Page 146 - Petroleum and Gas Field Processing

and the liquid retention time constraint is used to determine the height of the
vessel. For three-phase separators, however, a third constraint is added. This
is the requirement to settle water droplets of a certain minimum size out of
the oil pad. This results, as shown in the following subsection, in a second
value for the minimum diameter of the separator. Therefore, in selecting the
diameter of the vessel, the larger of the minimum diameters determined from
the gas capacity constraint and water settling constraint should be considered
as the minimum acceptable vessel diameter.

Water Droplets Settling Constraint
The condition for the settling and separation of water droplets from the
oil is established by equating the average upward velocity of the oil phase,
u o , to the downward settling velocity of the water droplets of a given
size, u w . The average velocity of the oil is obtained by dividing the oil flow
rate by the cross-sectional area of flow. If Q o is the oil rate (in BPD), then

3
Q o 5:61 ft 4 144 2
u o ¼ 2 ft
24 3600 s D
ft
Q o
u o ¼ 0:0119 2 ð20Þ
D s
Equation (7) gives the water droplet settling velocity:

2
6 ð
Þd m ft
u w ¼ 1:787 10 ð21Þ
s
o
For water droplets to settle out of the oil, u w must be larger than u o .
Equating u w to u o would result, therefore, in determining the minimum
diameter of the separator, D min , that satisfies the water settling constraint.
From Eqs. (20) and (21), it follows that

2 Q o o 2
D min ¼ 6686 2 in: ð22Þ
ð
Þd m
where D is the separator internal diameter (in.), Q o is the oil flow rate
(BPD), m o is the oil viscosity (cP),
is the difference in specific gravity of
oil and water, and d m is the minimum water droplet size to be separated
from gas (mm) microns.
Any diameter, larger than the minimum diameter determined from
Eq. (22) yields a lower average oil velocity and, thus, ensures water
separation.

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