Why Use a Preflash Drum or Preflash Tower Upstream of a CDU?

If the crude feed to an atmospheric tower is heavy and has virtually no light ends, then we can avoid using the preflash tower or preflash drum. However if the crude has some light ends we normally have a preflash drum/tower and we only send the flashed liquid to the fired heater before it goes to the Crude Distillation Unit (CDU). In this instance we re-combine the flashed vapor to the heated liquid leaving the fired heater. Apparently we do this to avoid the high vapor pressure of the liquid entering the fired heater and also we reduce the size of heater. High vapor pressure at heater inlet complicates heater design. There are also other implications at the preheat and desalters which are located upstream of the CDU. If preflash is not used the operating pressure in the CDU may increase and also we will need higher temperature to achieve the desalting. Coupled with this is the requirement of much higher water for the desalters. So frequently we use the preflash drum/tower to avoid these problems. Some old CDUs have been modified to include these preflashes before the main atmospheric tower. My question is at what deg API of the crude should we consider preflash equipment? Also are there any other considerations which I have not mentioned here, that must also be considered when deciding if there is a need for a preflash drum/tower?