FLASH VESSEL 4

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bureta:

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 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. For glycol or amine solutions, multiply above K values by 0.6 – 0.8.
3. Typically use one-half of the above K values for approximate sizing of vertical separators without mist eliminators.
4. For compressor suction scrubbers and expander inlet separators, multiply K by 0.7 – 0.8

Click to expand...

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.

I hope this helps you.

Milton Beychok
(Contact me at www.air-dispersion.com)
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An internet based sizing program is available here:


rmw

Could you please explain how the diameter is determined by the max vapor velocity? There is a 10 min liquid hold up in this flash drum. Thanks

cicottee:

The maximum vapor velocity that Milt Beychok refers to is a superficial velocity - i.e., the velocity calculated based on the over-all cross-sectional area of your separator, regardless of any internals. Therefore,

Separator X-sectional area, ft2 = (Vapor flow rate, ft3/min)/(vapor velocity, ft/min)

Make sure to use the proper units.

With the cross-sectional area known, calculating the inside diameter of a cylindrical shape is like falling off a log. The vessel volume you dedicate to hold-up is irrevelent to the diameter calculation. Simply make the vessel tall enough below the feed point (assuming you are designing a vertical separator) to contain the liquid you want to retain for subsequent drainage.

bureta:

The National Board has recomendations for sizing flash tanks (both horizontal and vertical). The calculations are rather simple, although the procedure they use relates to boiler blowdown. You can just use your known flowrate to size the flash tank diameter to check your vendors.


Hope this helps.

jproj

Carl L. Branan in his Rules of Thumb for Chemical Engineers (Gulf) in Chapter 8, titled Separators/Accumulators, addresses the issue.

He advices to select the k values that mbeychok listed for a vertical vessel as function of a separation factor:


which, depends on flow rates and P,T conditions, and provides a graph for that purpose.

As knowledgable people are tracking this thread, I would like to get some detail on the inlet nozzle design. Flash steam is mixed phase generated downstream of a control valve or steam trap, and in some cases can result in a high velocity and errosion.

1) What is the guidance on inlet nozzle velocity to the flash drum?
2) Sometimes I see nozzles which elbow down inside the drum, have deflection or impingement plates, or are tangential or other unusual designs to (presumably) enhance disengagement. Is there any guidance on this inlet detail?

Since this seemed an on topic detail of design, I thought I would ask. best wishes, sshep

Velocity is usually in the range of 50-150 ft/s. Inlet nozzles are brought in tangentially. A stainless steel baffle place is generally placed at least 180º (360º is common) around the vessel w/ a cutout for the inlet nozzle. The water outlet generally has a dip tube (elbow inside the drum) with a siphon breaker (hole) at the elbow.

regards,

jproj