Hi

I used to consider below load cases in stress analysis:
a)
L1 W+P1+T1(OPE)
L2 W+P1+T2(OPE)
L3 W+P1(SUS)
L4 L1-L3(EXP)
L5 L2-L3(EXP)

And another file for lift-off (Hot sustained) condition. Hot sustained stress is within allowable stress.

But recently our client asked us to use below load cases:

b)
L1 W+T1+P1(OPE)
L2 W +T2+P1(OPE)
L3 W+P1(SUS)
L4 T1(EXP)
L5 T2 (EXP)
L6 L1-L3(EXP)
L7 L2-L3(EXP)
L8 L1-L4(SUS)
L9 L2-L5(SUS)

In the later load cases(b), hot sustained L.C.s stress ratio increased by 20-30% relative to Lift-Off file of first L.C.s(a) at some nodes. These nodes are not near lift-off support positions. I don't understand reason of increasing stresses. Please advise me. (It should be mentioned that friction and gap considered in both models.)

In b)L4 and b)L5 there is no friction because there is no weight. Therefore, everything travels farther and lifts off higher.

Therefore, in b)L8 and b)L9, the change in displacements of piping is higher than when you calculate it in a)L4 and a)L5.

Thank you Michael.
Now two questions rise up:
1-Which method is correct and complying with B31.3 code?
2-Is it correct to consider same approach about friction and gap for these methods?

Method a is more prevalent, and more realistic, except when friction estimates/assumptions are very wrong. Both are compliant, as I understand it. It is generally accepted that you should not take credit for uncontrolled friction to save your piping and equipment from overstress. Instead, it should be used for design loads on supports, or when friction will have deleterious effects to piping and equipment.

However, I am given to understand that both methods are code compliant; they just use different design philosophies.

use the third method, the one that now is default on Caesar. Alt SUS.

The second method is superseded by this method.

Using the second method will lead to inclusion of friction effects in SUS which is not quite so.

I guess this can be confirmed.