Page 95 - Petroleum and Gas Field Processing

substitute 32.17 for g in the above equation to obtain the following
expression for the settling velocity:

1=2
o
u ¼ 0:01186 g d m ft=s ð3Þ
g C d
As mentioned earlier, in designing gas–oil separators, the smallest oil
droplet to be removed from the gas in the gravity settling section is
normally taken as 100 mm. Under such a condition, the mist extractor will
be capable of removing oil droplets smaller than 100 mm without getting
flooded.
There are other special separators known as gas scrubbers that are
normally used to remove liquid from gas streams, which have been
through regular gas–liquid separators. Such gas streams are mostly gas
with little liquid that may have formed due to condensation. Examples of
such separators are the gas scrubbers used at the inlet to gas compressors
and gas dehydration facilities. Scrubbers are also used on vents and flares.
Because the amount of liquid is very small, the design of such gas
scrubbers could be based on separation of liquid droplets up to 500 mmin
the gravity settling section with no danger of flooding the mist extractor.

3.6.4 Gas Capacity of Separators
The volumetric flow rate of the gas processed by a gas–oil separator is
directly related to the cross-sectional area of flow and the maximum
allowable gas velocity at which the oil droplets will be suspended and not
carried over. Mathematically,

3
Q g ¼ A g u ft =s ð4Þ
3
Equation (4) gives Q g in ft /s under actual separator pressure and
temperature. However, the volumetric flow rate of gas is normally

reported at standard pressure and temperature of 14.7 psia and 520 R,
respectively. Typically, Q g is reported in units of millions of standard cubic
feet per day (MMSCFD). Equation (4) could, therefore, be written as

P 520
6
Q g ¼ð10 60 60 24ÞA g u
14:7 TZ
ð5Þ
P

Q ¼ 3:056 uA g MMSCFD
g
TZ

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