Page 289 - Chemical process engineering design and economics

268 Chapter 6

VESSEL DESIGN

Although a mechanical or civil engineer normally designs vessels, the process
engineer should have some knowledge of the mechanical design of vessels. For
example, the process engineer may have to make a preliminary design of vessels
for a cost estimate. Reactors, fractionators, absorbers, heat exchangers, and some
phase separators are classified as vessels. What makes an absorber an absorber,
for example, is its internal design. A vessel consists of a cylindrical shell and end
caps, called heads. For safety, vessel design is governed by codes. An example is
the ASME (American Society of Mechanical Engineers) Boiler and Pressure Ves-
sel Code. Engineers who agreed on what is a safe procedure for designing vessels
formulated this code.
Most vessels in the process industries are thin-walled vessels, which have a
wall thickness of less than about 5% of the inside diameter of a vessel. Internal
pressure acting on the walls of a cylindrical vessel produces a longitudinal and
radial stress, also called hoop stress. For thin-wall vessels, it may be assumed that
the radial stress is approximately uniform across the wall. Rase and Borrow [1],
for example, showed that the radial stress, produced by an internal pressure, P, is
given by Equation 6.1.

PD
S = —— (6.1)
4t s
where the diameter of the vessel is D. The radial stress is larger than the longitudi-
nal stress, and thus it must be used to calculate the wall thickness, 1$. If a cylindri-
cal vessel fails, it will split longitudinally.
Vessels larger in diameter than about 30 in (0.672 m) and above are fabri-
cated from plates, which are formed into cylinders, called shells, and welded
longitudinally. Shells smaller than 30 in (0.672) may be extruded and thus will not
contain a longitudinal weld. Shells may then be joined by welding circumferen-
tially to form longer shells. After fabricating the shell, end caps, called heads, are
welded to the shell to form the vessel. Because the weld may have imperfections,
the radial stress will be less than its maximum value. Thus, S is multiplied by a
joint or weld efficiency, E, which depends on the type of x-ray inspection of the
weld. Thus,

PD M
o o ———— (0.^1
4t s

where the mean diameter, D, is the average of the outside and inside diameters.
M

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