A vapour power plant having a vapour generating section 24, a valved conduit 32 including a vapour and liquid separator 35, a superheating section 26a and 26b and a turbine 28, connected for series flow therethrough, and a valved conduit 44 by-passing the separator, is arranged to be operated with the pressure in the vapour generating portion of the power plant varying with load at subcritical and supercritical pressures. Up to a certain subcritical pressure in a subcritical pressure zone 59, Fig. 2, the working fluid flows through the separator 35 and at this pressure up to a supercritical pressure the working fluid flows through the bypass 44. The pressure may vary with load up to full load, line 66, Fig. 2, or alternatively the pressure may vary up to a partial load, line 46, with a constant pressure from partial to full load, line 65. The transition from flow through the separator 35 to flow through the by-pass preferably occurs with the condition of the steam between the limits of 10% moisture and 50 F. of superheat. In the embodiment shown, under subcritical operating conditions below the transition pressure, feedwater from a condenser 11 is delivered by a feed pump 20 into the evaporator 28 to pass into steam and water separator 35. Steam from the separator passes through the superheaters 26a and 26b to a highpressure turbine 28, a reheater 29 and a lowpressure turbine 30. Water from the separator is recirculated to the evaporator through conduit 36 by a recirculating pump 40. Operation under these conditions is controlled by a load controller (67), Fig. 4 (not shown), which sends a control signal to a boiler input controller (71) which in turn sends control signals to a feedwater controller (76), a fuel regulator (78) and an air regulator (80). The ratio of firing rate to feedwater flow is additionally adjusted by a firing rate-feedwater ratio controller (88) in response to a controller 92, Fig. 1, of the water level in the separator 35, and the ratio of fuel to air is adjusted by a fuel-air ratio controller (94). The steam temperature at the outlet of the superheater 26b is controlled by a controller 82, Fig. 1, which sends signals to a controller 84 operating a valve 85 in a spray water conduit. The superheated steam temperature may be controlled by tilting burners or a gas by-pass. Under operating conditions above the transition pressure and up to maximum pressure the separator 35 is by-passed, valves 34 and 43 being closed and valve 45 opened. Operation under these conditions is controlled by the load controller (67), Fig. 4, and boiler input controller (71), and the superheater outlet temperature controller 82 controls the firing ratefeedwater ratio controller (88) to regulate the superheater outlet temperature. With the power plant operating at constant steam generator pressure between partial load and full load the load controller (67), Fig. 4, sends signals to the boiler input controller (71). a turbine input controller (69) and a turbine governor (74). The turbine input controller (69) regulates the valves 46, Fig. 1, at the turbine inlet with supplementary control of the turbine input controller (69) by a controller 95 sensing the throttle pressure at the turbine inlet.
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