Page 201 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

A comparison of the value of bare module cost factor for Example 7.10 shows that it is the same as the
value of 3.29 evaluated using the individual values for α , given in Example 7.9.
i

7.3.4 Bare Module Cost for Nonbase Case Conditions

For equipment made from other materials of construction and/or operating at nonambient pressure, the
values for F and F are greater than 1.0. In the equipment module technique, these additional costs are
M
P
incorporated into the bare module cost factor, F BM . The bare module factor used for the base case, ,
is replaced with an actual bare module cost factor, F BM , in Equation 7.6. The information needed to
determine this actual bare module factor is provided in Appendix A. The effect of pressure on the cost of

equipment is considered first.

Pressure Factors. As the pressure at which a piece of equipment operates increases, the thickness of the
walls of the equipment will also increase. For example, consider the design of a process vessel. Such

vessels, when subjected to internal pressure (or external pressure when operating at vacuum) are subject
to rigorous mechanical design procedures. For the simple case of a cylindrical vessel operating at greater
than ambient pressure, the relationship between design pressure and wall thickness required to withstand
the radial stress in the cylindrical portion of the vessel, as recommended by the ASME [13], is given as

(7.9)

where t is the wall thickness in meters, P is the design pressure (bar), D is the diameter of the vessel (m),
S is the maximum allowable working pressure (maximum allowable stress) of material (bar), E is a weld
efficiency, and CA is the corrosion allowance (m). The weld efficiency is dependent on the type of weld
and the degree of examination of the weld. Typical values are from 1.0 to 0.6. The corrosion allowance
depends on the service, and typical values are from 3.15 to 6.3 mm (0.125 to 0.25 inches). However, for
very aggressive environments, inert linings such as glass and graphite are often used to protect the
structural metal. Finally, the maximum working pressure of the material of construction, S, is dependent
not only on the material but also on the operating temperature. Some typical values of S are given for
common materials of construction in Figure 7.5. From this figure, it is clear that for typical carbon steel
the maximum allowable stress drops off rapidly after 350°C. However, for stainless steels (ASME SA-
240) the decrease in maximum allowable stress with temperature is less steep, and operation up to 600–
650°C is possible for some grades. For even higher temperatures and very corrosive environments, when
the lining of vessels is not practical, more exotic alloys such as titanium and titanium-based alloys and

nickel-based alloys may be used. For example, Hastelloy B has excellent resistance to alkali
environments up to 850°C. Inconel 600, whose main constituents are Ni 72%, Cr 15%, and Fe 8%, has
excellent corrosion resistance to oxidizing environments such as acids and can be used from cryogenic
temperatures up to 1100°C. The maximum allowable working pressure for Incoloy 800HT, which also