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Petro0707

Petroleum
Apr 10, 2007
82
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...
 
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From your description I would say that the pinhole being located at the weld can be attributed to poor welding during manufacture followed of corrosion dew point because of steam purge along the time.

In our FCC, expansion bellows have not steam purge, nor nitrogen purge


lui marques
 
For your more clarity, here is the drawing image of the Bellow : Pl. see that towards both side steam leakage was observed before 6 months…which is shown by two dots ( 90 ° Angle )



 
Sorry, I forgot to mention in above that steam leakage it is shown by 90 ° Angle from the corner block as shown in the figure with dark red dots.

These are some more snapshots of the failed bellow including the photo of defective piece which is taken out for discussion purpose.Just to try to convey the location of this piece is exactly when you put this piece perpendicular on the black line (drawn by Marker pen in the drawing board 90 ° angle middle) from where bellow shape started.

From the below link photographs what can be the root cause can you pl. throw some light please ?








 
From a quick look at your images I would say that your expantion joint failure was due to dew point corrosion.

How old is your expansion joint? Actualy expantion joints are not aerated. Aeration-free expantion joints contain insulation between the bellow en inner wall which restricts catalyst deposits and for that reason do not neeed to be aerated.

In your case for corrosion reasons it will be better to aerate your expantion joint with nitrogen instead of steam.

Contact your licencer to have his feedback on this subject.

luis marques
 
Fine, We were just thinking that whether Nickel & Vanadium sulfates can cause dew point corrosion or at what pH & temperature NiS & VS2 can be converted in to their respective sulfates ?

Regards,
 
Hello...0707,Can you pl. explain what is the basis of dew point corrosion based on these photos because after again brainstorming we came to one point that there was weld leakage and this weld leakage may be due to :

1.Mechanical Failure/Vibration
2.Corrosion due to NiSo4,VSO2 (Nickel & Vanadium - Metal sulfides gets converting to Metal sulfates in Regenerator and again it may deposits to cause corrosion which can cause bellow weld leakage.

I have checked with licensor and as per them,Nitrogen is not the solution because it can cause coldness than steam.

As I mentioned earlier this bellow is on spent catalyst stand pipe and one thing to note that out of 8 nozzles of steam distribution the nozzle at 12 0 clock position was choked which can cause dead zone for liquid Hydrocarbon to condense and deposit to cause dew point corrosion ??

Still...It is very difficult to understand the bellow weld leakage is due to corrosion. Expansion joint life is around 8 Years.I think Best in class is 10-12 yrs.
 
Petro0707

Please post the photo where you conclude that you have had a failure on the weld expansion joint.

By far the most common is the packed bellows. A packed design incorporates a ceramic insulation pillow filling the annulus and a catalyst seal between the liner faces. Purged bellows are not as common today as they were 10-15 years ago. The purge is applied to the bellows annulus in the form of air, nitrogen or steam.

Each of purges has advantages and disadvantages Caution has to be taken so that the media used to purge the bellows is compatible with the process conditions and does not cause corrosion problems within the bellows element.

Uneven distribution of purge system due to coking plugging of steam inlet nozzles causes catalyst to aggregate inside convulsion plies and causes under deposit corrosion to dew point formation and high PH acid concentration underneath those catalyst deposits which led to bellows rupture due to corrosion.

Luis marques

 
Hello,0707

You can see the attached images about weld joint failure and one of the image (cross sectional view of bellow drawn on drawing board – black spots represents the weld leakage point.)





 
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