PetroStreet

Consider the following situation:

Working Fluid: Ammonia
Operating Temperature= 33 degrees Celcius
Recommended material= Killed Carbon Steel (can withstand up to -45 degrees Celcius)

At some points Carbon Steel has been used (cannot go below -10 degrees Celcius).

It is realistic to expect that temperatures in the range of -20 and below would be scaled during start-ups and shut-downs.

It is worthy to operate the line for around 45 days with some CS spools?

Please advise.

Kamran

You must have known the phenomenon of Brittle Fracture associated with low temperature operation of steels due to reduced toughness.
If your MDMT is coming out to be -10 deg C and the carbon steel in question is not impact tested, you cannot go below -10 deg C in any case as brittle fracture phenomenon is always faster than a ductile fracture. Even at lower stress values, brittle fracture can occur.
What's the stress ratio in your case as per ASME B31.3?

What's the CS material? ASTM designation?
Piping size and operating pressure?
Carbon steel pipes in API 5L gr. B and A106 gr. B can withstand temperature up to -29 deg C as per Curve B of Fig. 323.2.2A