ASME B31.3displacement stress clarification 1

[Jimbo2010]

I’m in the process of designing a new pipe line using ASME B31.3 and just want some clarification on the analysis of displacement strains/stresses. The code states the following should be achieved:

SE<SA Paragraph 302.3.5

And that SE is calculated as follows:

SE=(Sb^2+4St^2)^1/2 Paragraph 319.4.4

Are the values for Sb and St the same as what are used in the calculation for Longitudinal stresses and how are Thermal Stresses accounted for in the calculation for SE? It was my original intention to simply use Thermal Strain multiplied by Youngs Modulus to calculate stresses in a straight run of pipe?

I also appreciate that as the analysis begins I may determine that other sources of displacement may need to be considered.

Any clarification would be greatly appreciated.

Jimbo


Jimbo

EngCo Solutions Ltd

http://www.engcosolutions.com

 

[StevenHPerry]

B31.3 Appendix S should set you on the correct path. Appendix P for even more info.

- Steve Perry

http://www.linkedin.com/in/stevenhperry

This post is designed to provide accurate and authoritative information in regard to the subject matter covered. It is offered with the understanding that the author is not engaged in rendering engineering or other professional service. If you need help, get help, and PAY FOR IT.

 

[BigInch]

B31.3 isn't a pipeline code. Is there some reason you're using 31.3 rather than 31.4 or 8?

 

[Jimbo2010]

Thanks for the input, but I still dont fully understand how thermal loads are accounted for in 319.4.4, maybe I'm missing somerthing here??

Biginch when I say pipeline I meant a pressure piping system.

Thanks for any further help.

Jimbo

Jimbo

EngCo Solutions Ltd

http://www.engcosolutions.com

 

[StevenHPerry]

S_E is the range of longitudinal stresses due to a/the displacement cycle, so yes the bending and torsional stress are the same as when calculating a longitudinal stress.

Thermal displacements are the most common / most significant and should be considered when calculating S_b & S_t.

- Steve Perry

http://www.linkedin.com/in/stevenhperry

This post is designed to provide accurate and authoritative information in regard to the subject matter covered. It is offered with the understanding that the author is not engaged in rendering engineering or other professional service. If you need help, get help, and PAY FOR IT.

 

[Jimbo2010]

Thanks once again I think I might have it this time

But surely the fact that they are defined as two separate stresses there must be some differences.

So if I purely have a straight run of pipe fixed between two Anchors, for the S_L calculation I would exclude thermal effect and purely look at pressure and bending stresses.

Is it intended that I account for the thermal effects ( not considered in S_L) in calculating S_E, predominantly stresses due to restrained thermal expansion and possibly additional bending stresses due to temp changes. S_L can be subtracted from the displacement stress range (1.25(S_h+S_c)-S_L) while calculating S_A (equation 1b). because S_L has already been calculated previously and not considered in S_E. So in summary can I say that S_E are additional stresses above those calculated for S_L encountered as the temperature changes???

Jimbo


Jimbo

EngCo Solutions Ltd

http://www.engcosolutions.com

 

[StevenHPerry]

I think I'm catching up to you. Are you worried about the axial forces due to restrained thermal expansion? Axial forces, generally, contribute much less to the stress range than does bending. You're right that the paragraph you mention doesn't consider axial forces. They're usually ignored. You can use Appendix P as an alternative if you would like to consider axial force in your stress calculations.

But... if you have a situation with high enough axial forces to create a stress problem, the loads on your anchors (pumps, vessels, etc) are probably more of a concern than the pipe stress.

For additional clarification:

S_L is the longitudinal stress only due to sustained loads, a/k/a "sustained stress." 302.3.5(c)

S_E is calculated from longitudinal stresses exclusive of the sustained stresses.

For a simple system at constant pressure and varying between ambient and a higher temperature with no anchor displacements, considering only the longitudinal stresses:

S_ambient = S_L
S_E = S_operating - S_ambient


- Steve Perry

http://www.linkedin.com/in/stevenhperry

This post is designed to provide accurate and authoritative information in regard to the subject matter covered. It is offered with the understanding that the author is not engaged in rendering engineering or other professional service. If you need help, get help, and PAY FOR IT.

 

[LSThill]

Reference to ASME B31.3 edution 2008?

 

[Jimbo2010]

My system will have some straight pipe runs fixed at each end that will encounter thermal expansion stresses. For example a 4m straight pipe, working from 20oC to 40oC and with an expansion coefficient of 13x10^-6 m/m K between fixed supports, I estimated a stress of approximately 218MPa.

I also have bends etc that will suffer from additional bending moments as thermal expansion takes place. I’m assuming that the thermal moments discussed refer to similar moments that are calculated in standard beam theory??


LSThill - yes thats the code I have.


Jimbo

EngCo Solutions Ltd

http://www.engcosolutions.com

 

[Jimbo2010]

Correction 54.6MPa

Jimbo

EngCo Solutions Ltd

http://www.engcosolutions.com