2 replies to this topic

[rodolfoingeniero]

Hi Everybody,

I’m here again asking for your help.
This time the matter is related to the drop pressure inside the transfer line (i.e. the line which interfaces the vacuum heater and vacuum column).
According to my experience transfer line length must always be as short as possible.
At the company I work for, vacuum heater is located close enough to vacuum column so the length of the horizontal transfer line section is 15 meters while its size is 60”.
I want to know your opinion regarding the drop pressure along this transfer line, given that the pressure at the initial point is 208 mm Hg (close to vacuum heater outlet) while this line enters at the flash zone of the vacuum column where the pressure is 45 mm Hg.

I’ve already made a simulation of this situation but due to the flow is two phases (vapor fraction is 22%), I’m not sure of the results I’ve got.
The simulation reaches a very low drop pressure, would it be right?

Thanks in advance
Best regards.

[Andrei]

rodolfoingeniero,

You mention a 22% vapor fraction in your line. Where is that?
In the vacuum tower transfer line vapor fraction varies drastically from heater outlet to tower inlet due to liquid flashing. And there is a significant pressure drop inside the tower across the distributor; usually some special distributer is used there.
The frictional pressure drop is not high; most of the actual observed pressure drop is because of the volume expansion due to flashing.
So, in a simulation it is not important only the hydraulic equation used to solve the line (able to handle two phase), but also the thermodynamics, to accurately describe the flashing phenomenon along the line.
Another factor in obtaining good simulation results is accurately description of the actual layout of the line. Don’t expect to get good results when you input one pipe segment, an elbow and/or an expansion can be very important. Every geometry aspect has to be input accurately.

[djack77494]

Design of a vacuum transfer line and calculation of the system's hydraulics is tricky and critical. As Andrei said, much changes from the transfer line inlet (at the heater) to its outlet (in the column). The pressure changes are a very large fraction of the total absolute pressure, and all aspects of a vacuum column design are impacted by what happens in the transfer line. Do not attempt to generate a simulation of this critical part of the system's design "from scratch". Either find a previous model that has been verified or thoroughly research the topic or hire a consultant. Good luck.