I need to size a flash vessel to separate flash steam from condensate coming out from cooler bundles.

Although all suppliers provide easy charts to size them, I would like to check on the design equations by myself.

To avoid relying on vendor data, could anybody recommend where to look at for the design basis? I checked out already some of the most typical handbooks.

The size of a flash drum (or knock-out pot, or vapor-liquid separator) should be dictated by the anticipated flow rate of vapor and liquid from the drum. The following sizing methodology is based on the assumption that those flow rates are known.

Use a vertical pressure vessel with a length-to-diameter ratio of about 3 to 4, and size the vessel to provide about 5 minutes of liquid inventory between the normal liquid level and the bottom of the vessel (with the normal liquid level being at about the vessel's half-full level).

For the maximum vapor velocity (which will set the drum's diameter), use Souders-Brown equation:

Vmax = (k) [ (dL - dV) / dV ]^0.5

where:
Vmax = maximum vapor velocity, ft/sec
dL = liquid density, lb/ft3
dV = vapor density, lb/ft3
k = 0.35 (when the drum includes a de-entraining section)

The drum should have a vapor outlet at the top, liquid outlet at the bottom, and feed inlet at somewhat above the half-full level. At the vapor outlet, provide a de-entraining mesh section within the drum such that the vapor must pass through that mesh before it can leave the drum. Depending upon how much liquid flow you expect, the liquid outlet line should probably have a level control valve.

As for the mechanical design of the drum (i.e., materials of construction, wall thickness, corrosion allowance, etc.), use the same methodology as for any pressure vessel.

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