Tie-In Issue: 10”x10S SS316L to 24”x10S SS316L – Weld Clearance & Flange Type

[Suradach]

Dear Experts,
We are currently working on a tie-in for a 10" x 10S SS316L pipe to an existing 24" x 10S SS316L header in a chemical plant, designed in accordance with ASME B31.3.
Please see the attached sketch for two proposed concepts.
We are facing two main concerns:

1. Project Specification requires Weld Neck (WN) flanges only.
   However, due to site constraints, there is insufficient clearance to install the tie-in using a WN flange as shown in Concept 01 (required straight length ≥ 496 mm, which is not available).
2. Concept 02 uses Slip-On flanges, which allows us to perform the tie-in within the available space.
   However, it results in a very short distance between two circumferential welds – only 29.23 mm weld-to-weld. This raises concerns regarding:
   • Heat Affected Zone (HAZ) overlap
   • Residual stresses
   • Possible metallurgical degradation in the weld region

We would like to ask the community:

• Has anyone encountered a similar situation, and how was it resolved?
• Are there any acceptable practices under ASME B31.3 or recognized industry standards to justify this weld spacing?
• Would performing additional NDE and/or localized PWHT be sufficient for mitigation?

Any feedback, experience sharing, or technical references would be highly appreciated


Thank you very much.

 

[LittleInch]

So what is the current arrangement?

Why not use a weldolet, or split tee?

Option B looks horri⁴ble in terms of stress concentration. And generally anything that just looks terrible usually is for strict engineering reasons.

Why does project specification require flanges?

 

[akpipelineengr]

Neither option leaves any room for bolting those flanges or making the necessary welds, let alone the stress concentration issues (in Sch 10S!).

There has to be a better way. Move the tie-in up- or downstream to where you have some real pipe to work with. Use an appropriate branch connection.

 

[Piper Mike]

Suradach

The design is a stub-on branch connection on thin wall pipe.
There is 29 mm weld to weld clearance.
Your design implies it's a low pressure system with no mechanical or thermal imposed loads at this branch connection.

Welding this stub-on to the new 24" spool will not be of any fabrication concern.

FYI, Piping Fabrication Institute, or PFI, provides data for weld to weld spacing.

PS What is the red circle?

 

[Suradach]

So what is the current arrangement?

Why not use a weldolet, or split tee?

Option B looks horri⁴ble in terms of stress concentration. And generally anything that just looks terrible usually is for strict engineering reasons.

Why does project specification require flanges?

Thank you for your input.
We’re currently in the concept review stage to support the upcoming shutdown tie-in. Due to limited space, we’re exploring alternatives. I’m seeking expert suggestions on any better tie-in methods that meet ASME B31.3 and suit thin-wall SS316L pipe.
All ideas are welcome — especially if based on field experience.

 

[Suradach]

Suradach

The design is a stub-on branch connection on thin wall pipe.
There is 29 mm weld to weld clearance.
Your design implies it's a low pressure system with no mechanical or thermal imposed loads at this branch connection.

Welding this stub-on to the new 24" spool will not be of any fabrication concern.

FYI, Piping Fabrication Institute, or PFI, provides data for weld to weld spacing.

PS What is the red circle?

Yes, it's a low-pressure system.
Thanks for pointing me toward PFI – that’s really helpful!
As for the red circle, it simply represents the 10” branch pipe — no need to worry about it.

 

[Suradach]

So what is the current arrangement?

Why not use a weldolet, or split tee?

Option B looks horri⁴ble in terms of stress concentration. And generally anything that just looks terrible usually is for strict engineering reasons.

Why does project specification require flanges?

We initially considered using a weldolet or split tee, but the available space was insufficient — based on the layout shown in Concept 01.
That’s why we moved to Concept 02, switching from a WN flange to a Slip-On flange to gain additional clearance for fabrication and tie-in.

 

[LittleInch]

I'm confused. Do you have a straight run of pipe or not.

Why are you shoehorned in these flanges if they are not already there?

 

[Suradach]

I'm confused. Do you have a straight run of pipe or not.

Why are you shoehorned in these flanges if they are not already there?

There is no straight run of pipe in this area.
The flanges were proposed to allow easy disassembly during shutdown activities.
However, if a special, more compact flange design is feasible, it could help increase pipe clearance—we’re open to that alternative.

 

[GD2]

Dear Experts,
We are currently working on a tie-in for a 10" x 10S SS316L pipe to an existing 24" x 10S SS316L header in a chemical plant, designed in accordance with ASME B31.3.
Please see the attached sketch for two proposed concepts.
We are facing two main concerns:

1. Project Specification requires Weld Neck (WN) flanges only.
   However, due to site constraints, there is insufficient clearance to install the tie-in using a WN flange as shown in Concept 01 (required straight length ≥ 496 mm, which is not available).
2. Concept 02 uses Slip-On flanges, which allows us to perform the tie-in within the available space.
   However, it results in a very short distance between two circumferential welds – only 29.23 mm weld-to-weld. This raises concerns regarding:
   • Heat Affected Zone (HAZ) overlap
   • Residual stresses
   • Possible metallurgical degradation in the weld region

We would like to ask the community:

• Has anyone encountered a similar situation, and how was it resolved?
• Are there any acceptable practices under ASME B31.3 or recognized industry standards to justify this weld spacing?
• Would performing additional NDE and/or localized PWHT be sufficient for mitigation?

Any feedback, experience sharing, or technical references would be highly appreciated
View attachment 15057

Thank you very much.

Concept 02 looks fine to me. Although there is no explicit statement on distance between weld joints by any Code, it is generally be recommended to follow the 4x rule, i.e The minimum distance between welds should be at least four times the thickness of the thinner part joined, but not less than 1 inch (25 mm), which in your case is 29.23 mm.
The only thing I would worry about the WPS. You may be aware of the Sigma Phase formation in SS welding when exposed to temperatures in the range of 531 C - 927C.
SS typically exhibit sigma formation up to a max of 10%. Conversion to sigma in SS can also during PWHT at 691C, which can be de-sigmatized by solution annealing at 1066C and followed by a water quench.
Sigma formation will lower the ductility, in other terms the toughness.
Just make sure that slip-on flanges are allowed by the application. They have limitations (they are not recommended for severe conditions like, high pressure, high temp, cyclic and corrosive services).

 

[KevinNZ]

Option A welding to the thickened part the WN would likely have lower stress than the SO case. You can use FEA and compare local stress with conventual pipe on pipe weld.

The design pressure and pipe loads must very low to do this in any case with no reinforcing pad.

 

[LittleInch]

Can you use an 8"pipe as the connection then add a reducer up to the 10" flange?

Any pressure losses will be very low.