present status ? 1

[gnet]

severe uniform Corrosion in Amine Stripper column at a gas sweeting plant
after few years of service the amine stripper columns (A516-Gr70) started leaking from different points mainly near nozzle welds and seams of the tower.these leakages were stoppped by placing patches with mechanical binding. now during shutdown when these defects were being repaired by welding (preheating was looked after with care), they posed a tough problem , that was; lot of tranverse cracks about (1") started appearing (in radiography) where the seam was repaired. after rewelding we couldnt get rid of all the defects. one of the seam of 132" dia got its 45% length repaired. now the question why these defects appeared and how many times and how much area of the weld we can repair ? severe uniform corrosion was observed inside the shell.
Now the status is that shutdown of the plant is over. all repairs havebeen made.As i told you, and you can well imagine the repairs by one example that one of the seam of the stripper having 132" circumferential length has got 60" (in total) at different locations repaired, which also contains re-repairs. this seam is about bottom half of the almost 20 meter amine stripper column. well that is the worst example but other welds are getting towards the situation.(How much repair welding can be done and how many times on the same location?)
Hardness (175-210 HB) and Thickness (18-20 mm; at some points there is reduction of 1 to 2mm ) measurement at different locations havebeen made and found satisfactory.
The issue is how we can assess the fitness of the stripper for further operation because we know that there were lot of leakages earlier, so in future, we can expect the same as the material has degraded(we have yet to prove it). Can you kindly advice how to proceed for the evaluation for hydrogen or sulfide (as stripping of H2S) induced damage.Probably Metallography (details or reference from net), strain rate test, fracture analysis or some kind of ultrasonic inspection.(we can get 2-3 meter removed part of vapor return line of the same conditions.)
what would be remedial actions? (not going to use surface coating inside the shell, neither using nor have any plan of using inhibitors).

 

[metengr]

The 1" transverse cracks to me sounds like hydrogen cracking related to the absorption of hydrogen in the repair locations during welding or during service. Questions;
1. what is the typical service temperature of the vessel?
2. what was the preheat used for the weld repairs?
3. what is the corrosion mechanism in service?

Typically, transverse cracks are a good indicator of hydrogen cracking in carbon and low alloy steels. To avoid hydrogen cracking, it is imperative that the weld rod is a low hydrogen (E7018 H4) grade, and is either new from the can or has been properly baked and stored, prior to use. Second, you can increase the preheat to 400 deg F during welding to promote hydrogen diffusivity out of the steel. Once hydrogen has been removed, weld repairs using a low hydrogen electrode can be successfully completed.

The repair problem may NOT be an indicator of base metal degradation in service if it is only hydrogen cracking.

4. were the repair locations given a post weld heat treatment (PWHT)? PWHT will remove any absorbed hydrogen and will reduce residual stresses from welding. I would consider local PWHT to be performed during future weld repairs even if the Code does not require it.

To answer the second question in your post, I would initially consider a comprehensive ultrasonic examination (UT) of the shell material from the OD surface of the vessel at corroded locations, and in locations on the vessel that have not suffered from extensive surface corrosion, for comparison. Comprehensive ultrasonic examination can detect sub-surface hydrogen attack.

As far as base metal degradation of the vessel, depending on the service temperature in question 1, field metallography could be performed to assess the appearance of the shell metal microstructure on the ID surface at various locations within the vessel, again for comparison.

If you provide answers to the above questions, it may help to provide better direction in assessing the damage to the vessel base material.