thermal stress due to temperature change? 1

[YuJie_PV]

Hi all,

it's known that thermal stresses are stresses that result when a temperature change of the material. there is a general practice in this industry that people just consider the thermal stress resulting from thermal gradient.
we never consider thermal stress due to material elevated to operating temperature since my career?
but why? too small to be evaluated by FEA?

what's your thought?

 

[LittleInch]

I'm afraid this makes no sense to me.

"This industry"? Which industry?

"Thermal gradient"? Where? No diagram, no description nothing.

"We never..." who is "we"?

In my industry (pipelines and piping) we do consider it.

Mt thought is that this is a very vague question with no details so won't get you very far.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[davefitz]

Simply operating at a steady elevated temperature implies creep effects be considered above 750 F, but the thermal stress does not occur except in 2 cases: a ) the rate of change of temperature causes thermal stress during fast startups and shutdowns and b) if connected to another piping component ( including welds) that has a different coeficient of thermal expansion.

Welding two components together with different thermal expansion coeficients is treated in ASME sect VIII div 2, in the section detailing design of cladding the interior of a pressure vessel, but I think the same theory applies to thin dissimilar metal welds. The thermal strain would be proportional to Delta_T * Delta_a = thermal_strain, where delta_T= [max operating temperature - assembly temperature] ( usually room temp) , and delta_a= difference in the 2 materials coeficient of thermal expansion.

The thermal stress generated during fast startups and shutdowns is treated in the EU boiler design code ( for fatigue analysis) EN 12952-3 annex C and is of most importnace for those thick components greater than 4 inches thick ( > 10 cm thick) .

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[YuJie_PV]

@littleinch, sorry for the confusion, let me clarify:
all my question is limited to the scope of pressure vessel constructed to ASME VIII-1 or VIII-2.
the question come into my head when i was studying thermal stress in ASME VIII-2， which is defined as follows:
"thermal stress: A self-balancing stress produced by a non-uniform distribution of temperature or by differing thermal
coefficients of expansion."
does the definition imply, in case there is neither thermal gradient throughout the equipment, nor existing differing thermal coefficients of expansion, there is no thermal stress?
i know UG-20 of ASME VIII-1 mandates to consider "differential thermal
expansion", while in my experience, it's common practice that mechanical engineer don't consider thermal stress due to temperature elevated to operation condition.
it's my thought on the state of the profession, i just want to know something underlying.
i hope i have clarify my inquiry,
thanks in advance.

 

[DriveMeNuts]

All of the above.
Suddenly filling a cold vessel with a hot gas will cause the inlet nozzle to get hot faster than the surrounding shell, and therefore expand into the cold vessel's shell, causing potentially large thermal stresses around the nozzle, which can cause fatigue crack growth. This is and example that requires a Transient thermal FEM, or simply line the nozzle with a thermal sleeve.
Vessels expand in length when heated, meanwhile much of the connected pipework prefers to remain where it is. This results in stresses local to the connecting nozzles.