Level control valves are used to maintain liquid levels for different process equipment such as - Vessels, distillation column bottom section, distillation column side liquid draw, etc.
Liquid (or interface liquid) is in physical contact with gas or other liquid phase. Liquid is taken out from unit operations . Level measurement is done using level instruments and they send signal to a level controller. Controller compares the reading and gives inputs to throttle (open or close) the control valve.
Generally level control valve are found in liquid outlet line from vessel, liquid outlet from distillation columns, partial reboiler, partial condenser, flash vessel, separators, tank liquid outlet lines, etc.