Q & A > Question Details
I would like to know if when we design a transfer line of CDU or VDU heater then do we consider erosional velocity as a constraint? The mixed phase velocities in transfer line are frequently higher than calculated erosional velocity (from API-14E).
26/05/2015 A: Rajkumar Chate, Sulzer, rajkumar.chate@sulzer.com
Rather than velocity in transfer line pressure drop over the transfer line is much important, higher velocity and longer transfer line lenght will lead to higher pressure drop and hence drop in temperature from furnace outlet to column flash zone (it can be from 1 -20°C or even higher degree drop in temperature). This drop in temperature will reduce the yield of AGO from CDU or HGO from VDU.
Not only the pressure drop but velocity in transfer lines are also important, higher velocity in transfer line may create vibration in transfer line and may lead to some mechanical damage in transfer line or even to first few trays in wash section of CDU or wash bed/ wash bed chimney tray of VDU. Another possibility becasue of higher velocity in transfer line can be a higher entrainment from feed to the wash section and which may pollute the gas oil. Generally in VDU wash oil to wash bed is adjusted (if there is external control on wash oil, at most of the modern columns design generally have it to keep the wash bed clean)to meet the HVGO spec, incase HVGO is on spec and there is too much overflash rate because of entrainment ( measured overflash rate will be actual overflash + entrainment) then operator will draw more HVGO and wash bed may get dry and it may start coking.
Considering core process point, pressure drop, momentum and two phase velocity in transfer line are important design criteria to get the higher yield and to have a smooth operation.

It should be noted that transfer line design is verz important, even very well designed tower with proper internals but undersized/unproper design of transfer line may mess up the column.

Erosional velocity does not play much role in process parameter but it has a impact on metallergy, and this velocity depends on lot of things like corrosive or non corrosive system, presence of solid particles etc.
Generall rule of thumb for selection of transrer line size is to keep the velocity in transfer line <80% of the critical velocity. If the transfer line is much longer then the velocity across the length will be different becasue of different vapor and liquid fraction, velocity shall be calculated by dividing the total length in to several sections and estimating the properties across that section.

03/04/2015 A: Allada Veerabhadra Srinivasu, HPCL, vbsallada@hpcl.in
Both atmos and vac transfer line velocity plays vital role in design of transfer lines. Certain crudes mostly low sulfur crudes which have more diesel yield will have more than 20 % diesel carry over in RCO. Processing of such crudes have appreciable effect on transfer line velocity. Then, C-factor limit to be followed as per API guide lines and feed rate to be adjusted according for the given transfer line size. This will leak to erosion (if exceed acoustic limit), and leaks of transfer lines
28/03/2015 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
The atmospheric heater transfer line should be designed for a reasonable pressure drop. For the vacuum heater transfer line, at least 3 methods are used, all of which involve calculating the acoustical velocity, not an erosional velocity.
27/03/2015 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
I am not specifically familiar with API -14E, but normally these services would not be considered erosional. It is not unusual for transfer line velocities in vacuum units to be well over 100m/s. Unless naphthenic acid corrosion is a concern, velocity is not normally a major concern in these services other than the obvious impact on pressure drop, which impacts the achievable cut point. There can also be vibration problems in these lines. I once saw a crude unit transfer line moving back and forth several inches, which certainly was linked to the velocity and flow regime in the line.