The statement indicates to use Torsion in sustained stress when the temperature for the material is in the creep range.
For the temperature you can refer ASME Sec.III Sub.NH App.T Table T-1323.
As per this table, the allowable stress values becomes time dependent above a particular temperature which could be called as the temperature when creep initiates....
For 21/4Cr-1Mo ---- TEMP = 427 deg.C
SS304 ---- TEMP = 509 deg.C
SS316 ---- TEMP = 544 deg.C
Alloy 800H ---- TEMP = 573 deg.C
These temperatures are indicative...
OR
If you see the allowables indicated in B31.3 App.A, you will find a sudden drop in allowable stress above certain temperatures. This is the indication of Creep....hence that can be used as temp above which you can use Torsion in SL stress.
Also read B31.3 Code case-178, which indicates to use Torsion in SL Stress but it has expired on May 1, 2008.
Regards,
Suraj
_________________________
SKK