High pressure piping backout requirements

[ledon]

While most welded pipe joint designs have a requirement for backout at fit-up, I need to know if it is required to maintain that requirement after weld completion and where this is written.

 

[stanweld]

Are you refering to socket welds where you bottom the joint and then "back out" approximately 1/16" (1.5mm)as depicted in FIG. 328.5.2C and FIG. 127.4.4(B) of ASME B31.1? These are "typical details" only and the 1/16" spacing is not required after welding. Often times Engineers and Owners specify that you back out 1/16" and will require RT to be done a percentage of the welds to confirm; however, most understand that all that they are looking for is observing a space between pipe end and coupling.

 

[pipewelder1999]

The purpose for the bottoming clearance in a socket weld is usually to reduce the residual stress at the root of he weld that could occur during solidification of the weld metal.

In my opinion, looking for the "Gap" after welding by RT is an excess cost. No code I am aware of addresses this as a required inspection. A properly fitup socket weld would usually contract in a manner that would leave a smaller gap or no visible gap.

I have seen perfectly good socket welds cut out that were mistakenly interpreted during RT of an adjacent butt joint with no engineer or inspector argument. There is no code requirement or acceptance criteria for this in ASME Sec I, Sec VIII, B31.1, B31.3.





Gerald Austin
Iuka, Mississippi

http://www.weldinginspectionsvcs.com

 

[ledon]

Thank you for your response. Yes I am speaking of socket welds. Particularly, used under Naval Specifications. I have found no written guidance on this and one of my customers has raised the question. I have always maintained that a visable gap must be maintained after, but I have been directed to find some written guidelines.

 

[unclesyd]

Here is the codes/standards mentioned in a brochure from GAP-A-LET rings.

Design Data Commercial Construction Codes
ANSI. ASME
Military Std. NAVSEA - 250- 1500
Military Std. 278
Military Std. 22


All our Therminol (vaporized/liquid) piping 2 1/2" and under used socket welds with a 1/16" gap verified by RT. Along with the gap requirement all fillet welds were 2 pass, 6010 + 7018 on old pipe with all 7018 on new piping.

 

[pipewelder1999]

Unclesyd

Many codes refer to bottoming clearances on sockets welds. I worked a few years under the codes you mention above and do not recall any RT requirements for verification of bottoming clearance.

Does your company do this as an internal requirement or is it specified somewhere? It seems like an excess cost.


Gerald Austin
Iuka, Mississippi

http://www.weldinginspectionsvcs.com

 

[arto]

They usta stick in the pipe all the way & scribe a line a given standard distance [1"] back, then pull it back the gap distance & weld it. That way the gap can be easily verified externally with a scale.

 

[unclesyd]

A little windy.

I will try to get the piping standard we used and see if there is a reference to any particular code/standard. I do remember early on that the gap requirement was reinforced by several incidents at a competitor's plant and later two incidents at a sister site. All our piping internal standards were at or mostly above code requirements even though we were not a Code state and there no incentives other than the commitment to have piping systems you could live with and not worry.

You are correct when you mention the added cost. When we had an internal RT Crew and most of the piping was bent on a slab eliminating fittings the cost was negligible. When we went to RT by contract and low bid contractors, instead of resident, the cost was raised considerable and quality went south. We had started to lower the % RT requirements when we discovered the welder/fitter quality was terrible, not only could the welder not weld, but the fitters were using the sockets as a crutch. While we were attacking this problem the permissible reportable spill was lowered by 90%. Also with any vaporized DP/DPO heating medium there was ever present danger of a vapor or mist explosion or fire a big problem as the majority of the piping was located in an occupied building. Our piping standards also require a vacuum, pneumatic pressure test, and He Leak test prior to commission.

I still say that RT is not the cost it’s the rework that drives the cost escalation. It was our group’s
opinion why should we lower our piping standards, proven, to the lowest common denominator of the industry.

 

[metengr]

pipewelder1999/arto;
You are correct regarding socket welds. In the power industry, we fabricate socket welds most of the time for external piping and we have never performed an RT. There is no code requirement for volumetric inspection of socket welds (fillet welds).

Using a scribed line works well or using a water soluble gap-a-let contraction ring assures proper clearance for socket welds.