US3550562A

US3550562A - Control system for a steam generator

Info

Publication number: US3550562A
Application number: US773757A
Authority: US
Inventors: Charles Strohmeyer Jr
Original assignee: Electrodyne Research Corp
Current assignee: Electrodyne Research Corp
Priority date: 1968-11-06
Filing date: 1968-11-06
Publication date: 1970-12-29
Anticipated expiration: 1987-12-29

Description

References Cited UNITED STATES PATENTS 4/l963 Profos..........................

United States Patent [72] Inventor Charles Strohmeyer, Jr.

Bern Township, Reading, Pa. [2]] Appl. No. 773,757

pting a n Z VT. SG 87 66 99 ll oo 5 5 0, oo 3 3 ABSTRACT: The invention provides the means for ada single logic system for control of steam operating both in the once-throu e U 0: a f- D. s h am na u R K. m 6 nm H l m. E o mm .mm PA [22] Filed Nov. 6, 1968 Patented Dec. 29, 1970 [73] Assignee Electrodyne Research Corporation Reading, Pa.

[54] CONTROL SYSTEM FOR A STEAM GENERATOR generators when gh mode and during startup when bypass flow is extracted intermediately from the main heat absorption conduits to establish a minimum fluid flow rate in the heat absorption conduits which are exposed to high rates of heat input.

, 1/ 4/7 flm w b H 2 2 F F m u g m m .m m "m w m m m r m mh D m mm w m m& S n u m l. L 0 m d c s .n 5 U h l l] 2 0 5 55 l M m ..III III N T Illllh l l lllllullllllllllll l-lllllllllll i if 82 87 L as as PATEN1Enuaczsnm 1 '03550562 sum 2 or 2 INVENTOR. CHARLES STROHMEYER, JR.

his ATTORNEY CONTROL SYSTEM FOR A STEAM GENERATOR This invention provides a means for improving control of once-through-type steam generators in a steam electric generating plant whereby auniform mode of control can be employed both during startup when bypassing fluid inter mediately from the steam generator heat absorption conduits andduring normal online operation when operating solely in this once-through mode. The invention can be applied to standard control systems designed for the once-through mode of operation. Thus, this specification is primarily concerned with the special adaptions for the startup 'mode when the bypass system is in operation.

In the past, difficulties have been encountered in the control of once-through steam generators during startup. When the boiler is operating solely in the once-through mode reliance is placed upon parallel control of feedwater flow and firing rate for correction of steam pressure error. While such controls function satisfactorily when the bypass system is inoperative, they have not been able to be used successfully in conjunction with bypass system operations in the low load range. Instead, controls for this latter condition have assumed the form of inflexible programs which inhibit and discourage operation of the unit in the low load range except for starting up and shutting down of the unit.

The trouble has been: that the normal integral balance for steam pressure and temperature control is disturbed as super heater outlet flow is decreased and superheater bypass flow is increased. Transfer of the controls from manual to automatic requires that the output from the pressure and temperature integrals be in correct alignment with each other. Since the pressure and temperature integrals have long time constants when controlling feedwater .flow and firing rate, they cannot conwithout causing major upsets to steam generator fluid pressure and temperature. Thus, whenon bypass-system operation the regulating functions of the pressure integral is usually trans- I ferred to act solely upon the start up system flow control valves withoutcross tie to firing rate. This upsets the normal relationship between the startup and online modes of control and after the bypass system is shut down at sayone-third load it is especially difficult totransfer steam temperature control from manual to automatic for normal online operation.

This invention overcomes past difiiculties in that feedwater flow and firing rate may be employed to correct steam pressurerrorat allttimespil'ot-prevent a distortion in the output from the pressure or temperature integrals during startup when the bypass system-"is in operation, a measure of bypass fluid flow rate is combined with the demand for superheater outlet steam flow after" correction for temperature and/or pressure error. 1

The bypass system fluid flow control logic is superimposed upon and supplements then'ormal online, once-through con trol system in away to maintain all integral outputs within their normal operating rangeso that the controls can be readily placed on automatic soon after synchronizing the unit to the electrical system.

A specific object of this invention is to provide a steam electric generating plant having a steam generator comprising a feedwater inlet, superheated steam outlet and heat absorption conduits connected therebetween, a steam turbine driver as prime mover for electric generating means, first fluid conduit means serially interconnecting said feedwater inlet, heat absorption conduits, superheated steam outlet and steam turbine driver, a bypass conduit 'means connected intermediately between portions of said :heat absorption conduits and having means to flow fluid away from the portion of said heat absorption conduits which is upstream of said bypass conduit means, a source of feedwater supply to said feedwater inlet having fluid flow control means, a source of heat input to said heat absorption conduits having means to quantitatively regulate input, flow control means for p r oportioning fluid entering said feedwater inlet between said bypass conduit means and said heat absorption conduits which'are downstream of said bypass conduit means, the'method of controlling fluid pressure in at least a portion of said heat absorption conduits whereby said source of feedwater supply is controlled to maintain at least a minimum rate of fluid flow to said feedwater inlet and said control means for regulating. fluid flow through said bypass conduit means is responsive to (l) fluid pressure error as applied to correction of a representative measure of required output to' said turbine driver from said steam generator, in combination with (2) a representative quantitative measure of fluid flowing through said bypass conduit means, for comparison with (3) said minimum rate of fluid flow to said feedwater inlet, or equivalen t.combination thereof, high-pressure increasing fluid flow rate and low-pressure decreasing fluid flow rate through said bypass conduit means.

A further object of this invention is to provide correction of said representative measure of required output for steam temperature error at the superheated steam outlet.

A still further object of this invention is to employ pressure error to correct rate of heat input to said steam generator when the bypass system is in operation.

A still further object of this invention is to compare pressure corrected steam generator demand with required prime mover output as said equivalent combination.

A still further object of this invention is to employ said pro portioning flow control means valve position as a representa tive measure of fluid flowing through said bypass conduit means. g

The invention will be described in detail with reference to the accompanying drawings wherein:

FIG. 1 is an overall representation of a steam electric generating plant illustrating the basic cycle and control components essential to the invention,

FIG. 2 illustrates an optional heat transfer device which may be incorporated in the steam generator bypass system,

1 FIG. 3 illustrates an optional flash tank system which may be incorporated in the steam generator bypass system,

FIGS. 4 and 5 show control variations to produce representative equivalent functional results.

In FIG. 1, steam generator 1 is provided with feedwater inlet 2, superheated steam outlet 3 and

heat absorption conduits

4, 5, 6 and 7 there between.

Conduits

4, 5, 6 and 7 may be economizer, waterwall, primary and secondary superheater surfaces respectively.

Elements

2, 4, 5, 6, 7 and 3 are serially connected by conduit means 8..Conduit 9 connects outlet 3 with steam admission control valve 10 and high-preessure steam turbine driver 11 which exhaust through conduit 12 to reheater 13 in steam generator 1. Reheater l3 outlet is connected serially to conduit 14, intercepter valve 15 and reheat turbine 16 which exhausts through conduits 17 to condenser 18. Cooling water passes through conduits l9 condensing steam which collects as condensate in hotwell 20. Condensate pump 21 takes suction from hotwell through conduit 22 and discharges through conduit 23 to water purification equipment 24.

.Water purification equipment 24 discharges through conduit 25 to control valve 26 which regulates water level in deaerator storage tank 27 (controls not shown). Valve 26 discharges through conduit 28 to low-

pressure feedwater heaters

29 and 30 and through conduit 31 to deaerator 32.

t The shell side of low-

pressure heaters

29 and 30 receive steam I extracted from turbine 16 (not shown). Conduit 31 is provided with spray holes within deaerator 32 to distribute condensate fluid flow over trays 33 for heating and deaeration of the condensate. Deaerator 32 discharges through conduit 34 to storage tank 27. Conduit '35 equalizes steam pressure between storage tank 27 and deaerator 32.

Storage tank 27 feeds to boiler feed pump 36 through conduit 37. Pump 36 raises the fluid pressure to the working level of steam generator 1. Pump 36 discharges through conduit 38 to high-pressure heaters 39 and 40 in series to feedwater inlet 2. The shells of heaters 40 and 39 receive steam extracted from conduit 12 and turbine 16 respectively (not shown). Condensate collected in heater 40 drains through conduit 41 and flow control valve 42 to heater 39 shell space from whence condensate collected drains through conduit 43 and flow control valve 44 to storage tank 27.

Feed pump 36' is driven by auxiliary turbine drive 45 which receives steam extracted from main turbine 16 through conduit 47. Control valve 46 regulates the flow of steam to drive 45 which in turn regulates turbine and pump speed. The capacity of pump 36 in flow rate for any given fluid dynamic head is a function of pump speed. Turbine 45 exhausts to condenser 18 through conduit 48.

Turbines

11 and 16 drive electric generator 49 through shaft 50. During start up of the unit, a minimum fluid flow rate is required for circuits 4 and (i.e. one-third of rated flow) to assure fluid distribution stability and protect the circuits from overheating after lighting off burners 51.

Conduit 52 connects to conduit means 8 between

conduits

5 and 6. Conduit 52 could alternative or additionally connect to conduit means 8 between

conduits

6 and 7 within the intent of the present invention. Conduit 52 discharges through conduit 53 and flow control valve 54 to the shell side of heater 40, through conduit 55 and flow control valve 56 to deaerator 32 and through conduit 57 and flow control valve 58 to condenser 18.

The combined opening of

valves

54, 56 and 58 regulates flow through conduit 52. The opening of any one valve with respect to the others proportions the fluid discharging from conduit 52 among the deaerator 32 high pressure heater 40 and condenser 18. Thus, fluid flow rate through conduit 52 can be maintained at a preset value to assure adequate minimum fluid flow rate through conduits 4 and 5 during startup while at the same time fluid discharging from conduit 52 can be apportioned among

conduits

55, 53, and 57 within the capability of the fluid flow rate through conduit 52 to additionally regulate deaerator 32 steam pressure, heater 40 steam pressure or waste heat to condenser 18 as an overflow for

conduits

55 and 53. The apportionment of flow among

conduits

55, 53 and 47 is not an essential part of this invention. The essential element is the control of total fluid flow through conduit 52.

Fuel is supplied to burners 51 through feeder pipe 59 and flow control valve 60. Airflow is supplied to burners 51 parallelly with fuel flow (not shown). Thus, burners 51 may supply the necessary heat input to steam generator 1 to produce superheated steam at outlet 3.

F [(3. 2 shows a heat transfer means for raising the enthalpy of fluid exiting from conduits 5 before the fluid enters conduits 6 to be used during startup andwhich may optionally be included in bypass conduit 52 shown on FIG. 1. In such case flow control valve 61 is installed in conduit 8. Bypass fluid flows through conduit52. Heat exchanger 66 is equipped with supply manifold 63, tubular heat exchange surface 64 and collection manifold 65 serially arranged between portions of conduit 52. Fluid discharging from conduits 5 also may pass through conduit 67, pressure reducing valve 68, conduit 69 to the shell side of heat exchanger 66. High-pressure, high temperature fluid from conduits 5 when reduced in pressure through valve 68 has a reduction in temperature in conduit 69 although fluid enthalpy remains constant. Valve 61 would be closed or throttling at such time. Thus, a differential temperature exists between the fluid internal to heat exchange surface 64 and the fluid from conduit 69 passing through the shell side of heat exchanger 66. When fluid is passing through conduit 67. there is a fluid enthalpy decrease across heat exchanger 66 in conduit 52. Conversely, there is a fluid enthalpy increase across heat exchanger 66 from conduit 69 to conduit 70. Conduit 70 which connects to conduits 6 through conduit 8 completes the bypass around valve 61. The heat exchanger provides a means for furnishing dry steam to conduits 6 when the fluid exiting from conduits 5 contains some moisture and fluid flow rate through conduits 5 is greater than fluid flow rate through conduits 6.

FIG. 3 is a flash tank arrangement which may optionally be installed in FIG. 1 conduit 52. The flash tank system will produce similar results to those of heat exchanger 66. ln HO. 3, flow control valve 61 is installed in conduit8 between portions of heat absorption conduits between 2 and 3. Pressure reducting valve 71 reduces conduit 5 fluid pressure before the fluid enters flash tank 72. Water and steam separates in flash tank 72. Steam may pass to primary superheater 6 through conduit 70. (Conduit 70 could connect to conduit 8 between

conduits

6 and 7 or other equivalent location within the objectives of this invention. Valve 61 is upstream of the location where conduit 70 connects to conduit 8. Steam may pass to heater 40 through conduit 53 and flow control valve 54, to deaerator 32 through conduit 55 and flow control valve 56 and to condenser 18 through conduit 57 and flow control valve 58. The liquid drains from flash tank 72 pass through conduit 73 and flow control valve 74 to some convenient location in the feedwater cycle as the closed feedwater heaters, deaerator or condenser (not shown). Valve 74 is power operated in response to flash tank water level (not shown).

In the case of FIG. 3, when there is fluid flowing through conduit 70, the quantitative measure of fluid discharging from conduits 5 which bypasses the superheated steam outlet 3 is the totalized flow of fluid through

conduits

53, 55, 57 and 73.

The general principles and means of this invention are illus trated in FIG. 1. The control system employs means either of the electric or pneumatic type the components of which are of standard design and commercially available to perform the functions indicated. The control circuits are shown by dash lines and convey signals in the form of current, voltage or pressure which are calibrated throughout their range as a scalar value or quantative measure or representation of flow, pressure, temperature, megawatts or demand. I

A signal representative of plant MW demand is generated in unit 75. Circuit 76 conveys this signal to ratio relay 77. The output of relay 77 feeds to the firing rate controls through circuit 78, to the feedwater flow controls through circuit 79 and to the bypass system fluid flow control means through circuit Fluid pressure at some point in conduit 8 (upstream of valve 61 when used) is measured in transmitter 81 and is conveyed to difference unit 82 through circuit 83. A control signal is generated in set point unit 84 and transmitted through circuit 85 to difference unit 82 in which the circuit 83 signal is subtracted from the circuit 85 signal as the output signal in circuit 86. Pressure in conduit 8 above set point decreases the signal in circuit 86. Deviation of the signal in circuit 86 from a neutral value is the pressure error to proportional and integral unit 87. The output from unit 87 feeds to ratio relay 77 through circuit 88. Pressure in conduit 8 above set point lowers the output signal from relay 77 in circuits 78, 79 and 80 until pressure returns to set point from the integral action of unit 87.

Pressure corrected demand signals in circuits 78, 79 and 80 are ranged for fuel flow, feedwater flow and bypass systems flow respectively in

proportional relays

89, and 91. The outputs from relays 89 and 90 feed to ratio relays 92 and 93 respectively through circuits 94 and 95. The signals are temperature corrected in

relays

92 and 93. r

Temperature in conduit 9 is sensed by thermocouple 94a and conveyed to temperature transmitter 95a through circuit 96. Transmitter 95a gains the signal to the measuring level of the system. The output from 95a feeds to difference unit 97 through circuit 98. Set point generator 99 produces a signal representative of desired temperature in conduit 9 and is transmitted to difference unit 97 through circuit 100. In unit 97 the input' from circuit 98 is subtracted from the input from circuit 100. The output from 97 in circuit 101 is temperature error in conduit 9 which feeds to proportional and integral controller 102. When steam temperature in conduit 9 is below set point, the output from 102 in circuit 103 increases until the two are equal and vice versa. Circuit 103 feeds to ratio relay 92 through circuit 104.- As the scalar value in circuit 104 increases, the output from relay 92 in circuit 105 increases as the input from circuit 94 is multiplied by the input from circuit 104 in relay 92. Thus, steam temperature in conduit 9 below set point increases the demand for firing rate in circuit 105.

The output from circuit 103 feeds through circuit 106 to reversing proportional relay 107. In relay 107, the input signal is reversed about a neutral valve. Thus, as the input signal in circuit 106 is increased, the output signal in circuit 108 is decreased an equivalent amount. The gain of the error may also be proportioned in relay 107. The circuit 95 value is multiplied by the scalar value of circuit 108 in ratio relay 93. The output in circuit 109 is the temperature corrected demand for feedwater flow. Thus. steam temperature in conduit 9 below set point decreases the demand for feedwater flow in circuit 109 and vice versa.

Alternatively temperature correction in conduit 103 may be applied to only one of

relays

92 or 93, the unused relay being omitted. In such case, temperature error will interact with pressureerror.

Circuit 105 feeds to fuel flow controller 110 which regulates power operator 111 for fuel flow control valve 60 through circuit 112. Fuel flow rate in conduit 59 is sensed across orifice 113 and is measured in flow transmitter 114 and transmitted to controller 110 through circuit 115. Controller 110 compares the input from circuit 105 with the input from circuit 115 to determine the fuel flow error and integrates the output signal in circuit 112 to increase or decrease fuel flow until the error in flow rate is corrected.

The circuit 109 feeds to high selector 116. A minimum demand for feedwater flow is generated in setter 117 and is transmitted to selector 1 16 through circuit 118. High selector 116 selects the higher of the signals in

circuits

109 and 118 for transmission through circuit 119 to feedwater flow controller .120. Controller 120 through circuit 122 regulates power operator 121 for steam flow control valve 46. Valve 46 regulates steam flow to turbine 45 which in turn regulates pump 36 speed and feedwater flow rate in conduit 38.

Orifice 123 senses feedwater flow rate in conduit 38 which is measured and converted to a system working signal in flow transmitter 124. Circuit 125 conveys this signal to controller 120 which compares it to the demand signal in circuit 119 to develop a feedwater flow error. Controller 120 integrates this error to open or close valve 46 until the error is zeroed.

Firing rate error in circuit 105 is conveyed to air flow controls (not shown) through circuit 126. Control of airflow parallels control of fuel flow.

The plant demand signal generated in unit 75 is transmitted through circuit 126a to the turbine 11 steam admission controls for regulation of MW output from generator 49. Proportional relay 127 ranges the signal to the characteristics of the control loop. The output from relay 127 feeds through circuit 129 to difference unit 128. Stage steam pressure within turbine 11 is a measure of steam flow or turbine generator load and is sensed at point 130 and measured in transmitter 131 and transmitted through

circuits

132 and 133 to difference unit 128. Ratio relay 134 calibrates the stage pressure signal in accordance with MW error.

Difference unit 128 subtracts the input from circuit 133 from the input from circuit 129 and the output in circuit 135 is the turbine stage pressure error. The circuit 135 error feeds to controller 136. Controller 136 through integral action opens steam admission valves through circuit 137 and power operator 138 in response to stage pressure below the set point of circuit 129 and vice versa. Controller 136 functions until error in circuit 135 is zeroed.

Electrical MW output from generator 49 is sensed in means not shown, measured and transmitted through circuit 139 to difference unit 140. Unit 140 also receives a plant demand signal from unit 75 through circuit 141. The output in circuit 142 is the circuit 141 input subtracted from the circuit 139 input. Proportional and integral controller 143 increases or decreases the output signal in circuit 144 to relay 134 until MW error is zeroed. lf actual MW in circuit 139 is below demand in circuit 141, the circuit 142 error decreases the output in circuit 144 which in turn decreases the output of relay 134 in circuit 133. As the signal in circuit 133 decreases, the demand for steam flow in circuit increases opening valves 10. The reverse process decreases MW output.

The above-described systems are known and by themselves do not constitute the present invention. In the past when fluid was flowing through conduit 52 it has been customary to control pressure in conduits 8 solely by means of the flow control means associated with conduit 52 or in conjunction with valve 61 when used. As a consequence thereof, a control transfer was required whenever increasing or decreasing unit load above or below that value corresponding to the minimum required feedwater flow and in the once through mode of operation. The transfer has proven to be awkward and cumbersome.

This invention overcomes past difficulties in that for all unit loading conditions control of fluid pressure is a continuous and similar process. Firing rate may participate in pressure control even when the bypass system is in operation. At such time since feedwater flow is maintained at a predetermined minimum value (i.e. one-third of rated flow), the bypass system flow control valves participate in control of fluid pressure only to the same extent as is accomplished by changing feedwater flow rate when the bypass system is inoperative. Thus, when the bypass system is in operation, both firing rate and the bypass system flow control valves may be responsive to pressure error.

Plant demand from unit 75 is fed to function generator 145 through circuit 146. The output signal in circuit 147 is so characterized to provide a feed forward signal for total port opening of

valves

54, 56 and 58. This minimizes the degree of correction required from integral action in unit 148.

Pressure corrected boiler demand is fed through circuit 80 to proportional relay 91 where the signal is ranged to units of fluid flow. As an alternative to circuit 80, circuit 148a may supply a pressure and temperature compensated signal to relay 91. In such case circuit 80 would be omitted and vice versa for circuit 148a when circuit 80 is employed.

The output of relay 91 feeds to summer 149 through circuit 150. Fluid flow rate in conduit 52 is sensed by orifice 151 and is measured in transmitter 152 and conveyed through circuit 153 to summer 149. The output in circuit 154 is the total of the demand for steam flow to the main turbine plus the superheater bypass flow and is equivalent to feedwater flow. Circuit 154 feeds to difference unit 155. Minimum feedwater flow set point from 117 feeds through circuit 156 to unit 155. In unit 155 the input from circuit 154 is subtracted from the input from circuit 156 and the output in circuit 157 is the bypass system flow rate error. As pressure in conduits 8 increases above set point the output from ratio relay 77 decreases. Firing rate decreases and circuit 154 input to unit 155 decreases. Output in circuit 157 increases which feeds to proportional and integral unit 148 to increase the output in circuit 158 which combined with circuit 147 input to summer 159 increases the total port area opening of

valves

54, 56 and 58. The reverse occurs when the pressure in conduits 8 falls below set point.

The output from summer 159 feeds to valve 56 controller operator 160 through circuit 161, proportional ranging relay 162 and circuit 163. Relay 162 sequences valve 56 with respect to

valves

54 and 58. Valve 56 opens first. The output from summer 159 feeds to valve 54 controller operator 164 through circuit 165, negative bias unit 166, circuit 167, proportional ranging relay 168 and circuit 169. Relay 166 provides a negative offset which permits valve 56 to open a certain amount before valve 54 begins to open. Relay 168 sequences valve 54 throughout a specific range of change of conduit 167 input signal. Valve 54 opens second after valve 56 has opened a predetermined amount.

The output signal from summer 159 feeds to valve 58 operator controller 170 through circuit 171, negative bias unit 172 circuit 173, proportional ranging relay 174 and circuit 175 which functions similar to the valve 54 control loop. Valve 58 opens last after valves 56 and 54 have opened predetermined amounts.

In the case of FIG. 1 it is relatively easy to measure bypass system fluid flow rate separately from fluid flow rate through the superheater. Thus. bypass conduit 52 fluid flow rate can readily be totalized with steam generator demand corrected for pressure for comparison with minimum feedwater flow set point for determination of bypass flow control means error.

The same is also true where a heat exchanger is installed in conduit 52 as shown on FIG. 2. In such case flow orifice 151 is installed in conduit 52 downstream of heat exchanger 66.

When a flash tank system is installed in conjunction with bypass conduit 52 as shown on FIG. 3 there exists a problem of segregating superheater bypass system fluid flow from fluid flow through the superheater. It is possible to install flow orifices in both of conduits 52 and 70 (not shown) and subtract the flow in conduit 70 from the flow in conduit 52. Other combinations of flow meters (not shown) are possible to produce similar results.

An alternate control arrangements is shown on FIG. 4.

In the case where:

The control formula for summer 149 and difference unit 155 on FIG. I is:

BVe= MinF'F BPF- (D) pc.

lll'inFF BPF MW.

BVe= M W- (D) 11 Also:

Therefore:

Such formula is employed in FIG. 4 which is a modification of a portion of FIG. I. In FIG. 4 circuit 80 connects directly to difference unit 155. Where circuit 148a is used as an altemative for circuit 80, ranging relay 91 would be retained. Flow orifice 151, transmitter 152, circuit 153 and summer 149 are deleted. A branch from circuit 139 which is a measure of generator 49 MW electrical output is fed to difference unit 155 as a substitute for circuit 156. The output from unit 155 remains the same as for FIG. 1.

Thus, in the FIG. 4 modification, unit demand pressure corrected is compared with MW output to determine the bypass system flow control means error. In the case where a flash tank system is installed with conduit 52 as shown in FIG. 3, the flow control means can be valve 71 with operator controller 176. The output of summer 159 on FIG. I would feed to controller 176 via circuit 177 as a substitute for

circuits

165, 161 and 171 et al.

As pressure in conduits 8 sensing point increases above set point, the input to unit 155 from conduit 80 or conduit 148a in FIG. 4 decreases which increases the output in circuit 157 opening the bypass system fluid flow control valves. The reverse happens where pressure decreases below set point. Where the MW input to unit 155 from circuit 139 decreases with respect to the demand input from circuit 80 or circuit 148a, the output in circuit 157 decreases closing the bypass system fluid flow control valves. This is a correct relationship between demand and MW output. The inverse relationship is also a proper consequence. In the above discussion of FIG. 4 a measure of turbine stage pressure could substitute for actual MW output.

Valve position of the steam generator flow control means can be employed to approximate relative quantities of fluid flows through the respective bypass system and superheater conduits.

*An illustration may be found as described below wherein a FIG. 3 flash tank system is installed in conjunction with bypass conduit 52 of FIG. I. The position of valve 71 plus the position of valve 61 minus the position of steam admission valve 10 corrected for throttle pressure in conduit 9 is a representative measure of fluid which is bypassing the superheater in its recirculation path through conduits 5 and is equivalent to flow measurement at orifice 151 for FIG. I. FIG. 5 shows a control means for performing this function as a substitute for flow orifice 151 and flow transmitter 152 of FIG. I.

Valve position transmitters

177a, 178 and 179 are associated with

valves

71, 61. and 10 respectively.

Circuits

180, 181 and 182

connect transmitters

177a, 178 and 179 respectively to proportioning relays 183, 184 and 185. The latter range the respective inputs from the individual valves to an equivalent basis for totalizing port area in summer 186.

Circuits

187, 188 and 189 connect relays 183, 184 and to summer 186. In summer 186, the input of circuit 189 is subtracted from the inputs of

circuits

187 and 188. The output of summer 186 feeds to summer 149 as shown on FIG. I through circuit 153. The output of summer 159 on FIG. I feeds to valve controller 176 through circuit 177 as shown on FIG. 3. The control action is otherwise identical to that described for FIG. I.

In FIG. 5 ratio relay 190 compensates the input to unit 185 for variations in throttle pressure in conduit 9. Pressure transmitter 191 senses pressure in conduit 9 or other alternative location downstream of valve 61 of FIG. 3 (not shown), measures pressure and transmits the measurement to function generator 192 through circuit 193. Function generator 192 is characterized with pressure to compensate valve port area for any give valve position so as to give an equivalent measure of fluid flow for comparison with flows associated with

valve

71 and 61 positions. The characterization roughly follows fluid specific volume change with pressure change. Any or all of the valve positions may be compensated similarly for fluid temperature in approximating fluid flow quantities (not shown).

Thus, it will be seen that l have provided an efficient embodiment of my invention whereby a means is provided for improving control of once-through-type steam generators through use of the same control methods and devices both during startup when operating with the bypass system in use and later in the once-through mode with the bypass system shut down, wherein a method is provided for controlling fluid pressure upstream of the bypass conduits during startup at the time when feedwater flow rate is held to some preset minimum value through a comparison of pressure corrected steam generator demand and bypass system fluid flow rate with set point for minimum feedwater flow rate or other equivalent combination thereof, wherein said method may provide steam temperature corrections of steam generator demand, wherein said method includes variation of firing rate for fluid pressure control, wherein steam generator pressure corrected demand vs. MW electrical output is an equivalent combination for comparison, and wherein valve position may be employed to determine relative bypass system fluid flow rate.

While I have illustrated and described several embodiments of my invention, it will be understood that these are by way of illustrations only, and that various changes and modifications may be made within the contemplation of my invention and within the scope of the following claims:

Iclaim:

1. In a steam electric generating plant having a steam generator comprising a feedwater inlet, superheater steam outlet and heat absorption conduits connected therebetween, a steam turbine driver as prime mover for electric generating means, first fluid conduits means serially interconnecting said feedwater inlet, heat absorption conduits, superheated steam outlet and steam turbine driver, a bypass conduit means connected intermediately between portions of said heat absorption conduits and having means to flow fluid away from the portion of said heat absorption conduits which are upstream of said bypass conduit means, a source of feedwater supply to said feedwater inlet having fluid flow control means, a source of heat input to said heat absorption conduits having means to quantitatively regulate input, flow control means for proportioning fluid entering said feedwater inlet between said bypass conduit means and said heat absorption conduits which are downstream of said bypass'co nduit means, steam admission flow control means for said turbine driver; the method for low load operation of said steam electric generating plant which includes the steps of establishing and maintaining a minimum flow of fluid in said steam generator conduits upstream of said bypass conduit means, adding heat to said fluid from said source of heat, measuring fluid pressure representative of the pressure in said conduits upstream of said bypass conduit means, subtracting said measure of actual fluid pressure from a desired set point for fluid pressure to generate a measure of pressure error, adding said pressure error to a measure of required output from said generating means to generate a pressure corrected measure of demand, calculating the quantity of fluid flowing through said bypass conduit means from measurements representative of fluid flow rate in said system circuits, adding said pressure corrected measure of demand to said calculation of said bypass conduit fluid flow rate, subtracting the total from a measure of said minimum rate established for fluid flowing in said upstream steam generator conduits to generate an error for said proportioning means by use of which said proportioning means regulates the quantity of fluid flowing through said bypass conduit means.

2. In a steam electric generating plant as recited in claim 1, the method for low loadoperation of said plant which includes the steps of establishing and maintaining a minimum flow of fluid in said steam generator circuits upstream of said bypass conduit means, adding heat to said fluid from said source of heat, measuring fluid pressure representative of the pressure in said conduits upstream of said bypass conduit means, subtracting said measure of actual fluid pressure from a desired set point for fluid pressure to generate a measure of pressure error, adding said pressure ir'arto'a measure of required output from said generating means to generate a pressure corrected measure of demand, calculating power output of said turbine-generator set from a representative measure of said power output, subtracting said measure of demand from a measure of said power output to generate an error for said proportioning means by use of which said proportioning means regulates the quantity of fluid flowing through said bypass conduit means.

3. In a steam electric generating plant and method as recited in claim 1, the method including in addition measuring steam temperature at said superheater outlet, subtracting a measure of desired steam temperature from said actual measure of steam temperature to generate a measure of temperature error as applied to feedwater flow correction, adding said temperature error to said measure of required output in addition to adding said pressure error to generate a pressure and temperature corrected measure of demand before proceeding further with computations using said corrected measure of demand.

4. In a steam electric generating plant and method as recited in claim 1, the method including in addition, adding at least a portion of said pressure error to a measure of required heat input to said heat absorption conduits.

5. In a steam electric generating plant and method as recited in claim 1, the method including calculating the quantity of fluid flowing through said bypass conduit means from valve position measurements, said valves being a part of said fluid proportioning means.