Page 93 - Petroleum and Gas Field Processing

the separator should be equipped with sand jets and drains along the
bottom of the separator. Normally, produced water is injected though
the jets to fluidize the accumulated sand, which is then removed through
the drains.

3.6 DESIGN PRINCIPLES AND SIZING OF GAS–OIL
SEPARATORS

In this section, some basic assumptions and fundamentals used in sizing
gas–oil separators are presented first. Next, the equations used for
designing vertical and horizontal separators are derived. This will imply
finding the diameter and length of a separator for given conditions of oil
and gas flow rates, or vice versa. Solved examples are also given to
illustrate the use of these equations.

3.6.1 Assumptions
1. No oil foaming takes place during the gas–oil separation
(otherwise retention time has to be drastically increased as
explained earlier).
2. The cloud point of the oil and the hydrate point of the gas are
below the operating temperature.
3. The smallest separable liquid drops are spherical ones having a
diameter of 100 mm.
4. Liquid carryover with the separated gas does not exceed 0.10
gallon/MMSCF (M ¼ 1000).

3.6.2 Fundamentals
1. The difference in densities between liquid and gas is taken as a
basis for sizing the gas capacity of the separator ( o g ).
2. A normal liquid (oil) retention time for gas to separate from oil
is between 30 s and 3 min. Under foaming conditions, more time
is considered (5–20 min). Retention time is known also as the
residence time ( ¼ V/Q, where V is the volume of vessel occupied
by oil and Q is the liquid flow rate).
3. In the gravity settling section, liquid drops will settle at a
terminal velocity that is reached when the gravity force F g
acting on the oil drop balances the drag force (F d ) exerted by
the surrounding fluid or gas.

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