Use of Low Temp Carbon Steel piping material for 4500 kPa Super Heater Steam

[Meck91]

I need help from Piping Design Folks on this issue, please

For 14" 4500 kPa(g) super heated steam system, based on design conditions, the wall thickness required is 0.75" for NPS 14. Piping Material is A-106 Gr B
MDMT is -29 Degree C

Based on ASME B31.3 Figure 323.2.2A & combination of nominal thickness 0.75” (19.05 mm) & MDMT of -29 C comes below the Curve B, therefore, impact testing is required for A-106 Gr. B Material or I have to use low temp carbon steel A 333 Gr. 6 as per client specification.

I never heard of use of LTCS material for super heated steam line. At the same time the pipe will never be subjected to -29 C in service. This line is also insulated.
Yes if the line is out of service for considerable duration of time during extreme cold of -29 C, the pipe material can see this temperature but the line will not be pressurized at that point.

For impact testing, do we need to consider pressure as well extreme cold temperature of -29 C at the same time? How can I justify that the use of LTCS for superheated line is overkill.

Would highly appreciate your feedback.

Thanks,

 

[weldstan]

Please don't double post. Info has previously been provided.

 

[Meck91]

Weldstan,

Thanks for your reply and rest of the folks on my same post at Metal and Metallurgy forum. But the discussion was without any result. And it seems that piping engineering seems to be the relevant forum. Please accommodate.

 

[weldstan]

What else do you need to know? The designer has erroneously defined an MDMT that is irrelevant to the minimum temperature at pressure. The designer does not understand what MDMT means under B31.3. Have the designer select an MDMT relevant to the hydrostatic test temperature.

Furthermore, selection of A-333 Gr 6 to replace A-106 should not be done because of its decreased high temperature Creep properties and greater susceptibility to graphitization at the operating steam temperatures.