"eventhough i did't model the branch pipe simply i break the node and use weldolet connection SIF in that case it self it is failing"

Of course, it is the header that determine weldolet SIF, not the branch. (See Appendix D of B31.3)

In any case at 0.3 bar drag pressure, you can expect a lot of problems.

One solution is to increase the thickness of the line, but I think nobody want this.

The other is the one that PKU gives you i.e. to ignore the SIF for this particular case.

Regards,