Hi,
I am doing analysis of an existing line subject to slug flow. Client has requested us to do fatigue analysis using B31.3.
I am aware that B31.3 fatigue is applicable for thermal cycle. please advice how to consider this for slug cycles. we have slug frequency from which we can calculate number of cycles .
please advice if stress reduction factor can be used for slug loading (occasional load).

How are you using the term "stress reduction factor"?
I believe any sort of cycling stress not matter the source, can contribute to fatigue failure. B31.3 obviously points to the major source of such a stress.
You may want to use the welded joint design fatigue curves found in ASME Section VIII Division 2 Annex 3-F. See equation (3-F.4).