ASME allowable stress without pressure?

[howls88]

I am working on a calculation that someone else started and I will be cleaning up and finishing.

The calculation is to determine general pipe support spans for pipes of a certain material, maximum temperature of 110F, and 1" through 2" in diameter. He used a statement in the calc that the allowable axial stress is half ASME B31.3 code stress allowable. The reason this is being done is to not limit the calculation for certain pressures.

Is this a viable statement?

If so, I need to work it out in the calculation or have documentation that agrees with the above. Suggestions on this would be greatly appreciated.

If not, is there a way to keep the pressure out of the calculation and determine the pipe spans as described above while keeping the calculation conservative?

I was shown a section that I believe was located in ASME Section VIII (can't remember if Div 1 or 2) that had a couple of equations for the maximum allowable stress due to pressure and maybe bending. I cannot find that section again, and I don't remember if being the correct section to help me with this issue. Could this be of any help?

Thank you,

Geoff

 

[JStephen]

If you have a closed-end cylinder that is pressurized, the longitudinal stress will be approximately half the hoop stress, which normally controls the design. Sounds like the person was just using the other 50% for bending. In vessel work, the joint efficiency is also varied to reflect this, and so the details would vary. The stress equations you mention are near the front of Div. I, although I'm not sure if the pipe codes use the same equations.

 

[DSB123]

howls88,
This is a "double posting" of your question. You highjacked a posting on the "Pipelines,Piping and Fluid Mechanics Engineering" forum but received answers yet you have now listed the same question here. Do you not beleive the responses you received? The basis of the 50% of the Sh value is as JStephen says - the hoop stress limit is Sh and since the longitudinal pressure stress is 50% of this value then the longitudinal Pressure stress could be 0.5Sh. Since the piping codes allow the longitudinal stress to be a maximum of Sh then this then limits the longitudinal stress due to other loadings to be 0.5Sh. Notice I said longitudinal stress due to other loadings - not just bending stresses!!!!

 

[bhushan76]

Equations for internal pressure desgin are slightly different in B31.3 as compared to Sec Viii Div 1. For external pressure design B31.3 refers back to Sec Viii Div 1.

Piping components are out of scope Sec Viii Div 1 , see U-1(c)2(e)

 

[DSB123]

bhushan76,
Exactly where is External Pressure mentioned in the original posting so why bring external pressure into the discussion!!

 

[howls88]

I do believe the other responses, it is more that I wanted to stop my hijacking and start a new thread dedicated to this topic.

I was also hoping to get more information on how to prove that our statements are true or at least conservative.

It has been a short time since I posted the tread, but I think I will include a pressure limit along with a temperature limit that the support spacing will be good for.

Thank you for your assistance and input.