Temperatures for Stress Analysis to B31.3

[tote1399]

I have been trying to find in the piping design codes [ASME B31.3,B31.1] where its states the actual temperatures/pressures to be used in carrying out a stress analysis of a piping system. There appears to be 2 different schools of thought.
1. Use the design figures of actual piping materials quoted for the system.
2. Use the worst case operating temps/pressures that the pipe system is ever likely to see.
If you use the former surely the results (loads/stresses etc)will be unrealistic and lead to overdesign of supports and other equipment in the piping system.
In IGETD 12 it actually states 'Worst Case Operating Tempratures and pressures are to be used'

 

[DSB123]

tote1399,
Your piping system will have a Design Temperature and a Design Pressure associated with it. This is generally just above (or maybe the same) as the worst operating conditions. In any case the design of the system must be such that it can function under the Design Conditions. It may be that the Design Temperature and Design Pressure do not coincide in which case you need to consider the worst combination of "worst operating" conditions.

 

[tote1399]

Thanks DSB123
Obviously the system in question does have Design Temps and Pressures quoted of +75deg C and 45Barg. However, the worst case of coincident operating temp and pressure are +55deg C at 27barg.
If the design values were used in the analysis then the output would throw up inflated loads at restraints and stresses in the piping resulting in a possible code compliance failure - you would in fact be designing the system for a case which it will never see. This could culminate in excessive additional cost of manufacture to achieve a satisfactory design. There is also a paragraph in B31.3 which loosely states that "the most severe conditions of coincident pressure and temp shall be used to determine the design condition".
I have also had discussions with various stress engineers with differing views over which values should be used. Surely logic should prevail?

 

[NozzleTwister]

Don't confuse the requirements for design, "the most severe conditions of coincident pressure and temp shall be used to determine the design condition", as the conditions to be used for flexibility analysis. The design condition as defined above establishes the basis for flange rating and piping wall thicknesses and that could be at ambient temperature.

Flexibilty analysis should be performed at no less than the maximum or minimum temperature with thermal stresses being reviewed for the entire minimum to maximum temperature range. When using the max. or min. temp., use the max. pressure that occurs at that temp.

The design conditions for pressure design and the maximum/minimum line temperatures generally have some reasonable margin to account for operating excursions and upsets beyond the maximum anticipated conditions.

If your design conditions are excessive beyond reason, that's an internal problem in your organization that needs to be corrected and not an excuse to use normal operating temperatures.

I fight this same crap everyday and you will too. On my current project, I am bound by regulation to use no less than the Line List design values. Unfortunantly, Process has decided it was easier to list the Piping Specification limits as the Design Conditions rather than sharpen their pencils. They're sharpening their pencils now.

NozzleTwister
Houston, Texas