Steam separator sizing criteria

[chemieingineur]

I am looking at generating steam by using heat exchangers to utilize excess heat from the process. This 2 phase mixture then goes to a separator to produce high pressure steam (about 900 psig).

There is also a pump around which takes any liquid hold-up and pumps the by now hot water through exchangers to create additional steam.

Is there anything that I have to watch for when sizing the steam separator for this case?

Thanks.

 

[pkpbip]

I assume that the exchangers you are using to generate the steam already exist? If not, why not just use a steam generator to produce the steam rather than using an exchanger as well as a drum?

Otherwise,for sizing a vapor liquid separator first calculate the critical velocity of the 2 phase mixture (a function of the relative densities between the vapor and the liquid). This critical velocity determines the max vapor velocity in the drum allowed in order to separate the vapor and liquid. Assuming a certain liquid level in the drum (for horizontal drums) you can calculate the vapor velocity based on the cross sectional area. For a vertical drum this is easy because you just use the entire diameter of the drum. For a horizontal drum you have to calculate the area of the circle not occupied by the liquid. I believe that typically most use a vertical drum for steam service. Using the critical velocity and the max vapor rate you can then calculate the required drum size.

If you have certain liquid hold-up requirements for control purposes then you have take this into account as well. This depends on the stability of your process.

Also, for the pump you need to provide adequate net positive suction head so you may have to elevate the drum or designate a minimum liquid level.

 

[Montemayor]

chemieingineur:

I'm of the same opinion as pkpbip: why bother with a steam separator vessel when the process of generating steam with process heat exchangers is commonly done without a steam separator vessel. I've generated steam using process heat exchangers and I've used two exchanger schemes: one with vertical exchangers and the other with a horizontal exchanger. The vertical units had the process on the shell side and the horizontal unit had tube side process.

The vertical units (there were two of them) had their shell side connected to a common drum located between them. This drum was not designed as a steam separator, but rather as a water level control device to maintain the tubes totally submerged with saturated water. The horizontal unit incorported a level control on the shell side and had a steam "dome" on top of the shell. Depending on your configuration(s), you don't require to have a separate steam separator. Of course, you haven't stated it, but I have to assume you're planning on generating saturated steam - which is the usual case when using process heat for this purpose.

Based on the above, my simple comment is: you shouldn't have to watch for anything when generating steam using process heat. Simply employ some method of justifying a constant water level and allow sufficient vapor disengagement space above the water level. Perhaps this is what you are calling a "steam separator".

 

[25362]

Beside what has previously said mainly on the "mechanical" factors, watch over the "quality" of your water (is it condensate?). It should be free of solids and chemicals that may induce foaming and subsequent carryover.

 

[roker]

hello,

If you are circulating water through the steam separator vessel you should also have a blowdown stream from the separator, if the steam quality from the vessel is important
you have to use a mechanical separator at the steam outlet.
In horizontal vessels the normal level liquid is at the vessel centerline.

regards,
roker

 

[mbeychok]

Chemieingineur:

If, after reading the previous responses, you still decide to use a steam-water separator, here is my method for sizing almost any vapor-liquid separator:

The size of a vapor-liquid separator drum 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 this 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.

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

 

[brainstorming]

I agree with what proposed by mbeychole
but make sure that quality of vapour especially if it will be used for steam turbines and to ensure such quality recycling the water should be changed to continuous blowdown from the drum to maintain the quality.

Also, A demister pad mesh 144 kg/m3 is recommended to be installed at top of the drum to remove entrained liquid from the steam.

 

[MJCronin]

chem,

Here is my two cents.....

While I agree with all of the comments above, why wouldn't you employ the skills and tallents of those who design waste heat boilers ?

For many years, all of the subtleties of steam seperators, blowdown systems, ASME code materials requirements, and water level indication and alarms have been addressed by these people.

You are, of course, designing a high pressure boiler that must be inspected, certified and registered with the state....

Here are a couple of places to start......

http://www.neboiler.com/

http://www.aalborg-industries.com/ifs/files/AI/eng/Presentation/Website/index.jsp

By the way, the long time "godfather" of waste heat boiler design is an esteemed engineer named V. Ganapathy who has written books on this subject and has an informative website:

http://vganapathy.tripod.com/boilers.html

http://www.energy.rochester.edu/vg/

http://v_ganapathy.tripod.com/incinhr.pdf

http://www.allbookstores.com/browse/Author/Ganapathy, V.

Also, details of ASME Coded steam drum seperator design and mechanical design of boiler drum internals are contained in the book "STEAM" by the Babcock & Wilcox company ( 40th edition)

Hope that this helps.....


MJC