Welding 316L SS or 321 SS on 347 SS

[doct9960]

We have a 200 mm [~8"] thick vessel with a design temperature of 265°C [509°F]. The vessel is in hydrogen service (~10,000 kPaA H2PP) and wet sour service. The Material Selection Diagram (MSD) says the vessel needs a 347 SS lining, mainly due to the thickness of the base metal. But the internals material are 316L SS

Question:
1. Why 347 SS lining instead of 316L SS lining? How does the thickness of the base metal relate to the lining material?

2. We will be welding some internal clips on the lining. Can the clips be 316L or should they be 321 SS? What's the relative cost between the two?

Thanks.

 

[EdStainless]

321 and 347 should cost less than 316 because they don't contain Mo.
As long as your 316 is low C (say less than 0.02%) there is little chance of sensitization along the welds.
The stabalized grades are used to counter weld sensitization. This region of material is much more susceptible to SCC and other corrosion and cracking mechanisms.

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Plymouth Tube

 

[metengr]

Use 321 clips.

 

[stanweld]

If you weld the clips to the overlay after vesel PWHT, the clips can be 321, 347 or 316L.

 

[doct9960]

Thanks to you all.

I was checking a drawing that was done by another vessel designer and he specified 316L SS for the clips. My comment was to change the clip material to "321 SS or 347 SS" and explained to him that the 316L material in the MSD was intended for the removable internals and not for the fixed internals like clips, at least that's how I interpreted the MSD. And I think I've read somewhere, maybe the Avesta website, that E347 has better corrosion resistance than E308. Anyway, the drawing will still be reviewed by our material engineer and he will have the final decision if he wants 316L clips.