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4. For each combination of D and L s , the slenderness ratio, SR,
defined as the ratio of length to diameter is determined.
Separators with SR between 3 and 4 are commonly selected.

3.6.7 Sizing Horizontal Gas–Oil Separators
As with vertical separators, the size (diameter and length) of the horizontal
separator is determined by consideration of its required capacity for gas
and oil. It has been shown that the gas capacity constraint for vertical
separators determines the minimum allowable vessel diameter. For
horizontal separators, however, the gas capacity constraint yields, as
shown in the following subsection, a relationship between the diameter
and effective length of the separator. This along with a similar relationship
derived from the liquid capacity constraint are used in determining the size
of the separator. In reality, either the gas capacity constraint or the liquid
capacity constraint governs the design and only one of the two constraints
equations is used in determining the size.
In the following discussion, and as mentioned earlier, it is assumed
that each of the gas and oil phases occupies 50% of the effective separator
volume. Similar equations as those derived below could be obtained for
other situations where either of the two phases occupies more or less than
50% of the separator effective volume.

Gas Capacity Constraint
Because the gas occupies the top half of the separator, its average
flowing velocity within the separator, u g , is obtained by dividing the
volumetric flow rate, Q g , by one-half of the separator cross-sectional area,
A; that is,

Q g
u g ¼ 2
0:5½ð =4ÞD Š

Q g is usually reported in units of MMSCFD and should, therefore, be
3
converted into actual ft /s; also D, which is usually given in inches, should
be converted into feet in order to obtain the velocity in units of ft/s. The
above equation, therefore, becomes

Q g TZ ft

u g ¼ 120 ð16Þ
D 2 P s

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.