Material selection for flare stack and seal drum at 425 C degree 2

[sxz]

Hi, All,
The elevated flare has a liquid seal drum at bottom, stack, and flare tip.
95 m high total, drum diameter is 3.6 m, stack diameter is 1.7 m.
If the design temperature is 425 C degree, design pressure is 5 barg for seal drum and stack, does SA-516 Gr.70N still work because the creep concerns. As per ASME STS-1, SA-242 is recommended at this temperature for stack, but can we use SA-242 for drum, pressure vessel?
SS304 could be selected, but the cost will be much higher.
Any recommendations on the material selection are very appreciated.

Sam

 

[SNORGY]

If you have access to it, read Non-Mandatory Appendix A in ASME Section II Part D. My initial thought was a chrome-moly alloy if the concerns with carbon steel were creep or graphitization. Perhaps SA-387 Grade 2 (UNS # K12143), P-3 / G-2. (1/2Cr-1/2Mo).

In the late 80's / early 90's, I prepared a study (although it was for piping, not vessels) at a major Canadian refinery concerning the ramifications of temperature excursions into levels at or just above the accepted graphitization temperature thresholds for carbon steel. The scenario was desuperheater failure (or capacity shortfall) in a steam distribution system downstream of a 900# / 600# letdown station. Not much had been known or published about it back then (based on my knowledge at the time), but the approach taken was to consider the kinetics of graphitization as being slower than aggregate cumulative creep effects. The conclusion was that carbon steel was acceptable provided that temperature excursions into a creep / graphitization temperature range of 410 C to 427 C were monitored and a shutdown was forced after an estimated cumulative creep level was predicted. Some years later, I became aware of what is now ASME B31.3, Appendix V. I am not sure when that was first written into that Code. In any event, based on my past experience, if there is discomfort with carbon steel here, the next material upgrade I would think about is the chromium-molybdenum steels.

 

[sxz]

Snorgy,
Thanks for your input.
I used SA-387 Gr2 in Compress, PWHT is requried, and the MDMT couldn't meet -29C degree.
So, I have to use the SA-516 Gr.70N, and refer to API579 to find the allowable excursion time on the curves with stress. By the way, how to apply a creep monitor on vertical pressure vessel?

 

[SNORGY]

I don't think you need a physical creep monitor device, provided you monitor the duration and magnitude of excursions into the temperature ranges where creep will be a concern, as you propose.

That said, something appears unusual to me in your situation. Is everyone involved certain that the high design temperature will be as high as 425 C while the MDMT will be as low as -29 C? What pressure is coincident with -29 C? Sometimes, with the best of intentions, such temperatures are specified by designers when in reality they actually are not achievable.

In any event, I think your material selection is optimal in your circumstance. I also think, however, you (or others) might now need to decide whether it is brittle fracture or creep that is the higher risk to mitigate, since plate made to fine grain practice (from what I understand) has superior fracture toughness but inferior creep resistance compared with plate made to a coarse grain practice.