Vertical thermosiphon hydraulic calculation

[jotake]

Hi all,

First of all, sorry for my english. It is my first time posting in eng-tips and I am not sure how to explain my problem/doubt.

We are performing the hydraulic calculation of a vertical thermosiphon of a deethanizer column in order to establish the relative elevation of the thermosiphon from the bottom tangent line of the column. The elevation of the column has been previously fixed according to NPSH requierements of the bottom liquid pump.

The thermosiphon has piping in both the liquid inlet and the two-phase flow outlet (with this I mean that the thermosiphon outlet nozzle is not directly connected with the column nozzle).

The bottom of the column has a 4 meter vertical baffle in order to divide the bottom in two chambers: one for the thermosiphon feed and the other for the bottom liquid pump feed.

My main doubt is which liquid level to consider in the column bottom in order to perform the hydraulic calculation:

1st Option: considering the liquid level in the top of the baffle of the column bottom. In my opinion this is the most realistic option as in the normal operation this will be the available liquid level.

1st option problem: if due to any cause the level of the liquid decreases the available static head will also decrease, not allowing the desired circulation (less flow).

2nd Option: the most conservative option is to consider the liquid level in the bottom tangent line (less static head available for the calculation).

2nd option problem: as the normal liquid level will be always higher than the bottom tangent line, the available static head will be higher, leading to more flow circulation (less vaporization in thermosiphon as the heat input in it, is constant).

I do not know which is the most common way of performing the hydraulic calculation of these systems so I will be very pleased if anyone could help me with this issue.

I have considered installing a butterfly/globe valve in the liquid inlet pipe to the thermosiphon in order to have a controlling element of the flow (specially if the thermosiphon has been designed following the second option). I do not know if this is a common practice for these systems. Possible flashing in this valve may be possible problem of this solution?

Thank you very much in advance for your help and sorry once again for my english.

 

[EmmanuelTop]

You should base your hydraulic calculations on future normal operating level in the tower - since this will be the true governing case.

Sudden increase or loss of liquid level during operation inside the tower should not be considered as normal operating scenario and hence you can eliminate these.

The best way to control heat input is to install flow control valve on hot medium line. Sometimes, I encountered this valve being accompanied with an ordinary gate valve on reboiler return line. This is a safety margin and it allows you to pinch the circulation rate in case of excessive heat input.

Regards,

http://antwrp.gsfc.nasa.gov/apod/astropix.html