The rule of thumb is 1 to 2 wt% velocity steam but more is better. The problem with more is better is the high velocities produced in the coke drum and fractionator plus the sour water produced.
Crossover temperatures are typically between 365C to 377C. At these low temperatures there is no significant thermal cracking/polymerization so the velocities are not critical in the convection section but adding the steam to the crossover does not buy you much in pressure drop – usually.
If the feed has high inorganic solids (greater than .2wt%) there can be significant fouling in the convection section and upper radiant section. With solids more steam in the convection section might not be better.
