ASME BPVC Section II Part D

[TylerM]

I was reading over the codebook and came across a material (SA-312 TP304L) that has two different allowable stress values. One of them (The higher value) has note G5 attached to it.

G5 says “Use of these stresses may result in dimensional changes due to permanent strain.”

My question is: who in their right mind would use these allowable stress values for a pressure vessel? The note goes on to say that this material is not recommended in flanges because it can cause leaking (that's understandably). Is the risk of permanent dimensional change really worth the reward of a slightly higher allowable stress?

Your thoughts and opinions are greatly appreciated,
Tyler

 

[marty007]

We use the higher allowable stresses as normal practice...

Take that a step further, we always specify 304/304L dual certified material, and use the higher of the two allowable stress values for 304 straight grade...

 

[metengr]

There is a chance that use of higher allowables can approach the minimum specified yield at regions of local stress concentration, and result in local plastic strains that may not be visually apparent. This is a one time affect once the material locally strain hardens, it will not repeat in service unless the stress is higher than design.

 

[SnTMan]

Widespread practice is to use lower allowables for members of gasketed joints, higher allowables for all else. This practice is often codified in client specs, though a few specify use of the lower allowables only.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[TGS4]

TylerM - to answer your direct question:

who in their right mind would use these allowable stress values for a pressure vessel?

My reply is: basically each and every pressure vessel designer/engineer.

In order to understand where these allowable stresses come from, it is necessary to understand both the stress-strain response of these austenitic stainless steels as well the role of design margins (against yield and ultimate). The permanent dimensional changes are so small that they are not generally observable.

If that blew your mind - how about Appendix 44?

 

[CuMo]

Using the G5 version for shells and dished ends can save tons of material.
Recently estimated a vessel where difference made a saving of several tenths of thousands in material cost.
If you looked though at the MAWP of the vessel there was still pretty good margin compared to the design pressure.
So it's worth using where allowed.

 

[EdStainless]

And in heat exchanger tubes, who cares if there is some distortion, the reduced wall improves thermal performance and lowers weight and cost.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[TylerM]

Thanks for all of your input everyone, it's really helped me understand the mechanics of this problem more.

@metengr thanks especially for your reply, that was the one to help everything click in my mind.