Codes for steel tubing in hydrogen service?

[electromike]

I need to know which codes,API or ANSI ect., cover the installation of compresion and threaded steel tubing used in high pressure hydrogen gas service. This information is needed after an incident where a maintenance technican changed several 3/8" tubing valves in a valve/regulator tree fed by several "six packs" of high pressure hydrogen bottles.All involved threaded connections leaked a short time after the install. The technician used white PTFE thread tape as a sealant and swears he peformed a through leak test uppon completion of the install. Uppon investigation the question of the suitability of white PTFE as opposed to yellow PTFE was brought up as a possible cause.It was also suggested that these threaded connections need to be seal welded per some code concerning threaded connections used in hydrogen service. Any information on this topic would be greatly appreciated.

 

[SJones]

ASME B31.12. NASA might also have some tips:

http://www.hq.nasa.gov/office/codeq/doctree/canceled/871916.pdf

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[moltenmetal]

Hydrogen below 2500# class (~ 6000 psig at room temperature) is "normal fluid service" per B31.3. There is no CODE prohibition against the use of threads, compression fittings etc. nor is there any code requirement for welding.

There are plenty of practical pieces of advice that users may specify to minimize leakage. There are probably numerous H2 handling guidelines available from the Compressed Gas Institute and various gas manufacturers/distributors as well. Many avoid threads, even in cold services, just because of bad previous experiences- but that's a costly way to solve the problem. It is possible to seal threaded joints against hydrogen if care is taken in the selection of a sealant system and other good fitting practices are used.

As to the suitability of "white" versus "yellow" teflon tape, what matters is the specific gravity of the tape rather than its colour. You should be using teflon tape with an s.g. above 1.3. Some tapes have been stretched to an s.g. as low as 0.25, and these take a mile of tape (many wraps) to build a sufficiently thick layer. It is possible to get high density thread sealant tapes in white as well as yellow. We use anaerobic pipethread sealant in these services in addition to the tape or as a replacement for the tape, because tape alone is not as good at resisting thermal cycling.