Page 202 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

has excellent corrosion resistance in acidic environments, is shown as a function of temperature in Figure
7.5.

Figure 7.5 Maximum Allowable Stresses for Materials of Construction as a Function of Operating
Temperature (Data from Perry et al. [3], Chapter 10, and Ref [15])

The relationship between cost of a vessel and its operating pressure is a complex one. However, with all
other things being constant, the cost of the vessel is approximately proportional to the weight of the
vessel, which in turn is proportional to the vessel thickness. From Equation 7.9, it is clear that as the
operating pressure approaches 1.67SE, the required wall thickness, and hence cost, becomes infinite.
Moreover, the thickness of the vessel for a given pressure will increase as the vessel diameter increases.

The effect of pressure on the weight (and ultimately cost) of carbon steel vessel shells as a function of
vessel diameter is shown in Figure 7.6. The y-axis of the figure shows the ratio of the vessel thickness at
the design pressure to that at ambient pressure, and the x-axis is the design pressure. A corrosion
allowance of 3.15 mm (1/8 inch) and a value of S = 944 bar (13,700 psi) are assumed. It is also assumed
that the vessel is designed with a minimum wall thickness of 6.3 mm (1/4 inch). A minimum wall