HRSG / Deaerator Design

[threeS]

I found there are three different arrangement for the Deaerator in a Combined Cycle Power Plant:

1. Deaerating Condenser - Deaerating process is carried out inside Condenser.

2. Low Pressure Drum Deaerator - Deaerating process is carried out insde the LP drum.

3. Separate Deaerator - Deaerator is installed between Condensate Extraction Pump (CEP) and Boiler Feed Pump (BFP).

Obviously, arrangement in Items 1 & 2 can save the cost because no separate deaerator is required and LP feedwater can be directly supplied from CEP which no LP BFP is required.

But no all HRSG/Combined Cyle Unit supplier adopt "no Deaerator design". So I think there should be some problems in "no Deaerator design".

Can anyone tell the difference ?

 

[LSThill]

This line may help you

http://www.hrsgdesign.com./design0.htm

 

[rmw]

ericwong,

Not knowing exactly how you define the parameters of your category 1, deaerating condenser, I dont't know if you are referring to the deaerating function of every condenser, in that they are designed to scrub the droplets of condensate cascading throught the tube bundle and falling into the hotwell, and sweep the released NC gasses to the air removal zone of the tube bundle, or if you are referring to the "add-on" type vacuum condenser that deaerates incoming make up water as it enters the condenser, but before it gets into the hotwell, using a side flow of turbine exhaust steam, and having their own separate and dedicated air removal equipment.

In any case, I agree with your assessment. Deaeration is all about raising water to a temperature where the oxygen soluability is minimal, and scrubbing it in some way (mechanically, e.g. trays, packing, etc., or spray zones, or both) to get the DO to release and separate so it can be removed from the cycle.

If I remember my stuff from many years ago, somewhere in the range of 250F is the threshold point for oxygen soluability. That is why the majority of dearators operate at 15 psig, although it is not too uncommon to see them all the way up to 50 psig. (I am not going to go back and look it up, I will leave that for others.) So, obviously, a condenser operating in the range of 100F just doesn't get there (in my mind).

Plus, in a turbine surface condenser, conditions need to be optimal to attain the low dissolved oxygen levels that they are designed for. They have difficulties meeting those requirements at low loads, and/or very low CW temps. Not that they can't, just that it gets harder to accomplish.

Boiler/steam turbine power plants with the "no deaerator" design have been around for decades, although they are rare, in my experience. They suffer the same problems I mention above, difficulty with minimizing DO at low loads, loads that their initial designers would have believed not attainable 50 years ago.

The jury is still out in my mind as well regarding the "no deaerator" design. Especially now that some CCGT power plants are being compelled to operate at lower than maximum output levels that they were designed for.

rmw

 

[EdStainless]

One other catch is that many CCGT plants will not run steady state. This means that even if the condenser air removal was good, it would take many hours of operation to reduce the O enough. The plant may be shut off before this point is reached. And during this time damage is being done to the entire system.
In large base loaded plants, running steady and with little make up water, additional deariation may not be needed.
An external deariator has a capital cost, but you only run it when it is needed. And it will make the plant more responsive and flexible.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[davefitz]

The design without a separate dearator is normally selected because it has a better cycle efficiency and lower first cost, so it is evaluated at a better life cycle cost. The fact that the unit is 2-shift cycled and experiences rapid flow accelerated corrosion ( FAC) of the LP evap risers due to upsets in O2 and pH is nor considered by the MBA's that evaluate the bids.

The condenser will dearate to witin 7 ppb O2 if the unit is above a certain minimum load and there is not excessive subcooling of the condensate. The main ingress of O2 on these units is during restart operations: the makeup water is saturated with O2 and if not sprayed above the condenser tube bundle together with significant upflow of stripping steam ( from a steam sparger or LP bypass)then the make up water O2 will contaminate the condensate with O2. Also, most plants have ineffective control ogic for accurate feeding of hydrazien or carbohydrazide and the resulting overfeed of scavenger can also contribute to FAC.