Deaerator Pressure 2

[danieldecko]

Hello, I am a professional engineer in Ireland. I have little knowledge about deaerators. As far as I know, deaerators have a fixed pressure which can be taken as 1.2 bars. Does it mean that all streams coming to and going from deaerator should have the same pressure, for ex. for a cogeneration plant. Thank you. DD

 

[25362]

For a reasonable tutorial on pressurized deareators see:

http://www.spiraxsarco.com/learn/html/3_21_01.htm

 

[athomas236]

As far as I know there is no fixed pressure for a DA except that generally speaking it should operate above atmospheric pressure.

The function of a DA is to remove gases that might cause corrosion from the feedwater. The effectiveness of a DA is determined by how close the actual temperature of the water is to the saturation temperature. This is because the solubility of gases in water reduces as the actual temperature approaches the saturation temperature.

If the actual temperature equals the saturation temperature then there is no gas in the water. Think its called Henry's rule.

As a consequence, in coventional plant the pressure is selected on the basis of a cycle optimisation. I have worked on plant with DA pressures above 7 bar abs. With the advent of combined cycle plant operating at different pressures the DA pressure is dependent on the lowest boiler pressure.

The reason for keeping the DA pressure above atmospheric pressure is to prevent inwar air leaks into the DA.

Regards,

athomas236

 

[25362]

Google sites present interesting discussions on the advantages and disadvantages of atmospheric vs pressurizd deaerators.

 

[TBP]

Check out

http://www.kansascitydeaerator.com

for a variety of technical papers on deaerators.

 

[rmw]

Lots of plants, especially combined cycle use vacuum deaerators that operate at condenser pressure to remove non condensables from incoming make up water.

rmw

 

[MJCronin]

daniel,

What has not been mentioned in the threads above is the affect that DA pressure has on boiler feed pump NPSH. This operating pressure also indirectly affects required DA elevation in the plant layout

The DA, storage tank and the feed pump are part of a single system....

MJC

 

[rmw]

The DA pressure should have absolutely no effect on BFP NPSH because the vapor pressure of the saturated liquid in the DA storage tank is balanced off in the NPSH equation by the operating pressure acting on the liquid level in the storage vessel.

Only if the DA was not doing it's job, and the water in the storage tank was sub-cooled with respect to the operating pressure of the DA would there be any effect, and this adverse condition would actually have a beneficial effect on the NPSH.

rmw

 

[crawfishdaddi]

I work with DA's at a cogeneration plant and all of the ones I have ever delt with maintain an inlet pressure of 5 PSI (If of course you are using cogen steam to deaerate) and a constant Temp of 230F. Oxygen and other uncompressable gases (like CO2) are usually most insoluble under these conditions.

 

[mbeychok]

Crawfishdaddi:

Oxygen and other uncompressable gases (like CO2)

Yoy lost me with that statement. What did you mean by saying that oxygen and CO2 are uncompressible gases?

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

 

[hacksaw]

hopefully he meant non-condensible gases

 

[Montemayor]

I am trying to figure out if he means 5 psia or 5 psig. I think he means 5 psig & saturated conditions (which would give his 230 oF).

 

[crawfishdaddi]

Sorry for the previous confusion, Thank you hacksaw for the correction I did in fact mean non-condensible gases, primarly oxygen and CO2 (which under boiler conditions can form carbonic acid). Secondly, the congen plant I work at generates steam at 560 PSIG and 150 PSIG (from waste heat and heat recovery boilers). We have 30 PSIG extraction on both turbine generators and main blower turbines that is used for utilities and let down to 5 PSIG for use in our Deaerators. Our DA 's use a cross flow system and are maintained at 230F. The surplus steam is vented to the atmosphere at Sat. Temp.
I have been working with DA'S for about 3 months and am in no way an expert on the subject, thanks for the help.

 

[electricpete]

The steam pressure does in fact increase the suction head available to booster pumps and feedpumps, thus reducing pumping power requirements.

I work in a nuke plant. Most nukes don't have dearator tanks. As I understand one deciding factor during the design of our plant was that the addition of a D.A. meant that each of our 3 feedpumps would be capable of providing 50% flow so we have a built-in spare. Without the d.a. we would need all 3 feedpumps to reach 100% flow.

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[electricpete]

To clarify, it increases suction head, but not NPSH as pointed out above.

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