Sizing of a Blow Down Tank

[curtis2004]

Hi Everyone!
I'd like to get some guidance where to look for information about proper sizing of blow down tanks. I have 200 HP fire tube steam boiler with 7000 PPH steam @ 80 psi (100 psi design). It will be built to BPVC Section I. My boiler supplier quoted me a tank with 36" dia x 42" high, but didn't give an explaintion why do I need to have this size blow down tank. He simply said "for 200 HP boiler you need this size". Anybody can help where can I check this?

Thank you,

Curtis

 

[curtis2004]

Forgot to add that my boiler will have both continueos and intermittent blowdown. Can I furnish one for both?

Thank you,

Curtis

 

[Shmulik]

curtis,
This subject has been discussed in details:

thread124-277326

Yes, you may have one flash tank for both.

 

[curtis2004]

Shmulik,

Thank you for the link.
I still have questions:
1. If I provide one blow down tank for both continious and intermittent do I need to allow for both C+I or greatest?
2. How do I calculate continious blow down? What data do I need to use?

Thank you,

Curtis

 

[ione]

The blowdown rate is expressed as kg/h.
In order to evaluate the blowdown rate you need the following set of info:

F = Feedwater Total Dissolved Solids (ppm)
S = Boiler steam output (kg / h)
B = Required boiler water TDS (ppm). This value depends on type on working pressure and boiler’s type

Blowdown rate = (F*S)/(F-B)

As rule of thumb size the vessel volume at least twice the max. volume of discharged water (if your scheduled operation foresee an overlapping between continuous and intermittent blowdown take the two flows into account)

As marginal consideration, please note that valves for continuous blowdown could be a bit bothering as they are subjected to clogging because of their tight clearances.

 

[curtis2004]

ione,
Thank you for your quick response.
I thought I knew how to estimate intermittent blow down rate. It should be at least twice of volume removed from firetube boiler if water level reduced 4". Sounds very simple. However, in my case there is only 2" between normal level of water and lowest water level. In other words if my boiler loose 4" water it will trigger emergency shut down.
Should I still calculate a volum of 4" thick water from normal level? Does anyone faced the same problem?
Thank you all,

Curtis

 

[Shmulik]

Curtis,
It seems that you should verify the suitability of your water treating system to boiler's feed water quality requirements.

http://www.nationalboard.org/Index.aspx?pageID=238

http://www.nationalboard.org/Index.aspx?pageID=244