bottom blowoff ASME I PG 59.3.3

[davefitz]

Can anyone advise what is the official definition of "water space" in a power boiler?

Specifically ASME I PG 59.3.3 requires the boiler be provided with a bottom blowoff connected to the "lowest water space practicable". In our case we have a high pressure power boiler with a single steam drum, no mud drum, and downcomers feeding the evaporator inlet headers. The boiler vendor only supplied an "intermittent blowoff" connected to the upper steam drum, but this connection is 85 ft above the bottom of the downcomers.

Other similar boilers from other vendors have supplied a bottom blowoff connected to the bottom of the downcomers. What is the definition of "water space"?

 

[davefitz]

further:

see Marty Bernstein's "Power Boilers" p 96-97

code interpretation I-77-16 "... a bottom blowoff connection from the lowest water space available in the main downcomer"

(author) p 97 "...a regular blowoff from a water space somewhere low on the boiler, such as a waterwall header, would suffice."

Maybe a new Interpretation case should be filed "Q: Shall "water space" as used in PG 59.3.3 include downcomers and waterwall headers installed lower than the boiler's drums? "

Answer: Duh , yup.

 

[chicopee]

Impurities in the water will precipitate with chemical treatment and settle normally on mudd drums which is part of the water space of the boiler. Blow off activities is periodic in order to maintain a min. impurity concentration in the water. By virtue of your boiler design, it seems that water circulation thru downcomers and risers may not allow impurities to settle; however you still need to blow off to keep impurities within limits. Water space is the space below the water level in your steam drum.

 

[davefitz]

chicopee:
Per the code paragraphs, the bottom blowoff is used either for removing impurities or lowering drum water level, and is to be designed to be used under pressure.

It is true that the old mud drum boilers, which may have operated with crude water treatment and high blowdown rates, had bottom blowoffs on the bottom of the mud drum .That does not limit its use only to old mud drum boilers, and the code paragraph requires all boilers to have such bottom blowoffs installed at the lowest practicable water space. Just talk to an operator of a modern drum boiler which needs to undergo many startups per year- it is needed to control water level swells during fast restarts.

Today there are strict environmental time limits as to how many hours one is permitted to exceed gas emmissions limits during restarts. This effectively mandates a fast restart, and a large amount of water swell is the result. A drum boiler that does not have a botom blowoff cannot meet today's environmental requirements and also does not mee ASME I.

 

[davefitz]

chicopee;
Water space is all water space in the boiler below drum NWL, and it definitely is NOT limited to the drum. It includes downcomers and evaporator inlet headers. Read the referenced code inerpretations.