be as low as 1 ton/ft2
(1 X lo4 kg/m2). Intermediate values of 4 to 10 ton/ft2 (4 X lo4 to 10 X lo4
kg/m21 apply for mixtures of gravel with sand, hard clay, and hardpan.
A foundation may simply be a wall founded on rock or hardpan, or it may
be necessary to increase the bearing area by the addition of a footing. Plain
concrete is usually employed for making footings, while reinforced concrete,
containing steel rods or bars, is commonly used for foundation walls. If possible,
a foundation should extend below the frost line, and it should always be
designed to handle the maximum load. Pilings are commonly used for support-
ing heavy equipment or for other special loads.
Maintenance difficulties encountered with floors and roofs should be given
particular attention in a structural design. Concrete floors are used extensively
in the process industries, and special cements and coatings are available which
make the floors resistant to heat or chemical attack. Flat roofs are often
specified for industrial structures. Felt saturated with coal-tar pitch combined
with a coal-tar pitch-gravel finish is satisfactory for roofs of this type. Asphalt-
saturated felt may be used if the roof has a slope of more than $in./ft (4.17
cm/m).
Corrosive effects of the process, cost of construction, and climatic effects
must be considered when choosing structural materials. Steel and concrete are
the materials of construction most commonly used, although wood, aluminum,
glass blocks, cinder blocks, glazed tile, bricks, and other materials are also of
importance. Allowances must be made for the type of lighting and drainage, and
sufficient structural strength must be provided to resist normal loads as well as
extreme loads due to high winds or other natural causes.
In any type of structural design for the process industries, the function of
the structure is more important than the form. The style of architecture should
be subordinated to the need for supplying a structure which is adapted to the
proposed process and has sufficient flexibility to permit changes in the future.
Although cost is certainly important, the engineer preparing the design should
never forget the fact that the quality of a structure remains apparent long after
the initial cost is forgotten.

STORAGE
Adequate storage facilities for raw materials, intermediate products, final prod-
ucts, recycle materials, off-grade materials, and fuels are essential to the
operation of a process plant. A supply of raw materials permits operation of the
process plant regardless of temporary procurement or delivery difficulties.
Storage of intermediate products may be necessary during plant shutdown for
emergency repairs while storage of final products makes it possible to supply the