CONTROL ISSUES: SLIDING OR CONSTANT PRESSURE
The future generation of SC and USC plants in the US must be able to achieve high efficiencies not only in continuous base-load operation but also at part-load operation. The conventional approach suggests that, in the US, the new generation of SC and USC plants will operate, for the most part, at base-load conditions and constant steam pressure. However, for a plant with a life span of 30–40 years, it is inevitable that in the future these units will operate in load-cycling operation using sliding pressure.
Operation under constant pressure requires steady boiler and main steam line pressure over the entire load range. At part load, the steam turbine requires a lower pressure and flow to generate the amount of power needed. The operational system where the boiler provides only the required amount of steam and pressure to meet the demand without any throttling, is referred to as “sliding pressure.” Most steam generators and turbines in combined cycle applications operate in this mode.
For SC and USC, the sliding pressure method used in the industry (see Reference 14) is known as “modified sliding pressure.” In this case, a certain amount of pressure throttling is allowed, to provide a fast response to load changes. One advantage of the “constant pressure” mode of operation is a more robust load reserve capability at part load. Following a load increase demand, the throttling or admissions valves open and the pressures in the turbine and boiler begin to even out. The instantaneous reduction in pressure in the boiler creates a sudden increase in the steam-specific volume within the system and initiates an immediate discharge of steam mass towards the turbine. The additional steam mass provides an increase in the power output even before the firing or the fuel handling equipment is in operation.
It should be also emphasized that, below 70–75 percent base load, most steam generators operating in sliding pressure mode are at subcritical conditions.