Page 293 - Chemical process engineering design and economics

272 Chapter 6

Because the operating pressure in a vessel may fluctuate, for safety, process
engineers will use a design pressure in Equations 6.6 to 6.8 to calculate the wall
thickness. The design pressure is 1.10 times the expected operating pressure or the
expected operating pressure plus 25 psi, whichever is greater. For carbon steel, the
calculated vessel thickness is rounded off according to the rules listed in Table 6.2.
For high columns, the thickness at the bottom of the column may have to be in-
creased further because of wind load. Mulet et al. [8] describe a calculation proce-
dure to determine the effect of wind load on wall thickness. Other factors will also
affect the strength of vessels, such as nozzles and manholes. To take these factors
into account, an engineer must follow the ASME pressure vessel code. Table 6.3
summarizes the equations for calculating the vessel wall thickness, and Table 6.4
outlines the calculation procedure.

Table 6.3 Summary of Equations for Calculating Vessel Wall Thickness

P = 1.1P 0' — or (6.3.1)
P = P 0' + 25 psi — whichever is larger

For a shell

P
a s = ——————— (6.3.2)
2s s 'S'-1.2P

For a torispherical head:

1.104 P
<X H = ———————— — or (6.3.3)
2 E H' S' - 0.2 P

For a 2:1 ellipsoidal head:

P
«H = ———————————
2 e H' S' - 0.2 P

t s = a s D' + t c' (6.3.4)

t H = a H D' + t c' (6.3.5)

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