Fabricated 4way cross section

[Mechh007]

Dear Engineers,
We are using a fabricated Cross 6” x 6”,Sch 80 on the inlet and outlet side of the manifold,Working pressure is 1440 psi.Can you please tell me how to check(calculation) if this design is passing? MOC A234 WPB. Pic is attached for your ref

 

[XL83NL]

I guess it depends on your design code, so lets start with that. Is this a part for a piping system, e.g. falling under B31.3?

 

[Mechh007]

Yes it is falling under B31.3

 

[XL83NL]

Then you probably have to resort to para 304.7.2.

 

[Mechh007]

There is no detail at para 304.7.2

 

[XL83NL]

Thats absolutely correct. B31.3, as I recall it, is not a cook-book when it comes to such customs fittings. Youve made an unlisted component. Which as an example might require FEA by means of 304.7.2(d).

 

[yanuarsaja]

hi,
for special design, better to perform finite element analysis (FEA) to ensure stress integrity of the equipment/Component.
And to ensure the integrity of product that you have fabricated just follow normal inspection/testing, UT and hydrotest for welding strength.

 

[XL83NL]

If you really it that way, go for it, but why not try a forged 6" cross / standard pipe fitting? Im sure you should be able to get them somewhere.

 

[LittleInch]

I can't see why you can't analyse this as two branch connections. I suspect you will need some compensation plates for any appreciable reviewable pressure.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Mechh007]

Little Inch.u mean reinforcement pad??

 

[LittleInch]

Yes - see section 304.3.3 and appendix H.

What you're doing is a bit odd, but I can't see why this should be any different than welding two branches side by side, though there are rules somewhere in that section about minimum distance between them.

A forged component would solve all your problems....

At a design pressure of 1440 psig you can't afford to make mistakes.

You also need to talk to whoever is approving / authorising / inspecting this thing to make sure they are happy.

At the very least it's some sort of start point for your analysis.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[XL83NL]

That would be an interesting POV LI. Never thought of it that way before, I wonder if B31.3 allows to apply 2 branches at one point (180 degree located from each other).

 

[LittleInch]

I've had a reasonable look and currently can't see why not, but then neither does it say that it does.... The welds from the branches look far enough away to me and I've never seen anything saying you can't have a staggered set of branches, so why not directly opposite? I will stand to be corrected and maybe someone has asked the question before either to the 31.3 committee or here.

You can't write a code covering everything so need to apply some judgement from time to time.

The reinforcement pad is almost definitely needed on both branches, but that you can work out quite easily.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

Just because you can fabricate it, and the code doesn't say no, that doesn't mean that it's a good idea.

 

[LittleInch]

No, I agree, but why is it a bad idea? You can buy a forged cross much the same way you can but an integrally reinforced branch fitting or a tee, but if you can replace the O-let type fittings with your own branch and some reinforcement, why not a forged cross?

I'm happy to be pulled up on this, but I think it needs some explanation as to why it is not a valid approach?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

I didn't say you can't do it.

Interference in the HAZ?
Warping during fabrication?
Axial stress?
Bending Stress?
Torsion?
Stress concentration factors?

There's just very little metal left that can handle any kind of a load.
I'd tend to leave it only for someplace I would know will always be in a very low stress condition.

 

[MJCronin]

These people make forged sch80 fittings that you may desire

http://www.notokfittings.com/equal tee buttweld welded.html.

Unreinforced (stub-in) type intersections in this pressure and diameter range will fail ASME B31.1.

If pad reinforced intersections are used, it may become to expensive/difficult to fabricate.

Have weld-o-lets been considered ? They have slightly more cost but the pressure reinforcement is assured and fabrication is easy.

http://www.bonneyforge.com/resources/BC.pdf

I also have a lot of trouble with this " I just welded something up, now you engineers tell me why I can't use it" approach.

If your piping fabricator does not understand simple piping/pressure vessel concepts like pressure reinforcement, there is real trouble here.....

MJCronin
Sr. Process Engineer

 

[XL83NL]

A 6" WOL on a 6" 80 run pipe?? How much heat input will that require - what weld metal thicknesses will you get? I bet theyre HUGE!
Let alone the fact IF you can get a full WOL (i.e. 6" to 6").

 

[XL83NL]

PS: Im pretty sure your answer lies with these Indian guys that can provide e.g. tantalum unequal crosses (!) if they meet Code requirements ....

 

[Gator]

"Not OK" Fittings. LOL.

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