Page 147 - Petroleum and Gas Field Processing

Gas Capacity Constraint
As described in Chapter 3, the gas capacity constraint for a vertical
separator yields an expression for the minimum vessel diameter as follows:

1=2
TZ g C d 2

2
D min ¼ 5058Q g in ð23Þ
P o g d m
where D is the separator internal diameter (in.), T is the operating

temperature ( R), Z is the gas compressibility at operating pressure and
temperature, P is the operating pressure, g and o are the gas and oil
3
densities (1b/ft ), C d is the drag coefficient, and d m is the minimum oil
droplet size to be separated from gas (mm).
Any diameter that is larger than the minimum diameter determined
from Eq. (23) results in a lower gas velocity and, therefore, ensures settling
and separation of liquid droplets of diameters equal to and larger than d m
out of the gas.

Liquid Retention Time (Capacity) Constraint

The separator must provide sufficient volume for the oil and water to be
retained within the separator for the required retention times. The
retention times are determined to allow separation of the entrained water
droplets from the oil, separation of the entrained oil droplets from the
water, and for the oil to reach equilibrium with the gas. As explained
earlier, retention times are best determined from laboratory tests and they
normally range from 3 to 30 min based on fluid properties and operating
conditions. In the absence of laboratory data, a retention time of 10 min
may be used for both oil and water.
Let H o and H w be the heights of the oil and water (in in.),
respectively. Therefore, the volume of each phase within the separator is
given by

1 3 2 3
V ¼ D H o ft
o
12 4
and

1 3 2 3

V w ¼ D H w ft
12 4
Therefore,
4 2 3
V o þ V w ¼ 4:543 10 D ðH o þ H w Þ ft ð24Þ

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