Page 290 - Chemical process engineering design and economics

Separator Design 269

D + (D + 2 ts)
D M = ———————— (6.3)
2

If Equation 6.3 is substituted into Equation 6.2, the wall thickness,

PD
ts = ——— (6.4)
eS-P

Sivals [10] summarizes values of the weld efficiency in Table 6.1. Radio-
graphic examination locates imperfections in the weld using x-rays or gamma
rays. This technique is described by Gumm and Turner [2]. Shells are either
seamless or contain a longitudinal weld. As Table 6.1 shows, the weld efficiency
depends on whether the shell is seamless or not. To use Table 6.1, first decide if
the shell will be seamless or contain a longitudinal weld. Next select the type of x-
ray required to inspect weld.
Even in a thin-walled vessel the radial stress is not exactly uniform over the
vessel thickness. To correct for this, the internal pressure in the denominator of
Equation 6.4 is multiplied by 1.2 to obtain a more accurate formula. Thus,

PD
ts = ——————— (6.5)
2sS-1.2P

To account for corrosion, the vessel thickness is increased by adding a corro-
sion allowance, to to assure that the vessel operates safely during the lifetime of a
process. Therefore, Equation 6.5 becomes

PD
ts = ——————— fc (6.6)
2eS-1.2P

The minimum corrosion allowance frequently selected is 1/8 in (3.18 mm).
Wallace and Webb [3], however, point out that arbitrarily selecting 1/8 in can be
unnecessarily costly. There may be situations where there is no corrosion at all.
The corrosion allowance should be determined by past experience, laboratory
tests, or data taken from the literature.

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