Hi Anubis512

thanks for your reply, I think you perhaps did not read my question carefully; I wonder why the "allowable limits" of those two cases are different.
my system works in 100/-46 C with a material which has Sh of 207 MPa at both aforementioned temperatures so, when I calculate the Sa as per 302.3.5 of B31.3:

Sa= f(1.25x207+0.25x207) (f=1) so Sa=310 MPa

this limit should be considered for all cases including upper temp. to lower temp. range case and (Ope-Sus) case but, CII gives me a different value.