Foam is a major source of column operating problems. With foam, the level in the sight glass is lower than the level inside the tower. As the foam level inside the tower increases, the difference between the foam level and the height of the sight increases. Finally, the foam level rises above the upper level tap.
Once the foam rises above the upper level tap, the DP across
a DP cell reports the average liquid density across the distance
times the height. The DP only changes when liquid density. Sight
glasses only show changes of average density across the sight
glass taps. Liquid or foam can back up the tower all the way to
the overhead vapor line and a sight glass still shows a liquid
level and a DP cell level controller shows a liquid level.
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