What Is a Separator? Oil, Gas, Water

A separator is a pressure vessel used to separate a well stream into its individual phases: oil, gas, and water. Separators are the first major piece of process equipment downstream of the wellhead in oil and gas production facilities. They rely on gravity, centrifugal force, and coalescence to split the multi-phase flow into distinct liquid and gas streams for further processing.

When Separators Are Used

Separators are required at every stage of upstream oil and gas production:

• Production separators at wellhead platforms and gathering stations to split crude oil, gas, and produced water
• Test separators to measure individual well flow rates during well testing
• High-pressure / low-pressure separation trains to stage gas liberation from crude oil
• Inlet separators (slug catchers) to protect downstream equipment from liquid slugs in gas pipelines

Separator Types and Specifications

Feature	Two-Phase Separator	Three-Phase Separator
Phases separated	Gas + liquid (oil or water)	Gas + oil + water
Outlets	Gas outlet, liquid outlet	Gas outlet, oil outlet, water outlet
Weir plate	Not required	Internal weir to separate oil and water pools
Application	Gas wells, dehydration inlet	Oil wells with significant produced water
Level control	One level controller (liquid)	Two level controllers (oil/water interface + oil level)

Configuration	Horizontal	Vertical
Best for	High liquid volumes, three-phase separation, foaming crude	High GOR (gas-oil ratio), limited floor space, scrubbing
Liquid retention	Higher—more liquid surface area	Lower—smaller liquid pool
Footprint	Large horizontal footprint	Small footprint, taller
Sand handling	Easier—sand settles to bottom of horizontal vessel	More difficult

Separator Internals

Internal Component	Function
Inlet device	Reduces momentum of incoming fluid; distributes flow (diverter plate, cyclonic inlet, or half-pipe)
Mist extractor	Removes liquid droplets from gas before gas outlet (vane pack, wire mesh, or cyclone type)
Weir plate	Separates oil compartment from water compartment in three-phase separators
Coalescing plates	Promote droplet growth to accelerate oil-water separation
Vortex breaker	Prevents vortex formation at liquid outlets that could pull gas into the liquid stream
Sand jetting system	Nozzles for periodic sand removal in sand-producing wells

Common Mistake

Sizing a separator based solely on steady-state flow rates without accounting for slugging conditions leads to undersized vessels that cannot handle liquid surges. Always include a slug volume allowance in the separator sizing calculation, particularly for subsea tiebacks and long flowlines where terrain-induced slugging is common.

Separators are designed as pressure vessels per ASME Section VIII and include safety relief valves to protect against overpressure. Level, pressure, and temperature instruments are mounted on separator nozzles to control the separation process and provide safety interlocks.

Read the full guide to oil and gas equipment