Lummus design Cokers having same problem i.e. flaring from Blowdown overhead receiver during vapor heating as well as water quenching. I believe there are three PVs in your B/D overhead receiver i.e. one to flare,second for FG make-up and third to Fractionator O/H system for recovery.
To avoide above problem you can try the following options which are drastically reduce your flare losses:
1- Check the line throughness of Blowdown overhead receiver to u/p of fractionator O/H condensers. Also check for any loop formation in that line which cause the water seal and restrict the flow towards Fractionator O/H system.
2- Increase the B/D pressure slightly higher say 0.7 Kg/cm2 than Fractionator O/H, it facilitate all gases goes to Fractionator O/H rather than flare and it does not affect your vapor heating as you have sufficient time for vapor heating.