Is there a paradox between UCS-68(b) and UCS-68(c)?

[YuJie_PV]

hi, all
i encounter an issue when reading UCS-68.
UCS-68(c) states that "The resulting exemption temperature may be colder than −55°F (−48°C) when the
PWHT exemption in (b) is applicable.".
what does that mean?
i have an example offering where the issue lies:
a vessel constructed of SA-516 70N, required MDMT=-50C，coincident ratio>0.35. per UCS-66(b)(2), the reduced MDMT=-48C, thus, SA-516 70N is impacted test at MDMT, and will meet the absorbed engery indicated in UCS-68(b).
conclusively, the vessel is exempted from PWHT.

based on UCS-68(c), in case PWHT is performed as not required by other rules of the code, a MDMT reduction of 17C could be achieved, the resulting exemption temperature is -65C
-65C <-50C。
conclusively, the MOC could be exempted from impact testing.

it comes out a paradox. what's wrong with my understanding to the code?
Thanks.

 

[Maslov]

Here is my understanding:

UCS-68(b) requires PWHT for vessels with MDMTs below -48C and coincident ratios above 0.35, except for certain impact tested welds in UCS-68(b)(1) and (2).

UCS-68(c) only applies if PWHT is NOT required elsewhere in Div. 1, which would include UCS-68(b). The language of "The resulting exemption temperature may be colder than -55F (-48C) when the PWHT exemption in (b) is applicable" reinforces this idea, that if UCS-68(b) requires PWHT, you must PWHT, and therefore you cannot "opt" into PWHT.

It is only in the exemptions in UCS-68(b)(1)-(2) that PWHT is not required and thus you can opt into PWHT and use UCS-68(c) to reduce the MDMT below -48C (Which means that PWHT did not eliminate need for impact testing, you have to impact test as per UCS-68(b). It is only when your reduced MDMT is -48C or warmer, or your coincident ratio is smaller than 0.35 and MDMT warmer than -105C, that UCS-68(c) eliminates the need for impact testing).

I think this is where the confusion comes from: UCS-68(c) does not always swap impact testing for PWHT. If metal temperatures are below -48C and coincident ratios are above 0.35, you will have to impact test carbon steels.

 

[YuJie_PV]

Thanks so much for the reply, Maslov.

further to your point, i am just confused, if one can definitely benefit nothing from UCS-68(c), in the case of UCS-68(b) exemption of PWHT, why those experts of ASME formulate UCS-68 such way?
it's weird.