Hydrogen Service Reinforcement Pad

[Paulettea]

Dear all

I have seen some pressure vessel datasheets that mention the vessel is in Hydrogen service and there are some special considerations regarding their design.
What I can not fully understand is that for these vessels reinforcement pads are not used and instead in order to meet the area requirements integrally reinforced nozzles are used.
I can understand there could be hydrogen embrittlement due to diffusion of Hydrogen atoms but I do not know why this makes the re-pads useless. If the problem is with the welding of
the re-pads this problem is still there for the welding of an integral nozzle to the shell. Can you please help me understand this? Am I missing something regarding hydrogen service?
Is there any mention of hydrogen service in ASME BPVC and design issues regarding this situation?

Warm Regards,

 

[XL83NL]

Perhaps the 'collecting' or built-up of a hydrogen gas bubble between the repad and shell-/head-material is the reason why they dont like repads for hydrogen service. The bubble can cause hydrogen embrittlement, or a dangerous 'subatmosphere' which can act like a bomb. Just my throughts, not sure if this is the real reason.

 

[Paulettea]

@XL83NL
Thank you for your reply. I dont think that could be the reason after all the re-pads are provided with vent holes and can not hold pressure between re-pad and shell.
Furthermore, even if there could be any pressure built-up between the re-pad and shell this pressure cannot go higher than the pressure inside the vessel itself so if there is going to be explosion due to pressure it will happen from the location of the shell to nozzle weld.

Warm Regards,

 

[r6155]

Nozzle with Integral reinforcement can be examined with RT or UT.
See NACE 8X194.

Regards
r6155

 

[TGS4]

In addition to to the reasons listed by r6155, there is the potential for hydrogen gas built-up between the pad and the shell. Vent holes are provided for managing welding gasses only, and are typically filled with heavy grease (to prevent moisture ingress). Having a grease "plug" pop out due to a small build-up of hydrogen gas (remember that hydrogen auto-combusts, burns very hot iwht a colourless flame) is a safety hazard.

 

[EdStainless]

The H2 pressure inside of a bubble can exceed the internal pressure of the vessel.
It is only limited by sully of H and the strength of the metal.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[SnTMan]

Most specs I have seen state, for H2 service, no unvented spaces. Nozzle re-pads are thereby allowed, but usually frowned upon. Pads for the vessel supports are routinely furnished, always with vent holes. One difference between nozzle pads and support pads is that usually there are no welds beneath the support pads.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

Space between repad and shell is like a crack, hence must be avoided.

Regards
r6155

 

[Paulettea]

UG-37 (g):

(g) Reinforcing plates and saddles of nozzles attached to the outside of a vessel shall be provided with at least one telltale hole [maximum diameter 7/16 in. (11 mm)] that may be tapped with straight or tapered threads. These telltale holes may be left open or may be plugged when the vessel is in service. If the holes are plugged, the plugging material used shall not be capable of sustaining pressure between the reinforcing plate and the vessel wall.

As per this part of the ASME I do not think the pressure built-up can be a considerable amount leading to a safety issue.
Furthermore, even if it is possible that spontaneous combustion of hydrogen can be a safety hazard, the volume of hydrogen which can be trapped between the shell and pad is so less that can be safely ignored.

@r6115"Space between repad and shell is like a crack, hence must be avoided."
I do not think that this could be the reason behind this issue. After all, when you use a re-pad if the space between the pad and shell can be regarded as a crack, it is independent of the service of the vessel and this could limit the use of re-pads for all services. Secondly, as far as I know the space between the pad and shell can be regarded as a crack only if the stresses are of shear type but in case of tensile or compression membrane stresses these spaces have very limited effect on the stresses.

Warm Regards,

 

[Paulettea]

@XL83NL
"Nozzle with Integral reinforcement can be examined with RT or UT.See NACE 8X194."
Thank you for your reply. There are two questions regarding what you pointed about NDT examination of the welds in integrally reinforced nozzles. Here comes the first question: is it possible to examine all types of integrally reinforced nozzles in figure UW-16.1? I think that not all of these nozzles attachment to the vessel can be tested using RT or UT. In figure UW-16.1 Sketches (a), (b), (c), (d), (e), (f-1) through (f-4), (g), (x-1), (y-1), and (z-1) are examples of nozzles with integral reinforcement. However, I think only sketches (f-1) through (f-4) can be examined with RT or UT. And for example sketch (g) cannot be examined with these two methods (I think) however, I have seen this type of integrally reinforced nozzles used in hydrogen service vessels.
My second question is about the reason why we shall do these tests. The reason may be to find whether there is cracks present in the weld or not then after finding the crack you can take necessary actions to remove this problem. However, again this issue is independent of the vessel service. It does not matter that the vessel is going to be used in hydrogen service or not. The only motive to do these tests that comes to mind is that to suppose that for the vessel there is a time schedule to do these NDT examinations when the vessel is put in service. And since the vessel is exposed to hydrogen during operation, cracks may develop in the welds due to hydrogen embrittlement and by this method you can come to understand that there is a crack in the vessel welds. Now I want to know if there is such thing as a scheduled NDT examination for all nozzle welds during operating life of the vessels which are used in hydrogen service.
Correct me if my understanding of this matter is wrong.
Warm Regards,

 

[r6155]

@ Paulettea

1) See ASME VIII Div. 1
Figure UHT-18.1
Acceptable Welded Nozzle Attachment Readily Radiographed to Code Standards

Figure UHT-18.2
Acceptable Full Penetration Welded Nozzle Attachments Radiographable With Difficulty and Generally
Requiring Special Techniques Including Multiple Exposures to Take Care of Thickness Variations.

2) When do you think that integral nozzle must be used?

Regards
r6155

 

[SnTMan]

To help muddy thing further I'll just state that many, many vessels are in H2 service, but do not fall under Part UHT. Therefore Part UHT requirements have no relevance to said vessels.

Regards,

Mike


The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

@ SnTMan
I don't say that H2 is under Part UHT
Figure UHT 18.1 and 18.2 is only to show to Paulettea that integral welded nozzle can be radiographed,that's all.

Regards
r6155

 

[Paulettea]

Thank you r6115. I had not paid attention to that figure. It clarifies what you mentioned earlier.

 

[SnTMan]

r6155, I see, thanks for the clarification

The problem with sloppy work is that the supply FAR EXCEEDS the demand