Flash Vessel Sizing

[hadams417]

Please advise me on how to properly size a flash vessel. Here is my application:

Condensate inlet flowrate = 51, 348 lb/hr
Inlet pressure = 600 psig
Internal tank pressure = 160 psig

Which corresponds to 15.1% flash steam. Given this amount of flash steam for the given condensate flow, this corresponds to:


Steam Flow (from tank) = 51348 * .151 = 7753.6 lb/hr

Liquie Flow (from tank) = 51348 * (1-.151) = 43595 lb/hr

Now, using the specific volume data of the vapor and liquid (2.6 ft^3/lb and 53.6 ft^3/lb, respectively) and target velocities of 10 fps for the steam and 50 fps for the water exiting the tank, my calculations for the tank diamter are as follows:


Steam: Dia = [((7753.6*2.6)*4) / (3.14 * 36000)]^(1/2) = .844 ft = 10.1 in (dia. necessary to provide steam flow of 10fps)

Water: Dia = [((43595*53.6)*4) / (3.14 * 180000)]^1/2 = 4.07 ft = 49 in (dia. necessary to provide water flow of 50 fps)

Clearly these are substancially different diameters. I do not want to undersize my vessel and experience overflow as a result. However, I do not want to over size either. I am stuck at this point. How do I determine proper tank heigt? Ive read places that state "use a 3:4 ratio for dia. to height" and others that say "tank height = 3*dia or minimum of 36in). Please advise me on how to complete the sizing procedure.

 

[mbeychok]

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hadams417:

I realize this is a few months late, but perhaps it will still be of use to you or others:

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 the 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 mesh pad)

The GPSA Engineering Data Book recommends the following k values for vertical drums with horizontal mesh pads (at the denoted operating pressures):

0 psig: 0.35
300 psig: 0.33
600 psig: 0.30
900 psig: 0.27
1500 psig: 0.21

GPSA Notes:
1. K = 0.35 at 100 psig; subtract 0.01 for every 100 psi above 100 psig
2. Typically use one-half of the above K values for approximate sizing of vertical separators without mist eliminators.
3. For compressor suction scrubbers and expander inlet separators, multiply K by 0.7 – 0.8

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.

Milton Beychok
(Contact me at www.air-dispersion.com)
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