A separator is one of the first vessels well fluid reaches at surface, and one of the most important. Its job is to split the raw mix coming from the well - gas, oil, and water, often with sand - into separate streams that can be measured, treated, and sold. This guide explains what a separator is, the difference between 2-phase and 3-phase units, and how they are controlled.
Separator in one line: An oil and gas separator is a pressure vessel that separates produced well fluid into its phases - gas and liquid in a 2-phase separator, or gas, oil, and water in a 3-phase separator - using gravity, residence time, and internal devices.
Well fluid enters the vessel and immediately hits an inlet diverter that knocks the bulk gas out of the liquid. Inside, separation happens by density and time: gas rises to the top, liquid falls to the bottom, and if the vessel is 3-phase, the heavier water settles below the lighter oil. Giving the fluid enough residence time - seconds to minutes - lets these phases split under gravity.
Internals help the process along. A mist extractor (demister) near the gas outlet catches liquid droplets so they do not carry over with the gas. Weirs, baffles, and a boot or bucket help hold and separate the oil-water interface. The vessel can be horizontal or vertical; horizontal is common for high liquid volumes and where water must be separated cleanly.
A 2-phase separator splits the stream into gas and total liquid. It is used where the goal is simply to knock gas out of the liquid - for example ahead of a gas meter, or where water is handled downstream. A 3-phase separator goes further and splits the liquid into oil and water, producing three metered streams. Three-phase units are standard where produced water must be separated at the facility.
Test separators are a special case: a single well is routed through a dedicated separator to measure its individual gas, oil, and water rates - well testing - before commingling with the rest of the field.
A separator runs at a controlled pressure and controlled liquid levels. A back-pressure valve on the gas outlet holds vessel pressure, which sets how much gas flashes off and protects the vessel. Level controllers on the oil (and, in 3-phase, water) sections modulate dump valves to keep each interface in range - too high risks liquid carryover to gas, too low risks gas blowby into the liquid line.
Pressure, levels, and dump valve positions are read by instrumentation and controlled by an RTU, PLC, or dedicated controller. A cloud SCADA such as Merobix reads separator pressure and level signals over Modbus, so operators can watch vessel conditions and dump cycles across a facility remotely.
A 2-phase separator splits well fluid into gas and total liquid. A 3-phase separator splits it into three streams - gas, oil, and water - by additionally separating the liquid into oil and water. Three-phase units are used where produced water must be removed at the facility.
Well fluid enters and hits an inlet diverter that releases the bulk gas. Inside, gas rises, liquid falls, and heavier water settles below oil - separation by density over enough residence time. A mist extractor catches droplets in the gas, and level and pressure controls keep the interfaces and vessel pressure in range.
A test separator is a separator dedicated to measuring one well at a time. A single well is routed through it to determine its individual gas, oil, and water rates - well testing - before the stream is commingled with the rest of the field's production.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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