Gunbarrels are atmospheric tanks were originally developed in the oil and gas industry to dehydrate (remove water from) crude oil. With the advent of larger and larger produced water volumes they have also been used, albeit incorrectly, to separate oil from water. A gunbarrel tank has the inlet in a degassing boot, which is a concentric pipe extended up through the top, roof, or deck of the tank several feet. The inlet fluid enters the boot below its midpoint. Liquids fall down, while lighter gases separate. The gas is piped through the roof and into the top portion of the tank (the gas phase). The boot extends down through the roof to within a few feet from the bottom of the tank in what is refered to as the "downcomer" pipe. Oil and water exit the downcomer at the bottom. The downcomer may have a horizontal spreader plate attached at the bottom to aid in distributing the inlet oil, which naturally, rises when separated, from the water. Water exits the gunbarrel tank through a sidewall connection and enters an external riser pipe known as the water leg. The water rises up the water leg to a spillover point, and falls down through the spillover point in a second pipe back to grade, where it turns to enter the next tank. The spillover point establishes the water-oil interface (contact level) inside the gunbarrel tank. This spillover point can be calculated using a simply hydraulics formula:
Spillover Point = desired height of water inside the tank x the specific gravity of the water, times the desired height of the oil times the specific gravity of the oil, all divided by the specirfic gravity of the water.
The capacity of a gunbarrel tank used to dehydrate crude oil is shown in industry accepted nonographs like this one:
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Since about 1950 gunbarrel tanks have been misuded as oil-water separators in high water cut applications. Gunbarrels fail to perform in these applications because the dominant phase is water, which flows directly from the bottom of the downcomer out of the tank into the nearby water leg inlet, at too high a velocity for oil-water separation to occur. The lack of adequate retention time to achieve Stokes Law oil-water separation means that the majority of the inlet oil exits to the water leg and is carried in the downstream tanks, defeating the intended purpose of the gunbarrel tank for these applications. It is common for up to 99% of the inlet oil to remain in the effluent water in these applications.
A solution was patented in 1991. It is known as the HWSB Skim Tank. This tank is designed to separate very small quantities of oil from very large quantities of water. Since 1991 many thousands of these HWSB Skim Tanks have successfully replaced gunbarrel tanks, separating and capturing over 99% of all entrained oil in the produced water fed into them. The HWSB Skim Tank resembles the gunbarrel tank from the outside, but the internals are significantly different. The inlet fluid enters the HWSB and rises to near the top of the fluid level where oil separates. Water falls down through the majority of the tank's area. Two downward slanted internal spreader baffles maximize the water retention time so the oil has time to separate. Water exits near the bottom of the HWSB into a water leg much the same as in the gunbarrel. HWSB Skim Tanks routinely separate all oil from water leaving less than 50 parts per million of oil in the effluent water. HWSB is an acronym for "Hydrodynamic Water Separation Breakthrough". The HWSB patents are currently owned by KBK Industries, Rush Center, Kansas.
