Transfer Line Pressure Drop
A typical pressure profile (Figure 400-16) shows that pressure of the column feed decreases smoothly from the furnace exit through the transfer line to the column flash zone, unless the critical velocity (maximum velocity, discussed below) is reached. Critical velocity can be reached at the exit of the last furnace tubes (entrance to the transfer line), at the exit of a particular section of the transfer line, and/or at the exit of the transfer line (entrance to the column).
The maximum allowable pressure drop in the transfer line should be chosen such that the required flash zone conditions will be obtained without excessively high furnace outlet temperatures.
In most instances, the maximum allowable pressure drop should be limited to 15 psi through an atmospheric transfer line and 10 psi through a vacuum transfer line.