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Separator Design 279

Next, calculate the shell thickness from Equations 6.3.2 and 6.3.4. From Ta-
ble 6.1, with no x-ray inspection of the longtudinal weld, s = 0.7. Again, if we
neglect the corrosion allowance,

25 (36)
t s = —————————————— = 0.04235 in(1.08 mm)
2 (0.7) (15200) -1.2 (25)

Thus, the shell wall thickness is essentially the same as the head thickness.
According to Table 6.2, the minimum wall thickness is 3/32 in (2.38 mm) for
high-alloy steels. The application of this rule-of-thumb more than doubles the wall
thickness, which should be an adequate corrosion allowance. The selection of a
corrosion allowance in the final design must be based on past experience or from
laboratory and pilot plant tests.

PHASE SEPARATORS
Gas-Liquid Separators
As stated by Holmes and Chen [12], the reasons for using gas-liquid or vapor-
liquid separators are to recover valuable products, improve product purity, reduce
emissions, and protect downstream equipment. Gas-liquid separators are used after
flashing a hot liquid across a valve. In this case the separator is called a flash
drum.
A vertical gas-liquid separator is shown in Figure 6.4. The gas-liquid mix-
ture is separated by gravity and impaction. The mixture enters the separator about
midway where a splash plate deflects the stream downward. Most of the liquid
flows downward, and the vapor, containing liquid drops, flow upward. As the
vapor rises, large drops settle to the bottom of the separator by gravity. According
to Watkins [14], 95 % separation of liquid from vapor is normal. If greater than 95
% liquid separation is required, then use a wire-mesh mist eliminator, installed
near the vapor outlet. Very small drops are separated by impaction using a wire-
mesh pad located at the top of the separator. The mesh usually consists of 0.011 in
(0.279 mm) diameter wires interlocked by a knitting machine to form a pad from 4
to 6 in (0.102 to 0.152 m) thick [12]. Entrained liquid drops in the vapor impact on
the wires and coalesce until the drops become heavy enough to break away from
the wire and fall to the bottom of the separator. Because of the large free volume
of the pad - 97 to 99 % - the pressure drop across the pad is usually less than 1.0
in of water [13]. The separation efficiency of a pad is about 99.9% or greater.
The major objective in sizing a gas-liquid separator is to lower the gas ve-
locity sufficiently to reduce the number of liquid droplets from being entrained in
the gas. Thus, the separator diameter must be determined. The separator is also
designed as an accumulator for the liquid portion of the stream. Thus, the liquid

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