132.What is three element control in drum level control? Why it is necessary?

Drum level is controlled using three measurement parameters-- Steam Flow, Water Flow and Drum level signals. During rapid load changes, the water level in boiler drum swells or shrinks due to change in Drum pressure. This would result in erroneous reading in actual Drum level measurements. Hence level measurement alone will not be adequate for control purposes. In order to control level effectively, the control loop is designed to vary the water into drum by balancing the Stem flow (boiler OUT) and Feed water Flow(boiler IN).The controller signal is corrected thru the actual Drum Level deviations to continuously correct the steady state level variations.