allowable spacing between adjacent welds in 8" pipe 1

[FranciscoG]

Where can I find the minimum allowable spacing between adjacent welds for an 8" Sch 40 A-106 GR.B pipe? I need to make a small spool with WN flanges on each end and the flanges are resulting in 78 mm apart. Is this spacing OK for the welding?

 

[metengr]

FranciscoG;
Unfortunately, I have not been able to find any specific information related to placement of butt welds when installing "pup" pieces for piping or for boiler tube butt welds (like installation of dutchman). As a minimum, I would make sure weld placement should be suitable for unobstructed NDT (surface or radiographic testing), and the weld toes have enough separation to avoid a metallurgical "notch effect". You could consider a simple stress analysis using finite elemental modeling (FEM) to provide you with a sanity check for weld placement in piping.

Over the years, and in discussions with many experienced boiler/piping construction superintendents, I have used a 2X pipe diameter rule for minimum spacing between butt welds in boiler tubing and piping applications. Thus far, I never had any problems related to service adhering to this rule.

 

[DSB123]

FranciscoG,
On our site the general rule is 50mm between the toes of adjacent welds. BS2633 from memory states 40mm between the toes of the welds.

 

[FranciscoG]

Thank you all.
I see that there is no clear definition on this subject.
I have been looking at ANSI B31.3 and there is no reference about this.

Someone mentioned that in ASME SEC VIII DIV 1, UW-14 you were allowed weld-on-weld connections. Can anyone check this for me as I don't have the code?

 

[metengr]

Yes. Section VIII, Div 1, Part UW-14 b) describes the rules for openings located adjacent to welded joints (head to shell or Category B or C butt welded joints). You can place an opening, such as a welded nozzle, adjacent to a vessel butt weld. Paragraph d) in this section mentions that openings cannot be located less than 1/2" from the edge of a vessel weld (category A, B or C) for material 1.5" or less in thickness.

 

[metengr]

After getting back in the office earlier today, I was able to review one other source that I thought of regarding placement of adjacent welds on piping.

The source is the Pipe Fabrication Institute (PFI), specifically PFI Standard ES-7. This publication is titled "Minimum Length and Spacing for Welded Nozzles" and has some excellent reference information.

For 8" outer diameter nozzles with NO saddles or ring reinforcement, the minimum spacing between the nozzle outer diameters reported in Table 1 of ES-7 (1997) is slightly less than 1X the diameter of the nozzle. In other words, if you want to locate two 8" diameter nozzles side by side in a main pipe run, the minimum spacing between the nozzles (OD to OD) is 6" (152 mm).

 

[FranciscoG]

Thank you all. I think I will stick to what ASME VIII par. UW-14 b) says that provided the weld meets the radiographic requirements in Par, UW-51(m) you can make an openning in or adjacent to welds.

May be also testing the hardness of the welds, could give another assurance. A fitting manufacturer recommended a Brinell Hardness not more than 215 for the circumferential welds.

 

[stanweld]

FranciscoG
As memory recalls, I believe that B31.3 once limited a minimum of 2" between welds in welded miter bends (no longer the case). What you must consider is the effect of the opposed residual welding tensile stress on the pipe between the circ. welds. There is sufficient distance between the welds to assure that a design to ASME VIII or B31.3 will not be compromised. I would not be concerned with your design.