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 Post subject: Superheater Flues
PostPosted: Thu Jun 23, 2022 6:42 pm 

Joined: Fri Dec 22, 2006 12:44 pm
Posts: 51
Location: Cheyenne, WY
Hello,

The FRA 1472 day inspection criteria allows for super heater flues to remain in place if they are in good condition.

My question is, has anyone observed stress cracking at the interface of the flue to the rear tube sheet as shown in the attached photograph ?

Thank you,

Dave Griner

Note: The photograph is not of a super heater flue.


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tube crack 1.jpg
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 Post subject: Re: Superheater Flues
PostPosted: Thu Jun 23, 2022 8:22 pm 

Joined: Wed Jan 16, 2019 8:47 pm
Posts: 216
Looks to me like the issue might stem from the tube being over-rolled creating a stress riser just inside the tube sheet. A troublesome tube that just won't seal up and an adamant boilermaker's apprentice can over-roll a tube fairly quickly.


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 Post subject: Re: Superheater Flues
PostPosted: Thu Jun 23, 2022 11:59 pm 

Joined: Fri Nov 07, 2008 11:21 am
Posts: 473
This cracking, in this area, is common in most firetube boilers in industry. The typical Scotch Marine design that is now commonplace is used industry-wide, and I have seen this failure quite often. It is not necessarily a locomotive problem.


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 Post subject: Re: Superheater Flues
PostPosted: Sat Jun 25, 2022 7:40 pm 

Joined: Mon Aug 23, 2004 2:14 pm
Posts: 613
Location: Essex, Connecticut, USA
Greetings:
In my opinion, it would be difficult to over roll an 8 gauge, 5-1/2" flue.
We had a brand new 5-1/2" 8g flue that we had trouble getting tight and re-rolled it a couple of times. The last time we stalled a good IR vane type corner motor with a No.4 Morse hole. We ended up getting it tight with a beading tool and a No. 400 air hammer.
J.David


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 Post subject: Re: Superheater Flues
PostPosted: Tue Jun 28, 2022 7:59 pm 

Joined: Thu May 24, 2012 1:37 pm
Posts: 2213
Wouldn't a tube that is overrolled be larger than the expanded taper at the end of the flue? This end appears to have a reduced diameter with a sharp shoulder at the inner point of rolling -- I'd almost be surprised NOT to see a stress raiser form in that zone (look at the silhouetted profile across the top of the sample)

The type of failure appears unusual to me for a flue, where the inside pressure is dramatically lower than atmospheric. What accounts for the apparent bell-mouthing of the crack area?

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 Post subject: Re: Superheater Flues
PostPosted: Wed Jun 29, 2022 8:40 am 

Joined: Sun Aug 22, 2004 7:19 am
Posts: 6399
Location: southeastern USA
Prossering?

I think there must have been something beyond simply overrolling going on in this case, perhaps water chemistry or the batch of steel.

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 Post subject: Re: Superheater Flues
PostPosted: Wed Jun 29, 2022 10:19 am 

Joined: Tue Sep 14, 2004 7:52 am
Posts: 2560
Location: Strasburg, PA
Overmod wrote:
Wouldn't a tube that is overrolled be larger than the expanded taper at the end of the flue? This end appears to have a reduced diameter with a sharp shoulder at the inner point of rolling -- I'd almost be surprised NOT to see a stress raiser form in that zone (look at the silhouetted profile across the top of the sample)

What accounts for the apparent bell-mouthing of the crack area?
The inner end of the rolling appears to me to be just to the right of the apex of the "A". The portion of the tube that was inside the sheet is smaller due to its being constrained by the tube sheet, where to the left of the sheet, nothing was preventing the tube from freely expanding as the roller did its work.

Regarding the crack, perhaps the waterside of the tube hole didn't have its edge rounded before the tube was installed. Regarding the "V" shape of the crack itself, perhaps that is from stress corrosion, possibly from a long tube vibrating in service?


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 Post subject: Re: Superheater Flues
PostPosted: Wed Jun 29, 2022 11:18 am 

Joined: Mon Apr 23, 2012 7:10 am
Posts: 41
Looking closely at the pic. It appears to have a crack in the rolled area just to the right of the split. Steel defect?


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