Q & A > Question Details
Our furnace has 4 pass flow. Crude enters the furnace by 4" tube in the convection section. Then it changes its size by 5" X 4" reducer in the radiation section. It again changes its size outside the furnace and now this time by 8" X 5" reducer to a common header of 12" pipe line. This pipe line by a 16" X 12" reducer connected to the 16" pipe line that goes to column. My question is why we are using so many reducers in the process line?
16/12/2009 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
As has been stated, the increasing pipe diameters have to do with optimizing pressure drop. If your question has to do with the reducers themselves, there are two answers. The ones between the convection and radiant sections of individual tube passes are necessary. The ones into the intermediate collection header probably are not needed. It is a piping designers decision. They can be "stubbed in". To go from the 12" header to the 16" transfer line requires a "swage up" unless a dead section of 16" run is employed in the layout as part of the pipe support design, in which case the 12" can stub into the 16'' pipe.
16/12/2009 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
As the oil heats up in the furnace it begins to vaporize, which increases velocity. You need to gradually increase tube size in the heater very often to minimize pressure drop while avoiding fouling. If the tubes are too big the residence time can be too high in the front end of the furnace which can promote coking.
Outside the furnace as the oil passes through the transfer line and pressure drop occurs the oil continues to vaporize, which further increases the velocity. Expanding the size of the line minimizes the minimizes the delta P in the line, which allows a lower heater outlet temperature for a given flash zone T&P.
In atmospheric heater outlets you normally only see the line size increase where there are two or more streams coming together. The general rule of thumb when different flows are combining that the cross sectional area of the pipe for the combined flow should be greater than or equal to the sum of the pipe cross sectional areas for the lines feeding the junction. For a 4 pass heater with 5" tubes the minimum size of the outlet header would be 10" using this rule of thumb. In your case 12" sounds reasonable. I'm not sure why it goes to 16" after that.
In vacuum heaters the vaporization effects on velocity are magnified because of low pressure, so it's not unusual to see 3 or even 4 or 5 different tube sizes in the heater and multiple different sizes in the outlet line. I'm pretty sure I've seen at least 4 different piping sizes in outlet piping on an 8 pass vacuum heater. Proper design of the heater piping and transfer line piping is really critical on a vacuum heater to minimize coking, maximize flash zone temperature, and to avoid reaching sonic velocity in the transfer line.
16/12/2009 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
The number of swage ups has to do with ensuring appropriate two phase velocities and unit pressure drops [psi,delta per 100 feet linear coils plus psi,delta pressure drop going around U-bends] en route to the fundamental reference pressure [which is the downstream absolute pressure within the flash zone of the crude distillation tower].
The transfer line [extending from the coil outlet flanges of each separate pass exiting the crude heater] to the inlet diffuser device within the flash zone is active both from VLE thermodynamic perspective and hydraulic perspective.
For existing CDU+VDU units, performance benefits can frequently be identified and potentially implemented via a full hydraulic and computer simulation model-based VLE flash assessment should be conducted by specialists.