The Need for Transition Spools 1

[JohnRAA]

Can someone either point me to a resource or explain clearly when transition pieces are required between pipes of either different grade or wt?

1) Say I wanted to join an 8" Pipe .188 wt X-52 to 8" .500 wt X-52. Would I need a transition spool?

2) How about 8" .250 wt Grade B to 8" .500" wt X-52?

Thanks Ahead of time

 

[BigInch]

When the difference in wall thickness is around 3/16" or greater, or when the difference in yield strength is greater than about 20 ksi. Differing chemical compositions of materials making up the pipes to be joined may force additional considerations as well.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[StevenHPerry]

B31 codes (B31.1 at least) have limitations on girth buttwelds in the fabrication section.

API 1104 has a suggestion on alignment that applies to pipe of the same size and thickness, but it's not as explicit as B31.

In high temperature applications, beware of differential thermal expansion between different material grades. I'm not sure if this is explicitly addressed by any codes, but Peng's book on Piping Engineering has a method for analyzing these joints.

- Steve Perry

http://www.linkedin.com/in/stevenhperry

This post is designed to provide accurate and authoritative information in regard to the subject matter covered. It is offered with the understanding that the author is not engaged in rendering engineering or other professional service. If you need help, get help, and PAY FOR IT.

 

[Duwe6]

There is a simple solution, that is [slightly] documented in VIII, B31.1 & 31.3. Taper-bore a.k.a. counter-bore.

On at a 3:1 or shallower angle [slope], counter-bore the thicker member to match the thinner. Bore at least 3 wall thicknesses [5 preferred] into the thicker. Have the machinist put a small radius at the juncture of the thinned area into the slope of the counterbore -- eliminates a stress riser.

Your weld joint can never be any stronger than the thinner / weaker member. This moves the transition well away from the weld so that RT or UT will be easier to interpret, and the 'Hi-Lo' internal mismatch of the bores is about zero. As StevenPerry pointed out, there are strict limits of internal mismatch or weld joint fitups. They are limited to give the welder a good, weldable fitup and keep from making the weld joint very difficult to apply a sound told to.

 

[JohnRAA]

Thanks Guys.

This question would be for a crude pipeline.

When I read B31.4-2009 page 45 and 46 it indicates the need to taper the thicker material but it doesn't indicate any maximum allowable difference between wall thickness.

in fact the illustration seems to show that the wall thickness difference can be substantial, but for design calculation purposes td cannot be greater than 1.5t.

From this alone I do not see any mechanical code limitation to wall thicknesses that can be joined.
It seems like this would have been the perfect place to indicate any limitations.

Seems like the trouble starts if you have to bevel the thicker , weaker material as now the joint would be weaker than pipe itself reducing the MAOP.

 

[DLite30]

That's one thing you still have to be cognizant of...transition or bevel the pipe end to a wall thickness that still meets your pressure design requirements.

 

[SNORGY]

In CSA Z662 (which is probably similar to B31.4 / B31.8 in terms of rules, etc.), you would specify a transition when the wall thickness is equal to or greater than 2.4 mm in sweet service, or 1.6 mm in sour service. It would take the form of a transition spool if it became impractical to do the weld preparation on the pipe itself. The transition piece, when required, is always made from the material with the higher strength of the two materials being joined; the assumption is that the thicker material is the one with the lower yield strength (when different).

I should read B31.4 / B31.8 one day, but I would imagine that the wall thicknesses referenced in the OP clearly point towards the requirement for a transition or transition piece. But the 8" 0.250" wt Grade B to the 8" x 0.500" wt X52 seems...odd...if by "Grade B" you mean SMYS = 35000 psi.

Regards,

SNORGY.

 

[BigInch]

Nothing odd I see. "Grade B" usually refers to ASTM A106 Grade B

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[SNORGY]

The "oddity" I was referring to was welding thinner wall pipe with lower yield strength to thicker wall pipe with higher yield strength. Seemed counter-intuitive to me. But, there was probably more to the story.

Regards,

SNORGY.

 

[BigInch]

Then wouldn't it be equally as odd if the grades were the same?

Or the design factors have changed, or it is a "telescoped" pipeline and the thiner wall is at higher elevation and lower pressue, or there is a PCV and relief valve upstream, or he has a valve in stock with fat pups, or he's using up some pipe stock with a heavy design factor, or ....


Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[SNORGY]

That would be the "more to the story" part...

Regards,

SNORGY.

 

[JohnRAA]

Here is the rest of the story.

We had to an HDD to reroute an existing line.
Our HDD Spec requires the pipe wall to be .5" wt.
The only pipe I could find in time that met our spec was X-52.

Still don't see in B31.4 where there is a limit to the difference in wall thickness.

I Have heard multiple points of view in the office and from our engineering contractor on the need for the transition spools??

Thanks

 

[BigInch]

There is no limit specified in B31.4, however B31.4 references API 1104 for welding quality, so in other words, what .4 considers good is what will pass an 1104 inspection. That's going to translate into around 3/16" difference in WT before the weld inspection starts getting fuzzy enough to call for a cutout.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[BigInch]

P.S. That's to catch those that "search a pdf document" for an answer, rather than read the documents and those that they reference.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[Duwe6]

1104 U/G line installation usually has a Welding & Pipe Inspector. His decisions are usually unappealable unless they are outrageous. A good Weld Inspector will limit the ID mismatch to 1/16" to 1/8", depanding on pipe diameter and roundness.

To save a LOT of time grinding a transition bevel in a muddy, nasty ditch, just have a minimum-acceptable-length 'pup' of the thick stuff machined to a Code counterbore. Yes, any decent fitter can grind in a decent transition while laying on a mudboard, propped up in the weld bellhole in the ditch, while a trashwater pump tries to keep the water level down in the bellhole. But do you really want to engender that much work for your most expensive people? And have that work proceed at a painfully slow pace due to lousy working conditions??

Keep as much work in the shop as possible. Make it as fast and easy as possible for your high-dollar construction crew.

 

[JohnRAA]

Thanks Everyone for the input!