Page 100 - Petroleum and Gas Field Processing

The gas travels horizontally along the effective length of the separator,
L(ft), in a time t g that is given by
L
t g ¼ s ð17Þ
u g
This time must, at least, be equal to the time it takes the smallest oil
droplet, to be removed from the gas, to travel a distance of D/2 to reach
the gas–oil interface. This settling time, t s , is obtained by dividing the
distance (D/2) by the settling velocity from Eq. (2); therefore,
1
( )
D o g d m
1=2
t s ¼ 0:01186 s ð18Þ
2 12 g C d
Equating Eqs. (18) and (17), substituting for u g from Eq. (16), and solving
for the product LD, we obtain
Q g TZ g C d
1=2
LD ¼ 422 ft in: ð19Þ
P o g d m
Equation (19) provides a relationship between the vessel diameter and
effective length that satisfies the gas capacity constraint. Any combination
of D and L satisfying Eq. (19) ensures that all oil droplets having diameter
d m and larger will settle out of the gas flowing at a rate of Q g MMSCFD

into the separator that is operating at P psia and T R.
Liquid Capacity Constraint
As explained earlier, the separator has to have a sufficient volume to
retain the liquid for the specified retention time before it leaves the
separator. For a horizontal separator that is half full of liquid, the volume
occupied by the liquid is given by

D 2 3
V o ¼ 0:5 L ft
4 12
Substituting in Eq. (8), the following equation is obtained:
2
D L ¼ 1:428Q t ft 3 ð20Þ
o
Equation (20) provides another relationship between D and L that satisfies
the liquid capacity (retention) time constraint.

Sizing Procedure
For a given set of operating conditions (pressure, temperature, gas and oil
flow rates, gas and oil properties, and oil retention time), the size

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