Deaerator Pressure Curve vs Boiler Load 3

[hDaneshi]

Hi,
there is two different Curves for "Deaerator Pressure Curve vs Boiler Load" in my documents as per attached picture. please describe which one is correct?

http://files.engineering.com/getfil...2-43fc-ae67-2bc7d88ac042&file=untitled001.JPG

 

[rmw]

If you have to choose between them, the one on the left would seem correct to me. That said, my question is, why are you varying the deaerator with respect to boiler load? The deaerator removes oxygen as a function of temperature (pressure) and the higher the pressure the higher the temperature and the lower the oxygen content. Why would you want to put FW with a higher oxygen content in the boiler at certain loads.

If the curves relate to feed water heating and that has something to do with circulation flows within the boiler as a function of load, then ignore the above and change my answer to the opposite draft.

Still all in all, I do not have experience with deaerators that vary their operating pressure with boiler load.

rmw

 

[chicopee]

I thought deaerators were directly vented to atmospher and consequently were at atmospheric pressure.

 

[MJCronin]

chicopee,

Not all of them.

Some are designed to operate at low pressures (about 5 psig)

http://www.spiraxsarco.com/resource...s/the-boiler-house/pressurised-deaerators.asp

http://www.altairequipment.com/technical_files/DA_scrubber_specs.pdf

-MJC

 

[hDaneshi]

Hi,
These curves have been mentioned in "Deaerator Sizing Criteria" documents in two different CCPP projects, now i want to know that in lower HRSG load (25%,50%,75%) the deaerator pressure is lower or higher than the normal operation (HRSG load=100%)?
I think that the curve on the right should be correct but I need a good reason to be sure!

 

[chicopee]

Why would you think that pressure decreases as steam load increases?

 

[hrsgguru]

In conventional power plants (regenerative Rankine cycle), the deaerator is part of the feedwater heater train and thus sees an decrease in its operating pressure as the plant load drops unless specifically pegged to a constant pressure. These DAs routinely operate at 150 psig and higher at full load but may run down to 10 psig when the plant is at min load. This is due to the lower pressure from the steam turbine's extraction port.

In a CCPP, a changing pressure profile is also seen with integral deaerators except that as the gas turbine load is reduced, more heat load shifts to the back of the HRSG and the pressure in the low pressure section and/or DA goes up due to higher steam flow rates. So if your application is a CCPP, I'd say that the curve on the right is actually closer to what I'd expect although I'd think it would be a little smoother drop with increasing load. Then again, I don't know what types of pressure controls are being utilized for either the deaerator or the steam turbine throttle flow. Either of these could have an effect on the curve's shape. For example, if the turbine and/or duct burner are using a high sulfur fuel, the integral DA may require pegging steam from a higher pressure source (IP or HP drum) to keep it's temperature comfortably above the acid gas dew point in the exhaust to avoid gas side corrosion. This could cause a flat operating pressure curve as shown in the right side curve. This curve seems to flatten at about 4 kg/cm2-A or about 60 psia. This would give a saturation temp of about 290F which might be high enough to deal with relatively small amounts of sulfur.

 

[MJCronin]

Excellent !!! HRSGGURU !!!!

So.... a CCPP deaerator is a certain "special animal" and operates unlike the ancient deaerators that all of us are familiar with !!??

 

[hrsgguru]

A CCPP deaerator operates, in theory, just like all deaerators (direct contact heating of feedwater with steam and venting of non-condesables). By "ancient deaerator" I assume that you are referring to a STANDALONE deaerator whose source of heating steam is from a pressure controlled source. In theory, the two (CCPP vs. "ancient") are the same. But the "ancient" will typically operate at a fixed pressure whereas an Integral Deaerator will often operate at a pressure determined by the LP Steam generation rate. The higher the LP steam rate, the higher the pressure. And as I'm sure you're aware, as you reduce CTG load (or duct firing) in a combined cycle plant with a 2- or 3-pressure level HRSG, the LP steam rate goes up. My point about pegging steam applies to both of these deaerators also - if you're worried about corrosion on the boiler's gas side due to condensation in the cool end, you peg it up. In HRSGs you also have the option for feedwater recirculation as a bonus.

The question in this thread dealt with typical pressure profiles for deaerators vs. boiler load. My post notes that an Integral Deaerator in a CCPP may likely be inversely related to regenerative Rankine power plant unless the Integral DA is pegged to reduce gas side condensation. If the DA is of the "ancient" variety and its steam supply is pressure controlled directly by the operator, then its pressure profile doesn't change with boier load.

 

[rmw]

I'll admit that my deaerator experience is mostly limited to standard Rankine cycle type plants or industrial steam boilers with stand alone constant pressure deaerators operated at 15 psig or stand alone deaerators as one of the heaters in a string of FWH's, some of which were operated all the way up to 50 psig in supercritical plants.

The only CC deaerators I ever messed were were vacuum deaerators which were an integral part of the condenser and operated at a negative pressure.

Thanks hrsgguru for your explanations.

rmw