A gun barrel tank - also called a wash tank or settling tank - is a low-cost, atmospheric vessel that separates oil and water using nothing but gravity and time. It is a staple of smaller and older tank batteries, especially on stripper and low-pressure wells. This guide explains what a gun barrel is, how its internal wash section works, and where it fits alongside separators and treaters.
Gun Barrel Tank in one line: A gun barrel tank is a tall, atmospheric-pressure settling tank that separates oil from water by gravity: emulsion enters through a central downcomer, rises up through a water layer (the wash) that helps break the emulsion, and clean oil accumulates at the top to overflow to the stock tank while water is drawn off the bottom.
The name comes from the tall, slender central conductor pipe - the "gun barrel" or downcomer - that runs down the middle of the tank. Incoming emulsion is piped to the top and drops down this central pipe, then is released near the bottom into a standing layer of water. As the oil droplets rise slowly up through that water layer, water-wetting and coalescence break the emulsion, a process called water washing. The freed oil continues to rise and collects as a clean oil pad at the top of the tank.
Clean oil overflows a top outlet to the stock tank, while separated water is drawn from the bottom. The oil-water interface level is held by an external water leg or siphon whose height sets where the interface sits - raise the water-leg outlet and the water level in the tank rises, and vice versa. Because the whole vessel operates at atmospheric pressure and typically ambient (or gently warmed) temperature, a gun barrel is inexpensive to build and run.
The heart of the gun barrel is the water wash. By forcing rising oil to pass through a deliberate water layer, the tank uses the water itself as a coalescing medium - the emulsion droplets contact the continuous water phase and merge, releasing their oil. A spreader or flume at the base of the downcomer distributes the incoming fluid across the tank so it rises evenly rather than channeling.
Effective separation depends on retention time. A gun barrel is sized to hold the fluid long enough - often many hours of settling - for the small droplets to rise and coalesce. This generous retention is why gun barrels suit low-rate wells: the volumes are modest, so a reasonably sized tank still delivers hours of quiet settling. Higher-rate or tighter-emulsion streams usually need a heated, faster heater-treater instead.
A gun barrel typically sits between the separator and the stock tanks in a smaller battery, often replacing a heater-treater where the emulsion is loose enough to break cold. It doubles as both a treating vessel and surge volume. Because it is atmospheric, it is fitted with a thief hatch and pressure-vacuum protection like any stock tank, and vapors are routed to a vent or vapor-recovery system.
Operators watch the oil and water levels, the interface, and any temperature if the tank is heated. A cloud SCADA like Merobix reads those level and temperature tags from the site controller over Modbus, DNP3, or OPC UA - it does not connect to the tank directly. Trending the interface and oil level remotely helps catch a slug of water, a plugged water leg, or a rising water cut before oil is lost to the water line or water carries over to the oil sales stream.
It separates oil from water by gravity in a tank battery, usually on lower-rate wells. Emulsion rises through a water wash layer that helps break it, clean oil collects at the top and overflows to the stock tank, and water is drawn off the bottom. It acts as both a treating vessel and surge storage.
The name refers to the tall, narrow central conductor pipe - the downcomer - that runs down the middle of the tank like a gun barrel. Incoming fluid drops through this pipe and is released near the bottom into the water layer where the oil begins its rise. The distinctive tall, slim profile also earns the name.
A gun barrel separates oil and water cold, at atmospheric pressure, relying on long gravity settling and a water wash. A heater-treater adds heat (and often chemicals or an electrostatic grid) to break tighter emulsions faster in a pressurized vessel. Gun barrels suit low-rate, loose-emulsion wells; treaters handle higher rates and stubborn emulsions.
By an external water leg or siphon. The height of the water-leg outlet sets the balance point between the oil and water columns, which fixes where the interface sits inside the tank. Raising the water leg raises the water level; lowering it drops the interface. It is a simple, valveless way to hold the interface at atmospheric pressure.
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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