UG-101 component part new question wording

[buggybubble]

We understand that the minimum MAWP component part is to be used in UG-101 for the sample being burst. However, our sample is essentially an o-ring/diaphgragm seal which is sandwiched between a welded 316L flange and an aluminium plate. The sandwhich is fastened by UNC bolts which thread into the aluminum plate. Got the picture?

The burst mechanism was observed to be the failure of the o-ring due to displacement by the pressure. Niether the aluminum nor the 316L flange fractured, and in fact, shear calcs and UG-27 calcs determined both the flange and the Aluminum to be more than adequate for the design pressure.

So... the question is, in UG-101 (m), which component part shall be used to determine the MAWP when the o-ring, which is the limiting component part, doesn't have a minimum specified tensile sength at room temperature? The aluminum is clearly the weakest, and since it is cast a casting factor f is applied of 0.8, which would yield an acceptable MAWP. Ironically, the flange, which is the strongest component, is welded, but too small to be radiographed, so the Efficiency factor is E=0.7 (which severely limits the MAWP under UG-101 to levels which would involve much cost addition an re-design). The bolts are the strongest element.

Can the fact that the flange has been proven under UG-27 to be strong enough justify the exclusion of the 316L flange in the determination of the MAWP under UG-101?

 

[gr2vessels]

buggy,
I am sensing somewhere a missinterpretation of the efficiency factor required by the code, but I'm unable to point to it from reading your post. What sort of welded flange with E=0.7 are you using? Where is the weld? I am trying also to figure out the microscopic size of your flange, recalcitrant to radiography. It might be more suited perhaps to other NDE's, in order to elevate the E to a decent 1.
Then again, the o-ring being extruded by the internal pressure sounds like design error, since the design pressure would determine at the very initial stages of design, the suitability of rubber o-ring for this application. As for the last question, the above issues once clarified will clarify this one also.

 

[buggybubble]

Thanks for your carefully considered reply.

The flange is under NPS 10 (no kidding!). Per UW-11 (a) (4) it is exempt from radiography.

We have figured out that the weld type is a corner joint, so the E=1 (table UW-12 type (7) - corner joint.

We identified the thinnest material (by inspection) with the weakest properties and used that in UG-101 as the component part for which E = casting factor instead of efficiency factor as that part is cast.

As for your o-ring insight...that raises an interesting question. These units are build for low pressures, but at burst (over 4 times the design pressure) the o-rings do indeed extrude. I've always expected that of o-rings, but you would seem to suggest that perhaps something metallic should fracture before the o-ring. If I may ask, what is your experience with this phenomena?


Thanks!

 

[gr2vessels]

buggy,
I don't mind to answer your last question, is over 35 years of experience in this field (design and fabrication of pressure vessels and the like..). However, my experience is irrelevant to the topic.
The UW-11(4)(a) refers to the nozzle neck thickness calculations, not the flange thickness calculations. That's where the E is relevant. Is there any part you are welding the flange to?
Also, the ASME code is fairly clear on the definition of design pressure and Maximum Allowable Working Pressure (UG-21 and UG-98). The design pressure should contain the "burst" pressure if that operating condition leads to the loss of pressure containment. That will either limit the use of the o-ring or will perhaps move it into a suitable "tongue and groove" arrangement where it cannot be extruded. Obviously, the current design does not prevent the extrusion and the design pressure doesn't contain the burst pressure. However, once the seal/gasket is removed from equation as the weakest link, then any of the metallic items could be the limiting MAWP, most likely exceeding the design pressure.
Cheers,
gr2vessels