Vertical Three-phase Separators Liquid Phase Separation
Flow of Liquids
Oil and water flow downward to the left of the horizontal baffle in Figure 300-7. Bulk oil passes through the coalescer pad; water droplets are coalesced and settle from the oil in the oil drawoff compartment to the right of the coalescer pad. Bulk water flows downward to the left of the coalescer pad and collects in the bottom head below the oil-water interface.
The high interface level (HIL on Figure 300-7) is determined by the maximum velocity allowed for oil flow through the coalescer pad (see the next paragraph). The normal interface level, NIL, is Dimension “J” (usually 1 foot) below the HIL and Dimension “I” (usually 6 inches) above the lower edge of the pad. The low interface level, LIL, is 6 inches below the bottom tangent line, below the pad. The interface might have to be held at the LIL if the coalescer became plugged.
Coalescer Pad Area
The pad is not necessarily located on the vessel centerline. Pad width (not thickness) will be equal to or less than the vessel diameter. If the interface were at the HIL, bulk oil would flow only through that portion of the pad between the HIL and the horizontal baffle at the top of the pad. The HIL is placed to make the oil velocity through that cross section equal to 5 ft/min per Section 335. The total pad height is the height just determined plus the sum of Dimensions “I” and “J.”
Elevation of Horizontal Baffle
Dimension “C,” height of the baffle above the bottom tangent line, is the total pad height plus any additional height left for flow of oil if the coalescer should plug (usually zero but up to 1 foot). The minimum recommended value of Dimension “C” is 3 feet 0 inches.
Separation of Water Droplets from Oil
Placement of the pad in the horizontal direction is shown by Dimension “G” in Figure 300-7. The oil holdup volume for separation of water droplets is bounded by the horizontal baffle supporting the pad, the NIL, the vessel wall, and the left (upstream) face of the pad. Oil residence time is the holdup volume divided by oil flowrate. Settling distance is from baffle to NIL. As in Section 353, use Equation 300-5 or 300-7 to estimate the water droplet size which will move through the settling distance during the oil residence time. Apply the quality of separation criteria in Section 334.
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