Stainless steel pressure vessels 1

[Corgas]

I came across following spec :

"For uninsulated 304L and 316L stainless steel pressure vessels with design temperature less than 450 °F and not stress relieved, the external attachments may be of carbon steel material, welded directly to the stainless steel pressure part."

1.- Why CS is allowed to be welded directly to the stainless steel? I've always thought that an isolation pad of stainless steel is needed.
2.- I failed to see reason in specifying design temp? I wouldn't recommend operating 300 series above 750 °F anyways.
3.- Does it make a difference wether it is stress relieved or not?

Any comments or suggestions are welcomed

 

[metengr]

Corgas;
You don't indicate the source of this specification. If it is a customer's specification, so be it.

Here are my replies to your questions

1. Because the customer desires it this way. The less than 450 deg F is probably the customer’s requirement to avoid elevated temperature failure of the dissimilar metal weld (DMW) joint in service. In this case, an isolation pad is not required.

2. Design or even service temperatures should be stated in any pressure vessel specification.

3. Well, once again, if the customer requires a lower temperature stress relief of an austenitic stainless steel vessel, so be it. In this case, having an isolation pad avoids having to weld directly to a stress relieved pressure vessel. If the vessel is not stress relieved, welding directly is specified as above.

 

[Corgas]

Metengr :

Yes, it is a customer's spec and cannot disclose name of company.

1.- Yes, that was my initial thought. However, API RP 571 indicates 510 °F as the temp where DMW can occur (due to thermal expansion/thermal fatigue stresses). I guess customer is being conservative (450 °F).
2.-OK!!
3.- Yes you're correct...It's monday!!!. For 300 series SS, isn't a solution annealed ( 1hr/in. of thickness)followed by rapid quenching recommended (even for cold formed parts) rather than stress relief?

Thanks again metengr

 

[metengr]

Corgas;

For 300 series SS, isn't a solution annealed ( 1hr/in. of thickness)followed by rapid quenching recommended (even for cold formed parts) rather than stress relief?

I am not a believer in low temperature stress relief of austenitic stainless steel pressure vessels. Regarding solution anneal - yes, I would recommend it if any cold work is introduced from fabrication, and the object is exposed to either elevated temperature service or aqueous service with contaminants. Keep in mind that the Code does have maximum cold work limits without the need for a re-solution treatment, which I do not necessarily agree with, but is based on service temperature.

 

[strider6]

usually the requirement is that the external attachements shall be of the same materials of the shell.. a dissimilar weld introduce also the possibility to have galvanic corrosion..

S

Corrosion Prevention & Corrosion Control

 

[MikeMet]

metengr;

Regarding your statment:

Regarding solution anneal - yes, I would recommend it if any cold work is introduced from fabrication, and the object is exposed to either elevated temperature service or aqueous service with contaminants.

If that was the case than every vessel in high temp or corrosive service that had rolled shells plate would have to be solution annealed. Also, many welds made in stainless steel vessels have residual stresses very close to yield and are in services you describe with no problems.

Bottom line is every installation needs to be evaluted on its individual merits.