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Technical Discussions
Jim J.
Jim J.
07:14 Oct-12-2007
UT in Lieu of RT per ASME

I have a question regarding the use of UT in lieu of RT for ASME B31. ASME B31 committee has developed B31 Case 181 to permit the use of UT, particularly TOFD in lieu of RT for B31.3 applications. ASME B31 case 181 was apparently taken from ASME Code Case 2235, which addresses the UT examination in lieu of RT for vessels and not piping.

The major difference from what I can see between B31 case 181 and ASME Code Case 2235 is the minimum wall thickness requirements and maybe the acceptance criteria.

As we all know ASME B&PV codes looks at flaws by aspect ratios. a/l or vertical height divided by length. That gives you the aspect ratio which is then used to determine the maximum flaw size permitted. You can actually accept cracks per the ASME B&PV code as long as they are within the acceptable limits, but not with ASME B31.3.

After looking at the acceptance criteria for B31 Case 181, I see that they are using the same philosophy.

Where ASME B31.3, Table 341.3.2 does not allow any cracks for RT and in most cases no IF. Why would cracks be permitted with UT and not RT? I believe the B31 Case 181 was taken from ASME Code Case 2235 and the reference to cracks was inadvertently missed. When using TOFD you can differentiate between linear indications and cracks. Does anyone know of any reference by ASME B31 committee regarding this issue?



 
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David Mackintosh
Engineering,
Acuren Group Inc., Canada, Joined Feb 2011, 85

David Mackintosh

Engineering,
Acuren Group Inc.,
Canada,
Joined Feb 2011
85
09:13 Oct-12-2007
Re: UT in Lieu of RT per ASME
----------- Start Original Message -----------

: Where ASME B31.3, Table 341.3.2 does not allow any cracks for RT and in most cases no IF. Why would cracks be permitted with UT and not RT? I believe the B31 Case 181 was taken from ASME Code Case 2235 and the reference to cracks was inadvertently missed. When using TOFD you can differentiate between linear indications and cracks. Does anyone know of any reference by ASME B31 committee regarding this issue?
------------ End Original Message ------------

According to the paper "Technical basis for ASME Section VIII Code Case 2235..." (Transactions of the ASME Journal on Pressure Vessel Technology Aug 2001 vol 123, p 338 - 345) "Experience and fracture mechanics show that many conditions rejected by RT do not affect in-service integrity. ... With RT the worst-case depth must be assumed, through wall." But not all cracks need repair. Using UT, the crack can be sized and assessed as acceptable or rejectable by fracture mechanics, based on the criteria in 2235 and 181.
Hope this helps.


 
 Reply 
 
Jim J.
Jim J.
02:10 Oct-15-2007
Re: UT in Lieu of RT per ASME
----------- Start Original Message -----------
:
: : Where ASME B31.3, Table 341.3.2 does not allow any cracks for RT and in most cases no IF. Why would cracks be permitted with UT and not RT? I believe the B31 Case 181 was taken from ASME Code Case 2235 and the reference to cracks was inadvertently missed. When using TOFD you can differentiate between linear indications and cracks. Does anyone know of any reference by ASME B31 committee regarding this issue?
: According to the paper "Technical basis for ASME Section VIII Code Case 2235..." (Transactions of the ASME Journal on Pressure Vessel Technology Aug 2001 vol 123, p 338 - 345) "Experience and fracture mechanics show that many conditions rejected by RT do not affect in-service integrity. ... With RT the worst-case depth must be assumed, through wall." But not all cracks need repair. Using UT, the crack can be sized and assessed as acceptable or rejectable by fracture mechanics, based on the criteria in 2235 and 181.
: Hope this helps.
------------ End Original Message ------------

That makes sense but has the B31 committe accepted this as well? Do you really want cracks in a 80" diameter 1 kilometer long flare line when you are performing pneumatic testing?



 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1303

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1303
03:54 Oct-15-2007
Re: UT in Lieu of RT per ASME
It may be useful to point out that the applications of both these referenced Code Cases (2235 and 181) are in the "construction" of pressure retaining components (i.e. vessels or pipe). These acceptance criteria are based on conservative assumptions detailed in the 2001 ASME paper by Rana et al explaining the technical basis for the 2235 acceptance criteria. The use of the fracture mechanics-based acceptance criteria in CC2235 and CC181 is not implied, nor I think should it be used, with inservice conditions (these would require different calculations).
However, the concern for crack or IF (or IP) is not relevant when we depart from the old "workmanship" based acceptance criteria that RT is based on. The radius of the flaw is a critical parameter and for the fracture mechanics-based calculations it is assumed that all the flaws are planar (e.g. this would include IP, IF and cracks...all having very small radii) and the flaw is assumed to be contained in the rectangle that defines the flaw extents.
Trying to compare the workmanship-based acceptance criteria used by RT and the old UT codes to the fracture mechanics-based codes such as these Code Cases is a bit like comparing apples to oranges....
Remember, CC181 is an ALTERNATIVE acceptance criteria. You are required to use it. However, if opted for, it is used by agreement and within the context and limitations stated in the Scope of B31.3. (read Para 300 (c) Intent of the Code)


----------- Start Original Message -----------
: :
: : : Where ASME B31.3, Table 341.3.2 does not allow any cracks for RT and in most cases no IF. Why would cracks be permitted with UT and not RT? I believe the B31 Case 181 was taken from ASME Code Case 2235 and the reference to cracks was inadvertently missed. When using TOFD you can differentiate between linear indications and cracks. Does anyone know of any reference by ASME B31 committee regarding this issue?
: : According to the paper "Technical basis for ASME Section VIII Code Case 2235..." (Transactions of the ASME Journal on Pressure Vessel Technology Aug 2001 vol 123, p 338 - 345) "Experience and fracture mechanics show that many conditions rejected by RT do not affect in-service integrity. ... With RT the worst-case depth must be assumed, through wall." But not all cracks need repair. Using UT, the crack can be sized and assessed as acceptable or rejectable by fracture mechanics, based on the criteria in 2235 and 181.
: : Hope this helps.
: That makes sense but has the B31 committe accepted this as well? Do you really want cracks in a 80" diameter 1 kilometer long flare line when you are performing pneumatic testing?
------------ End Original Message ------------




 
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