Water Separation from Hydrocarbon

If water separation is not a requirement, any of the vessel sizes meeting the surge volume and L/D requirements in Section 352 may be chosen. Otherwise, water separation must be checked. Using Equation 300-5 or 300-7 depending on the Reynolds number, compute the settling velocity for a 100-micron water droplet in hydrocarbon. Hydrocarbon residence time (seconds) is surge volume divided by the hydrocarbon flow rate. The settling distance is from maximum liquid level to minimum liquid level. If a 100-micron water droplet will fall through the settling distance within the hydrocarbon residence time, separation will be “good.”

Check the various trial sets of drum dimensions found in Section 352. For constant surge volume, settling distance decreases and separation improves as L/D increases. Increase L/D until water separation becomes satisfactory or until the maximum L/D of 6.0 is reached. In the latter case, increase vessel size at constant L/D. Hydrocarbon liquid surge will be in excess of the requirement.

Water will collect along the bottom of the vessel before running into the boot. The hydrocarbon liquid outlet is therefore raised above the bottom in order to avoid drawing water. The clearance is 2 to 6 inches depending on the volume of water expected.

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