144.What is Constant Pressure mode and Sliding Pressure mode?

Large units will be operated at reduced steam pressure at loads less than 60% from 25%. The pressure will be maintained constant at rated value from 60 to 100% load. Typically for a 550 MW Unit, pressure will be 120 bar at 25% load and increased proportionally to 160 bar up to 60% load. This mode of unit operation is known as Sliding/Variable pressure mode. The governer valve will remain at a fixed value except for any emergency. From 60% to 100% the pressure will be maintained constant at 160 bar by modulating the governer valve, it is called constant pressure mode.In sliding pressure mode an appreciable amount energy is saved in the BFP.