The most commonly used single-well lease treater is the vertical treater as shown in Figure 6-8. Flow enters the top of the treater into a gas separation section. Care must be exercised to size this section so that it has adequate dimensions to separate the gas from the inlet flow. If the treater is located downstream of a separator, this chamber can be very small. The gas separation section should have an inlet diverter and a mist extractor.
The liquids flow through a downcomer to the base of the treater, which serves as a free-water knockout section. If the treater is located downstream of a free-water knockout, the bottom section can be very small. If the total wellstream is to be treated this section should be sized for 3 to 5 minutes retention time for both the oil and the water to allow the free water to settle out. This will minimize the amount of fuel gas needed to heat the liquid stream rising through the heating section. The end of the downcomer should be slightly below the oil water interface to “water wash” the oil being treated. This will assist in the coalescence of water droplets in the oil.
The oil and emulsion rises over the heater fire-tubes to a coalescing section where sufficient retention time is provided to allow the small water particles in the oil continuous phase to coalesce and settle to the bottom.
Treated oil flows out the oil outlet. Any gas, flashed from the oil due to heating, flows through the equalizing line to the gas space above. Oil level is maintained by pneumatic or lever operated dump valves. Oilwater interface is controlled by an interface controller, or an adjustable external water leg.
The detailed design of the treater, including the design of internals (many features of which are patented) should be the responsibility of the equipment supplier.
Figure 6-9 shows a “gunbarrel” tank, which is a vertical flow treater in an atmospheric tank. Typically, gunbarrels have a gas separating chamber or “boot” on top where gas is separated and vented, and a downcomer. Because gunbarrels tend to be of larger diameter than vertical heater
treaters, many have elaborate spreader systems to try and create uniform (i.e., plug) upward flow of the emulsion to take maximum advantage of the entire cross section. Most gunbarrels are unheated, though it is possible to provide heat by heating the incoming stream external to the tank, installing heating coils in the tank, or circulating the water to an external or “jug” heater in a closed loop. It is preferable to heat the inlet so that more gas is liberated in the boot, although this means that fuel will be used in heating any free water in the inlet.
Gunbarrels are most often used in older, small flow rate, onshore facilities. In recent times vertical heater-treaters have become so inexpensive that they have replaced gunbarrels in single well installations. On larger installations onshore in warm weather areas gunbarrels are still commonly used. In areas that have a winter season it tends to be too expensive to keep the large volume of oil at a high enough temperature to combat potential pour point problems.