Temperature load

[beastgod]

Hello!

I have some questions about temperature in pressure vessel.
Suppose that I have pressure vessel which operation (or design - it doesn`t matter) loads only P, Ps, D and T.
to evaluate the vessel I use elastic-plastic analysys acc. ASME VIII div.2 part 5.
So I should use load combinations from table 5.5. They are:
1)2.4*(P+Ps+D)
2)2.1*(P+Ps+D+T)
3)2.1*(P+Ps+D)
*cases with P=0 we will not consider for simplification

Questions:
a) Can I don`t consider the third combination because 2.1<2.4 ?
b) Should I use material properties according temperature = T even there is no T in load case?
c) Why is T in only one load case although P can`t exist without T as a rule.

Thanks!

 

[TGS4]

a) of course, if you have no snow load, live load, or wind load, then of course Design Condition 3 would be a subset of Condition 1, and if condition 1 were acceptable, then condition 3 would be acceptable by inspection.

b) it would be best for you too better understand the relationship between temperature and thermal loads. All design conditions must be evaluated at the Design temperature, including material properties, such as Young's Modulus.

c) T is not temperature. See Table 5.2.

Just to confirm, you are also satisfying ALL of the other failure modes? Thermal loads typically become important for fatigue and ratcheting.

 

[beastgod]

c) I think the load T acts in a case when fixing the object does not allow to compensate temeperature free.
Thus, in cases where the load T is not involved, I still use temperature to find the temperature distribution to automatically take into account the desired properties of the material, and assign alpha=0 (linear expansion coefficient) to remove the load from the non-free fixing.

Is it right? Or could I overlook something?

And of course other failures are satisfying.
Thank you!

 

[TGS4]

Setting alpha=0 is definitely what I do for the design conditions without T.

 

[beastgod]

"c) Why is T in only one load case although P can`t exist without T as a rule.
c) T is not temperature. See Table 5.2."

Koefficients with loads only show us whether this load dominate in this case or not (or this load absent in this case at all) and probability its appearance.

So...let we have a pressure vessel with not free fixing. Then I will have thermal loads anyway. Probability= 100%. So I think we should consider load T in all cases (koeff I think should be the same as P and D have in appropriate cases).

For example, why in case (4), when W is considerable, thermal loads from not-free fixing don`t act?

I`m looking forward the answer.

 

[TGS4]

This discussion is starting to veer into the realm of justifying the load factors in the LRFD approach used for the EP in determining Protection Against Plastic Collapse. I would highly recommend that you read ASME PTB-1 and the referenced documents therein to better understand these concepts.

The easiest explanation that I can provide (and it is worth exactly what you paid for it) is that it is rare that thermal loads cause plastic collapse.