Control valves limit turbine temperature swings

This paper reports that daily cycling of large generating units that were designed for base load now is an economic necessity for many utilities. And repeated temperature changes common to cycling can result in thermal stress which may shorten the life of turbines and boiler parts. The problem was recognized years ago in supercritical and subcritical once-through steam generators that were designed for minimum loads of 30% to 50%. Redesign solutions were developed, with varying degrees of success, to permit frequent cycling to much lower loads. One goal had been to reduce turbine temperature change during wide load swings. As for drum-boiler units, the jury is still out. Some were redesigned to hold boiler and turbine temperatures relatively constant. Other power plants resorted to sliding-pressure operation to relieve the problem in turbine-generators only, but relatively lengthy ramp times to reach full load still persist.