ASME VIII Div.1,UCS-66(d) interpretation 1

[Elvie]

I have to deal with a Shell and Tubes HEX where tubes in LTCS (SA334 Gr.6)are proposed by Contractor for coincident design conditions of 67Barg @-75C.This MDMT is based on ASME VIII Div.1,UCS-66(d)interpretation for which I cannot agree as:
a) - § UCS-66(b)(2) does not include UCS-66(d) as exception to impact testing for minimum design metal temperature (MDMT) colder than -48C;
b) – § UCS-66(d) allows impact testing exemption for materials 2.5mm in thickness (first sentence) and for vessels or components made from NPS 4” or smaller tubes/pipes… down to a metal temperature of (MT)-104C (second sentence)as a function of SMYS and WT.
c) But also in first sentence covering all this paragraph UCS-66(d) it is stated :
“but such exempted UCS materials shall not be used at design metal temperature (DMT) colder than -48C”.This conditions should apply to both sentences of 66(d) paragraph.
c)it is a clear difference betwenn DMT in c) and MT in b) above
Bottom Line:LTCS tubes 2.5mm or lower in thickness may be used below -48C (MDMT) down to a MT of -104C provided stress ratio follow the curbes in UCS-66.1.
MDMT of -104C may be acceptable if stress ratio is o.35 or lower only.
ASME B31.3 and PD5500 have similar philosophies as the one I exposed above.

I sould appreciate your opinion on this delicate subject as No Code Interpretation deals with this case.

 

[metengr]

Ok, this is one of those Code questions where you need to lay out the sequence of pages on your desk or table top to create a "road map". First off, UCS-66 (d) is a continuation of paragraphs UCS-66 paragraphs (a) thru (c) with regards to rules for impact testing of UCS materials. When you read thru each paragraph, one needs to determine which of the paragraphs is applicable to their design conditions.

For example simply using UCS-66 (b) applies to impact test and associated exemptions related specifically to Fig 66.1 and the subparagraphs and sentences in (1)(a), (b), (c) or (2), (3) would apply.

UCS-66 (b)(2) states that impact testing shall be performed for minimum design temperature less than -48 deg C, with the exception in b(3) and UCS-68 (c).

UCS-66 (b)(3) states that when the minimum design temperature is colder than -48 deg C, and the coincident ratio defined Fig UCS-66.1 is less than or equal to 0.35, impact testing is not required.

IF UCS-66 (b) does not apply to your design, you move on to UCS-66(c) (which is for impact test exemptions for flanges). If this does not apply, you move on UCS-66 (d), which is in reference to thin wall materials like tube or piping.

So, yes one could go directly to UCS-66(d) for thin walled tubing or pipe that is P-No 1 material, 4 NPS or less in diameter and not have to perform impact testing provided the material specified minimum yield strength values fall into one of the ranges specified in this paragraph for the maximum thickness.

 

[Elvie]

Thank you Metengr for your clarification which at this point is not convincing me.As I was saying in UCS-66(d)first sentence refers to all UCS mat'ls, 2.5mm and thinner to which a MDT of -48C applies.
For the vessels/components NPS 4" or smaller in P-No.1 (which by the way is a particular case of UCS mat'l)the exemption are more generous in WT (up to 6.0mm) as related to lower SMYS .But the limit is the same:2.5mm for high strength mat'l.
And the -104C stated refers to "metal temperature" and not to MDT -minimus design temp-(which is the one to be associated with the Pd(Design Pressure).
So why we shall limit the MDT to -48C for thin UCS materials (sheets=OK, but pipes,tubes are not also in UCS materials ??)and -104C to thin tunes/pipes for P-No.1 only?The risk is not the same if these MDT are coincident with Pd?
I do not think here is a consistent logic in this interpretation.

 

[metengr]

Elvie;
If you carefully read UCS-66(d), the first sentence of this paragraph is intended for all UCS materials, kind of a blanket endorsement with thickness and minimum temperature limitations for impact test exemptions. The second sentence in this same paragraph gives specific guidance for additional impact exemptions applicable to only thin walled tubing or pipe 4 NPS or less in diameter. In my opinion, the rationale for this additional exemption is that it would be difficult at best to even perform impact testing, and it would serve no purpose for the lower strength P-No1 materials. You would not have plane strain conditions or restraint that would increase susceptibility to brittle fracture in a lower strength, thin-walled component.

 

[Elvie]

Metengr;
I really appreciate you answer but your explanation fully applies also to the first sentence of UCS-66(d)i.e. thin UCS-materials.
The bottom line is that under the entire UCS-66(d)rules it is hardly acceptable to endorse the use of these tubes at a MDMT of -104C and this high coincident Pd (design pressure).
YES for temperature excursions below -48C down to -104C for LTCS thin tubes caused by a depressurization,may be also YES for an immediate repressurization (rapid re-heating of the thin walls) but again NO for -104 C @ 67 Barg for steady design conditions.
My stance hereabove is supported also by PD5500 -Appendix "D" and ASME B31.3 provisions in its ChapterIII "Materials" (which by the way deals also with same tube materials-A179 and A 334 Gr.6).
My point is that any Design Code must be very comprehensive in all its statemnts when the brittle fracture is at stake as any approximative interpretation may be very risky.