boiler "lowest water space practicable" ? bottom blowoff
boiler "lowest water space practicable" ? bottom blowoff
(OP)
Can anyone advise the official definition of "water space" as used in the ASME code sect I PG 59.3.3 ?
Specifically, the code requires the supply of 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 intermitent blowoff connected to the steam drum, but this connection is 85 ft above the bottom of the downcomers. In my opinion, the downcomer should be included in the definition of "water space", but can anyone else confirm this interpretation.
Specifically, the code requires the supply of 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 intermitent blowoff connected to the steam drum, but this connection is 85 ft above the bottom of the downcomers. In my opinion, the downcomer should be included in the definition of "water space", but can anyone else confirm this interpretation.





RE: boiler "lowest water space practicable" ? bottom blowoff
RE: boiler "lowest water space practicable" ? bottom blowoff
That may be the same position the boiler vendor is taking also, but it does not seem to meet the intent of the Code and is not an adequate design for the expected use of the boiler.
The boiler is a "HRSG" downstream of a gas turbine, hence it must startup and increase pressure very quickly due to the fast startup rate of a gas turbine. The resulting swell of water in the HP drum cannot be blowndown fast enough using a surface blowoff, since this blowoff does not have any significant "gravity subcooling " to limit its choking in the blowoff connection and blowoff valve. Any attempt to compensate by use of the downcomer "drain" system will result in an unsafe condition to the operators, which is exactly what the Code wordings on bottom blowoffs is supposed to prevent.
When push comes to shove commercially, it comes down to what is the official interpretation of "water space".
RE: boiler "lowest water space practicable" ? bottom blowoff
RE: boiler "lowest water space practicable" ? bottom blowoff
That is a well-worded reply.
It is clear that if there is no code interpretation on the matter, then the vendor will provide an answer which limits his costs. By calling a surface blowdown a bottom blowdown, he has avoided the supply of a bottom blowdown, and probably reduced the size of the blowdown tank that was in his scope.
If no code interpretation exists, either it will be up to the 3rd party NB inspector's judgement or an interpretation case would need to be submitted to ASME.
RE: boiler "lowest water space practicable" ? bottom blowoff
Per Marty Bernstein's "power Boilers" p96-97:
interpretation I-77-16 specifically "provides a bottom blowoff connection from the lowest water space available in the main downcomer"
elsewhere p 97 (author) " ...a regular blowoff from a water space somewhere low on the boiler, such as a waterwall header, would suffice"
I think that if another interpretation needs to be filed, it would probably require the bottom blowoff to be , well, a bottom blowoff.
RE: boiler "lowest water space practicable" ? bottom blowoff
seems to support this design. However it is for u-bends and not a boiler with downcomers. This boiler is probably in compliance with SecI if it is Stamped.
Have an R stamp holder install additional blowdowns if you wish. I suggested you contact the designer as the flow may be such that a "true bottom blow" may not gain you anything.
RE: boiler "lowest water space practicable" ? bottom blowoff
Case I-77-16 includes the implied position that the downcomers are considered to be part of the water space, and a telecon with former subcommittee member included his opinion that it was a no-brainer- the evaporator inlet headers and downcomers are part of the water space.
RE: boiler "lowest water space practicable" ? bottom blowoff
assume that a stamped unit is correct. Good luck.
RE: boiler "lowest water space practicable" ? bottom blowoff
I agree the AIA is the quicker route, but this matter needs an imprimatur from ASME since I think (a) there may be other mfrs that have been cutting corners and misreading the code to save a few bucks and (b) many people, including AIA's , make the same assumption that because a boiler has a stamp, that by itself guarantees that it meets code- not a good assumption if one has prima facia evidence that it cannot be operated safely.
RE: boiler "lowest water space practicable" ? bottom blowoff
RE: boiler "lowest water space practicable" ? bottom blowoff
I worked over 20 years for a major boiler manufacturer, and trust me, a stamp does not guarantee no errors or ommissions were included in a shipped boiler.