Three Phase Horizontal Separator Sizing

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For sizing a horizontal three-phase separator it is necessary to specify a vessel diameter and a seam-to-seam vessel length. The gas capacity and retention time considerations establish certain acceptable combinations of diameter and length. The need to settle 500-micron water droplets from the oil establishes a maximum diameter.

Gas Capacity
The gas capacity constraints provide the following formula,

 Three Phase Horizontal Separator Sizing

Retention Time
Retention time constraints give another equation that provides acceptable combinations of d and Leff.

 Three Phase Horizontal Separator Sizing

where Qw = water flow rate, bpd
(tr)w – water retention time, min
Q0 = oil flow rate, bpd
(t.r)0 = oil retention time, min
Derivation of Equation 5-4
t is in S, Vol in ft3, Q in ft3/s, D in ft, d in inches, Leff in ft

 Three Phase Horizontal Separator Sizing

Settling Equation
The requirement that 500-micron water droplets be capable of settling out of the oil pad establishes a maximum oil pad thickness given by the following formula:

 Three Phase Horizontal Separator Sizing

This is the maximum thickness the oil pad can be and still allow the water droplets to settle out in time (t.r)0

 Three Phase Horizontal Separator Sizing

For a given oil retention time and a given water retention time the maximum oil pad thickness constraint establishes a maximum diameter in accordance with the following procedure:
1. Compute (h0)max.
2. Calculate the fraction of the vessel cross-sectional area occupied by
the water phase. This is given by:

 Three Phase Horizontal Separator Sizing

Any combination of d and Leff that satisfies all three of Equations 5-3,
5-4, and 5-5 will meet the necessary criteria.

 Three Phase Horizontal Separator Sizing
Coefficient "|3" for a cylinder half filled with liquid.

Seam-to-Seam Length and Slenderness Ratios
The seam-to-seam length can be estimated from the effective length using the same formulas as for two-phase separators. Where the gas capacity governs, the slenderness ratio should be limited to less than 4 or 5 to prevent re-entrainment of liquid at the gas-liquid interface. If the separator sizing is based on liquid capacity a higher slenderness ratio is acceptable. There is the possibility of generating internal waves at the oil-water interface. Unless a more elaborate study is performed, it is recommended that slenderness ratios of less than 4 be chosen. Most common horizontal three-phase separators have slenderness ratios between 3 and 5.

Procedure for Sizing Three-Phase Horizontal Separators

 Three Phase Horizontal Separator Sizing

 Three Phase Horizontal Separator Sizing

For separators other than 50% full of liquid, equations can be derived similarly, using the actual oil and water areas. The equations are derived using the same principles.

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