Hello xinquan,
nozzle stress concentration factor is set as 2.25 as a default value is under PD 5500.
if you chose calculation method under the nozzle local stress analysis input as PD-5500 then the window of nozzle stress concentration factor becomes active.
it is a factor for increasing the allowable stress for membrane+bending stresses.
Hope this helps.
Regards
asd23789