Hydrogen Bakeout

[Dpearcey]

Where can I find reference material when a hydrogen bakeout is required for a repair? Presently, I'm working on a procedure to replace a 2" nozzle in the water boot of a DHT steam receiver, sour with H2S. The design is 135 psig @ 375F. The 2" nozzle is xxs and the boot itself is 24" OD with 1/2" wall. All mateials carbon steel, sa105, 106 gr.b, and SA515 gr 65. Comments appreciated.

 

[AEF]

Good question. I've also recently tried to find information on this subject without much success.
First, is the process actually hydrogen service, > 690 kpa H2 partial pressure, and was the vessel or boot stress relieved for service requirements? If not, you probably don't need to be too concerned and normal preheat should suffice. We're planning nozzle repairs on a gas scrubber in H2S service this summer and the bake out procedure submitted to us by our repair contractor calls for soaking at 260 C for at least 4 hours. I've been searching info sources trying to verify this procedure but no luck so far.
Hydrogen is pretty mobile and will migrate quite quickly. Also, a very smart old scottish marine engineer once told me that a rough rule of thumb is that 260 C of preheat is worth 800 C of PWHT. Good luck.

 

[PAN]

AEF,
800 C of PWHT seems too high for carbon steel. Could you please describe about holding temperature in this case or give some references?

 

[twjag]

AEF,

Most oil refineries have company standards that specify required bakeout temps.
On past projects I have seen bakeout temperatures for ferrous materials in the same ranges specified for PWHT in ASME Section VIII, Div 1, UCS-56. Most U.S. refineries we deal with requires the standard PWHT temperature (based on 1hour/inch), & do not allow reduced temperature PWHT for longer holding times.

You might check out the publication section of API at

http://api-ep.api.org.

They may have a standard or technical publication that covers this subject.

 

[Dpearcey]

Thx for the info. After considerable discussion in-house and with metallurgists, we settled on a 400 F bakeout for 2 hours. A colleage later showed me minutes of a NPRA conference where a refinery specified 450F for 2 hours for wet H2S service (1/2" thick material).