At FCC unit expander inlet, in the weld between 48” flue gas pipe and expander by pass valve, we experienced a few cracks on the weld and heated affected zone. This pipe is subjected to intermittent steam injections for thermal shocks, to remove catalyst-aggregated particles on blades of expander rotor. Steam injection is made 1 meter downstream the cracked weld. Can we assume that those cyclic thermal shocks were responsible for the fatigue cracks on the above weld? Valve body and 48” pipe material is A312 TP304H Regards Luis Marques

I totaly agreed.

Luis / Chakra I would like to clarify the following on my views on the likely failure cause for the particular FCC down stream piipng and valve components. Answering the point raised by Chakra on the possibility of thermal fatigue and a view expressed of innumerable failures in operating plants - we should recognize the effects of thermal fatigue on different materials (before citing instances of failures). I am sure that you would recognize that thermal fatigue effect for ferrtic material is vastly different from that on austenitic S.S. I believe that for a S.S piping in F.C.C service to fail assuming this to be purely by thermal fatgue, the F.C.C should have seen cycling effect of atleast a couple of upset start ups and shut downs - continuously - over a period of several years. For Ex: if thermal fatigue is the likely cause ( assuming the refinery FCC had several upset shut-dwons and start ups of something like 20 upset problems in a month to have a likely reasoning of thermal fatigue as a contributory problem in addition to basic SCC). The cumulative effect of such frquent upsets in plant operaton on other equipment including the FCC unit would be much severe. Refinery operation would have come to a stand still under these conditions of frequent upset problems. We have to also recognize that the refinery or the FCC unit could not have been operated successfully with such cyclic upset start-ups and shut-downs over several years (say for an example an upset start up and shut down of 20 times in a month if you consider thermal fatigue is the likely cause for the failure experienced by Luis in his question) It is known that thermal fatigue will be more severe in muti-layer or single heavy wall thickness material than on a Sch 40 or 80 - 48" piping even assuming that there is a drop in temperature profile across the wall thickness is from ambient to the expected 980 deg F for FCC unit operation. For your information : I have done detailed failure analysis on SCC trends and catastrophic effect of SCC based on actual and practical studies with quantitative and qualitative tests and data (metallography, residual stress measurement by X-ray, TEM studies on micro-alloys and their contribution to SCC in these materials). I have also done detailed studies on effects of residual stress ( quantitatively measured both tensile and compressive stress values by X-ray back reflection methods), influence on welding parameter ( i.e., welding current, welding speed, welding design, welding technique, welders' quality of doing) on residual stress contibution, threshold limit of chloride /caustic SCC with or without stress relief anneal on Austenitic S.S, Inconel, Incoloy, Cast Fe-Cr-Ni - HK / HP type alloys. Based on my experience on what is SCC and the catatrophic effect due to SCC in these materials, i still believe, that the likely predominant failure cause for the particular F.C.C unit piping components - could be a combination of SCC (IGSCC / TGSCC) than thermal fatigue related failure for a 48" Sch 40 o4 SCh 80 piping. Finally, Luis would do well by doing metallographic examinaation of the failed piping components to establish the likely predominant cause. Contributory causes from other related effects and their role in accelerating the failure will have to be analyzed separately from both quantitative and qualitative failure analysis evaluation. A single method and guess work in discussion forum - without doing a detailed failure analysis may not be a solution for a particular problem. I am sure that readers forum will appreciate and recognize that detailed analysis is important for unexpected failure problems faced by users. Regards C.V.Srinivasan Nishi Engineers Pvt Ltd India August18m, 2005 E-mail: nishi@vsnl.com >The cracking reported might have resulted from SCC or even >from Thermal fatigue (as guessed by LIMA). > >The reasoning provided by CV Srinivasan is quite convincing. >However, the cracking reminded me of a failure Analysis I >saw few months back in internet. The failure study discusses >cracking of an FCC overhead line operating at 980 F which >had a steam connection (steam temp - 300 F). The cracking >was reported from nearby the area of the steam nozzle. The >report is available at ctlweb@corrosionlab.com. > >Regarding the cracking of static pipeline from thermal >fatigue, there are innumerable references in the internet >whre cracking resulted in similar circumstances. A simple >internet search for thermal fatigue failure in pipes shall >lead there. > >However, this is all theory. A simple metallographic study >should be able to decide the root cause for cracking. We >hope that LIMA will let us know the exact cause of cracking. > >'Hope this helps. > >Regards