MOC for a tower with MDMT -70C ? 1

[YuJie_PV]

hi, all

i've a question regarding MOC of a tower, which is constructed to ASME VIII-1
it's a deethanization tower with diamter of 2.5m, and height of about 50m，Design pressure 4.6MPa, Design Temp 150C， MDMT-70C.
i would like to fabricate the tower with a low alloy material.
i don't like to use the MDMT reduction as indicated in UCS-66(b), therefore i think it's hard to achieve exemption of impact test for the material.
if i do impact test to the material at temperature -70C, which material would meet the toughness requirement?

thanks in advance.

 

[Geoff13]

I have designed pressure vessels for northern Canada where the MDMT was specified as -40°C or -45°C. I may have had one that was even -50°C, but I can't remember for sure. Where would this vessel be built that needs -70°C?

Why don't you like the MDMT reduction permitted by UCS-66(b)? It may not be helpful for the main shell but I can't think of any reason not to use whatever the Code permits.

I would probably ask your question of the steel mill where you would likely purchase the plates. They would have the best understanding of what materials they can produce that will have adequate impact test values at -70°C.

If I wanted to take a guess for suitable material I would look at the SA350 flanges and see which grades have impacts at -70°C (LF3, LF9 and LF787 Class 3) and look for matching plate specifications.

 

[SJones]

3.5Ni steel

https://nickelinstitute.org/media/1716/low_temperaturepropertiesofnickelalloysteels_1238_.pdf

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[YuJie_PV]

thank you all.
Geoff, -70C MDMT is due to service, instead of min. atmospheric temperature .
BTW，i'd like to deviate a little from the original question, if i may. you mentioned the -40/-45C MDMT in northern Canada. i guess it's based on atmospheric temperature. i wonder what atmospheric temperature do you regularly use in establishing the minimum design metal temperature. minimum?
i've heard some company use minimum monthly mean temperature.
do you know which standard/code contain such guidance ?

Thank in advance.

 

[Geoff13]

The MDMT is specified by the Owner, and is used plant wide. I expect it was picked in the 70's and no one has thought about it since.

I know the nearest town to the south has recorded temps as low as -50°C.

 

[YuJie_PV]

Thanks Geoff,
i am the designated agent of owner now,
i just think using lowest recorded temperature as MDMT is overconservative.
Thanks

 

[weldstan]

3 1/2% Ni alloy steel per Steve.

 

[akpipelineengr]

I've seen supplementary CVNs specified on A350 LF2 and A333 Gr 6 down to -80°F, but getting plate material of similar metallurgy that will pass is likely to be a tall order.