Looking for other's experiences with welding of piping in H2 service.
At our refinery, we have 2 current disciplines regarding such welding.
The first is a written "Outgassing" procedure for piping in hot H2 service, operating over 400F, with a H2 partial pressure greater than 40psia. This applies to carbon steel and Chrome piping (up to 9Cr). Although there are several variables to this procedure, the basics include heating the pipe to 600F and holding for 1 hour to allow for proper H2 diffusion. This procedure has never been disputed.
The second discipline is an "old-school, rule-of-thumb", which applies to piping in our Fuel Gas and Flare services, as well as some other services known to contain H2 (neither temp or pressure dependent). Although, these services do not meet the criteria of the 1st discipline, there has always been a request to "bake-out" the piping to 800F for 10 minutes to again allow for H2 diffusion.
Recently, there are questions as to the requirement for a "bake-out" of this piping (as per discipline 2). Questions include: a) Is it even necessary?, b) If necessary, is 800F/10 minutes adequate or even potentially detrimental?, and c) if it is necessary, and the 800F/10 min is deemed detrimental to integrity, should the "outgassing" procedure of discipline 1 be followed (or is this "overkill")?
Any thoughts, experiences, references, links, etc would be greatly appreciated.
thanks for your time...
JW
The 10 minutes at 800F will do relatively nothing for any hyrogen present in base metal. The diffusion curve for H2 peaks near 600F, so going hotter than 600F actually slows down the diffusion of hydrogen. Hydrogen also diffuses slowly, so I don't think 10 minutes will do anything at all. When making HY80 castings, they can be baked for days on end before enough hydrogen is outgassed to make a difference in the elongation. However, on carbon steel, you will not do any metallurgical damage at that time and temperature. Are you sure that the 800F for 10 minutes is not just to burn off the remnants of any volitile elements, such as oil or grease residue, which would in turn reduce the chance of H2 cracking?
HI GR,
thanx for the reply. Originally, the "bake-out" was for the volitile residuals in our dirty services (Fuel gas, Fuel Oil, Flare, etc), however sometime in the past, the procedure was adopted for use on clean systems (ie. clean vent gases, and low press/temp recycle H2 lines, etc), the reasoning being: H2 entrainment in the basemetal. Conveniently, no one can remember who or when this was incorporated.
Anyways, your statements coincide with out "Outgassing" procedure. We shall review our useage of our "Bake out" procedure. If you have anything else to add that would be greatly appreciated.
Thanks for your time...
JW
I agree fully with GRoberts on the point. Our DHT criteria for steel materials fabrication is the range 600F, +-25F, 2 hrs per inch, 2 hr. min.
I interpret there is H bake-out after welding, and there is H bake-out after service during a shut-down for maintenance. The time at temperature depends on the expected H concentration in the metal and the metal thickness. H concentration from service can be significantly greater than H concentration after controlled welding, and the cold craking risk that much greater, not to mention the embrittling effects of hydrogen attack and methane or H2 blistering of the steel matrix.
In the refinery maintenance practice, I would expect there are very specific procedures for bringing steel materials out of high temp H2 service. But the 800F for 10 minutes regime appears to be misplaced from an original practical context is an "old-supervisors-tale" that seems more a cultural artifact than a technical one.
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