Low temperature service 3.5Ni steel - 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

 

[unclesyd]

We had a lower limit of -150°F (-101°C)mainly for use for storage of liquid butane and propane.

The materials manual from US Steel for "Low Temperature and Cryogenic Steels" talks about its use down to -175°F(-115°C)
for land based storage tanks, liquified gases down to ethylene.

One thing is that it does reference is that welds should be stressed relieved/PWHT @ 1150°F. There is about a 40% gain in the keyhole impact test values for everything except the weld metal.

The impact curves for normalized and tempered .5" specimens goes to -200°F(-129°C) but have a band of normal expectancy that gets close to bottom line. The mean value for V notch @ -200°F(-129°C) is 10 ft•lbs and the Keyhole value is also 10 ft•lbs.

There is also a family of curves based on thickness and thermal history that carry the impact values to lower temperatures and none show an NDT as is shown in the 2 1/4% Nickel curves.

This is rather old data so don't use these numbers without verification.

 

[robsalv]

Thanks UncleSyd. Very helpful.

Looks like we may be up for an expensive replacement job.

Cheers

Rob