Patent Number: 060211695
Section: summary

BACKGROUND OF THE INVENTION The current System 80+ Feedwater Control System (FWCS) automatically controls the feedwater flow to the steam generators between 5% and 100% power. However, feedwater control below 5% power is a manual operation. The current FWCS uses input signals of steam flow rate, feedwater flow rate, and steam generator level to develop output signals that control the position of the feedwater valves and the speed of the feedwater pumps. There is a large economizer feedwater valve, a small downcomer feedwater valve and a very small startup feedwater valves. The startup feedwater control valve is located in parallel with the downcomer feedwater valves. The large economizer feedwater valve is automatically controlled by the above three signals of the FWCS between 20% and 100% power. Also, the small downcomer feedwater valve is automatically controlled by the steam generator level signal of the FWCS between 5% and 20% power. Furthermore, the very small startup feedwater valve is manually controlled by an operator when the power level is between 0% and 5%. It is an object of the present invention to simplify the FWCS design, which previously had two automatically functioning systems, and one manually functioning system, according to the existing power levels. It is another object of the present invention to provide a more reliable feedwater system than is currently provided, especially with the System 80+ Feedwater Control System, by using additional input signals when the power level is between 5% and 20%, and by increasing the automatic feedwater control operating range, thereby relieving operators of monitoring and operating of the FWCS at operation power levels below 5%. SUMMARY OF THE INVENTION The above-described needs and others are met by a feedwater control system for a pressurized water reactor steam generating system, which includes a first input signal, which is determined by a downcomer feedwater flow differential pressure, a second input signal, which is determined by a reactor power level, a first output signal, which is determined by the first and second input signals, and, when combined with a third input signal which is determined by a steam generator level, which automatically controls at least one feedwater pump and designated valves which regulate water flow from the one or more feedwater pumps, to a steam generator, when a steam generator steam load and a reactor are operating at a first predetermined power level, and a second output signal, determined by a steam generator water level, which automatically controls at least one startup feedwater control valve when the steam generator steam load and the reactor are operating at a second predetermined power level. In a preferred embodiment of the invention, the first predetermined power level is between about 5 and about 20 percent, and the second predetermined power level is between zero and about 5 percent. Also in a preferred embodiment of the invention, the first designated valves include at least one economizer feedwater control valve, and at least one downcomer feedwater control valve. Furthermore, in a preferred embodiment of the invention, the startup feedwater control valve stops controlling feedwater at the first predetermined power level, and the feedwater control system includes a delay device, in series with the second output signal. The delay device ensures that the startup feedwater control valve does not reach a closed state until the downcomer valve reaches an opened state, thereby ensuring continuous feedwater supply to the steam generators. The above objects and others are also provided by a method of controlling a feedwater supply to a steam generator in a pressurized water reactor, which includes the steps of providing a first input signal which is determined by a downcomer feedwater flow differential pressure, and providing a second input signal which is determined by a reactor power level. The method also includes the steps of automatically controlling at least one feedwater pump and a designated set of valves which regulate water flow from the one or more feedwater pumps to a steam generator, using a first output signal which is determined by the first and second input signals and combined with a third input signal which is determined by a steam generator level, when a steam generator steam load and a reactor are operating at a first predetermined power level, and automatically controlling at least one startup feedwater control valve, using a second output signal which is determined by a steam generator water level, when the steam generator steam load and the reactor are operating at a second predetermined power level. As noted before, in a preferred embodiment of the invention, the first predetermined power level is between about 5 and about 20 percent, and the second predetermined power level is between zero and about 5 percent. Also in a preferred embodiment of the invention, the first designated valves include at least one economizer feedwater control valve, and at least one downcomer feedwater control valve. The method can also include the steps of providing a signal to close the startup feedwater control valve at the first predetermined power level, providing a delay device to the second output signal, and automatically operating the delayer to ensure that the startup feedwater control valve does not reach a closed state until the downcomer valve reaches an opened state.