reinforcing pads 3

[beachcomber1]

Is it allowed to use nozzel reinforcing pads on pressure vessels in sour service manifactured to the requirements of ASME 8 Div1 or is it purely at the clients/engineer discretion. Are there any mandatory requirements?

 

[Fluque]

It´s mandatory. That´s what ASME VIII Div 1 says for welded vessels:

Openings in vessels not subject to rapid fluctuations
in pressure do not require reinforcement other than
that inherent in the construction under the following conditions:
*welded, brazed, and flued connections meeting
the applicable rules and with a finished opening not
larger than:
3 1?2 in. (89 mm) diameter in vessel shells or heads
with a required minimum thickness of 3?8 in. (10 mm)
or less;
2 3?8 in. (60 mm) diameter in vessel shells or heads
over a required minimum thickness of 3?8 in. (10 mm)

I recomend you use reinforcements in all openings larger than 3in.

I hope it would be useful for you.

 

[jehan17718]

beachcomber1,

Code (ASME VIII-1) does NOT address requirements of reinforcement pads based on service (NACE service, H2S service, Caustic service, amine service, etc) requirements.

For sour service, generally pads are not restricted unless specifically not allowed by applicable project specifications.

However, for hydrogen service, it is generally a good engineering practice to avoid pads altogether considering the risk of HIC (hydrogen induced cracking) in the welds connecting pads to shell. Nozzles are self-reinforced correspondingly. Pads are however ok for componemnts such as support lugs, cleats foro piping, ladder and platforms in equipments in hydrogen service. Many BUT NOT ALL project specification adhere to this practice.

Hope this helps.

-Jehan

 

[wsmith22]

Fluque,

Pads are not required by Code for the conditions you stated. ASME requires that you check for reinforcement for those conditions. Repads are just one option if additional reinforcement is required.

 

[robsalv]

Jehan, can you elaborate further on your HIC comment please.

Reinforcing pads are on the outside of the vessel and not exposed to the high hydrogen partial pressure environment. Why would you avoid them?

Thanks.

Rob





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[TGS4]

The hydrogen molecule, because of it's extremely small size, will diffuse through the solid steel wall. In fact, there are formulae and curves to describe this phenomenon. However, it depends on the hydrogen partial pressure. if you have a repad on the outside of a vessel, hydrogen will diffuse into the space between the pad and the vessel shell. Normally, the weep-hole that was drilled into the repad should be sufficient to allow the hydrogen to escape. However, these weep holes often get forgotten or filled, and hydrogen pressure can build up. Not a big deal, until the vessel de-pressurizes, and the hydrogen tries to escape, and ends up pressurizing the vessel shell under the repad. After a few cycles of that, your fillet welds will likely crack, and your re-pad will no longer be "attached" to the shell. It's a similar process as you would see in hydrogen blistering...

 

[jehan17718]

Robsalv,

TGS4 has provided an excellent description of what you wanted.
I am not from a materials background but nevertheless, just want to add that as per my understanding atomic hydrogen i.e. H+1 is extremely "mobile" within in the metal crystal lattice. However, molecular hydrogen i.e. H2 is not. Once hydrogen atoms combine to form molecular hydrogen they are not mobile. Molecular hydrogen occupies the space in the metal structure (voids, etc) and over a period of time will build up considerable pressure leading to eventual failure of the equipment.

Also the quality of the fillet weld connecting the pad to shell cannot be radiographed which makes this joint a potential source of failure due to HIC.

My apologies for "missing" out to reply on this post earlier.

-jehan