Deaerator Sizing - Continued

[Pavan Kumar]

Hi All,

Further to discussion on the below thread, I developed a calculation spreadsheet to perform some sizing calculations for a Pressurized Deaerator.

Deaerator Sizing - Chemical process engineering

Hi All, Can anyone provide me the references or documents that I can use to learn about Pressurized Deaerator sizing. Any help is appreciated. Thanks and Regards, Pavan Kumar

www.eng-tips.com

I am however facing mass balance issues at the Boiler( please see the sketch pasted below, also in the attached spreadsheet). I calculated the following from the provided Boiler Rating and Feed water temperature and the O2 Concentration in the Deaerated water.

1. Steam Generation Rate of the Boiler.
2. Make-Up Water Rate ( Considered Equal to the Boiler Steam Generation Rate as there is no condensate returned).
3. Vent Gas Flow Rate ( Air and Steam Venting Rate).
4. Deaerator Storage Tank Volume and Dimensions.

The issue I am facing is that since I considered the Make-Up water flow rate equal to the Boiler Steam Generation Rate, the input to the Boiler is not matching with the output. The difference is due to the Steam that is condensed in the Deaerator. I want to understand how a Deaerator operates in practice and how this mass balance question can be answered meaningfully. Your inputs in this regard will be very helpful to me.




Thanks and Regards,
Pavan Kumar

 

[pierreick]

HI,
Download MEASUR you will find example of deaerator.
Are you sure about the operating conditions of the deaerator (Pressure and temperature)? what is the quality of the steam at the discharge of the PRV (steam), pressure and temperature?
Your schematic is wrong, steam is supposed to bubble within the deaerator sump meaning steam will condense partially, the trays are there to provide contact between water and steam to help for the realize of O2 through the vent.
Pierre

 

[goutam_freelance]

Mass flows to and from boiler are wrong. Outgoing flow can not be more than incoming.

 

[Pavan Kumar]

Hi Mr. Goutam,

I knew the Mass Balance at the Boiler is wrong and that was my question above.

Hi Mr. Pierre,

Yes I will download MEASUR today and look for the Deaerator example. The sketch is only a schematic, plus the Stripping steam is added on a side nozzle on the dome close to the bottom. There are variations where steam is added into the Deaerator storage Tank as well.

Thanks and Regards,
Pavan Kumar

 

[pierreick]

Pavan,
your mass balance is the following:
Make up water 98.4 t/h
vent : 1.14 t/h
steam to deaerator : 16.47 t/h
feed water to boiler : 113.73 t/h
steam to process : 97.26 t/h
to be tuned.

Pierre

 

[goutam_freelance]

Hi Pavan Kumar,

On which basis you are calculating?
For example , is it not possible to reduce the boiler steam flow or increase the make up water flow?

 

[Pavan Kumar]

Pavan,
your mass balance is the following:
Make up water 98.4 t/h
vent : 1.14 t/h
steam to deaerator : 16.47 t/h
feed water to boiler : 113.73 t/h
steam to process : 97.26 t/h
to be tuned.

Pierre

Hi Mr. Pierre,

I found that my adjusting the Make-Up water flow rate in ratio with Stripping Steam flow rate, I can get the Mass Balance corrected. I was able to do that. My updated spreadsheet is attached.



My final values are as follows:

1. Make-Up water Flow Rate = 98.6 Tonne/hr
2. Stripping Steam = 14.96 Tonne/hr
3. Total Venting Rate = 0.013 Tonne/hr
4. Steam to Process = 98.6 Tonne/hr
5. Deaerated Water to Boiler = 113.54 Tonne/hr

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

Hi Pavan Kumar,

On which basis you are calculating?
For example , is it not possible to reduce the boiler steam flow or increase the make up water flow?

Hi Mr. Goutam,

The Make-Up water control valve operates in the level in the Deaerator storage tank and thus to balance to the output the Make-up water flow rate has to adjust. This is exactly what I did and posted my corrected calculation spreadsheet in my response to Mr. Pierre above.

Thanks and Regards,
Pavan Kumar

 

[TBP]

What continuous blow down rate are you considering? At 100% make-up, I'd guess about 10%? If that's the case you'll need to send about 125,000 kg/hr feedwater to the boiler. I'd allow at least 130,000 kg/hr feedwater to the boiler so as to allow for bottom blow down, as well as the gauge glass and water column once per shift.

Is there any seasonal variation in make-up water temperature? The colder the water, the greater the steam demand by the deaerator.

 

[Pavan Kumar]

What continuous blow down rate are you considering? At 100% make-up, I'd guess about 10%? If that's the case you'll need to send about 125,000 kg/hr feedwater to the boiler. I'd allow at least 130,000 kg/hr feedwater to the boiler so as to allow for bottom blow down, as well as the gauge glass and water column once per shift.

Is there any seasonal variation in make-up water temperature? The colder the water, the greater the steam demand by the deaerator.

Hi TBP,

I have not considered Boiler Blow down in my calculation. I will consider 10% of BFW as B/D and recalculate the Feed water input. Also my present spreadsheet considers only make-up water as the input and no condensate return. I need to include the Condensate return and correct the size of the Deaerator. The question is condensate returns at 80 Deg C while the make-up water is at 20 Deg C. Do I calculate the stripping steam requirements separately and add up?. Since Condensate is already stripped off of the dissolved gases, how does this affect the tray sizing?. I have not considered seasonal variation in the Make-Up water's temperature to keep the calculation simple at this moment.

Thanks and Regards,
Pavan Kumar

 

[TBP]

The reason I mention variations in temperature of the make-up water, is that is something that needs to be considered when using surface water sources in colder climates. For most of my life, I lived and worked close to the Great Lakes in Canada, and that was the water source. In Aug, the water temp might hit 17 or 18 C. In early spring, with the snow & ice melt running into the lakes, we would see water temps about 1 deg C. The temp of ground source water is typically pretty consistent.

Any condensate return typically goes directly to the storage section of the DA, and normally there isn't a level control valve on this line, because you won't get more condensate back than you send out as steam. There might be a few minutes on a start-up or other issue where the storage section will overflow slightly, but it's not normally worth the cost of trying to control it. The deaeration section needs to be designed to handle the maximum flow - and lowest temperature - of make-up water you anticipate.

 

[Pavan Kumar]

Hi TBP,

You seem to have a good experience in sizing Deaerators!. So the Deaerator tray sizing and stripping steam has to calculated for the maximum make-up water flow at minimum temperature. That makes sense.

If the condensate return is coming at 80 Deg C and the BFW to be sent to the Boiler is required to be at 105 Deg C then how can you still put the Condensate into the Deaerator storage section, will it not have to pass over the trays to get heated back up to 105 Deg C?. I have seen the nozzle for Condensate return on the dome section and not on the storage tank in some cases.

Thanks and Regards,
Pavan Kumar

 

[TBP]

There are a number of factors - and various combinations of those factors - that come into play with DA design and configuration. I've even seen a vacuum deaerator (just one in about 50 years) installed in a boiler house. I worked in a central station where the DAs operated at 150 PSIG - which is about 10 bar.

It all depends...