When I model bend (type-single flange)..Does the weight of flange is included in the model? or do i need to model it separately?..thanks in advance

You should really do a small test run to answer this type of question for yourself.

Selecting "single" or "double" flanged bends only changes the SIF and k-factor as per Appendix D (of B31.1/B31.3). The weight of the system is not affected.