Cycling of combined cycle units

Base loaded units run at or near full load for extended period. They are warmed to operating temperature “slowly” (restricted ramp rates) and come offline only for maintenance (running continuous between outages). Cycling units experience varied loads, are warmed to operating temperature more quickly per demand. Frequent starts and stops happen as required by the grid (i.e. load variation from renewables).

In Power Gen International, Daniel J. Azukas, senior management consultant at Sargent & Lundy summarized the impact of cycling.

Thick-walled components face increased thermal stresses. Rapid startup times are stressful for steam turbine as it is typically slowest to warm up. Increased usage of drains/vents, more frequent condensate generation upon shutdown are seen. If not cleared before next start, can be damaging

Here’s how the plant can be made more capable of handling cycling: Maintaining heat in the unit during shutdown; stack dampers with insulated stack; steam sparging; maintaining vacuum during shutdown and keeping steam turbine warm; liquid ring vacuum pumps and auxiliary steam source; steam turbine bypass; starting the combustion turbine and ramp independent of ST

Ensure plant drain system is adequate. Frequent cycling will lead to more condensation which needs to be removed for safe and reliable operation. Water chemistry upsets can affect FAC and ID corrosion. Ensure appropriate water chemistry control for cycling demand. Implement an FAC inspection program for problematic areas.

Cycling has a significant impact on bottoming cycle, said Mr Azukas. More output from an existing combustion turbine = potential for significant impacts on bottoming cycle. More exhaust gas flow/higher temperature, changing exhaust gas flow profile, more steam production/higher steam temperature, and duct firing for supplemental fired units may need to be considered.

On steam systems, more steam flow/higher steam temperature would be needed. Increased velocity in piping system, noise and wear concerns (FAC); higher desuperheating spray requirements; increased demand from spray water system and pumps would all be observed.

On the feedwater system, more feedwater flow, increased velocity in piping system/higher pressure drop through existing piping, feedwater pump running back on it performance curve could be necessary. Pump modifications may be needed too.

More condensate flow; increased velocity in piping system/higher pressure drop through existing piping (not as significant as feedwater due to energy profile of HRSG); condensate pump running back on its performance curve would be observed.

The balance of plant would need to handle higher heat loads and more steam condensing. Similarly, the auxiliary cooling system would have to handle increased heat load from equipment, while fuel systems would face fuel flow and pressure requirements