3.5Ni steel low temperature service - How low?!

[robsalv]

Hiya folks.

Our process guys are asking for an opinion on how low we might be able to run some 3.5Ni piping. Currently the piping has an impact tested MDMT of -101DegC.

They're asking whether it can be used with -157DegC liquid at our flare pressure - for all intents and purposes, slightly above atmospheric pressure. I think the shock chilling issues knocks this on the head right away, but interestingly my looking through codes has not found a specified absolute lower limit.

Best that I can make out is API RP579 fitness for service guide brittle failure chapter, where the low temperature reductions for reduced pressure stop at -104DegC.

I'd be very interested in your thoughts.

Cheers.

Rob

p.s. please excuse the cross posting in the piping forum.

 

[metengr]

robsalv;
For the service temperature you are interested in the 3.5% Ni steel is not recommended. The lowest recommended service temperature I have seen for this steel is -101 deg C, as you stated. For good engineering practice, you should not use steels below their minimum design metal temperature (MDMT) in pipe or pressure vessel applications.

For lower service temperatures you need to look at higher nickel compositions that promote low temperature toughness - for example, 9% Ni steel is acceptable for use to -195 deg C. Service temperatures below -200 deg C require the use of austenitic grades of stainless steel.

 

[robsalv]

I totally agree about the recommendation of an alternative material Metengr. 304/316 Austenitic SS would be my choice.

B31.3 says that 3.5Ni steel requires impact tests below -101degC, this would set the MDMT. It doesn't prohibit it's use below -101DegC though, so how low can 3.5Ni go?? I don't know. Someone in the forum might have experience.

Anyway, I suspect the answer is that at -157DegC the existing line wont be suitable, even at 1% design pressure... but I can't seem to find info that says under no circumstances do not go below -xxx DegC.

It's encumbent on me to investigate whether 3.5Ni could be exposed to the temperature... the alternative is a $1m upgrade.

Thanks for your input!

Cheers
Rob

 

[metengr]

robsalv;
I could not find specific impact values for 3.5% Ni steel below -101 deg C. The ASME Code uses an "adequacy of toughness" design basis. You can use actual Charpy impact values or drop weight tear tests to determine a specific, minimum toughness threshold with obvious risk.

For the 3.5% Ni steel, I would expect that you would be somewhere around 5 ft-lbs at -157 deg C, which is 100% brittle fracture behavior.

 

[robsalv]

Thanks Metengr. Very helpful info.