Patent Number: 046506330
Section: description

DETAILED DESCRIPTION The invention as described herein, is employed with a water cooled and moderated nuclear reactor of the boiling water type, an example of which is illustrated in simplified schematic form in FIG. 1. Such a reactor system includes a pressure vessel 10 containing a nuclear fuel core 11 submerged in a coolant-moderator such as lightwater, the normal water level being indicated at 12. A shroud 13 surrounds the core 11, and a coolant circulation pump 14 pressurizes a lower chamber 16 from which coolant is forced upward through the core 11. A part of the water coolant is converted to steam which passes through seperators 17 which are inside a dryer seal skirt 9, dryers 18, and thence through a steam line 19 to a utilization device such as a turbine 21. A portion of the steam is diverted from turbine 21 through preheaters 92 and 93 in a feedwater flowline 26. Condensate formed in a condenser 22, along with any necessary make-up water, is returned as feedwater to the vessel 10 by a condensate pump 30, a subsequent feedwater pump 23 and through a control valve 24 in the feedwater line 26. A plurality of control rods 27, containing neutron absorber material, are provided to control the level of power generation and to shutdown the reactor when necessary. Such control rods 27 are selectively insertable among the fuel assemblies of the core under control of a control rod control system 28. For proper reactor operation, it is necessary to maintain the water level in vessel 10 within predetermined upper and lower limits. A general approach to such water level control will now be discussed. A first aspect of such control is a comparison between the steam outflow from the vessel with the feedwater in-flow. A signal proportional to the steam flow rate is provided by a steam flow sensor 29, which may be a differential pressure transmitter that senses the differential pressure from a pair of spaced pressure taps in a flow measuring device 31 placed in the steam line 19. Similarly, a signal proportional to the feedwater flow rate is provided by a sensor 32 which may be in the form of a differential pressure transmitter connected to a flow measuring device 33 in the feedwater line 26. The signals from flow sensors 29 and 32 are transmitted to a feedwater control system 34 wherein one is subtracted from the other. A difference of zero indicates that outflow and inflow are the same and the water level will remain constant. If the difference is other than zero, a signal corresponding in sign and proportional in amplitude to the difference is applied to valve controller 36, which adjusts the valve 24 in a manner to bring steam outflow and feedwater inflow toward balance. This arrangement provides rapid correction and normally maintains vessel water level within the bounds of a relatively narrow deadband. However, it does not sense or control the position of the water level in the vessel. Thus, a second aspect of water level control is the provision of an upper water level pressure tap 37 and a lower water level pressure tap 38 which provide signals from which the position of the water level can be determined. The pressure taps 37 and 38 communicate with the interior of the vessel 10 and are connected to a differential pressure transmitter 39 which converts the difference in pressure at taps 37 and 38 to an output signal indicative of the position of the water level 12. This signal is applied to the feedwater control system 34 and is employed therein to modify the control signal to valve controller 36 whereby the valve 24 is controlled to adjust the feedwater flow rate and thereby maintain the position of the water level 12 within the prescribed upper and lower normal operating limits. (Although not shown here for clarity of drawing, it is noted that the usual system employs two or more sets of pumps 23 and 30, valves 24, and controllers 36 connected in parallel. See FIG. 2.) If for some reason, such as component failure, the water level control system 34 fails to maintain the water level within normal limits, the water level may become excessively low or high. A level detector 40 is provided to detect an excessively low, out-of-limits water level, and to produce a signal OL.sub.1. Similarly, a level detector 41 is provided to detect an excessively high water level and to produce a signal OL.sub.h. These signals are received by a Reactor Protection System 42, which responds to an out-of-limits condition by signaling the control rod control system 28 to insert the control rods and scram the reactor. These and other water level control systems, to which the present invention can advantageously be applied, are set forth in detail in U.S. Pat. No. 4,302,288, incorporated above. Referring to FIG. 2, an overview of a typical condensate delivery system is shown. Elements used for active control of feedwater temperature or pressure are schematically depicted. Condenser 22 collects condensate from a power turbine and from preheaters 92 and 93. Condensate is delivered to three condensate pumps 30 through a one into three manifold 150. The condensate is delivered as feedwater through the condensate pumps, and its temperature is raised by passing it through preheaters 92. The preheaters utilize steam extracted from the power turbine. The feedwater is then brought into a 3 into 2 manifold 151 for delivery to two feedwater pumps 23. Preheaters 93 are provided after the outputs of the feedwater pumps. If the condensate delivery system incorporates motor driven feedwater pumps, flow control valves 24 are incorporated in each flow line immediately after the last preheat stage. A two into one manifold 152 then delivers the feedwater to the reactor vessel. As noted above, a typical condensate delivery system comprises a plurality of centrifugal pumps. The use of groups of pumps connected in parallel provides benefits of redundancy in case one pump fails. Polyphase electrical motors and/or steam driven turbines are utilized to provide motive force to the various pumps. Where turbines are used, flow control means for steam delivered to those turbines can be substituted for flow control means 24 in the feedwater flow lines. Condensate is typically at a temperature of 10.degree.-20.degree. F. above ambient temperature and at a pressure of 20-25 inches of mercury. The condensate pumps boost the pressure of the feedwater to approximately 700 psig. Preheaters 92 raise the water temperature to about 375.degree. F. The feedwater pumps then boost the water pressure to about 1075 psig. All of the above figures are for normal operation and under certain circumstances can be expected to vary. In FIG. 3, a preferred embodiment of the present invention is set forth. The condensate delivery system is depicted as having only two inline pumps for the sake of clarity. The positions of manifolds 150, 151 and 152 are shown. Each feedwater pump in a condensate delivery system will have a pump system protection system. Accordingly, each flow control valve 24 is independently controlled. Feedwater flowline 26 comprises the various pumps, pipes and valves used to connect condenser 22 to the reactor vessel 10. Condenser 22 is directly connected to the condensate pump 30. The condensate pump leads into the feedwater pump 23. The feedwater pump 23 communicates with the pressure vessel 10 through the flow control valve 24. The pumps 23 and 30 typically are centrifugal pumps. Drive motors 50 and 52 drive the condensate and feedwater pumps respectively. Generally, a three phase, non-synchronous induction type motor is used. The flow control valve 24 is adapted to be selectively positioned by valve controller 36. Preheaters 92 and 93 use steam diverted from the turbine 21 to raise the temperature of the feedwater being introduced to the reactor vessel. Preheater 92 heats water flowing in the feedwater line 26 between the condensate pump 30 and the feedwater pump 23. Preheater 93 heats water received from the feedwater pump. A temperature sensor 56 and a pressure sensor 58 are provided in the intake 54 of the feedwater pump 23. Each sensor develops an electrical signal proportional to the value of the physical condition measured. The temperature signal is thus proportional to the temperature of the feedwater in the pump intake. The pressure signal is proportional to the water pressure in the pump intake. The water temperature during normal operation is typically 375.degree. F., although it will be lower when the reactor system is not operating at full power. Normal water pressure in the intake is about 700 psig. The temperature signal and the pressure signal are processed by appropriate circuitry in a subcooling processor 60. The subcooling processor may include a microprocessor adapted to perform a table lookup operation. The temperature signal and the pressure signal are processed by individual analog to digital converters. Subcooling values for the matrix of discrete pressures and temperatures are provided in memory. The microprocessor determines the appropriate address in memory from the temperature and pressure indications and thus generates a subcooling level indication. A digital to analog converter processes the subcooling indication from the accessed memory register. A signal value, correlated with the subcooling of the water in the pump intake, is thus provided. The correlated signal is transmitted to the non-inverting terminal of a summer 62. The subcooling function is non-analytic and is depicted graphically in FIG. 5. The limit signal generator 64 receives the temperature signal from the feedwater pump intake. The limit signal generator is a function generator which matches the measured temperature to a required predetermined value of subcooling needed to prevent cavitation in the feedwater pump at that temperature. Such subcooling values are provided from test data supplied by the manufacturer. A representative set of values is depicted graphically in FIG. 5. The circuit can be realized with a calibrated constant current source and a summing node. A particular quantity of subcooling required at a given temperature implies a certain minimum pressure for that temperature. A signal proportional to the subcooling required is transmitted to the inverting input terminal of the summer 62. Summer 62 develops a signal proportional to the subcooling margin of feedwater entering the feedwater pump 23. A negative signal indicates a negative margin and the consequent possibility of cavitation. This signal is transmitted to a subcooling limit trigger 98. Subcooling limit trigger 98 generates a constant valued, positive "on" signal should the subcooling determined by subcooling processor 60 be less than the minimum required; that is should the signal from summer 62 be negative with respect to ground reference. This occurs when the subcooling processor 60 generates a signal smaller than the required subcooling signal from subcooling generator 64. The limit trigger can be realized using a Schmitt trigger with following inverter. Any signal generated by limit trigger 98 is transmitted to a first input terminal of an OR GATE 80. The output signal from OR GATE 80 is applied to a valve position control signal generator 84 for control of flow control valve 24, as described hereinafter. As mentioned above, three phase induction motors may be used provide motive force to the pumps in the feedwater flow line. Such motors draw electrical current at a constant voltage and frequency and convert it to mechanical power and torque in response to the load imposed on the motor. Such motors are adapted to draw increasing current to produce increasing mechanical power and torque throughout their useful operating range. Such motors also include power limit switches, which disconnect the motor from its supply lines should electrical power consumption rise above a predetermined limit. The electrical power consumption of the motor is given by the relation: EQU P=(3).sup.1/2 Cos .phi.V.sub.11 I.sub.b where Cos .phi. is the inphase component of the current drawn (power factor) PA1 V.sub.11 is line to line voltage PA1 I.sub.b is branch current The power factor, Cos .phi., in the operational area of the motor can be treated as a constant for operating values of interest here. Also, the line to line voltage is assumed to be constant. Thus, I.sub.b varies almost directly with power consumed and this is correlated with the load driven by the motor. Current drawn is monitored as an indication of power consumed. Other conditions could be monitored as such an indication, e.g., motor rotational velocity, or power could be calculated by monitoring the above values and using the above relationship. However, a current monitor provides a reliable, easily resolvable, and relatively inexpensive indicator. Accordingly, a current transformer 66 is applied to one of the three power input lines 68 of a drive motor 52. This is proportional to the total power as the time average current drawn in any one of the three lines of a symetrical motor is equal to that drawn on any one other line. A signal proportional to that of current drawn is induced in the current transformer and transmitted to a current scaler 61, which reduces that signal to a signal appropriately scaled to the subsequent limit trigger 70. The scaled current is introduced to the inverting terminal of trigger 70. A second signal, a steady current limit signal from a calibrated current source, is provided to the non-inverting terminal of limit trigger 70 from current limit generator 65. Should the indicative signal from the current scaler 61 exceed the current limit signal, the limit trigger 70 will produce a fixed, positive valued output signal. This signal is transmitted to a second input terminal of OR GATE 80. OR GATE 80 operates conventionally and transmits a signal to an integrator 82 in the valve position control signal generator 84 in response to either indication signal. The valve position control signal generator 84 receives and sums input signals from both an existing water level control system 34, such as described hereinbefore, and the pump system protection system. The signal from the water level control system 34 is introduced to the valve position control signal generator 84 through a signal limiter 88 which limits a positive indication (i.e., an indication to begin opening the flow control valve) to a predetermined maximum value. Such a limiter can be built using an operational amplifier with a resistive negative feedback loop. The integrator 82 produces an output signal which increases with time for as long as an output signal is received from OR GATE 80. Integrator 82 can be realized using an operational amplifier with capacitive feedback. The output signals from signal limiter 88 and integrator 82 are introduced, respectively, to the positive and negative terminals of a summing amplifier 90. Summer 90 generates the actual valve position control signal which is applied to valve controller 36. Integrator 82 and limiter 88 are provided so that when conflicting demands are made by the respective systems, i.e. the pump system protection system and the water level control system, the pump system protection system eventually prevails. This arrangement maintains pump operation in case of a heavy demand for feedwater flow. A time delay shutdown trigger may be incorporated, as a backup shutdown device, into the aforedescribed pump system protection system. The subcooling margin signal generated by summer 62 is transmitted to an analog to digital converter 113. A/D 113 provides the data input to time delay calculator 105 which is adapted to transmit a trip signal to relay 104 which, in turn, can cut off power to drive motor 52 under circumstances to be described below. Calculator 105 incorporates a microprocessor programmed to trigger a timing mechanism should the subcooling margin become negative and fall below a first minimum value, for example -10 BTU/LBM. As subcooling initially falls through the first minimum, the timer begins a 30 second countdown, which, should it come to completion, will cause a trip signal to be transmitted to relay 104. A series of secondary minimums are provided in memory, which if passed result in set quantities of time being subtracted from the aforesaid timer. For example, if the subcooling margin falls below -20 BTU/LBM, 10 seconds are subtracted from the running timer. If the subcooling margin falls to -30 BTU/LBM, 15 additional seconds are subtracted from the timer. A sudden decline in subcooling from a safe positive level to -30 BTU/LBM allows the pump protection system a maximum of 5 seconds to restore satisfactory operating margins. The timer is stopped and reset should subcooling margin recover to a predetermined minimum, for example, -5 BTU/LBM. Referring now to FIG. 4, a second preferred embodiment of the invention will be discussed. The specific embodiment of the invention depicted is a primarily analog realization of the invention. As before, a pressure sensor 58 and a temperature sensor 56 are introduced to the inlet of a feedwater pump 23. The signal generated by the temperature sensor is transmitted to a saturation pressure function generator 161. The saturation pressure function generator 161 is a one input function generator which generates a signal proportional to what the pressure sensor 58 would generate if the water were saturated at that temperature. Function generator 161 is realized with a calibrated current source and a summing node. Accordingly, the signal generated by function generator 161 is equal to or less than the signal produced by pressure sensor 58. The saturation pressure signal is subtracted from actual pressure at summer 160. The resulting pressure difference signal is the pressure margin which is correlated with pump inlet subcooling. The pressure difference signal, from junction 160, is introduced to the positive terminal of a summer 162. Function generator 164 provides a temperature dependent, required pressure difference signal which correlates with adequate subcooling at each operating temperature. Function generator 164 is a one input generator and may be realized as a calibrated current source and summing node. The signal generated by function generator 164 is transmitted to the negative terminal of summer 162. Should the value of the difference signal fall below the signal from function generator 164, the signal from summer 162 will become negative. Again a subcooling limit trigger 98 is provided to generate a fixed, positive valued control signal should summer 162 generate a negative valued signal, indicative of an inadequate pressure margin needed to assure an adequate subcooling margin. The depicted condensate delivery system utilizes a steam driven turbine 132 to drive the feedwater pump 23. Control of flow through the flowline 26 is effected through control of the motive force driving turbine 132. Control is achieved by controlling the quantity of steam introduced to turbine 132. A flow control valve 124 is included in the steam to turbine delivery line for this purpose. Valve controller 84 performs the same function in the embodiment in FIG. 4 as in the previously discussed embodiment of FIG. 3. The signal produced is applied through a summer 138 to a valve position controller 136, which controls steam flow to turbine 132 by positioning flow control valve 124 according to the demands of the water level control and pump protection systems. Accordingly, a demand for increased feedwater flow will result in opening of the steamflow control valve 124. An overriding signal that pump cavitation is threatened results in progressive repositioning of valve 124 to reduce steam flow. Such variation in steam flow controls turbine energization and thereby controls feedwater flow through pump 23. The reduced flow through the pump allows the condensate pumps to restore pressure to the pump inlet reducing the danger of pump cavitation. As in the case of the embodiment of FIG. 3, a time delay shutdown trigger may be incorporated as a backup shutdown device in the embodiment of FIG. 4. An analog to ditigal converter converts the pressure margin signal from summer 162 into a digital input for time delay calculator 105, which is the same as calculator 105 described for FIG. 3. Note, however, that pressure margin levels are substituted for subcooling margins as minimum trigger levels for the timer. Trip generator 204 is connected to receive a trip signal from calculator 105. On receipt of a trip signal, trip generator 204 develops a valve position signal of sufficient magnitude to dominate all other inputs to summer 138. The resulting signal from 138 is transmitted to valve position 136 and closure of flow control valve 124 is effected. A turbine cannot draw power in a manner analogous to an electrical motor. Accordingly, it is not necessary to monitor the power consumed by the turbine. The power monitoring aspect of the invention is not used in the second embodiment. It will be understood that the analog based embodiment described immediately above may be substituted for the microprocessor based embodiment described in relation to the motor driven feedwater pump. Likewise, the microprocessor based embodiment can be applied to a turbine driven pump system. The operation of the invention is hereinafter elaborated upon with reference to FIGS. 1, 2, 3, 4, and 5, as appropriate. FIRST EXAMPLE Consider the first preferred embodiment. Condensate is collected in condenser 22 at approximately atmospheric pressure. The condensate pump 30 boosts pressure to approximately 700 psig. The feedwater pump 23 further boosts this to approximately 1075 psig for reintroduction to the pressure vessel. Suppose water temperature at the feedwater pump inlet is 375.degree. F. Flow is controlled through the aforementioned flow control valve 24. This is normal operation. Required subcooling is about 75 BTU/LBM. Suppose that the water level control system detects a steam flow greatly in excess of feedwater flow. This condition may be a consequence, for example, of a leak in the feedwater line upstream from the feedwater flow measuring device 33. If not responded to, it portends a coming reduction in water level within the reactor vessel. Accordingly, the water level control system transmits a signal to the valve position control signal operator which generates a command to the valve position controller to begin opening the valve to increase feedwater flow. Increasing flow is associated with decreasing pressure at the inlet of the feedwater pump. System operating conditions will begin to move downward on the curve denoted "MARGIN" in FIG. 5. As flow increases, the load on the motor 52 driving the pump 23 increases. Consequently, current drawn by the drive motor 52 increases. As can be observed from FIG. 5, subcooling will decrease as pressure falls (water temperature remains constant). Should the point marked "minimum" be crossed, a signal will be provided by the feedwater pump system protection system through control signal generator 84 to valve controller 36 to move valve 24 toward its closed position maintaining the minimum subcooling necessary to prevent pump cavitation. Likewise, if current drawn by motor 52 becomes excessive, a signal will be generated to close the valve 24 to reduce flow and thereby reduce load. Integrator 82 assures that these signals dominate the signal from the water level control system. EXAMPLE 2 Suppose operation of the same plant as above, but under partial power. Referring to FIG. 5, an exemplary partial power operating point is so labeled. If the condensate delivery system is operating normally, feedwater pump inlet pressure will be uneffected from the full power operating point. However, pump inlet temperature will be substantially reduced. The system would be operating with approximately 225 BTU/LBM subcooling. The required minimum subcooling would be about 70 BTU/LBM. A prior art pressure trigger would trigger a motor shutdown at a pressure, which would yield subcooling of about 155 BTU/LBM. A variety of causes could result in a rapid reduction in feedwater pump inlet gauge pressure below the 375 psig level at which pressure triggers have been set to activate. A failure of a condensate pump could reduce pressure below the previously employed pressure trigger level but not put the pump into actual danger of cavitation. The condensate delivery system could tolerate one condensate pump failure and remain operational. An unnecessary reactor scram would be avoided. In the exemplary embodiments of the invention described above and shown in FIGS. 4 and 5, the invention is shown as applied to a condensate delivery system in a nuclear power reactor. It will be readily apparent that the invention is not so limited and that it may be used as a reliable method and apparatus to protect pumps used in various settings, e.g. hydraulics. Various substitutions and modifications may also be made in the types of components used. While certain embodiments of the present invention have been disclosed herein, it will be clear that numerous modifications, variations, substitutions, changes and full and partial equivalents will now occur to persons skilled in the art without departing from the spirit and scope of the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.