Thermal stress relief in nozzle welds 1

[ll23]

Greetings, we have certain doubts about the stress relief in the welds of nozzle in the shell of an AST, in API 650 2013 in 5.7.4, states in 5.7.4.2 and 5.7.4.3 that the connection "shall be prefabricated into the shell plate or thickened insert plate, and the prefabricated assembly shall be thermally stress-relieved within a temperature range of 600 °C to 650 °C (1100 °F to 1200 °F) for 1 hour per 25 mm (1 in.) of thickness prior to installation."

My doubts are first the PWHT apply to the internal weld of the nozzle and external reinforcement and internal welds, or second we have to do the internal weld first, next the PWHT, and then complete all the external welds without the PHWT.

There is a great risk about having deformations in the shell during de PWHT, that's why we may have to do a bigger hole in the shell for the neck of the nozzle.

Or we just make the Nozzle with the Reinforcment or the thickened insert, PWHT, and then put it in the shell, and then make the internal weld, without the PWHT.

 

[IFRs]

PWHT is performed after all welding is done and the assembly fabrication is completed.

 

[ll23]

thank you very much IFRs for your reply, but this means that we have to do a PWHT to the reinforcement fillet weld, in the case of a tombstone shell reinforcement, in the 5.7.4 says, "The stress-relieving requirements do not apply to the weld to the bottom annular plate", which make me doubt about the PWHT in the outer weld.

 

[IFRs]

"Or we just make the Nozzle with the Reinforcment or the thickened insert, PWHT, and then put it in the shell, and then make the internal weld, without the PWHT."

This statement is almost correct but I'm not sure what you mean by "internal weld".

Are you working on a flush type fitting (5.4.7.1) or a nozzle meeting (5.4.7.2) ?

For flush fittings 5.4.7.1 you PWHT an assembly that includes a part of the bottom.
For nozzles meeting 5.4.7.2 the assembly you are PWHT does not include a part of the bottom.

 

[ll23]

well, I mean the internal weld of a nozzle like in the image, so is this the case, first the reinforcement, sorry for the misspelling, to the neck (first weld), then put it to the Shell outside the tank (the second weld), we do the PWHT, and then do the internal weld. In our case our nozzles and manhole meet the 5.4.7.3

 

[IFRs]

Nope. The assembly must include the shell plate ( or thickened insert plate ). The welds you have marked "First Weld" and "Second Weld" as well as the weld you have marked "After the PWHT" must ALL be made as part of the assembly and PWHT with the assembly. The corner weld that you have marked "We have to do PWHT to this weld" is done after the assembly PWHT and when it is installed in the tank. The corner weld - inside or outside does not get PWHT. The portion of the corner weld in-between the shell plate and the reinforcing plate needs to be done in the shop and gets PWHT with the assembly.

Just checking - why are you PWHT this nozzle?
Is it over 12" NPS and the shell plate over 1" ?
Is the shell plate material grade over IV or higher ?

 

[ll23]

ok no doubts at all, thanks a lot, and yes we have to weld some nozzles and manholes in a 26 mm, 33 mm and 41 mm thick shell, of S-355-J2 Steel plates, that belongs to a IV A group.

 

[ll23]

Greetings, first thanks for your answer, but I have new doubt for the stress relief in our nozzles, in Technical Inquiries for API Standard 650, Welded tanks for Oil Storage last updated August 2016, it discuss the stress relief issues in the page 6, they refer about if it's possible to do the stress relief with local heating or with electrical resistance, also mentions that it should do it in all the parts of the assembly, not only the welds.

Since API don't mention how to do the heating, clearly states that shall do it prior to the installation of the nozzles. My doubt is how we can pre heat the preassemble , doing all the welds, without complete the installation of the nozzle in the shell, if we shall going to use an insert plate, it would be easy to do, using a furnace, but we are using reinforcements.

 

[IFRs]

You install the nozzle with the repad in an appropriately sized portion of a shell plate, which I will call a "shell insert plate". That assembly is PWHT in a furnace, brought to the field and welded into the rest of the shell plate where there is an appropriately sized hole or gap. You will size the "shell insert plate" to maintain proper weld spacing to all welds including the butt welds between the shell plate and the "shell insert plate". The "shell insert plate" may be just large enough for weld spacing so it fits in a small furnace, it can also be a full-height plate.

 

[ll23]

Thanks IFRs, I just realizing that we got a serious confusion about the nozzles, I thought that the condition in paragraph 5.7.4.2, the one that states "The

stress-relieving requirements need not include the flange-to-neck welds or other nozzle-neck and manhole-neck

attachments, provided the following conditions are fulfilled." applied also for those specified in the 5.7.4.3, apparently not, if I don't mistake.

This mean that we need to have to stress relief all the assembly, flanges, neck, reinforcement pad and put into an insert shell plate, and then weld this "pre-assembly" into the Shell?

 

[IFRs]

Yes, there is still confusion. You must include the nozzle neck to reinforcing plate weld and nozzle neck to shell insert plate weld in the assembly that is PWHT. You can't PWHT the neck and repad without the shell insert plate. The assembly that is PWHT must include all welds that go around hole in the plates and the fillet weld around the repad. The neck to flange weld does not have to be included in the PWHT assembly because it is outside of the area used for reinforcement of the shell opening. Can you post a drawing of your assembly showing the shell insert plate?

 

[ll23]

Good day, yes it's really a confusing matter, our design doesn't involve any shell insert in nozzle, except for the flush type connection that will be mounted in an insert.

There is a matter about the local heating, in the inquiries forbid any local heat at least it is made in all the tank, and then the definition of pre-assembly and complete assembly it's not quite clear, how I state it before, IMHO, we should weld the neck of the nozzle first, in the reinforcement pad or thickened insert, then weld over the shell opening, finally we complete the weld of the reinforcement pad in the shell, and do the PWHT.

The problem comes when you can't do the pre assembly in the shop, and we must install the nozzle in field.

 

[IFRs]

If you can't pre-assemble and PWHT then you won't be in compliance with the code. The choice of shell plate material does not help. I don't find the code to be confusing on this subject, but I understand trying to make your situation fit. Check out API 653 section 11.3 which may provide an alternative.

 

[ll23]

Again thanks very much IFR for sharing your knowledge