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I am working as a superintendent in CDU/VDU unit.
We experience increase in the VDU off gasses due to cracking. We inject the Turbulising steam in the inlet and also before exit of the Vacuum heater pass flows. We never adjust the steam flows to see the reduction of cracking. We set high flow in the first tube pass and low flow in the last before tube of the exit.
Please share your operational experience in reduction of cracked off gasses from the VDU unit.
 
Answers
08/01/2010 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
As others have mentioned, formation of cracked gases is a function of feedstock and operating conditions (velocity steam and temperature). In addition to these parameters, design and operation of the atmospheric unit can effect cracking. Running a high atmospheric bottoms temperature, especially when combined with a long residence time in the bottom of the tower, can serve as a high temperature heat soak, which promotes cracking.
24/11/2009 A: keith bowers, B and B Consulting, kebowers47@gmail.com
I agree completely with the answer provided by Mr Ragsdale. In addition a Question: Have you determined the composition of the 'cracked gasses?' Perhaps an air leak has developed somewhere in the system.
24/11/2009 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Test runs by varying steam rates and furnace outlet temperature can result in a shortened run, unfortunately, as you know, because coke deposits remain until removed. You mentioned deep cut operation in your previous question. You are aware, I assume, that maximum furnace outlet temperature without cracking, regardless of steam rates, varies crude by crude, and cannot be determined accurately in the laboratory. We learned, for example, that Bachaquero crude must run 20 to 25 degrees F lower than Arabian crudes at the vacuum furnace outlet. Hopefully, someone running the same crude as you will compare notes on steam rates and furnace outlet temperature.