Patent Number: 060211695
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT The above described needs are met by a system wherein automatic control of feedwater flow to pressurized water reactor (PWR) steam generators is enabled, over the full range of reactor power from 0 percent to 100 percent. FIG. 1 shows a functional configuration of a Nuclear Power Feedwater Control System (FWCS) with the features that enable such control. Two nearly identical FWCS's, one for steam generator A 10 and one for steam generator B 11, are provided with a steam supply system such as an ABB Combustion Engineering Nuclear Power (ABB-CENP) nuclear steam supply system. Either FWCS is exemplified by FIG. 1. The FWCS's control feedwater flow by processing signals from the transmitters and controlling the pumps and valves shown in FIG. 2. First, each FWCS receives input signals from various sources, the signals being numerically represented in FIG. 1. These signals convey data involving the total feedwater flow differential pressure (DP) 16, the downcomer feedwater flow DP 15, the steam flow DP 12, the steam generator level 13, the reactor power 14, and the reactor coolant system average temperature instrument channels 17. Upon receiving and processing these signals, each FWCS generates an output flow demand (FD) signal 18. The FD signal is processed into output signals 27, 28, 29 that are respectively received by the feedwater pumps 19, the economizer feedwater control valves 20, and the downcomer feedwater control valves 21. A high speed logic processor 22 also receives the FD signal from each FWCS, and sends the FD to the other FWCS. The FD is processed by the high speed logic processor, and used to determine the pump speed demand. Manual/automatic control stations 23, 24, 25, for the feedwater pumps, downcomer feedwater control valves, and economizer control valves, respectively, are placed in automatic mode when the FWCS determines the output demand signals based on the input signals. When any one of the manual/automatic control stations 23, 24, 25 are placed in manual mode, its output signal is determined and controlled by an operator. Each FWCS can be placed in automatic mode when the feedwater pumps are running and the control valves can control feedwater flow such that stable SG water level is maintained. However, for existing FWCS's, such as the ABB-CENP FWCS, reactor power and steam load must be approximately 5% or greater, typically 15 to 20%, to establish these conditions. In order to provide automatic control of SG water level when the reactor power and steam load are below 5%, a new output signal 30 is provided for each startup feedwater control valve 26. The startup feedwater control valves are automatically controlled, based on the SG water level, from 0 percent power up to the lower range of the existing FWCS, which in the preferred embodiment is approximately 5%, although the principle of the invention is applicable to situations where the lower power level is higher or lower than 5%. In this embodiment, however, during operation between 0 and 5% power, the constant speed startup feedwater pump 31 is the expected source of flow. Because the pump 31 runs at constant speed, no automatic control signal is needed. Transfer of flow control from the startup feedwater control valves 26 to the downcomer feedwater control valves 21, at 5% power in the preferred embodiment of the invention, will occur at a point of change of an output state in a bistable 32. Also, a time delay 33 is added to the output signal 30 for the startup valves 26, so that they will close after the downcomer valves 21 open. Because there is a time delay in complete shutdown of the startup valves 26, continuous feedwater flow to the steam generators 10, 11 is assured. The present invention also improves the control of supply to the steam generators 10, 11 between 5 and 15% power. In existing systems, a bistable triggered by the reactor power level signal 14 introduces the steam flow 12 and total feedwater flow DP 16 input signals to the FWCS logic. Use of these signals with the steam generator water level signal 13 is sometimes called 3-element control. The present invention makes use of the downcomer feedwater flow DP signal 15, used only for indication in the existing FWCS's. The present invention also makes use of the reactor power input signal 14, used for transfer and lead/lag logic in the existing FWCS's. Using these available signals, a new 3-element control scheme 34 is provided, at lower power levels. At higher power levels, between 15 and 20%, the new 3-element control scheme 34 switches from the new downcomer feedwater flow-reactor power-steam-generator level combination to the existing total feedwater flow-steam flow-steam-generator level scheme. Having described an embodiment of the invention, it is to be understood that the invention is not limited to any of the precise embodiments described herein. Various changes and modifications could be effected by one skilled in the art without departing from the spirit or scope of the invention as defined in the appended claims.