The rate of vapor heating a coke drum is controlled by the pressure balance of the system. To force more flow into the heating drum, you have to change the pressure balance in favor of the flow that direction. For instance you could….
– pinch overhead vapor valve
– open the condensate valve fully
– line up the condensate to a low pressure system (blowdown tower over the frac or condensate pot)
– other options may exist depending on your configuration.
In continuatuon to what Evan mentioned, one can also try maintaing low level of coke condensate drum and blowdown tower during vapour heating so that vapour flow is not restricted due to high liquid levels in these vessels.