Dear All experts,

We have faced one serious problem in our FCC spent catalyst standpipe bellow leakage problem which has caused Plant S/D and major financial loss too.Actually, this bellow is metallic Inconnel 625 with 1.5 mm thickness double ply.As you all of know that purge steam flow through LMP control valve around 6.5 kg/cm2 pressure followed by RO followed by 8 distributors is there and before 6 months we have observed steam leakage so, on nipple we have made clamping but recently sudden leakage found and Hydrocarbon vapours as well as condensed Hydrocarbon was also seen during leakage. On subsequent investigation and inspection it was found to us that the leakage started on weld joint of metalic expansion joint from pinhole to 2.5 inches opening - at the time of sudden failure.But how pin hole has occurred and how Hydrocarbon gets out still we are not very confirm and sure though we did Root cause analysis but one thing that is observed is Bellow design pressure and temperature ( 6.5 kg/cm2 including catalyst head & 538 C was adequate and actual pressure temperature was also under limit i.e. it has not gone beyond that limit also,(we don't have any online temperature monitoring system on the bellow).Vibration phenomenon of structure was also brainstormed but subsequently it was ruled out,One observation that made is two sided steam leakage (before 6 months)from the equalization ring/ bellow joint.

Whether it is Dew point corrosion, uneven steam purge flow due to RO was choked , or Weld leakage ( Manufacturing defects during bellow design or poor weld strength) , whether it is Fatigue etc. is again not confirmed.
One thing we can say that there was not corrosion and bellow was designed for 200 cycles and life was under limit.

Still what can be the root cause ...can you please throw some light and how to prevent such in future..?

Thanks in advance...