Minimum bending radius for a tube

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Discussion Overview

The discussion revolves around the calculation and practical considerations for the minimum bending radius of a stainless steel tube, specifically in the context of creating a U-shaped bend for high-pressure hydrogen applications. Participants explore various methods, materials, and challenges associated with bending the tube.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • Some participants suggest a standard minimum bend radius of 4 times the outer diameter (OD) of the tube, while others mention specific ratios like 3:1 for certain tube sizes.
  • One participant notes that bending a 7/32 OD tube into a .41 diameter curve for a 180-degree bend may be too tight, leading to kinking issues.
  • Concerns are raised about hoop stress during bending, with suggestions to consider wall thickness and pressure ratings.
  • Participants discuss alternative methods, such as using fittings, mitre joints, or creating a mandrel to assist with bending.
  • Some propose filling the tube with low-temperature solder or sand to prevent kinking during the bending process.
  • There is mention of the importance of adhering to piping codes, although one participant humorously claims they create their own code.
  • One participant expresses uncertainty about the strength implications of various bending methods and suggests machining or casting the bend as an alternative.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best method for bending the tube, as multiple competing views and approaches are presented. The discussion remains unresolved regarding the optimal bending technique and the associated risks.

Contextual Notes

Limitations include uncertainty about the wall thickness of the tube, the specific pressure conditions, and the effectiveness of the proposed bending methods. The discussion reflects a range of practical experiences and theoretical considerations without definitive conclusions.

Who May Find This Useful

This discussion may be of interest to engineers and technicians involved in fluid dynamics, piping design, or those working with stainless steel tubing in high-pressure applications.

xJuggleboy
How do you calculate the minimum bend radious for a tube?

Its a SS tube and we want to bend as small a radius as possible...
 
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The standard I always follow is 4 times the OD. I'll see if I can't dig up a relationship that is a bit more accurate to include wall thickness.

BTW...what size tube? Is there any reason you can't use a 90° fitting?
 
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I have 3 Swagelok tube benders under my desk. The 1/4" and 1/2" are both the standard 3:1 radius, and the 3/8" is a standard also with a bend radius of just a bit more than 3:1. I believe the Parker ones are the same.

If you can't use an elbow as Fred suggests, there are weld fittings, or you might even consider making it a mitre joint, though that's very uncommon and generally not good practice.
 
The Tube is a 7/32 OD I am not sure of the Wall thickness but the bend we need to make is about a .41 diamiter curve and it has to go 180Deg.

Bassiacly a U shape. the .41 is the id of the U.

But so far what I hear is the bend is way to small for the tube :cry:
 
Why not just make up a mandrel and try it? You don't have much to loose. I'd be a bit concerned about hoop stress because when you bend a tube, the ID will thicken but the OD will thin. If you're trying to meet a piping code, and especially if this is a mass produced item, it would be wise to apply minimum wall thickness to the thinned OD of the tube. That said, what pressure are you going to, what material is the tube made from and what are you flowing through it?
 
Questions:

First of all...7/32? Couldn't you make this easy and make the tube Ø.250?
What's the set up like? Where is this tube going? If you can give us a general idea of what the plumbing is doing we may be able to find an alternate method.
 
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FredGarvin said:
we may bea ble to find an alternate method.
If it's a low-pressure, non-reactive environment, plastic might be easier to use... but it seems unlikely if the first choice was stainless.
 
Q_Goest said:
I have 3 Swagelok tube benders under my desk. The 1/4" and 1/2" are both the standard 3:1 radius, and the 3/8" is a standard also with a bend radius of just a bit more than 3:1. I believe the Parker ones are the same.

If you can't use an elbow as Fred suggests, there are weld fittings, or you might even consider making it a mitre joint, though that's very uncommon and generally not good practice.

What about taking a look at the Crane paper?? :smile: :smile: :smile:
 
Hardee-har-har. Go back to your tube now.
 
  • #10
Well to answer some of the questions...

The size and material of the tubs can not be changed... Its for high pressure hydrogen.

I have been trying it. I made a few mandrels and the tube always kinks. Even with Annelling it.

Im pritty positive it can't be bent as tight as I am trying... I still have a few more things to try.

Now I am trying to fill the tube with a low temp solder before I bend it. Ill see how that works out...
 
  • #11
What about taking a look at the Crane paper??
LOL Maybe we need to perform an FEA analysis on it . . . <not>

Now I am trying to fill the tube with a low temp solder before I bend it. Ill see how that works out...
I've also heard of people packing sand inside. That might be a bit more practical. But if this is for high pressure hydrogen, I'd suggest doing some burst testing (hydrostatic) to ensure you still have the proper strength after forming. Are you familiar with ASME B31.3? I'd assume that was the right code if you're in the US. There are some others depending on what kind of system it is. If you're in Europe, they have an equivalent code for pressure rating.
 
  • #12
I tryed sand first... It didnt turn out so well...

And as far as coad goes... We make up our own coad! We don't need no stinking ASME! =-P
 
  • #13
I think you may be stuck having to use some fittings and doing a little rerouting. You could possibly use a small manifold...
 
  • #14
FredGarvin said:
I think you may be stuck having to use some fittings and doing a little rerouting. You could possibly use a small manifold...


I keep trying to tell the head engener that
 
  • #15
I have no idea what strength issues this will insult, but what about either machining or casting the bend and then welding it to the rest of the tube?
 
  • #16
Danger said:
I have no idea what strength issues this will insult, but what about either machining or casting the bend and then welding it to the rest of the tube?


That would be a nice thing... But time and money won't allow for that =-(




I would like to thank you all for your input :smile: I think we have come up with a solution. We are filling the tube with a low tep solder then bending it to a slightly larger diamiter. It seems to be working! :approve:
 
  • #17
xJuggleboy said:
It seems to be working! :approve:
Please keep us posted regarding your progress. Happy hunting.
 
  • #18
Well after my 2 week vacation... I decided that the best thing to do was bending the tube after filling it with solder.

The bends came out pritty good. It is just a pain in the ass to make. and I need to make a bunch of them. Thanks fer the help everyone! =-)
 
  • #19
Hopefully you can get a small assembly line set up to ease the pain a bit. Glad it's working out.
 

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