Separator Definition / Meaning

A separator in the oil and gas industry is a pressure vessel used to divide a wellstream into its main components: gas, oil, water, and sometimes solids. It is a vital piece of equipment in upstream production facilities, ensuring each phase can be processed, measured, or disposed of safely and efficiently. Separators come in various shapes—vertical, horizontal, or spherical—and are designed for different flow rates, pressures, and fluid properties. Their proper operation directly impacts downstream equipment performance and overall production efficiency.

Overview and Function

The primary function of a separator is to remove liquid droplets from gas and gas bubbles from liquid, as well as separate oil and water phases. This is achieved through four mechanisms: gravity settling, coalescence, centrifugal force, and impingement. Separators are typically classified by the number of phases they handle: two-phase (gas and liquid) or three-phase (gas, oil, and water). They are also rated by operating pressure and temperature. A well-designed separator maximizes separation efficiency while minimizing pressure drop and turbulence.

Types of Separators

• Vertical Separators: Often used when space is limited (e.g., offshore platforms). They have a smaller footprint and handle solids better because debris can fall to the bottom. However, they are less efficient for liquid-liquid separation due to shorter settling distances.
• Horizontal Separators: Preferred for high liquid loads and three-phase separation. The elongated shape provides longer residence time for oil and water to separate by gravity. They are common in onshore facilities.
• Spherical Separators: Compact and robust, used for high-pressure, moderate-flow applications. They are less common due to limited internal volume and less flexibility in internals.

Key Components

All separators contain essential internals to enhance separation:

Operating Principles

Separation relies on density differences. In a three-phase horizontal separator, the wellstream enters through the inlet diverter, which spreads the flow and allows initial gas breakout. The gas rises and passes through the mist extractor to remove entrained liquid droplets. The liquid falls into the gravity settling section, where oil (lighter) rises and water (heavier) settles. An oil-water interface controller and weir system maintain distinct layers. The separated gas, oil, and water exit through separate nozzles. Key design parameters include residence time (30 seconds to 5 minutes for liquid, 1-2 minutes for gas), droplet size (typically 10-500 microns), and allowable gas velocity to avoid re-entrainment.

Design Considerations

Engineers size separators based on flow rates, fluid properties, and operating conditions. For vertical vessels, the diameter is determined by gas capacity, and the height by liquid holding time. For horizontal vessels, the length and diameter balance liquid retention and gas flow. Important factors include:

• Pressure and Temperature: Affect phase densities and viscosities.
• Emulsion Formation: Requires chemical injection or heat to break.
• Sand and Solids: May need jetting or sand removal systems.
• Corrosion and Erosion: Influences material selection and internal coatings.
• Safety: Pressure relief valves and blowdown systems are mandatory.

Industry standards (e.g., API 12J, ASME Section VIII) guide separator design and fabrication.

Usage Example

In a typical onshore gathering station, a three-phase horizontal separator receives production from multiple wells. It separates the wellstream into sales gas sent to a dehydration unit, crude oil routed to storage tanks, and produced water directed to a disposal well. Proper separator sizing ensures efficient separation under varying flow rates, preventing carryover of liquid into gas pipelines or oil into water treatment systems.