The column base runs empty and there is just enough liquid head in the line connecting the column base to the reboiler to overcome the pressure difference between the kettle reboiler and the column base. To protect the tube bundle in the event that boilup temporarily exceeds downflow, a head measurement must be made on the tube bundle chamber. This can be connected to an interlock, or, as shown on Figure 4.7, to suitable overrides.
Shown here are two overrides (pneumatic devices illustrated). One has a latch-up circuit with a gain 25 relay, which provides a high signal the first time the reboiler is filled with liquid; the steam valve is held closed until the level covers the tubes. Once latched up this circuit does not function again until it is unlatched by switching to "shutdown."
The other circuit is intended to shut the steam valve if the total head drops below a certain amount after normal operation has been in effect. This requires
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