Requirements for field bending of pipe 4

[GD_P]

We are thinking to bend the pipe in field / shop instead of buying and welding the 90 deg bend, I have following doubts
1) Are these requirements apply to bending of pipes at room temperature?
2) For heat treatment requirements after bending as per ASME B31.3, which elongation of grade is to be compared to fiber strain due to bending? Longitudinal or transverse? As the pipe is deformed longitudinally, so longitudinal shall be compared. Am I right?
3) ASME B31.3 gives requirements of flattening as 3 & 8% for external and internal pressure respectively, but if we refer the factory made bend as per B16.9 there is no flattening allowance. As the spec for such bends for eg A403 allow manufacturing of bends using bending of pipes. Still there is no such allowances.
4) Welded seam orientation- If we keep it along extrados or intrados weld seam which is brittle than parent metal will be subjected to plastic deformation. Where as if we keep it along the middle or neutral axis of bending, it will be subjected to max shear due to bending. So I think it should be between extrados and neutral axis. Is my interpretation right? Since ASME B31.3 do not give any requirement regarding the orientation.

One more point is there any requirement of min thickness of pipes such as ASME Sec VIII div. 1 require min 1.5 mm shell thk for carbon steel.
Please share your opinion and any help would be appreciated.


GD_P

 

[racookpe1978]

You MUST provide service fluid, pressure, temperature, alloy of pipe, controlling standard, pipe support method, buries/above ground/pipeline or industry.

Diameter. You did list wall thickness = Very low pressure.

Reason for the bend, reason why a simple elbow is not acceptable.

 

[GD_P]

Code - ASME B31.3
Fluid service - Normal fluid & Category D
Pressure - Max 15 bar
Temperature - Max 400 deg C
Pipe MOC - A106 Gr b & A312-304, 304L, 316, 316L & rarely 316Ti.
Piping for process plant & some times for food processing as well and hence above ground supported with U clamps
Diameter - Max DN 150

Currently Butt welded fitting are used, but as it involves welding which consumes lot of time and off course costlier than bending, we are compiling the requirements to use field bends.
Also I have been reading articles on use of field bending of pipes instead of Welded fitting for food processing, i think it will be very helpful rather than using orbital welding for every fitting.
But ASME BPE code doesn't allow bending of tubes of dia more than 1/2" without prior permission of client, may be due to change in surface finish on bended surfaces and wrinkling on intrados.
Hope this information will suffice.

GD_P

 

[MJCronin]

Some more thoughts about Field Pipe bending:

1) When you make a mistake and either the pipe wall thinning is excessive or the bend becomes kinked or knuckled, you are done. The piece has been ruined and must be discarded. Commonly, the "bending team" will ask the young field engineer if the kinks can be hammered out, he must say no.... Welded elbows are slightly more forgiving, and may be cheaper in the long run.

2) Smaller diameter pipe, of heavier wall thickness (say, up to 2"NPS schedule 40/80) is resistant to wall thinning and kinking. Equipment is available to use in this pipe size range. This would be a reasonable place to start. Having a group of newbies trying to bend 6" NPS, Sch 10S stainless will be a disaster.

3) Field pipe bending of larger diameter pipe can require the use of plug mandrels and packing sand. Things must be done right. The tendency of engineering MBAs to assemble a group of fitters with no experience is always a receipe for disaster.

4) If, for some reason, the piping system has a computerized stress analysis on it and you substitute bends for elbows, be aware that the bends will increase the calculated nozzle reactions on equipment terminations. This can be a problem for hot systems.

5) 3D and 5D bends on piping systems will cause some system redesigns because bent pipe has a much larger minimum length run. System with bends are much more "loopy" than those with elbows.

More information here:

https://www.tubeformsolutions.com/hubfs/doc/TFS_Tool_School_-_Tube_Bending.pdf?t=1481048596950

MJCronin
Sr. Process Engineer

 

[LittleInch]

I'm not at work at present so can't see what B 31.3 thinks about bends as opposed to elbows, but it probably doesn't really like the idea.

Pipeline codes such as B 31.4 and B 31.8 will have much more guidance on the requirement for what I suspect you are thinking of as cold bends, not "hot" or Induction bends. I've never heat treated a cold bend on pipelines ever.

Bending as opposed to using elbows is possible, but requires more space as you won't be at less than 3D or 5D as opposed to 1.5D and bend thinning and issues such as wrinkling get much more problematic the tighter you go, especially will thin wall pipes like stainless as you seem to have.

Small bore (2" and lower) should be OK, but things get much more complex the bigger diameter you go.

Why do you think the weld seam is brittle? It should be better than the parent metal if it's done correctly.

MJ Cronin has it explained well.
The smallest 6" radius cold bends I ever got made were 10D and they were rammed with sand to prevent wrinkles. Induction bending is the only way to go and that isn't at all cheap.

I think you have an interesting idea, but when you add it all up you'll find out why it isn't done on any sort of scale above instrument tubing size / 1" low piping.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MJCronin]

More information here:

https://www.eng-tips.com/viewthread.cfm?qid=425015

https://www.eng-tips.com/viewthread.cfm?qid=359480

Consensus seems to be that, depending on pipe diameter, a system with all cold bends can be more expensive and difficult to install in an existing plant than a system with welded elbows ...

MJCronin
Sr. Process Engineer

 

[GD_P]

Great Information....

I was missing the other side of the coin and only had commercial perspective.
Thank you very much for the practical info.
@MJCronin thanks for the document from tubeformsolution.....
Perfect documents to start with...
I haven't gone through it completely, but soon I will.
@LittleInch, space requirements will be one of the main issue, as in skids, routing piping with large bend rad will be difficult. And thums up for your status...

Also there are hardly 25% of total piping over size 2", so bending requirements can be categorized as per size, really good idea to work upon.

GD_P

 

[EdStainless]

I have done a lot of bends in SS tube 2" and smaller in 0.065" and 0.083" walls.
If you go long radius such as 8d or larger they are fairly easy.
We didn't fill or use mandrels, but we used shaped shoes and curved roller guides.
When given a choice I like the welds mid-way between the NA and Into.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[GD_P]

Hello forum once again,

I have a doubt regarding cold bending of pipe.

If I have to bend a pipe of SS (say 304 or 316, P-Number 8) designed as per ASME B31.3 for temperature greater than -29 deg C. And it's fibre elongation is 50% (assumed, for example). Does this cold bend requires post bending/forming heat treatment?

As per the clause 332.4.2 a & b, No HT required.
Now my doubt is, what does the point c) mean? (In broad sense, apart from this particular example)
Since when we refer to design of cold bend as per 31.3, there is no relationship between design parameters (thk, allow stresses etc) and heat treatment. So how to justify whether the HT is required or not?

The point c) definitely mean something, otherwise no SS comp operating at temp above -29 deg C will not require post bending HT.

Can we refer UHA-44 of sec VIII div-1 for this?

Please share your opinion.
Thank you.

GD_P

 

[XL83NL]

It means that if you're doing a job where the design prescribes that in engineering you need to do PFHT, you need to do it, simply. The engineering design is a broad term for e.g. client specifications (like Shell DEP's or internal compnay standards) that are applicable to the engineering.

You may refer to ASME VIII-1 table UHA-44 for limits and applicability of PFHT for high alloy steels.

However, 50% elongation, even though it's an example, is a lot, if not too much. Do you have any idea what A182, A403 or A312 specify as minimum elongation requirements for similar materials?

 

[GD_P]

Thank you XL83NL for your comments.

You are right, most of SS has 30% max long elongation.





GD_P