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mrgaee (Mechanical)
15 Mar 07 11:52
Severe cracking has occured in a power station gas turbine exhaust duct, 30mbar pressure and 500C temperature with turbulent conditions and high temperature ramp rates. The cracked pressure boundary is 8mm Coreten A steel material. The duct has internal and external insulation ('warm' casing). Cracking has been happening over a number of years and even repaired (welded) cracks have reopened at the repair. The duct is rectangular and approx. 7m by 8m with a 90 degree bend and has three banks of LF/HF internal baffles.

Has anyone seen this behavior before? short term repair method? long term solution?
metengr (Materials)
15 Mar 07 13:23
Yes, I have seen gas turbine exhaust duct problems on our aged fleet of CT's. Although our duct casing material is ferritic stainless, in lieu of Corten.

Getting back to your repairs, for duct cracks that have been repaired and continue to fail, it might be that the method of repair was incorrect the first time around. Many times on duct repairs, I have seen where the crack is simply pad welded over with no attempt to locally excavate the crack and weld repair to restore structural integrity. Check to make sure that when cracks are found, the proper weld repair is performed. Also, prior to after weld repair perform nondestructive testing to assure all cracks in the repair area have been totally removed. It might require a little effort and coordination, but the results will be beneficial.

Regarding the prevention of future cracks, you could look at adding external stiffeners to reduce resonance conditions in service.
MikeHalloran (Mechanical)
15 Mar 07 22:27
I've seen photos of cracking at bent corners in marine gas turbine exhaust ducts 0.5m x 0.5m, made of 12 gage Inconel and  heavily reinforced.

I don't think resonance is necessary to explain failure.  Check the sound pressure levels.



Mike Halloran
Pembroke Pines, FL, USA

0707 (Petroleum)
16 Mar 07 6:59
It seems to me that your cracking problem is related with the following:

1-vibration of the duct due to variations in fluid with turbulent conditions pressure.

2-As the duct has internal and external insulation ('warm' casing) if the duct is exposed to weather conditions I also presume that cracks found in welded sections of the seals were caused by corrosion due to rainwater entering from gaps.

In these conditions, (vibrations and thermal oscillations) repaired welds will always crack because tensions induced by rewelding.

Luis marques
mrgaee (Mechanical)
16 Mar 07 7:42
Thanks for your interest and comments. I'll have more information next week but it seems that the interior insulation has gaps and the exterior insulation is not that good. The cracks I mentioned before are significant long complete fractures of the external skin (between the insulations). This would indicate a thermal loading. Certainly the forces required to do the damage would be significant. Stay tuned.
gerhardl (Mechanical)
16 Mar 07 11:41
..interesting! Is there something more to observe about pattern, type, location and occurrence of the cracks?

All causes mentioned could of course alone or combined in some way contribute to cracking. If I remember my metallurgy correctly we are at 500 deg C at a temperature were the material is weakened or suspect to structural changes. Even more so if uneven heated and cooled rapidly in different ares near each other.

Direct cause could be different to pinpoint, but self explaining short term:
- Isolation (even out temperature) as indicated
- Outer support to alter/better vibrations and possible resonance frequencies
- Inner 'smoothen out' air turbulence and look at baffel support, girders and corners and other possible max stresspoints (welds, different materials or structure forms meeting) to observe for cracking starting at stresspoints.
- Check temperature distribution (camera?) also to look for stresspoints, cool/warm ares meeting.
-Traces of corrosion or condensate at (if relevant) at possibly (timewise) cooler areas
-Check for how structure is cooled down by shutdowns.

Longterm: newest material and technology from material suppliers if above is not sufficient. Thickness?

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