A common hydrocarbon vapor line velocity guideline which sets the line size in a new design is based on pressure drop during normal conditions. The guidance says that the pressure drop should be less than 0.1 psi per 100 equivalent feet of line. A typical maximum velocity in ft/sec is 100 divided by the square root of the vapor density in lb/ft3.
The overhead vapor line should be insulated and sufficient HCGO added as quench to lower the vapor temperature down to less than 800 F. The HCGO quench should wet the walls all the way to the fractionator. If the walls of the pipe dry out, coke will begin to form and restrict the overhead line.
Some refiners remove the insulation on the coke drum overhead line as the heat loss keeps the line wet and prevents coking of the overhead line. The heat loss changes day to night and summer to winter or during a rain storm. This excessive heat loss causes excess amount of recycle and will increase coke yield. If you are operating the Coker to maximize liquid products, the recycle should be closely controlled which will require the overhead vapor line to be insulated and enough quench added to keep the line clear.