High pressure stainlees steel tubing Maximum Working Pressure Vs. Bend radius 1

[themad1]

Hello everyone!

We are facing a new challenge in our company.
We are manufacturer of high pressure equipments from 15Kpsi to 60Kpsi included valves and fittings.

We also deal with high pressure tubes from 1/4" to 1" o.d. 316 high strenght material (usually 100.000PSI 700N/mm2 Rp 0.2 minimum)

One of our customer is asking us to provide him with a chart showing the maximum allowable working pressure as a function of the bend radius of tubes.
As a matter of fact, tubes are often cold bended to be installed. This bending cause an impoverishment of the mechanical features includin the yield value in fuction of the size of the bending. So MAWP decreases.

Diameters, thikness, material, benda radius and mechanical features are known.
Tubing thikness is calculated according to ASME B31.1

How can we calculate the MAWP?

Thanks

 

[LittleInch]

As per the re-arranged formula in section 104.1.2

P = (t x (SSE+Py)) / Do See B 31.1 for the meaning of the various parts.

I've never seen this done before, but it will reduce it compared to straight tubing.

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

 

[bimr]

The tube wall is stretching on the outside radius and compressing on the inner radius of the bend. This causes thinning on the outer radius and thickening on the inner radius of the bend. Because of the thinning, there may be a decrease in working pressure at the bend. However, because of cold working and the resulting toroidal shape, the working pressure may actually increase at the bend.

Wall thinning caused by bending of tubing must be accounted for when performing MAWP calculations, as defined in ASME B31.3 Chapter II, Paragraph 304.2.1 the minimum wall thickness on the extrados of the bend can be thinner than the minimum wall thickness required for a straight of the same length (section 304.1.2), conversely on the introdos the wall is required to be greater than that of the straight for the same design pressure..

Bend minimum wall thickness by calculation 3c; using 3d and 3e for intrados and extrados.

Also look at:

http://www.eng-tips.com/viewthread.cfm?qid=383484

http://www.eng-tips.com/viewthread.cfm?qid=115925

http://www.eng-tips.com/viewthread.cfm?qid=25004

http://r.search.yahoo.com/_ylt=A0LE...Ec2VjA3Ny/RV=2/RE=1437604054/RO=10/RU=http://

http://www.kasa.com.au/AS4041%20ASM...ness.xls/RK=0/RS=FMYlLvNpeD.1COOk.sqHjzWAEb8-

 

[themad1]

Ok, now I start to understand!

But my problem now is that I'm working with pipes MAWP from 10Kpsi to 60Kpsi and a t>D/6
If I got right I shall use ASME B31.3 chapter IX par. K304.1.2 for wall thikness calculation (or MAWP)
Is Paragraph 304.2.1 still applicable? No longer I think...

 

[racookpe1978]

So, bend a few pipes (at the customer's expense) and take several slices of the bend cross-sections. You need to look for out-of-roundness distortion, as well as the wall thickness changes mentioned above.

On three others, bend the pipes, hydrostatically test them to failure (yielding); then measure the failure pressure in terms of actual (original) wall thickness. Note! Calculate failure based on actual ORIGINAL wall thickness, not nominal wall thickness.

 

[israelkk]

As far as I understand, the bent part of the pipe is actually a part of the torus. The stresses in a torus are half the stresses in a straight cylinder (same as a ball shaped pressure vessel need half the wall thickness compared to same diameter cylindrical pressure vessel). Therefore I believe the bent part is even stronger than the straight parts of the pipe, as long as there are no stress raiser as cracks, nicks, etc.

 

[racookpe1978]

Remember though, that torus pressure rating calc is derived from a "theoretically perfect" comparison of perfectly round pipe to perfectly round and continuously-thick-walled torus.

I'm trying to get the OP to measure the actual distortions in his actual bent pipe to see if that assumption is valid.