Three-phase separators are used to separate a mixture of three phases, typically oil, water, and gas, into their individual components. The separation process takes place through a combination of physical processes such as gravity separation, coalescence, and centrifugal forces.
The working principle of a three-phase separator is based on the physical differences between the three phases (oil, water, and gas) that are present in the mixed stream. The separation process takes place through a combination of gravity separation, coalescence, and centrifugal forces.
The following is a detailed explanation of how a three-phase separator works
Inlet Section: The mixed stream of oil, water, and gas enters the separators through an inlet nozzle and flows into a large vessel. The flow rate is slowed down to allow for initial gravity separation, with the larger and heavier droplets of the liquid phases beginning to settle due to gravity.
Gas Separation: The gas phase, being the lightest, rises to the top of the vessel due to buoyancy and forms a gas layer. The gas is then removed from the top of the vessel through a gas outlet nozzle.
Liquid Separation: The remaining liquid mixture, consisting of oil and water, flows to the bottom of the vessel. Here, it undergoes secondary separation through a variety of mechanisms such as gravity separation, coalescence, or centrifugal force, depending on the separator design. Internal devices such as baffles, weirs, or mesh pads can be used to enhance separation.
Oil-Water Separation: The oil and water phases are then separated through a process such as gravity separation or coalescence. The oil floats to the top of the liquid layer and is collected in an oil outlet nozzle, while the water sinks to the bottom and is collected in a water outlet nozzle.
Discharge: The separated phases are discharged from the separators through their respective outlets and sent for further processing or disposal.
Separators are devices used to separate two phases (e.g., liquid-liquid or liquid-gas) in a process stream. The straight length requirement for separators depends on several factors such as the type of separator, the flow rate, the fluid properties, and the desired separation efficiency.
In general, a straight length of piping is required upstream of a separator to ensure that the flow is fully developed and uniform before entering the separator. This straight length allows the fluid to stabilize and reduces turbulence and swirl, which can negatively impact the separation performance of the separator.
Separator straight length requirement
The straight length requirement can vary from a few pipe diameters for small separators to over 10 pipe diameters for larger separators. For example, a typical rule of thumb for a horizontal two-phase separator is to have a straight inlet piping length of 5 to 10 pipe diameters upstream of the inlet nozzle.
It’s important to note that the straight length requirement may also depend on the type of flow conditioning devices used upstream of the separator, such as flow straighteners or screens. These devices can help reduce the required straight length and improve the separation performance of the separator.
Size the Vessel
When designing a three-phase separator, vessel sizing is a critical step to ensure proper separation efficiency and safe operation.
Main Criteria
Gas Capacity – Souders–Brown Equation
The maximum gas velocity can be determined using the Souders–Brown equation:
v_max = K × sqrt((ρL − ρG) / ρG)
Where:
- vmax = Maximum gas velocity (m/s)
- K = Souders–Brown constant (typically 0.1–0.35 m/s for 3-phase separators)
- ρL = Liquid density (kg/m³)
- ρG = Gas density (kg/m³)
Liquid Retention Time
A retention time of 3–5 minutes is commonly used for oil–water separation. This ensures sufficient time for the two liquid phases to separate under gravity.
Weir Height and Boot Size
Weir height is set to maintain a stable oil–water interface inside the vessel. Boot size (for vertical separators) is chosen to handle water accumulation without affecting oil separation efficiency.
Frequently Asked Questions (FAQ) – Three-Phase Separator Design & Sizing
1. What exactly is a three-phase separator?
It’s a specialized vessel used in oil and gas operations to split a production stream into three distinct outputs — gas, oil, and water — for further processing.
2. How does a three-phase separator achieve separation?
The unit works by using the natural density differences between the three phases. Gas moves upward, water sinks to the bottom, and oil stays in the middle. This process may be enhanced with baffles, coalescers, or centrifugal action.
3. Why is correct sizing of the separator so critical?
If the vessel is too small or too large, efficiency drops. Proper sizing ensures each phase is separated cleanly without contamination or product loss.
4. What role does the Souders–Brown equation play?
It helps determine the highest gas velocity that still allows liquids to drop out before exiting with the gas stream.
5. How much retention time should be provided for liquids?
A retention period of about 3 to 5 minutes usually gives oil and water enough time to naturally separate.
6. Why is weir height an important design factor?
The weir controls the depth of the liquid layers, keeping the oil–water interface steady and preventing mixing during discharge.
7. What is meant by a separator “boot”?
In vertical designs, a boot is a small side or bottom section that collects water separately from oil to maintain clean phase separation.
8. How do fluid densities affect the design?
If gas density is low or liquid density differences are small, the separator must be larger to allow sufficient settling time.
9. Can separators handle stubborn emulsions?
Yes, but they may need extra design features such as heating, chemical injection, or specialized coalescing devices.
10. What problems occur if the separator is too small?
An undersized unit can cause oil in the gas stream, water in the oil, high maintenance needs, and reduced production quality.
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