Low temperature reached by tubing material during blowdown

[SILVERSTONE]

I've to design an Emergency Depressurization System (blowdown valve + orifice + tubing) for a methane tank (80 bar a. at 35°C). A study seems to show that very low temperatures can be reached by the tubing steel (from -40°C to -100°C).
I want to use some low temperature resisting materials but, because they are more expensive, I need to estimate the length wich will have to be made in this low temperature steel.
Is there anybody having an idea or some informations ?

 

[jsummerfield]

Some show piping specification breaks to include something like 15 meters downstream of an individual blowdown device then back to steel. Many stainless steel alloys are suitable to -150 degree C. Typically the piping branch reaches a flare header downstream of the blowdown valve. The mass of the flare system may prevent one small blowdown valve from severe temperature impact. However, if the design includes a unit or facility blowdown such as required during a fire on an offshore platform, much of the system would be refrigerating. Study the system for the resulting outlet temperature if it can be simulated.

BTW, if two-inch in Class 600 or higher consider a choke instead of an orifice to take the blowdown pressure drop.

John