I have seen 3 element and single element used in more applications than just FW (boiler) level control. Although it seems that most boiler level controls use these terms, any tank that maintains level control by level error, flow in, flow out to control level could have a term of 3 element level control.
Simple terms that help me...
Single Element - Level Setpoint compared to Level Actual (Level error) to control level.
3 Element - Level error, Steam/water out compared to Water/steam in.
M1Ark
I never understood why the DCS couldn't autoswitch at power levels below 25%. It has all the inputs it needed, and switched to single level for everything else. (Steam flow errors, feed flow errors, post scram feed water logic, etc.) Or maybe I'm missing something with the startup level controls since we switched to that train at powers less than 25%.