Changing Fossil Unit Startup Process Reduces Rotor Stresses

The changes in the energy market dispatching and pricing, have increased the need to start fossil steam turbines faster to meet demand and save fuel. Exelon worked with a consultant to optimize the start up times of their steam turbines resulting in greatly reduced start up times, increased dispatching frequency, and reduced thermal stresses on the turbines. This optimized start up process was achieved by utilization of the Valve Open Start (VOS) and Accelerated Hot Start (AHS) process. VOS utilizes condenser vacuum aligned to the steam generator to produce superheated steam at much lower temperatures and pressures than usual. This steam is drawn through the turbine to warm the unit while the boiler increases in temperature and pressure. The AHS changes the startup sequence of operations by setting up the turbine in a manner that allows the turbine to roll at precisely the time that a perfect temperature match is obtained between the main steam temperature and first stage metal temperature. The use of these processes significantly increases profitability of the units and meets all OEM criteria for unit protection and results in a reduction in rotor thermal gradient, temperature mismatch, and thermally induced vibration.