Gun Barrel
A gun barrel in oil production operations is a vertical cylindrical settling vessel used for gravity separation of crude oil from produced water and entrained gas, named for its characteristic tall, narrow shape that resembles the barrel of a gun; in a gun barrel treater (also called a wash tank or gun barrel tank), the oil-water-gas emulsion from the producing wells enters near the bottom of the vessel through a spreader that distributes the incoming stream horizontally, allowing the lighter oil to rise and the denser water to settle by gravity alone without the application of heat or chemical demulsifiers in simple field operations; the oil flows upward through the water phase (which acts as a wash leg that coalesces and drops out water droplets that were entrained in the oil phase), then separates from the gas that is released from solution as pressure drops inside the vessel, with clean treated oil exiting at the top through an overflow weir or outlet nozzle, produced water discharging from the bottom through a water leg that maintains the oil-water interface at the designed level, and gas venting from the top of the vessel to a gas gathering line or flare; gun barrel tanks were the primary oil treatment vessels on many shallow, low-pressure onshore oil fields from the early 20th century through the mid-20th century, and simpler versions remain in use on small-volume field operations where the low capital cost and simple operation of a gravity separator is preferred over the higher cost and complexity of heated treaters, electrostatic treaters, or horizontal three-phase separators.
Key Takeaways
- The separation mechanism of the gun barrel relies entirely on Stokes' law gravity settling of droplets whose rise or fall velocity depends on the density difference between the oil and water phases, the droplet diameter, and the viscosity of the continuous phase: for a water droplet in oil, the settling velocity increases with the square of the droplet diameter and with the oil-water density difference, and decreases with increasing oil viscosity; the tall vertical profile of the gun barrel tank maximizes the settling path length for oil droplets rising through the water wash leg and for water droplets falling through the oil layer, improving separation efficiency compared to a horizontal vessel of the same volume; the water wash leg (the column of water below the oil-water interface through which the incoming oily emulsion must pass before reaching the oil layer) provides a coalescence zone where small water droplets contact and merge with the continuous water phase, reducing the water content of the rising oil; gun barrel treaters work best with low-viscosity crude oils (API gravity above 25 degrees), stable oil-water density differences, and low flow rates that provide adequate residence time for gravity settling.
- Residence time is the critical design parameter for gun barrel separators, with typical designs providing 4 to 6 hours of oil residence time in the vessel to allow adequate gravity settling of the water droplets entrained in the oil phase: a gun barrel tank treating 500 barrels per day of oil-water emulsion must have a volume of approximately 83 to 125 barrels of oil-phase capacity (500 bbl/day divided by 24 hours times 4 to 6 hours) plus the water-phase volume below the oil-water interface and the gas-phase volume above the oil; in practice, gun barrel tanks in the United States are often sized as 250 to 1,000 barrel vertical tanks (55 to 220 inch diameter by 20 to 40 foot height) that provide several hours of residence time at typical small-to-medium field production rates; oversizing the gun barrel (providing more residence time than the minimum required for acceptable oil treating) is common practice because the low cost of a larger tank is justified by the insurance against upset conditions (higher water cuts, colder temperatures, or emulsions with unusual stability) that could temporarily require longer settling time to maintain oil quality.
- Chemical demulsifier treatment is often added to gun barrel systems to improve separation performance when the crude oil forms stable emulsions with produced water that do not settle adequately by gravity alone within the available residence time: demulsifiers (also called emulsion breakers) are surface-active chemicals that adsorb at the oil-water interface, displace the natural emulsifiers (resins, asphaltenes, and heavy organics) that stabilize the emulsion droplets, and promote coalescence of small droplets into larger drops that settle more rapidly; the demulsifier is injected at the wellhead or at the gun barrel inlet at concentrations of 50 to 500 parts per million of the emulsion volume, with the optimal dosage determined by bottle tests (laboratory settling tests that compare treated and untreated emulsion samples at field temperature) and field optimization at the separator; the combination of demulsifier treatment and adequate residence time in a gun barrel can routinely achieve oil BS&W (basic sediment and water) specifications of 0.5 to 1 percent water content required for pipeline acceptance, though heavy crude oils with high asphaltene content may require heated treaters or electrostatic treaters in addition to gun barrel gravity separation to meet the 0.5 percent spec.
- Gun barrel maintenance and operational monitoring focus on controlling the oil-water interface at the designed level, preventing oil carryover into the produced water outlet, and preventing water carryover into the oil outlet: the oil-water interface level is controlled by a float-operated dump valve on the water outlet (which opens when the interface rises above the set point and discharges water until the interface returns to the set level) or by a fixed height water leg pipe that discharges water by gravity when the interface rises to the pipe inlet elevation; maintaining the correct interface level is critical because too high an interface (oil-water boundary too close to the oil outlet) allows water-contaminated oil to exit to the stock tank, failing the BS&W specification, while too low an interface (boundary below the water wash leg inlet) allows the incoming emulsion to bypass the wash leg and enter the oil phase directly without the coalescence benefit of passing through the water; interface level measurement using sight glasses, level gauges, or electronic level sensors (float-type, magnetic, or ultrasonic) provides the operational feedback needed for dump valve control.
- Gun barrel displacement by modern horizontal three-phase separators reflects the limitations of vertical gravity separation in high-volume, high-gas-liquid-ratio, and heavy oil applications: horizontal three-phase separators (which separate gas, oil, and water simultaneously in a horizontal vessel where gas rises to a separate gas phase, oil occupies the middle liquid phase, and water settles to the bottom) provide higher throughput per unit volume than vertical gun barrels because the horizontal profile maximizes the cross-sectional area available for gas-liquid separation at the inlet section; heated treater-separators, which combine heat application and residence time to break stable emulsions from heavy crude or from fields with high paraffin content, also address the gun barrel's fundamental limitation of relying on gravity at ambient temperature for phase separation; the gun barrel remains in service in applications where simplicity, low capital cost, and minimal operational complexity are paramount, particularly in small onshore fields, produced water handling downstream of primary separation, and remote field locations without access to power for heated treaters.
Fast Facts
Gun barrel tanks have been used in oil production since the earliest days of the American petroleum industry in Pennsylvania and Ohio in the late 19th century, when producers discovered that allowing oil and water to stand in tall vertical tanks produced better separation than in short horizontal tanks of the same volume. The gun barrel name came into common use in the oil patch vocabulary of Texas and Oklahoma in the early 20th century, appearing in oilfield slang literature by the 1910s. The design remains largely unchanged from its earliest iterations, a testament to the simplicity and effectiveness of gravity separation when residence time is adequate and the crude is light enough to settle without heating.
What Is a Gun Barrel?
A gun barrel is a tall, narrow vertical settling tank used in crude oil production to separate produced water, oil, and gas by gravity without the application of heat or electricity, relying on the density difference between oil and water and adequate residence time (typically 4 to 6 hours) for the phases to separate. The incoming emulsion passes upward through a water wash leg that coalesces entrained water droplets, clean oil overflows from the top, and produced water discharges from the bottom through a float-controlled dump valve. Gun barrel treaters remain in service on small onshore fields where their low capital cost and simple operation are preferred over more sophisticated heated or electrostatic treaters.
Synonyms and Related Terminology
Gun barrel is also called a wash tank, gun barrel tank, or gun barrel treater. Related terms include three-phase separator (a pressure vessel that separates gas, oil, and water simultaneously in a single vessel using gravity settling and, in horizontal designs, maximizing the cross-sectional area for gas-liquid separation, which has largely replaced gun barrel gravity separators in high-volume field applications where the superior throughput and phase separation efficiency of a three-phase design justify its higher capital cost), demulsifier (a surface-active chemical injected into the oil-water emulsion at the wellhead or separator inlet to promote coalescence of small water droplets in the oil phase and accelerate gravity separation in gun barrel and other settling vessels, typically applied at 50 to 500 ppm of emulsion volume with the optimal dosage determined by laboratory bottle tests at field temperature), basic sediment and water (BS&W, the pipeline acceptance specification for crude oil water and solid content, typically 0.5 to 1 percent by volume, which is the treating target that the gun barrel or other separator must achieve before crude oil can be transferred to a pipeline or storage tank), water cut (the fraction of the produced fluid stream that is water, expressed as a percentage of total liquid production, which is the primary operational variable governing gun barrel performance because high water cuts require longer residence time or additional treating to separate the water phase from the oil product to pipeline specification), and heated treater (a pressure vessel that combines heat application with residence time to break stable crude oil emulsions that cannot be resolved by ambient-temperature gravity settling in a gun barrel alone, using a fire tube or steam coil to raise the emulsion temperature to 120 to 200 degrees Fahrenheit where oil viscosity decreases and water droplets coalesce more rapidly).
Why the Gun Barrel's Simplicity Continues to Earn Its Place in Oilfield Operations
In the sophisticated modern oil and gas industry of sophisticated multi-phase metering, electrostatic treaters, and automated process control, the gun barrel's persistence as a working piece of equipment reflects a durable truth: simple gravity separation, given enough time, works remarkably well for a broad range of crude oil systems. On small fields where the capital cost of complex treating systems cannot be justified by production volumes, on battery operations where reliable local personnel are not available for complex equipment operation, and as a polishing vessel downstream of primary separation that catches residual water not resolved by upstream equipment, the gun barrel continues to earn its place in production facilities. Understanding when simple gravity separation is adequate and when more sophisticated treating is required is one of the core competencies of production and facilities engineering.