Ideally, there should be flow controller / control valve and FSLL on each pass. Second preference is balancing valve with flow indicator and FSLL on each pass, with balancing valve adjustment to be done during first startup. Do you have a PID for this furnace showing passes? What type of fractionator is this? - brief intro required.
Diagram is not clear, but I can see only 2 control valves which appear to be on flow control. What about the other 6 ?
In any case, either you adjust pass flow by FIC or by manual balancing valve. Keep flow at above low flow limit for each pass. Startup flow should be equivalent to min firing rate at this furnace. See ops manual for the furnace for min permissible flow and min firing rate for startup. Ramp up rate for firing should be within permissible ramp rate for product leaving the furnace; it may be 50degC per hour or similar.
Presume the furnace pass flow controllers are FC 0811 to FC 0818. Check the ops manual for min permissible total flow to this furnace. Looks like there is some flow ratio control incorporated, so flow to each pass is automatically set to be the same.
