Oil Separation Pressure Vessels - Two and Three Phase Separation Explained

Separation is where wellstream or process flow becomes saleable product. A separator has to split gas from liquid and, where water is present, split water from oil - reliably, at the design flow, and without carrying liquid into the gas line or gas into the liquid outlet. When separation is poor, the problems appear downstream in compressors, pumps and meters, which is where they are most expensive to diagnose.

Engineered by Shijiazhuang Zhengzhong Technology Co., Ltd. (Center Enamel), oil separation pressure vessels are sized from the actual flow rates, fluid properties and operating pressure, with internals selected for the separation duty rather than from a standard arrangement.

1. What Does an Oil Separation Vessel Do?

It separates the incoming stream into gas, oil and - in a three-phase vessel - water, using gravity settling assisted by inlet momentum control and a mist extractor.

· Primary Separation: An inlet device removes the bulk of the liquid from the gas by momentum change.

· Gravity Settling: The liquid section holds the liquid long enough for droplets to separate by density.

· Mist Extraction: A mesh pad or vane pack removes fine liquid droplets from the gas before it leaves.

· Water Separation: In three-phase service, water settles below the oil and is drawn off separately.

· Level Control: Level and interface control keep each phase at its designed outlet.

2. Two Phase or Three Phase - How Do You Choose?

By whether water is present in quantity. If the stream carries produced water or free water that must be removed, a three-phase vessel with interface control is required; if not, two-phase separation is simpler and cheaper.

· Two Phase: Gas and liquid only; simpler internals and control.

· Three Phase: Gas, oil and water, with a weir and interface level control.

· Water Content: The deciding factor is how much free water arrives with the stream.

· Downstream Requirements: What the next process step can tolerate dictates how sharp the separation must be.

· Future Flexibility: Some vessels are specified with three-phase capability for later field conditions.

3. How Is a Separator Sized?

Gas capacity is sized by the settling velocity of the smallest droplet to be removed; liquid capacity is sized by retention time. The two often give different answers, and the governing case is the larger vessel.

· Gas Capacity: Based on the terminal settling velocity of the target droplet size in the gas phase.

· Liquid Retention: Based on the time needed for oil and water to separate at the design flow.

· Mist Extractor Selection: Mesh or vane type, sized for the gas flow and the droplet spectrum.

· Operating Pressure: Pressure affects both gas density and separation behaviour, so it is part of the sizing.

· Design Code: Separators are designed to the applicable pressure vessel code, with API 12J commonly referenced for oil and gas separators.

Engineering Assurance and Project Support

Every welded oil tank delivered by Shijiazhuang Zhengzhong Technology Co., Ltd. (Center Enamel) is designed to API 650 with shell courses sized by the one-foot method and checked for wind, seismic and hydrostatic load cases, or to EN 14015 and the Eurocodes where the site is European. Shell plate is shot-blasted to Sa 2.5 and coated to a documented dry film thickness, longitudinal and annular plate welds are examined by radiography or ultrasonic testing to the acceptance level written into the purchase order, and every tank is hydrostatically tested and dimensionally surveyed before hand-over. Separators are supplied to the applicable pressure vessel code with API 12J sizing, internals selected for the duty, level and interface control provisions, relief devices and full material and testing documentation.

Frequently Asked Questions (FAQ)

What is the difference between a two phase and three phase separator?

A two-phase separator splits gas from liquid and needs only level control. A three-phase separator also splits water from oil, which requires a weir and interface level control in addition. The deciding factor is whether the stream carries free water in quantity that must be removed.

How is a separator vessel sized?

Gas capacity is sized from the settling velocity of the smallest liquid droplet that must be removed, and liquid capacity from the retention time needed for oil and water to separate at the design flow. The two calculations frequently give different answers, and the larger result governs.

What does a mist extractor do?

It removes fine liquid droplets from the gas stream before it leaves the vessel, using a mesh pad or a vane pack. Without it, droplets that gravity settling cannot capture are carried into the gas line, where they cause problems in compressors, meters and downstream equipment.

Can the vessel be customised for our stream?

Yes. Diameter and length, orientation, two or three phase configuration, internals selection, design pressure and temperature, material and corrosion allowance, nozzle schedule, level and interface control, relief devices, instrumentation and supports are all engineered to your flow rates and fluid properties.