ASME B31.3 Flexibility analysis - Operating Conditions

[ENG2194]

Looking through B31.3 all the examples in APP.S seem to allow you to run the various load cases under the operating conditions and compare the results to allowable stresses at the operating temperature. However the over pressure protection section only requires you set your relief pressure at the design pressure, and there doesn't seem to be a section providing defining the value for your operating condition used in the flex analysis.

Theoretically i could have a system that standardly operates at 500 psi but could see spikes up to 1000 psi. I could then run my flex analysis at 500 psig with my design pressure and relief pressure set at 1000 psi. This would probably not turn out well if the system was at max stress due to the way its supported. I know it talks about occasional variation in pressure above the design pressure but not above the operating pressure.

This seems like an unsafe practice to be able to run stress calculations at a much lower pressure than the system could see. Am I missing any other protections from doing this in the code, or do they just expect common sense and good judgment from the design engineers?

 

[goutam_freelance]

A plant with 500 psi operating pressure and 1000 psi spike pressure is very rare. Even when it happens you need to calculate your design pressure
considering occasional spike also. Design pressure is always higher than operating pressure which sustains for a significant amount of time.

Regarding flexibility analysis, you need to take one additional run with sustained + occasional loads. The allowable stresses for this condition are specified. The stresses from the flexibility run should not exceed the allowable.

The thermal stress range is treated separately as it is known that thermal stresses relax due to localised yielding at various high stress points.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[KevinNZ]

Maybe the "operating conditions" should read the pressure and other conditions under consideration? For sustained case the pressure/temp will be design pressure/temp.

Normal operating pressure is used for EQ and wind cases.

 

[tiz99]

The system operates at 500 psi and has an upset condition of 1000 psi, so it still "operates" at 1000 psi, so this is the number to use in the analysis. Design Temperature = MAX(500,1000)