Heat Exchanger U-bend Under External Pressure 1

[pvwander]

Hi All,

I am just new in this forum and I have these questions;

Currently, I am calculating the max. span of U-tube under external pressure (shell side press.)

1. Do I need to add the U-bend length to the straight length for the computation of L/Do?

2. Can we consider the U-bend as stiffener?

Thanks for your help.

 

[sjrfc2]

I don't think you'd need to include the U-bend length. The U-bend provides inherent stiffening and the adjacent straight legs will buckle from external pressure first. There is a book on pressure vessel design by Harvey that shows the results of a U-bend and adjacent legs of piping subjected to external pressure. The legs become completely flattened by external pressure while the U-bend remains intact.

 

[SnTMan]

pvwander, what sjrfc2 says is true, however the approach used by TEMA is to treat it as a straight tube of length equal to the bend length plus the two straight lengths from the baffle or support plate. See RCB 2.31 if TEMA applies. Way conservative, but...

ASME Section VIII, Div. 1 does not address the calculation of external pressue for toroidal type shapes so far as I know, therefore the TEMA approach.

Regards,

Mike

 

[MJCronin]

sjrf....

With regard to your comments about the bend...

The bend also has a certain degree of work-hardening on the tube material which makes it more resistant to collapse than the adjacent (probably annealed) straight tube.

The Harvey book is a classic......

http://www.amazon.com/Theory-Design-Pressure-Vessels-Harvey/dp/0442232489

-MJC

 

[pvwander]

Thank you for your responses guys about my question. sorry my reply is a bit late.

I have checked the book of Harvey "Theory and design of Pressure Vessel" and I saw the photo of flatten cylinder legs while its U-bend was still intact. But the book still has not mentioned about the reason why we shouldn't include the U-bend length in calculating the L/Do under external press.

By the way, here is the details of the U-tube I am calculating;

Tube OD = 25.4 mm
Tube Th'k = 2.11 mm
Ext Press. = 90 KG/cm^2
Material = SA-179

Can you help me calculate the maximum span(length) for such given thickness? case 1: without U-bend, case 2: including U-bend.

Thanks again,
-pvwander

 

[pvwander]

I forgot to include the design temperature, it will be evaluated at 400 deg.C

-pvwander

 

[SnTMan]

pvwander, I now believe I mis-understood your original question. For calculating external pressure of a U-bend tube, it is common to use the straight length from bend tangent to the oppposite end.

For calculating external pressue of the U-bend itself, the length is commonly taken as the sum of each straight leg from tangent to baffle or support plate, plus the length of the bend along the centerline.

External pressure is calcuated for the U-bend to check that the wall is sufficient after thinning due to bending, although I believe this method, treating is as a straight tube, is super conservative.

The inner rows thin the most, but have the shortest length, it is usually necessary to check several rows.

Regards,

Mike