In the three-element level control system of steam drum water is one which uses the measured water level, the steam flow rate from the boiler, and the BFW flow rate into the boiler to regulate the flow of BFW into the boiler.
In this control philosophy, there are three process variables used to control the level of steam drum. 1. Steam drum or Boiler Level, 2. Feed water flow to drum 3. Steam Flow
1. Boiler Level measured by LT (LT0281), as the water density changes with pressure, So density compensated with pressure (PT0281) of the boiler. . Then the level signal is sent to Summation switch (LY0281)
2.The second signal of steam flow which is compensated with temperature and pressure of the steam is sent to the same summation switch or calculation box (LY0281).
3. BFW flow transmitter PV is sent to the flow controller (FC-0281).
4. Flow control takes two signals SP (Set Point) from LY0281 (Summation) and PV from FT-0281.
Then the controller (FC-0281) commands the final control element according to the set point requirement. Level controller (LC0281) is the master controller of the system.
Further, During lower boiler loads normally, ~30% steam flow, single element control system is applied. Drum level signal LT and the fixed local set point SP are compared in LC and the controller output is fed to feed water control valve FC.
For further explanation please see the link here, Boiler Three Element Controller Philosophy.