UCS-68 (c) and Div. 2, temperature reduction for impact test exemption

[jtseng123]

Dear all,

Div 1, UCS-68(c) saying if PWHT is mandatory per code (normally is based on material and thickness), 30F reduction can not be taken. I have many thick wall vessels greater than 1.5" thick, but I can not take 30F reduction since PWHT is mandatory by code.

Does Div 2 have the same requirement? I saw para 3.11.2.3 and many tables and figures in Part 3 showing "parts subject to PWHT", but not knowing if this PWHT is in line with UCS-68(c): reduction can only be taken if PWHT is not mandatory but end user chooses to do so.

 

[pdiculous963]

Division 2 has separate curves for PWHT and non-PWHT. See Figures 3.7 and 3.8 of VIII-2. It has nothing to do with if PWHT is required or not. If your vessel is PWHT, then you use the PWHT curves. If it is not, then use the non-PWHT curves.

The intention of these curves were to explicitly consider weld residual stress which is critical when evaluating the potential for brittle fracture. The toughness rules in VIII-2 were created using a fracture mechanics approach in accordance with API 579. WRC Bulletin 528 documents the background to the toughness rules in VIII-2.

 

[jtseng123]

pdiculous963, thanks for the reply. However, I am still puzzling:
I can see PWHT can get a lower MDMT from Div 2 tables and figures, that means PWHT do enhance the toughness regardless it is mandatory or not. But why in Div 1 I can not take the reduction if PWHT is mandatory.

We order the same material and using the same welding procedures regardless Div 1 or 2. Metals and welds themselves can not tell if "I am Div 1 or Div 2". They will exhibit the same properties. If I have trouble in Div 1 for impact test exemption due to mandatory PWHT, then I can re-classify vessel to DIv. 2 and it qualifies for exemption. Is it something weird?

 

[pdiculous963]

Division 2 does not consider the PWHT to enhance the toughness, but rather to decrease the driving force to cause a fracture (weld residual stress).

I completely agree with your second statement that the shell/welds cannot tell if they are PWHT or not. We have actually used similar wording in our office that the shell doesn't know if it is Div. 1 or 2. There does not appear to be a technical basis for the 30°F reduction, similar to there being no technical basis for the UG-20(f) exemption. However, those are just the Code rules. Unfortunately, when it comes to the two Codes, the rules are different, and therefore you will not get the same results for impact test exemption between the Codes.

Remember, the inspection requirements for Div. 2 will be more stringent, and that should lead to a lower likelihood of having a flaw that would lead to brittle fracture. While this doesn't answer your question, it is worth noting.

 

[jtseng123]

pdiculous963,
More inspection requirement in Div. 2 does not help the impact test. You still need to do impact test on base metal and welds, and it must be passed regardless more NDE or not. Cutting a sample is just cutting a sample, it can't tell it is Div 1 or Div 2. Metal is just metal, will have the same behavior regardless Div 1 or 2.

Anyway, it seems I am out of luck for Div 1 vessels to take exemption. or can have a lower MDMT curve like Div. 2 when PWHT is mandatory.

 

[pdiculous963]

It does appear you are out of luck.

Perhaps I did not get across my last point well. It was aimed at the actual failure mode rather than the Code rules. The impact test rules are there to ensure that a brittle fracture does not occur. To have a brittle fracture, you need cold enough temperature, a stress (residual or applied), and a defect (notch, crack, etc.). All I was saying with my last point was that if I have 2 identical vessels, but I performed twice as much inspection on the second, I would be more comfortable in operating at a lower temperature. Things like flaw detectability are considered when creating brittle fracture rules such as those in Div. 2.

 

[Mec.tec]

For Div I,In my opinion the additional pwht when it is not required by code may settle residual stresses and enhance the grain refining thus increasing toughness and reducing the chance of brittle fracture. That's why code give us the benefit of additional 17 deg C reduction in impact temperatures.