Three-phase separators are commonly used in the oil and gas industry to separate the mixture of gas, oil, and water. This article deals with the basic theory behind three-phase separators, their general types, and the design of each type of three-phase separators.

Working of Separator

There are three stages of separation in a separator. The first stage uses an inlet device to break the momentum of the incoming vapor/liquid mixture. This causes the largest droplets to impinge on the diverter and then drop by gravity. The secondary separation is the gravity separation of smaller droplets as the vapor flows through the disengagement area. In the liquid collection section of the vessel, the oil separates, forming a layer above the free water. Generally, a weir maintains the oil level, while an interface liquid level controller maintains the water level. The final stage is mist elimination where the smallest droplets coalesce so that larger droplets are formed which will fall down due to gravity.

Separator Types and Selection

Three-phase separators may be oriented either vertically or horizontally. In some cases, it is necessary to compare both designs to determine which one of these two is more economical. Moreover, Separators can be designed with or without mist extractors. The vapor/liquid ratio is used to select the appropriate separator. Horizontal separators are preferred for low vapor/liquid ratios while verticals are suitable for high ratios.


  1. Vertical terminal vapor velocity can be calculated by:

K value can be calculated using table 1.

As long as Uv < Ut, the liquid droplets will settle out. Typically, Uv is taken as 75% of Ut. Equation 1 can be written as:

2. Calculate the vapor volumetric flow from the mass flow (if required).

3. Vessel internal diameter can then be calculated by:

By rearranging

If there is a mist extractor, then add 6 in to the vessel diameter and round off the value to the next 6 in.

4. Heavy liquid settling velocity can be calculated using Stokes’ law in the light liquid phase.

Ks can be estimated using table 2.

5. Light liquid rising velocity can be calculated using Stokes’ law in the heavy liquid phase.

6. Calculate light and heavy liquid volumetric flow rate (if required).

7. Assume HL=1 ft and calculate the settling time for the heavy liquid drops to settle through this distance.

8. Assume HH=1 ft and calculate the settling time for the light liquid droplets.

9. To calculate the baffle plate area (if present)

a. Calculate density difference

b. Assume HR=9 in and calculate HL+HR

c. Obtain G from figure

d. Calculate AD using equation 13.

e. Assume WD=4 in and calculate WD/D

f. Use table 3 to determine AD/A and calculate A with equation 14.

h. Calculate the area of the baffle plate.

10. The residence time of light and heavy liquid can be calculated by their occupied volume.


increase diameter and repeat from step 7.

11. Height of the light liquid above the outlet can be estimated by:

This value should be approximate to the assumed value in step 9.

12. Surge height can be estimated with surge time.

The minimum height should be 6 in.

13. Vessel height can be estimated by:

The nozzle diameter can be estimated using the method described in the Two-Phase Separator Design.

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