Most oil treating on offshore facilities is done in vertical or horizontal treaters, such as those described in “Crude Oil Treating System“. Figure 2-8 is an enlargement of the oil treater in Flow Sheet. In this case, a gas blanket is provided to assure that there is always enough pressure in the treater so the water will flow to water treating.
At onshore locations the oil may be treated in a big “gunbarrel” (or settling) tank, as shown in Figure 2-9. All tanks should have a pressure/vacuum valve with flame arrester and gas blanket to keep a positive pressure on the system and exclude oxygen. This helps to prevent corrosion, eliminate a potential safety hazard, and conserve some of the hydrocarbon vapors. Figure 2-10 shows a typical pressure/vacuum valve. A pressure in the tank lifts a weighted disk or pallet, which allows the gas to escape. If there is a vacuum in the tank because the gas blanket failed to maintain a slight positive pressure, the greater ambient pressure lifts another disk, which allows air to enter. Although we wish to exclude air, it is preferable to allow a small controlled volume into the tank rather than allow the tank to collapse. The savings associated with keeping a positive pressure on the tank is demonstrated by Table 2-3.
Figure 2-11 shows a typical flame arrester. The tubes in the device keep a vent flame from traveling back into the tank. Flame arresters have a tendency to plug with paraffin and thus must be installed where they can be inspected and maintained. Since they can plug, a separate relieving device (most often a gauge hatch set to open a few ounces above the normal relieving device) must always be installed.
The oil is skimmed off the surface of the gun barrel and the water exits from the bottom either through a water leg or an interface controller and dump valve. It must be pointed out that since the volume of the liquid is fixed by the oil outlet, gun barrels cannot be used as surge tanks. Flow from the treater or gun barrel goes to a surge tank from which it either flows into a barge or truck or is pumped into a pipeline.