Hydrogen Diffusion Question

[jrjones]

I am looking at a repair on a high pressure high temperature separator vessel (2 1/4Cr base material ~ 6.75" thick, with 309/347ss overlay ~ 0.3" thick). The repair that we are considering would leave a small "void" in the behind the overlay as we plan to weld a small patch to the surrounding overlay so that we do not have to PWHT the repair.
The vessel is in hydrocarbon service (sour) at ~1600 psig and 850°F. Hydrogen partial pressure is about 1200 psig.

What I am trying to find out is what the pressure behind the patch could reach due to hydrogen diffusion. I wouldn't think that it would be higher than the partial pressure of hydrogen, but I don't know for sure.

Thoughts?

jrjones

 

[metengr]

Please explain.

The repair that we are considering would leave a small "void" in the behind the overlay as we plan to weld a small patch to the surrounding overlay so that we do not have to PWHT the repair.

 

[jrjones]

We have a local area that has some thinning in the overlay to the point where the base material has been exposed. In order to repair this area without PWHT, we are considering welding a patch (1/2" 347 plate or similar) over the thinned area such that all the welding will be between the patch and the surrounding full thickness overlay. Therefore the HAZ will not penetrate into the base material.

Now we could vent the patch to get away from worrying about pressure buildup due to diffusion, but that would cause some minor corrosion of the exposed base material (very little as the void would likely fill up with corrosion product and stop). We did want to consider the case that we don't vent the patch...what could the pressure reach?

Hope that clarifies...jrjones

 

[metengr]

In effect what you are proposing is to use a wall paper type cladding, in lieu of an integral cladding on the surface of the vessel. I get nervous when I hear of voids used in pressure vessel repairs.

If you do this correctly, there should be no build-up in hydrogen pressure at the interface between the cladding and steel because the diffusivity of hydrogen thru austenitic stainless steel is very low in comparison to steel.

 

[EdStainless]

I agree with metengr, the hydrogen diffusion rate in austenitic alloys is so low that I can't see this being an issue.

Why do you have local thinning?
Right now the overlay is 347 on top of 309, right?
Are you going to set the patch in so it is sort of flush?

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[jrjones]

Yes to the 347/309 overlay...309 against the shell, then 347. We are going to set the patch into the area a bit, but we want to make sure we are only welding to the overlay.
jrjones