High pressure boilers Hot blow 1

[smahsan]

Hi every body,
My question is that why we hot blow the High pressure Boilers(Drum pressure169 bar and 1000t/h steam generation) and when (As per pressure/boiler condition)
My informations is not much so i m asking experts.
Regards
S.M.Ahsan

 

[Goahead]

For a review of the process and for blowdown heat recovery you may wish to see:

http://www.nrel.gov/docs/fy02osti/31697.pdf

and

http://www.ptm.org.my/mieeip/boilerblowdown.html

Regards,

http://www.welding-advisers.com/

 

[metengr]

For a boiler of this size 169 Bar (2451 psi) and ~2 million pounds of steam per hour, this sounds more like a large Power Boiler for electric generation?

If so, hot blowdowns are not required if you have feedwater heaters. Typically, for large power boilers, the blowdown is used to control excursions in water chemistry. It is not something that should be done on a continual basis unless you do not have HP or LP feedwater heaters to preheat feedwater. For most small recovery boilers, this is a different matter.

I would strongly suggest you review ASME Section VII, Recommended Guidelines for the Care of Power Boilers. This is a comprehensive source book for operation, care and maintenance of Power Boilers.

 

[davefitz]

As Meteng states, large high pressure drum boilers used for power generation usually do not need to utilize bottom blowoffs except at part loads to control water chemistry upsets related to "phosphate hideout".

The original purpose of the bottom blowoff was to eliminate accumulations of crud that developed in low pressure boilers with primitive , non-volatile water chemistry treatments. Also, older boilers had rolled tubes in teh drums, which led to accumulations of metal shavings that needed to be removed ( which would plug up anything except an wye pattern globe valve). The boiler bottom blowoff could also be used to lower the drum water level in an emergency situation, and for that reason the B31.1 contains specific rules for the design of the blowoff drain piping ; it requires those drains to be super-strong to prevent failure due to high momentum 2-phase flow upsets.

Regardless of size and pressure rating of the boiler, ASME sect I pg 59.3.3 still requires each boiler be supplied with a boiler bottom blowoff connection " in direct connection with the lowest water space available" This means the bottom of a waterwall header or bottom of a dwoncomer.

One large vendor of HRSG's has interpreted the pg 59.3.3 to mean the bottom of the steam drum, but that is clearly an incorrect interpretation and it led to a failure of the blowoff drain pipig at one of our sites. The steam drum bottom drain was not formally defined as the ASME required boiler bottom blowoff connection on contract P+ID's, so its drain to the flash tank was not designed as strong as B31.1 would require for such a connection. When the operator opened the drain to blowdown excess phosphates due to hideout , the darin pipe failed. Fortunately no-one was hurt.