Titanium Cladding Vessel Repair

[sahsanb]

We have a pressure vessel being used to recover highly concentrated acetic acid in our process chain. Vessel has a 6 mm thick Titanium CP 1 cladding on 30 mm thick SA 516 Gr. 70 base metal. 15 mm thick steam jacket gap is between the SA 516. Gr. 70 base metal and external shell. Tell tale holes are available in vessel to monitor any leakge in cladding as well as provide backing purge gas in case repairs are to be undertaken on titanium cladding.

In past we observed some process abnormalities which resulted in cracks development in titanium cladding longitudinal weld seam, particulary in top shell dealing with vapor phase. This resulted in process ingress towards tell tale holes and since it was not immediately picked, while the process evaporated its solid residue remained and has now choked entire tell tale hole path hence a leakge cannot be detected and back purging could no longer be carried out.

A repair strategy was devised a few years back as per which titanium cladding weld seam was removed and then rewelded. Welding was then flushed followed by application of 3 mm thick fillet welded titanium batten strip which stretched beyond the weld seam by approximately 2" on each side over the weld seam. This repair method served us well for 5 years as no further cracks appeared on batten strip weld seams.

However for the past 2 years cracks have been observed on batten strip weld seam. Their is evidence of process having gone through the cladding and into the base metal corrodig as process is observed to be coming out of cracks.

We are temporarily repairing these cracks by grinding them and rewelding. However since there is no back purging and continous contamination by process that has ingressed into the base metal, weld quality is poor and cracks reoccur in 2-4 months time again. We also believe adverse metallurgical effect would have also happended since the welding is being carried out on more or less same the weld seam repeatedly.

Based on your experience would request you to share some ideas as to what may be the best possible way of repairing this given no back purging is possible. Can we remove this batten strip install another batten strip that is wider than the orignal one? Are there any NDE techniques available through which we can check state of base metal without removing the cladding since there is no direct access to the base metal from both internal and external side.

I am attaching some pictures for ease of understanding and recommending a suitable solution.

Regards

 

[gr2vessels]

From my experience, I can tell that the repair has not really followed the code;- you were suppose to remove a much larger area down to the substrate. Clean metal.
Drill a 3 mm hole in the middle of the CS area for back purging. Place your strip on the edges of the titanium, leaving the thickness of the existing clad as a gap for gas circulation. Complete the fillet weld of the strip to the titanium clad, while circulating in the gap the backing gas. Seal weld the hole after the completion of titanium weld.
Cheers,
gr2vessels

 

[sahsanb]

The problem is there is no direct access to CS base plate without removing titanium cladding since there is 15 mm steam jacket space betweeen base metal and external shell. Secondly there is a rotor installed in the vessel, and its original clearance with shell is 10 mm which was reduced to 7 mm after installation of 3 mm thick batten strip.

I am sharing equipment General Arrangement Drawing along with rotor to shell clearance. Please review it and based on your experience and knowledge please what are the possible repair options available to us.

Regards

 

[gr2vessels]

sahsanb,
Given your vessel deteriorated over some period of time, the repair is very difficult, if not impossible. The contamination behind the clad could be extensive. However, you are looking for a quick fix of those cracks only. It appears that the titanium welding you performed in past was contaminated, hence the cracks. The cracks could be the result of mixing contaminants into the weld pool, insufficient gas shielding, inappropriate WPS, etc.
I don't believe the external jacket can prevent a good preparation of the cladded shell for back purging of the titanium weld. Remove a small section of the jacket to access the inner shell and at the end repair the jacket. Remember, there is no cheap and quick fix, but good engineering to fix the titanium clad.
Obviously, you have to decide how much of the titanium cladding and all previous strips you need to remove, down to clean carbon steel metal. Then apply a new strip, wider if needed. this weld of titanium to titanium must be performed by a qualified welder per your WPS/PQR. Repair the hole drilled in the CS for back purging of the titanium weld and then repair the jacket where you removed the small section.
Again, all this is a fix for the local cracks, it does nothing for the previous leaks and the extent of damage done to the CS elsewhere in the shell.