Quantifying lower MDMT limit for lower hoop stress

[LeFonz]

thread378-347097

Hi everyone,

I was working with the information provided by the earlier posts to understand how a Cat I pipe may be used under Cat II (lower MDMT) conditions.

Background information on the subject is as follows:
Material: A106-B
NPS: 4 (OD 4.026in; Thickness 0.237in)
Schedule: 40
MOP: 275 psi
I calculated the hoop stress as 2340 psi (16 MPa).

Based on CSA Z662, proven notch toughness not required for pipe for which the design operating stress is less than 50 MPa.
My question is how do I quantify the lower MDMT limit based on ASME B31 and data provided?

Please feel free to contact me if there is any information missing or advice on how I could answer my query.
Thanks in advance for all feedback received.

 

[SNORGY]

LeFonz,

I think that CSA Z662 might be somewhat silent on this, but I am not aware of a specific MDMT reduction mechanism similar to B31.3 323.2.2 or Section VIII Div. 1 UCS-66 of the ASME Codes. However, my own personal approach would be to rationalize as follows:

(1) Scope of CSA Z662 ==> -70 C to +230 C; accordingly, MDMT for any material must be -70 C or higher.
(2) CSA Z662 Table 5.3 ==> the "cross" symbol, footnotes and, in particular, Note 2, imply that ASTM A-333 Grade 6 seamless would not be permitted for use below MDMT = -45 C for Category III.
(3) CSA Z662 Clause 7 allows for WPS's / PQR's in accordance with ASME Section IX.

Arising from the above, it would make sense, to me, to follow the same approach for MDMT qualification as adopted in the ASME Codes. In other words, I would be disinclined to recommend or allow for the use of ASTM A-106 Grade B for MDMT below -30 C, regardless of stress level, unless, as a minimum, the same rationale is applied as would be for B31.3 pipe in accordance with 323.2.2.

 

[LeFonz]

Thanks for the feedback Snorgy - much appreciated1
I will review the applicable codes in further detail.