Boiler control system

BACKGROUND OF THE INVENTION

This invention relates to systems for controlling the boiler inputs and the turbine throttle valve in accordance with the energy demand established for the turbine to provide the desired electrical output of the generator driven by the turbine. More specifically, this invention is concerned with the establishment of separate demand signals each indicative of the desired value of one of the inputs to the boiler and the control of the turbine throttle valve to maintain a balance between the boiler inputs and the turbine input. The level at which that balance is established is controlled in accordance with the energy required from the system. The invention is particularly concerned with the control of the boiler inputs and the turbine throttle valve under those conditions in which one or more of the inputs is limited, particularly as to magnitude or rate of change.

For the purposes of this description, the term “energy” is used to mean energy per unit of time, more accurately referred to as power.

The boiler control systems used in the past have generally incorporated complicated arrangements for maintaining the desired relationship between the boiler inputs under conditions in which the magnitude of the individual inputs or their rate of change may be limited. The present invention provides a simplified and more comprehensive means for controlling the boiler inputs and the turbine throttle valve under such conditions.

It is an object of this invention to provide an improved boiler control system.

It is another object of this invention to provide a simplified control system for maintaining the required boiler inputs and the required throttle valve opening for the turbine connected to the boiler so as to maintain, under conditions of limitation as to the magnitude and/or extent of change which can be made in any one of the inputs, the proper relationship between each of the inputs as well as the proper relationship between those inputs and throttle valve opening itself.

SUMMARY OF THE INVENTION

In carrying out this invention there is provided a boiler control system which utilizes a measure of the boiler load to produce a signal representing the desired boiler demand. There is derived from that signal separate demand signals associated with the separate inputs to the boiler so as to control those inputs to have a predetermined relationship as well as to meet the boiler load while maintaining the desired steam pressure at the boiler output. The improvement comprises means for modifying the desired boiler demand signal to produce a signal that represents a required boiler demand and to produce from 5 that signal separate demand signals for each boiler input.

This means would include means for producing an error signal which varies in accordance with the difference between a signal responsive to a measured value of a boiler input and the corresponding separate demand signal. 10 When that error signal exceeds a predetermined allowed deviation, the modification of the desired boiler demand signal is such that the magnitude of the required boiler demand signal which is produced changes the separate demand signals derived therefrom so as to maintain the 15 boiler inputs in their predetermined relationships. Additionally, means can be provided which are responsive to the difference between the desired boiler demand signal and the required boiler demand signal and which are operable to control the steam flow to the turbine so as 20 to tend to maintain the difference between the desired and required boiler demand signals within a predetermined tolerance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a boiler control system.

FIG. 2 is a detailed circuit diagram of that portion of FIG. 1 which provides for the improvement in the operation of the control system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 the boiler 10 is provided with the usual inputs such as a fuel flow through line 12 which is com„_r bined at the burner 14 with the air flowing through line ⁰⁰ 16 so as to provide the necessary heat input to the boiler while the feedwater flow is provided through line 18. The air flow input through line 16 is subject to control by the adjustment of valve 20 by the controller 22 which is shown as a controller providing both proportional and integral action as noted by the notation PI in block 22. The air flow through line 16 is measured by the flowmeter 24 which by means of the associated pressure taps responds to the pressure differential established across the flow ₄₅ restriction 26 in line 16.

Similarly, the fuel flow in line 12 is subject to control by the adjustment of valve 30 by the controller 32 which is similar to controller 22. Also, the fuel flow is measured by flowmeter 34 in response to the pressure drop 50 across the orifice plate 36, the assumption being that the fuel supplied is either a liquid or a gaseous fuel.

The feedwater flow through line 18 is subject to control by the valve 40 in response to the action of the controller 42 which is a controller of the same type as _5g those utilized to control both the fuel and the air flow, namely controllers 22 and 32, respectively. Similarly, the feedwater flow is measured by the flowmeter 44 in response to the pressure differential across the orifice plate 46.

The concentration of oxygen in the fuel gases is measured at the boiler stack 48 by the oxygen concentration measuring device 50.

Still other measurements are made of the boiler outputs. For example, the temperature measuring device 65 52 responds to the temperature detected by thermocouple 54 which is located in the steam line 60 from the boiler 10, Additionally, the steam pressure produced by the boiler in the output line 60 at the turbine throttle is measured by the pressure measuring device 62 by means 70 of the single tap 64 connected to the output steam line 60.

Another measurement which is necessary for the

3,545,207 particular control system shown in FIG. 1 is a measurement of the pressure in the first stage or impulse chamber of the turbine 68 which is shown as being made by the pressure measuring device 70 which is connected by the tap 72 to the first stage or impulse chamber of turbine 68. The pressure measured by the device 62 is noted as P_t while the pressure measured by the device 70 is noted as P,.

As shown in FIG. 1, the turbine 68 which is supplied with steam from boiler 10 by way of line 60 is mechanically coupled to the generator 74 to produce an electrical output on the output lines 76. The power output on the lines 76 is measured by wattmeter 80 to provide on line 81 a signal representative of the actual output of the generator, namely G_a.

For the purpose of establishing the desired output of the generator 74 there is provided at terminal 82 a signal G_d representing the desired generation for generator 74. The signal G_d may be obtained from a simple hand-set potentiometer, for example, or as an output from a complicated load dispatch computer.

As in most power systems, the mechanical coupling between the turbine and the generator, namely the coupling 84, rotates at a speed which is utilized as one input to a governor mechanism represented by the block 86. The mechanical output of 86 by means of the mechanical coupling 88 controls the position of the throttle valve 90 in the steam output line 60 and the valve 90 in turn controls the steam flow to the turbine 68.

The speed governor 86 is normally subject to adjustment of its characteristic in response to signals from the load dispatch system which establishes the signal G_d so that the particular opening which exists in valve 90 for a particular frequency or a particular rotational speed of mechanical coupling 84 may be modified so as to change the electrical power output in response to the changes in the signal G_d while maintaining a predetermined frequency in the electrical system. The modification of the governor characteristic is provided by the input signals on lines 91 and 92 which input signals are provided to the governor motor in the governor 86 so as to modify its speed response characteristic.

In providing the required signals on lines 91 and 92 the signals G_d and G_a are compared in the comparator 96. The difference between those two signals then appears on line 98 as an input to controller 100 which is a control having proportional, integral and derivative response, as indicated by the notation FID in block 100. That controller provides an output on line 102. The output on line 102 is introduced as one input to comparator 104. The other inputs to comparator 104 are the signal G_d which is supplied on line 106 and a signal Ρχ/Pt which is supplied on line 108. The signal on lines 106 and 108 are compared and the difference between them is added to the signal on line 102 so as to provide the output of comparator 104 on line 110. That output is then the input to the controller 112 which is shown as having both a proportional and integral response.

The signal provided on line 108, namely Pi/Pt, is a signal which is indicative of the effective opening of the throttle valve 90 since it is derived by dividing the output of the first stage or impulse chamber pressure measuring device 70 by the output of the throttle pressure measuring device 62 in the dividing network shown at block 114. The output of the dividing network 114 appears on line 116 which is connected to line 108. Reference should be made -to the description in the U.S. Pat. 3,247,671 which issued to James H. Daniels on Apr. 26, 1966, for a more complete description of the significance and the manner for producing the signal Pi/P_ton line 116.

The controller 112 may be of the type disclosed in U.S. Pat. 3,008,072 and may be designed so as to produce on its output lines 120 and 121 electrical pulse signals whose duration depends upon the extent of control action called for by the controller 112. The line 120 supplies pulses to relay operator 122 by way of relay contact 124α which is normally subject to actuation by relay operator 124 under conditions to be described later. When the relay operator 122 is energized, its relay contact 122α is closed so that the potential supplied at the terminal 126 can be connected through relay contact 128α and relay contact 130α to line 91 and hence to the governor motor in governor 86. The relay contacts 128α and 130α are subject to being opened under conditions which will be described later. Any pulses which are provided from controller 112 to line 91 causes the governor to act in such a way as to decrease the opening in the valve 90 for operation of the turbine at a particular speed and hence at a particular electrical frequency on the lines 76 so as to decrease the power output of the generator 74.

In a similar way the output from the controller 112 through line 92 to governor 86 will cause the governor to open the valve 90 to a wider opening so as to provide for an increased output from generator 74. The pulses provided to line 92 from the controller 112 are supplied by way of line 121, relay contact 134α, which is subject to the actuation by the relay actuator 134 so that the pulses supplied on line 121 when the relay contact 134α is closed causes an energization of the relay actuator 132 to close the relay contact 132α and thereby connect the potential supplied at terminal 126 through the closed contact 132α, and the contact 1306 as well as contact 1286 to line 92. In FIG. 1, the contacts 130α and 1306 are shown in the state which is normal when their actuator 130 is de-energized. Also, the contacts 128α and 1286 are shown in the state which is normal when their actuator 128 is de-energized.

Having described the manner in which the electrical output of the generator 74 is subject to control in response to the signal supplied at terminal 82, there will now be described the manner in which the feedwater, fuel and air inputs to the boiler 10, as supplied by lines 18, 12 and 16 respectively, are controlled so as to provide the desired steam pressure P_t in line 60 as well as the desired steam temperature T_s while maintaining the desired oxygen concentration in the stack 48 and preserving a balance between the boiler inputs and the turbine input.

The signal provided on line 116, namely Pj/Pt which is representative of the effective opening of valve 90, is therefore also representative of the energy demand established for the turbine 68 in accordance with the desired generation from generator 74 as established by the signal G_d. The turbine energy demand signal supplied on line 116 is subject to modification in accordance with the deviation of the throttle pressure P_t measured in line 60 from its desired value or set point. This modification is accomplished by utilizing the output signal of the pressure measuring device 62 which is supplied on line 140 as an input to controller 142 where it is compared with the set point for steam pressure as set in the controller. The controller has proportional, integral and derivative action so as to provide an output on line 144 which acts as an input to the multiplier 146 whose other input is derived from line 116. Then the output of the multiplier on line 148 is added to the signal on line 116 by the summing unit 150 so as to provide an output on line 152 which is a signal representative of the desired boiler demand. In the usual control system the desired boiler demand signal, DBD, on line 152 would correspond with the required boiler demand signal, RBD, which is shown in FIG. 1 as appearing on line 154. That signal on line 154 would then be utilized as one of the separate demand signals for controlling the inputs to the boiler 10 and in particular it would be utilized to control the feedwater control to the boiler 10, thus the signal from line 54 is supplied over line 156 as a feedwater demand signal to the comparator 158 where it is compared with the output of the flow3,545,207 meter 44 which is supplied on line 160 to the comparator 158. There is then produced as an output from comparator 158 on line 162 an error signal e_w representing the deviation of the feedwater flow in line 18 from the desired feedwater flow as established by the demand signal on line 156. The error signal e_w is utilized as an input to the controller 42 which then establishes in response to that error signal a change in opening of the valve 40 so as to attempt to control the feedwater flow through line 18 to a value corresponding to that represented by the signal on line 156.

In order to maintain the desired relationship between the feedwater flow and both the fuel flow and air flow, the signal RED on line 154 is modified by the multiplier 166 in response to the deviation of the steam temperature T_s from its set point. The steam temperature measuring instrument 52 supplies a signal on line 168 representing the measured temperature while the set point is established in the controller 170. The signal on line 168 is supplied to the controller 170 which is a controller providing proportional, integral and derivative action in response to the deviation of the measured steam temperature from the set point to produce the output signal which appears on line 172.

The output line 172 is introduced as one input to the multiplier 166, the other input being from line 154 so that the output of the multiplier on line 174 to the summing unit 176 provides a modifying signal for establishing on the output line 178 of the summing unit 176 another separate demand signal which can be utilized to establish the required fuel flow to the boiler 10 as needed to maintain the desired temperature in the steam line 60 whenever the feedwater flow in line 18 is in accordance with that value represented by the signal on line 156. The signal on line 178 is introduced by way of line 180 to the comparator 182 where it is compared with the fuel flow signal on line 184 derived from flowmeter 34. The result of the comparison provides an error signal e_t on line 186 which then provides an input to controller 32 by way of line 188 so that the controller 32 can adjust the valve 30 to maintain the fuel flow in line 12 at a value corresponding with the fuel demand established by the signal on line 180.

The desired relationship between the fuel flow and the air flow is normally maintained by responding to deviations of the oxygen concentration in the stack 48 of the boiler 10 from its desired value; thus there is provided a multiplier 190 which has as its other input a signal from line 192 which signal is the output of controller 194 whose input is derived from line 196 and whose input is therefore representative of the deviation of the oxygen concentration in the stack 48 from its desired values as determined from the oxygen concentration measuring instrument 50 and the set point adjustment so as to provide for the establishment of the desired oxygen concentration. The output of the multiplier 190 on line 200 is summed with the signal on line 178 by the summing unit 202 so as to produce on the output line 204 of the summing unit 202 a separate demand signal representative of the demand for air flow through line 16 as required to maintain the desired oxygen concentration in the boiler stack 48 when the fuel flow in line 12 is in accordance with the demand established by the signal on line 180. The signal on line 204 is compared with the signal on line 206 which is derived from the flowmeter 24 and represents a measure of the air flow in line 16. The signal on line 206 is compared with the signal on line 204 by the comparator 208 so that there is provided on the output line 210 an error signal e_awhich provides an input to the controller 22 so that the controller can adjust the valve 20 to modify the air flow in line 16 so as to maintain as closely as possible the air flow demand as established by the signal on line 204.

In order to provide for a balance between each of the inputs to the boiler, namely the feedwater, fuel and air, and to regulate those inputs in accordance with the variable turbine input as established by adjustment of the throttle valve 90 under conditions when any one of the boiler inputs may be limited as to the rate at which it can change or to its magnitude, it is necessary to provide a means for establishing a signal RBD which is different from the signal DBD so that the boiler inputs will be maintained in proper balance. It is also necessary to provide means for modifying the throttle valve 90 when any one of the inputs to the boiler is limited to its magnitude or rate of change so that the input to the turbine is related to the boiler inputs. The means for providing the signal RBD different from the signal DBD and for providing a modification of the throttle valve 90 is shown in FIG. 1 as a block 220 which has as its inputs the several error signals e_w, e_t and e_a supplied from lines 162, 186 and 210, respectively, as well as an input from line 152 representing the signal DBD which signal would correspond with the signal RBD provided as an output on block 220 whenever the respective error signals e_w, e_f and e_a are zero or are less than a predetermined allowable magnitude. Other outputs of the block 220 are the signals supplied on lines 222 and 224 which, respectively, serve to actuate the relays 134 and 124 to open their respective contacts 134α and 124α. Opening the contact 134α, for example, would prevent the controller 112 from causing an increase in the output of generator 74 while the opening of the contact 124α would prevent the controller 112 from causing a decrease in the output of the generator.

Other outputs of the block 220 are the signals O_Msupplied on line 228 and the signal O_bu supplied on line 230, which respectively energizes the relay actuators 130 and 128 so as to provide for the decrease of the opening of valve 90 when the signal O_M appears on line 228 and so as to increase the valve opening when the signal O_buappears on line 230. It will be evident that if a signal appears on line 228 to energize relay 130, the contact 1306 will be opened and the contact 130α will be switched to connect to contact 130c so as to provide a path from the pulse generator 240 to line 91. On the other hand, upon the appearance of the signal O_bu on line 230 to energize relay actuator 128, the contact 1286 will be connected to the contact 128c and the contact 128α will be opened. The closing of the contact 1286 on contact 128c will allow for pulses from the pulse generator 240 to line 92 so as to cause the governor 86 to increase the opening of valve 90 and hence increase the turbine demand. The opening of the contact 128α will prevent any pulses generated by the controller 112 from causing a signal on line 91 just as the opening of the contact 1306 in response to the appearance of the signal O_M prevented any signal from appearing on line 92 as a result of the output of the controller 112.

The particular circuit which may be used for the unit shown as block 220 and identified as the flow balance interlock unit, FBI, may be any one of a number of circuits, of which FIG. 2 is an example.

Referring to FIG. 2 it will be evident from the following description that if the error signals on lines 162, 186 and 210 are either zero or within predetermined values, then the signal appearing on line 152, namely the desired boiler demand, DBD, will cause a comparable signal to appear on line 154 as the required boiler demand, namely RBD. This results from the fact that the operational amplifier 300, which is of the differential type, has an input resistor 302 at the inverting input equal to the feedback resistor 304 which connects the output of the amplifier 300 to the inverting input by way of diode 308 so as to form a unity gain amplification from the line 152 to the line 310. Similarly, the operational amplifier 312 has the input resistor 314 connected to the inverting input from line 310 with a value equal to that of the feedback resistor 316 which connects the inverting input with the output line 318 of amplifier 312 by way of diode 320 to form another unity gain amplifier so that there appears on line 322, and

3,545,207 hence on line 154, a signal of the same polarity and the same magnitude as that appearing on line 152.

The amplifier 300 has its non-inverting input connected by way of resistor 324 to a ground connection. Similarly, the amplifier 312 has its non-inverting input connected by way of resistor 326 to the ground connection.

If, for example, the signal on line 152 has a range from zero to +8 volts, the signal which will appear on line 154 will also have a similar range. Since the signal on line 152 would be positive in polarity, the signal appearing on line 306 would be negative in polarity and feedback current would flow through resistor 304 and line 310 and diode 308 so that the line 310 would be negative in polarity and of approximately the same potential as line 306, assuming no drop across the diode 308. With a negative signal on line 310 current flow through the resistor 316 will be in the direction shown by the arrow, likewise current flow through the diode 320 will be in the forward direction, namely that shown by the arrow. The source of the current through resistor 316 and the current through diode 320 is the potential -]-£ which is shown as supplied at terminal 330 which is supplied through a small resistor 332 to line 322. The amplifier 312 will serve to provide a signal at output line 318 of positive potential but of such magnitude as to be less positive than the signal on line 322. Assuming no drop through the diode 320, the potential at line 318 will be the same as that on line 322 and will be maintained by the amplifier 312 such that the current in the feedback circuit through resistor 316 maintains the junction point between resistors 314 and 316 at zero potential; hence the potential at line 322 will be maintained equal to that on line 310 but of opposite polarity.

Whenever one of the error signals e_w, e_f or e_a exceeds a predetermined value, it is desirable to cause the signal RBD on line 154 to represent a modified value of the signal DBD on line 152 rather than the same value and as previously mentioned. If, for example, the error signal e_wwhich appears on line 162 can vary from —10 to -f-10 volts, the network which includes operational amplifier 338>v can be so arranged that when the signal on line 162 deviates from zero in a negative direction or in a negative polarity by an amount in excess of a predetermined value, the diode 340w can become conductive so that the line 310 will become more negative than the line 306 thus backbiasing the diode 308 and causing the amplifier 338w to be the amplifier which is effective to determine the potential on line 310.

To establish the predetermined allowable limit which the error signal e_w must exceed in order to cause the amplifier 338w to be effective in establishing the potential on line 310, there is provided an adjustable contact 342iv on potentiometer 344w which is adjustable in accordance with the magnitude of that predetermined limit. It will be noted that one of the inputs to the non-inverting input of amplifier 338>v is by way of resistor 346w from line 152 to the summing junction 348w at the non-inverting input. Another input to the summing junction 348w is by way of resistor 350w from line 351 which is connected to the terminal 330 and hence to the potential source +E. The input from the contact 342w to the summing junction 348w is by way of input resistor 352iv. Effectively, the potential -j-E supplied by way of line 351 and input resistor 350iv represents the predetermined allowable limit which should not be exceeded by the error signal e_w, and the input signal from line 152 through input resistor 346iv to the summing junction 348iv modifies the predetermined limit which the error signal e_w should not exceed, which modification is made in accordance with the magnitude of the required boiler demand signal, so that the predetermined limit, which the error signal e_w must exceed before amplifier 338w determines the potential on line 310, varies with the magnitude of the desired boiler demand signal on line 152.

Since the summing junction 348iv at the non-inverting input of amplifier 338iv is effective to compare the inputs through the three input resistors 346iv, 350vv and 352w, it is possible to effectively modify the predetermined limit which the error signal must exceed by modifying that portion of the error signal itself which is utilized as an input to the resistor 352>v, namely by the adjustment of contact 342iv rather than by making a similar adjustment of the other inputs. Thus, since the inputs on lines 152 and 351 are both positive in polarity, the signal on line 162 must be negative in polarity by a certain amount to cause the summing junction 348w to go negative so that the diode 34(hv will be conductive to cause the amplifier 338>v to determine the potential on line 310 by back-biasing diode 308. When the error signal e_w causes the amplifier 338>v to determine the potential on line 310, the feedback circuit through capacitor 354w and resistor 356w to the inverting input of amplifier 338w from line 310 will have a current flow in the direction shown by the arrow causing the network including amplifier 338w to act as an integrating network so that the potential on line 310 is constantly increased in value with a negative polarity as long as the error signal e_w is negative by a sufficient amount to keep summing junction 348w negative. The inverting input of amplifier 338w is also connected as shown by a resistor 358iv to a ground connection.

The above operation of the circuit including amplifier 338w would occur if the signals e_£ and e_a are either zero or are within the predetermined limits established for them.

Each of the other error signals e_t and e_a, is connected to a similar network as is error signal e_w; thus the error signal e_f is connected to a network which includes amplifier 338/ while the error signal e_a is connected to a network which includes amplifier 338α. The other elements of the network have reference characters comparable to those shown in the network including amplifier 338iv with the exception that the suffix letter “/” or “a” is used depending on whether it is associated with the network for the error signal e_f or the network for the error signal e_a.

The networks which include the amplifiers 300, 338iv, 338/ and 338α and their associated diodes 308, 340iv, 340/ and 340α not only are effective to compare the error signals e_w, e_f and e_a with the predetermined values representing their allowable limits but also by virtue of the diodes 308, 340iv, 340/ and 340α act as an auctioneering circuit which is effective to produce a negative potential on line 310 which is the maximum of the negative potentials produced at the outputs of the amplifiers 300, 338w, 338/ and 338α.

If, for example, the feedwater flow to the boiler exceeds the feedwater demand as established by the signal on line 156 of FIG. 1, then the error signal appearing on line 162 is negative and when that signal becomes negative to a sufficient degree that it exceeds the predetermined allowable limit as established by the setting 342iv for the particular signal appearing on line 152 at the time, then the amplifier 338w will be effective to establish the potential on line 310 and that potential will be effective through the operation of amplifier 312 and its associated network to produce on line 154 a higher positive potential than would have been produced had the error signal e_w not exceeded its predetermined allowable limit. Hence, the required boiler demand is increased if the feedwater flow exceeds the demand previously established, as for example by the signal from line 152 by way of amplifier 300 and amplifier 312. By increasing the required boiler demand signal on line 154 it will be evident from FIG. 1 that there will be a comparable increase in the separate demand signals produced on lines 180 and 204 for the fuel and air flow so that an increased fuel supply and an increased air flow will be called for to match the excessive feedwater flow and to thereby maintain the balance between the several inputs to the boiler, namely the feedwater, fuel and air as re3,545,207 quired by the existing temperature in the output steam line 60 as well as the oxygen concentration in the stack 48.

When all of the error signals e_w, e_£ and e_a are either within the predetermined allowed limits at the particular desired boiler demand value established on line 152 or when those error signals are positive in polarity, the diodes 340w, 340/ and 340α are back-biased and the amplifier 300 and its associated network determines the potential on line 310. As pointed out previously, the potential on line 310 determines the potential on line 322 and hence the signal RBD by virtue of the conduction through diode 320, as long as the error signals do not exceed their predetermined allowable limits. However, if those predetermined allowable limits are exceeded in such a direction that one of the error signals, as for example e_w, is positive in polarity and its value in excess of the value of the set limit, then the amplifier 360w and its associated network will be effective to determine the potential on line 322 rather than that potential being determined by the amplifier 312. This is accomplished by a network which is somewhat similar to that shown for amplifier 338w.

A predetermined portion of the error signal determined by the adjusted allowable limit as established by the contact 342w on the resistor 344w provides a positive potential on line 36Iw which is introduced through the input resistor 362w to the summing junction 364w at the inverting input of the operational amplifier 360w which is an amplifier of the differential type as are all of the other amplifiers in FIG. 2. Another input to the summing junction 364w is provided from line 310 by way of the input resistor 366w while the third input to the summing junction 364iv comes from the terminal 367 which is supplied with a potential —E and is introduced through the input resistor 368w to summing junction 364w. The input connection to the non-inverting input of amplifier 360w is from ground through resistor 365w.

The potential —E is representative of the allowable limit on the error signal e_w while the input signal from line 310 represents the amount by which the allowable error signal is modified as the desired boiler demand signal on line 152 varies, since the potential on line 310 will vary with the signal on line 152 but will be of opposite polarity. Thus, the signals provided from line 310 and from terminal 367 are negative in potential while the signal supplied on line 361w is positive in polarity. Whenever that positive polarity input signal causes the summing junction 364w to go positive, the amplifier 360w will begin to determine the potential on line 322 since the diode 370w will become conductive and current will flow also in the integrating feedback circuit consisting of capacitor 372w and resistor 374w so that the potential on the output line 376w of amplifier 360w will become less positive than the potential at output line 318 of amplifier 312, thus drawing more current through diode 370w and resistor 332 so as to cause the potential on line 322 to decrease and hence provide a required boiler demand signal on line 154 representative of a lower required boiler demand and hence a lower feedwater flow demand that was previously required before the error signal e_w exceeded its allowable limit. With the integrating feedback circuit consisting of capacitor 372w and resistor 374w, the potential on line 154 will continue to decrease as long as the error signal e_w exceeds its allowable limit and hence the feedwater demand signal Will constantly decrease until the deviation of the feed water flow from the feedwater demand is at or within the allowable limit.

If any of the other error signals e_£ or e_& exceeds its predetermined allowable limit by an amount such that the potential on the corresponding lines 361/ or 361α exceeds the signal on line 361 w, then one of the other amplifiers 360/ or 360α will be operable to determine the potential on line 322 since it will establish on its output line 376/ or 376α a potential of lower positive value than that previously established, thus making the associated diodes 370/ or 370α conductive and causing the other diodes connected to line 322 to be back-biased. Thus, the amplifiers 312, 360w, 360/ and 360α with their associated diodes 320, 370w, 370/ and 370α form an auctioneering circuit which is effective to produce on line 154 a signal for the required boiler demand which represents the lowest boiler demand which would be required, in other words the required boiler demand determined by the error signal e_w, e_t or e_a, whichever is the greatest positive value. Since the error signals e_w, e_tand e_a are positive when the particular flow is less than the associated demand signal, when any one of those error signals exceeds its predetermined allowable limit, the RBD signal is decreased in its positive value so as to become more nearly equal to the flow of the particular quantity which has been limited and, of course, since RBD has decreased in its positive value, all of the inputs to the boiler will receive decreased demand signals and the decrease will continue to change in magnitude so as to constantly reduce the demand signal RBD until all of the error signals e_w, e₍ and e_a are at or within the predetermined allowable limits. In other words, the appropriate one of the amplifiers 338w, 338/ and 338α or 360w, 360/ and 360α will integrate to force the signal at their respective input junctions (such as 348w for amplifier 338w) to zero and thus clamp the controller errors to their limit whenever the error tries to exceed the limit. As has been set forth, this action results from the loops established. For example, the output of amplifier 338w or 360w connects through lines 154, 156 and 162 back to the input circuit of 338w and 360w.

When by virtue of one of the error signals e_w, e_£ or e_a exceeding its allowable limit in either a positive or negative polarity, the potential on line 310 or the potential on line 322 is established by an amplifier other than amplifier 300 and amplifier 312, respectively, there will be produced a signal on either line 380 or 382 to the corresponding relay amplifiers 384 or 386 so as to produce a signal on either line 222 or 224 which will be effective, as mentioned in the description of FIG. 1, to disconnect the controller 112 to prevent a change in the turbine input by adjustment of valve 90. In other words, whenever the error signals: exceed the predetermined limit, the turbine is prevented from having its input modified from the load control circuit. For example, if the signal on line 306 goes positive, that indicates that one of the signals e_w, e_£ or e_a is sufficiently negative to cause the potential on line 310 to be determined by an amplifier other than amplifier 300 and hence it is necessary to prevent the controller 112 from causing a decrease to occur in the opening of the valve 90.

Likewise, when one of the error signals e_w, e_£ or e_ais sufficiently positive to cause the potential on line 322 to be determined by an amplifier other than amplifier 312, then the signal from line 224 should prevent the controller 112 from causing an increase of the opening of valve 90.

Whenever the signal DBD on line l52 exceeds the signal RBD on line 154 by an amount in excess of a predetermined allowable limit as established by the adjustment of contact 392 on potentiometer 394, then an output appears on line 228, whereas if the signal DBD is less than the signal RBD by an amount in excess of that allowable limitation set by the positioning of contact 392, then a signal will be produced on line 230.

The signal DBD is introduced to the comparator 390 by way of line 396 and the signal RBD is introduced by way of line 398.

The diode 399 is connected between line 322 and ground so as to prevent line 322 from going negative.

In the circuit of FIG. 1 the controllers 22, 32 and 42 may be of the type shown and described in U.S. Pat.

3,545,207

2,666,170. The controllers 100, 142, 170 and 194 can be of the type described in U.S. Pat. 3,092,321.

The various components of FIG. 2 may have the following values:

Component: Value

302 40K

304 40K

324 20K

31440K

31640K

326 20K

358w, 358/ and 358α100K

356w, 356/ and 356α 200K

346w, 346/ and 346α10M

350w, 350/ and 350α 15M

352w, 352/ and 352α100K

368w, 368/ and 368α15M

366w, 366/ and 366α10M

362w, 362/ and 362α100K

365w, 365/ and 365α100K

374w, 374/ and 374α 300K

354w, 354/ and 354α4/if.

372w, 372/ and 372α 4μ£.

344w, 344/, 344α and 394 2K

It will be evident to those skilled in the art that the method disclosed can be carried out by a properly programmed digital computer. Furthermore, the logic functions shown by the relays 124, 134, 122, 132, 130 and 128 and their associated contacts may also be carried out by other logic devices including solid state logic, fluidic logic or any other type of logic.

It will also be evident to those familiar with boiler control that the multiplication of the signals on lines 116 and 144 may be replaced by an addition of those signals or a subtraction depending on the desired mode of modification. Likewise, the multipliers 166 and 190 may be omitted if desired thus making the temperature measurement T_s and the O₂ measurement unnecessary in the control system shown. Still another approach would be to introduce preset values on lines 172 and 192 based on the desired fuel-feedwater flow ratio and the air-fuel ratio, respectively.