Startup Valve trays

Column Bottom Temperature. The bottom temperature is often controlled on the reboiler outlet line with a control valve in the heating medium line. The control point can also be on a bottom section tray. Care must be exercised in location of the temperature control point. It is recommended, especially for large columns, that a cascade arrangement be used. The recommended scheme has a complete flow recorder/controller (FRC) in the heating medium line including orifice and control valve. The set point of this FRC is manipulated by the temperature recorder/controller (TRC). This eliminates the TRC from manipulating the control valve directly (recall that temperature is the most difficult parameter to control). This makes for smoother control for normal operations. Also, it is handy for startup to be able to uncouple the TRC and run the reboiler on FRC for a period. 

A once-throu thermosiphon reboiler could not be started up because tray weeping at startup starved the reboiler of liquid. A valved dump line cotmect-ing rdxriler liquid and bottom sump was added and solved problem. 

To shut down a column or train, it is only necessary to switch the master switch to shutdown this shuts oflF the steam, feed, and drawoflf valves. Liquid then drains down into each column base. This procedure has worked well for. small columns, say, 6 feet or less in diameter. For larger columns there have been some cases where the lower trays were damaged. It is probably safer, therefore, before startup to pump column-base contents down to a level well below that of the bottom tray. In doing this one should also allow for displacement of liquid from a thermosyphon reboiler once boilup has been established, most of its mbe volume will be vapor. 

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