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Low temperature reached by tubing material during blowdown

Low temperature reached by tubing material during blowdown

Low temperature reached by tubing material during blowdown

(OP)
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 ?

RE: Low temperature reached by tubing material during blowdown

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

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