Bending of Stainless steel pipe

[gravitate]

We have a design of stainless steel pipe with a bend radius of 1.5 The OD of the pipe is 47mm and the wall thickness is 3mm. Can anyone tell me if this is possible please? I am pretty sure the SS is austenitic. I thought that as a rule the minimum radius was 2.
Thank you

 

[gravitate]

I forgot to say that this is in a nuclear reactor and it is bieng built to ASME b31.3

 

[gravitate]

OK I am really stuck here. Will how the stainless steel is formed effect this at all?

 

[Q_Goest]

Is the bend radius 1.5 meters? Is that to the centerline of the pipe? Or is that 1.5 times the pipe OD?

 

[gravitate]

Is the bend radius 1.5 meters? Is that to the centerline of the pipe? Or is that 1.5 times the pipe OD?

Hi it is 1.5 th OD of the pipe.

 

[jehake12]

I don't have my copy of ASME B31.3 within arms reach, but there should be sections that correlate stress intensification factors (SIFs) and flexibility to the bend radius of pipe. It also shows up in B31.1 (Power Piping). A larger bend radius will result in a lower SIF and increased flexibility characteristic.

Typically 1x nominal pipe size is considered a "short radius" bend and 1.5x nominal pipe size is a "long radius" bend with regards to piping.

 

[jehake12]

I forgot to suggest that if determining whether or not 1.5xdia was acceptable is under your scope of work, you should really buy a copy of the standard that governs your project. I think ASME sells them for around $250-500.

 

[Q_Goest]

I think to answer your question, ASME B31.3 doesn't prohibit bending a pipe to any radius as far as I know. For a bend that tight however, the most common approach is to weld in an elbow. I'm not sure if austenitic stainless steel pipe can be bent to that radius or not without compromising the pipe, but I suspect it's possible. The 300 series material is very forgiving. But the pipe will thin on the point furthest from the center of curvature so you should consider derating the pressure accordingly.

 

[Vadar2012]

There is a simple way of calculating the maximum stresses on a bent pipe with a force applied to it, but I've never had much success with it. The mothod always gives me weird answers. In terms of Australian Standards, I haven't seen any maximum/minimum bend radii either. I just go with 4 times the pipe OD.

 

[gravitate]

Thats great. Well I can get a copy no problem through my company. How would I work out these values though?

 

[Vadar2012]

I went looking through my old documets and found one that specifies the allowed minimum radii for each pipe OD given the method used to bend it. According to this document, for a 47mm OD pipe, the minimum radius should be limited to 11.1mm and 17.5 for grooved bending tools. So 1.5*OD should be fine.
EDIT: Misread part.

 

[jehake12]

I went looking through my old documets and found one that specifies the allowed minimum radii for each pipe OD given the method used to bend it. According to this document, for a 47mm OD pipe, the minimum radius should be limited to 11.1mm and 17.5 for grooved bending tools. So 1.5mm seems kinda small.

1xDia = 47mm
1.5xDia=70.5mm

Where did you get 1.5mm from?

1.5mm/47mm=0.032

I don't think anyone is suggesting using 0.032xDia as a bend radius... Maybe we're looking at different dimensions.

 

[Vadar2012]

Yeah, I miss read it, my bad. I was in a hurry.

 

[Vadar2012]

I read it as 1.5mm, obviously it was 1.5xOD.

 

[gravitate]

I have a copy of asme b31.3 now so what info do I need?

 

[jehake12]

I'll paraphrase (ie not word for word) two sections from a good reference, Piping Handbook 7th Edition by Mohinder L. Nayyar. The section of the book is an attempt to provide an overview of B31.3 and serve as a basic guide to the design of process piping. Its a thick read, but worth owning as you will digest it section by section over the years.

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From Section C.320 Process Piping Systems: Pressure Design of Piping Componnents: Bends and Elbows:

With regards to pressure design of piping components (in process piping systems)... the minimum required wall thickness at a pipe bend after bending should be determined as for straight pipe. The bend radius is typically a minimum of 3 x nominal diameter, but bends as tight as 1.5 times the diameter are possible with appropriate heat treatment methods.

Elbows that are manufactured in accordance with any of the standards in Table 326.1 of B31.3 are suitable at their rated temperature and pressure. Any elbows made to nominal pipe thickness (like ASME B16.9) are allowed for use with pipe of the same nominal wall thickness.

From Section C.350: Fabrication, Assembly, and Erection of Process Plant Piping: Bending and Forming:

It is allowable to bend a pipe to any radius that will result in bend arc surfaces free of cracks and buckles, but recommended to have bend radii equal to at least 3 times the pipe diameter AND fabricated in accordance with ES-24, Pipe Bending Methods, Tolerances, Process and Material Requirements of the Pipe Fabrication Institute. Tighter bends approaching 1.5 x dia are possible but will probably need heat treatment (induction bending methods).
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In summary, you are permitted to bend the pipe to 1.5 x dia. If you don't have the means, you could buy elbows that require welding or bolting. The tighter radius comes with a price in the overall pipe design (stresses/flexibility/constructability). Check out the sections on design and fabrication for your fluid service in B31.3.