OG&E's seminole plant operates under free pressure

Boiler turbine/control using free pressure mode operation at OG&Es Seminole Plant improves heat rate while maintaining load response capabilities. Free pressure mode combines a variation on the valves wide open (VWO) operation mode with hybrid variable pressure operation, resulting in an elaborate throttle pressure versus load profile. In VWO operation, the turbine valves are fixed at their full open position, and the control system varies the firing rate to achieve the desired megawatt output rate with no regard for operating pressure. In free pressure mode, the control system drives the turbine valves toward predetermined positions at steady state load conditions, forcing the steam pressure to vary to maintain the load.