Control Global

Voices: McMillan & Control Talk

Make sense of sensitive level

Radar offers increasing opportunities to improve control at a reasonable price.

May 31, 2019

*Greg:* It’s not well recognized that a sensitive level measurement can be extremely valuable. For inventory, a fraction of an inch change in level in a large-diameter tank is a lot of material. For closure of material balances, these seemingly small changes in level can make a huge difference in accounting and in process control of some key unit operations. For crystallizers, evaporators and reactors, tight residence time control depends upon an accurate level measurement. Many multi-effect evaporators measure and control the product density in the last stage by manipulating a product or feed flow. These multi-effect evaporators depend upon tight level control to make sure changes in flow in and out of each effect are equal. For distillation columns where the distillate receiver level controller manipulates reflux flow, extremely tight level control enables much better temperature control, particularly through inherent internal reflux control. Tight level control translates manipulated changes in distillate flow by the temperature or composition controller to the necessary equal changes in reflux flow. While ratio or feedforward control can help here, changes in overhead vapor flow are unmeasured and the correction by temperature or composition controllers is playing catch up. Translation of these unmeasured changes in overhead vapor flow to reflux flow by tight level control enables internal reflux control. Tight level control self-corrects for decreases in upper column temperature (e.g., blue northerner rain storms) that cause a sudden large decrease in vapor flow and upset to the column. When distillate receivers are horizontal vessels, level sensitivity is even more important because the cross sectional area is so large.

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