make up to steam cycle

[Zehnder]

what are the advantages/disadvantages of adding the make up water into the feedwater storage tank deaerator versus adding it into the condenser?
our dearator tank pressure will always be above ambient pressure. the condenser of the backpressure turbine will be operating in vacuum and also above ambient pressure under certain conditions. how are the non condensible gases usually being removed in such a condenser?
the amount of makeup in relation to the nominal steam flow in the cycle is about 4% (little process steam extraction).

 

[JEB66]

Make up to the condenser does not have to be heated.

The non condensible gasses are removed from the condenser via some sort of air ejector, like a steam jet air pump, a nash vacuum pump...

 

[Zehnder]

hello jebb66,
is there also a disadvantage in putting the condensate into the condenser instead of the deaerator? is the deaeration capability of the condenser sufficient for a make up supply of 4% total cycle turnover?
are the ejectors or vacuum pumps also working with a condenser operating in a pressure range from 0.3 bara to 2 bara?
if not, what other system can be used?

 

[JEB66]

I work for the power industry and my company has system ranging from 90MW to 1300MW. All of the cycles put makeup in the condenser to avoid a separate preheat.

The system also has the ability to go beyond the 4%, but rarely do we use that amount for very long because it would mean we have a large tube leak.

The condenser is a very good deaerator. It is called a condenser beacuse of the main roll in the process. The air ejectors work on condensers that operate at 0.5 inches Hg. ~13 inches Hg would be a perfect vacuum.

 

[Zehnder]

i was also working in the utility power business with "normal" condensing turbines where we always put the make-up into the condenser. this is now a different business: the turbine is of the backpressure type where the exhaust pressure is ranging from 0.3 bara to 2.0 bara.
the question is: are there ejectors or vacuum pumps on the market, which can handle this pressure range and still are able to remove the noncondensible gases from the condenser? what would be other solutions? i'd like to avoid putting the make-up water into the dearator on top of the feedwater tank.

 

[engy74]

Dear Zehnder,
I 'm not sure to have understood the trouble, so I need more details.
First how can your turbine operate in both backpressure and condensing mode?
Second: why do you avoid to feed the make up water to the deaerator tank?
Third: which kind of plant (cogeneration, thermal power plant, CCPP etc.) are we talking about?
with the informations I have I can note that, operating the turbine in backpressure mode no vacuum pumps are generally need (if the condenser is pressurised You have probably and escpaoe of steam, but external gas will not enter in your system) so the pumps may be designed only for the operation in condensing mode
with refence to feed the deaerator instead the hot well under the condenser, for Cogeneration Plants sometimes it's necessary to feed the deaerator; but this for control purposes, as if the factory need great quantity of steam for a little time, it's easier to manage the feed water tank level (which depends on the control valves donwstream condensate pumps and the demi water pumps ) then the hot well level (which depends on a lot of paramters difficult to evaluate).
Clearly, if You are in a CCPP (or in plant with great quantity of make up water, and this probably isn't the case we are speaking about...) You can preheat the demi water with the feed water at the outlet of the deaerator so optimising the thermal recovery of the HRSG (low feed water temperature = low exhaust temperature at stack) and diminishing the steam consumption in the deaerator, but these solutions may decided only during the design phase of the steam generator...
best regards,
Engy 74