boiler "lowest water space practicable" ? bottom blowoff

[davefitz]

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.

 

[deanc]

It may be that the water level in the steam drum is very low by design. What appears to be the surface blow may indeed be the bottom blow. Check with the Manuf. see what they have intended. Check the Data report. The flow may be such that it is the best place. Would expect to see a drain at the bottom of the downcomers.

 

[davefitz]

deanc:
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".

 

[deanc]

Understand your problem and I have no answer for you. This may be a"use" concern and not a code issue. Could not find an interpretation. Suggest you contact the manufacturer's design eng. They should be happy to help you.

 

[davefitz]

deanc:
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.

 

[davefitz]

further:

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.

 

[deanc]

I looked more and found an interp. I-77-16 which actually
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.

 

[davefitz]

It looks like we will go the route of filing for a written interpretation. The current design did not provide for the proper design of the downcomer "drains" as proper "blowoff" connections, and the drain piping failed when they were used to blowoff excess water during a cold startup. It is clear the Code requirements were based on the need to provide a safe method of operating a boiler, and the boiler vendor's position is sufficiently audacious that it should be met head-on.

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.

 

[deanc]

Very interesting! Interps. are slow. Suggest you call the AIA that has the shop and speak with the AIS. Have to always
assume that a stamped unit is correct. Good luck.

 

[davefitz]

deanc:
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.

 

[deanc]

List your email so I can contact you directly. I will start an inquiry with more info. if I am able.

 

[davefitz]

Thanks anyway, but I have it under control.

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.