Impact Test of Stiffening Rings

[lmacallndong]

According to UCS-66, attachments that are essential to the structural integrity of vessel shall be evaluated for impact testing. Are vacuum stiffening rings considered to be one of these attachments?

Are stiffening rings required to be evaluated for impact testing per UCS-66?

Vessel MDMT is -20F and shell material is SA-516-70. Stiffening material is SA-36.

 

[fegenbush]

Are vacuum stiffening rings considered to be one of these attachments?

Yes, unless the vessel is designed in such a way as to not require the vacuum rings when the temperature is below the MDMT that would require impact testing. Example: MDMT of -20°F at -1psi AND MDMT of 20°F at -5psi where the vessel does not require vacuum rings at -1psi.

Recommended options:
- Impact test the SA-36. If fully killed, acceptable results at -20°F should be readily obtained.
- Fabricate the rings from SA-516-70 plate. The cost increase for this material may be minimal, if the rings are able to be fabricated from plate.

 

[lmacallndong]

fegenbush,

Thanks for quick reply.

Stiffening rings are 6" x 6" x 5/8" angles. We specified SA-516-70N for material. The vessel fabricator requested to substitute with SA-36 angle bars.

Material substitution is acceptable to me, but I would like the fabricator to impact test the SA-36 angle bars. I would like to know if my request to impact test is valid. It seems to be valid if rings are "essential to the structural integrity of vessel."

However, when I ran Compress pressure vessel software, it says that MDMT for ring is N/A. Anybody know the reason why? I will also post this question in Compress forum group.

Thanks.

 

[fegenbush]

SA-516 is a specification for plate only, so it is not surprising that you cannot find angle to meet the requirement.

If you are concerned about the impact testing, it is possible to do one of the following:
- Fabricate an equivalent section modulus angle from plate (SA-516-70). The dimensions will not be the same as your 6x6x5/8 since the sharp heel will not be present. This section could then be rolled to the proper radius.
- Fabricate an equivalent bar section from plate (SA-516-70) and cut it into segments so it need not be rolled.
- Change the material to SA-276 304, if allowable.

 

[Duwe6]

"Change the material to SA-276 304, if allowable."

Using a stainless ring on a carbon vessel [and vice versa] is a bad idea. If the vessel operates much above ambient temperature, the joint will be highly stressed due to differential expansion. This gives a pretty good probability for stress-corrosion cracking. If the vessel temperature cycles up and down much, you are almost guaranteed SCC.

 

[fegenbush]

Duwe6,

That's why I said "if allowable" and made it the third option, as opposed to the first.

 

[EngAddict]

Duwe6, do you mean low cycle fatigue (ratcheting)? I wouldn't expect the stress to be high enough just above ambient. Is it a particularly corrosive environment to develop SCC?

I would accept the substitute if they impact test, if they don't want to pay for testing then they should fabricate it out of plate.