Petroleum and Gas Field Processing

Lecture 3
Sizing Vertical Gas–Oil Separators
• Capacity for gas and oil are considered when the size (diameter and height or length) of a separator is
normally determined.

1) Gas Capacity Constraint

For vertical separators, the upward average gas velocity should not exceed the downward terminal velocity
of the smallest oil droplet to be separated. This condition is expressed mathematically by equating Eqs. (3)
and (6)

where D is the internal diameter of the separator in inches, and then solving for D, we obtain

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• Equation (9) provides the minimum acceptable diameter of the separator.
• Larger diameters yield lower gas velocities and, thus, better separation of the oil droplets
from the gas.
• Smaller diameters yield higher gas velocities and, therefore, the liquid droplets will be
carried over with the gas.
• In solving Eq. (9), the value of the drag coefficient Cd must first be determined. Cd is related
to the Reynolds number, Re, according to the following formula:

where u is settling or terminal velocity, the µg is the gas viscosity. 2

The velocity, u, is given by Eq. (3) and is a function of Cd. Therefore, Cd could only be
determined by an iterative procedure as follows:

1. Assume a value for Cd (a value of 0.34 could be used as a first assumption).

2. Calculate the terminal velocity, u, from Eq. (3).
3. Calculate Re from Eq. (11).
4. Calculate Cd from Eq. (10) and compare to the assumed value.
5. If no match is obtained, use the calculated value of Cd and repeat steps 2–4 until
convergence is obtained.

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2) Oil Capacity Constraint
• The oil has to be retained within the separator for a specific retention time, t.
• The volume of separator occupied by oil, Vo, is obtained by multiplying the cross-
sectional area by the height of the oil column, H (in.).

Equation (8) could, therefore, be rewritten as

Vertical separator
schematic

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Sizing Procedure

The size (diameter and seam-to-seam length or height) of a vertical separator is determined
as follows:

1. Check for gas capacity constraint, Eq. (9) which is used to determine the minimum allowable vessel
diameter (Dmin).

2. Check for liquid capacity (retention time constraint), Eq. (13)

3. Select values for D larger than (Dmin) and substitute in Eq. (13) to calculate the values of H.
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4. The seam-to-seam length, Ls, is determined using one of the following expressions as
appropriate:

5. For each combination of D and Ls, the slenderness ratio, SR, defined as the ratio of length
to diameter is determined. Separators with SR between 3 and 4 are commonly selected.

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Note: Z, µ, ro and rg will be given in the exam, so you do not need to calculate them. 8
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both are acceptable. A preferred selection will be a 36-in. diameter by 12-ft separator as
compared to a 42-in. by 11-ft separator. This is because the 36-in. separator is a standard
unit and would probably be less costly than the larger-diameter, shorter separator.
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Sizing Horizontal Gas–Oil Separators

• As with vertical separators, the size (diameter and length) of the horizontal separator is
determined by consideration of its required capacity for gas and oil.

• In the following discussion, it is assumed that each of the gas and oil phases occupies 50%
of the effective separator volume.

• Similar equations as those derived could be obtained for other situations where either of
the two phases occupies more or less than 50% of the separator effective volume.

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1) Gas Capacity Constraint

Because the gas occupies the top half of the separator, its average flowing velocity
within the separator, ug, is obtained by dividing the volumetric flow rate, Qg, by
one-half of the separator cross-sectional area, A; that is,

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Horizontal separator schematic
Qg is usually reported in units of MMSCFD and should, therefore, be converted into actual ft3/s;
also D, which is usually given in inches, should be converted into feet in order to obtain the
velocity in units of ft/s. The above equation, therefore, becomes

The gas travels horizontally along the effective length of the separator, L(ft), in a time tg that is
given by

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. 14
This settling time, ts, is obtained by dividing the distance (D/2) by the settling velocity from Eq. (3); therefore,

Horizontal separator schematic

Equating Eqs. (18) and (17), substituting for ug from Eq. (16), and solving for the product LD, we obtain

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
dm and larger will settle out of the gas flowing at a rate of Qg MMSCFD into the separator that is operating at P
psia and ToR.
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2) Liquid Capacity Constraint
• 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
half of a cross sectional area multiply by the length of separator as shown bellow:

Horizontal separator schematic 16

Substituting in Eq. (8),

the following equation is obtained

Equation (20) provides another relationship between D and L that satisfies the
liquid capacity (retention) time constraint.

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Sizing Procedure
The size (diameter and seam-to-seam length) of a horizontal separator
is determined as follows:
1. Check for gas capacity constraint, Eq. (19):

2. Check for oil capacity (retention time), Eq. (20):

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 Cont. Sizing Procedure
3. Assume various values for the separator diameter, D.
4. For each assumed value of D, determine the effective length, Lg, that satisfies the gas capacity
constraint from Eq. (19) and calculate the seam-to-seam length, Ls, from

5. For each assumed value of D, determine the effective length, Lo, that satisfies the liquid capacity
constraint from Eq. (20) and calculate the seam-to-seam length, Ls, from

6. For each value of D used, Compare the value of Ls for the oil capacity to those for the gas capacity to
determine whether the gas capacity constraint or the oil capacity constraint governs the design of the
separator. Of course, the larger required length governs the design.
7. Select reasonable combinations of D and Ls such that the slenderness ratio SR is in the range of 3–5.
The cost and availability would then determine the final selection.
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Example 2: Design problem, horizontal separator

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(19):

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12*Ls (oil)/D

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