US3545207A

US3545207A - Boiler control system

Info

Publication number: US3545207A
Application number: US843946A
Authority: US
Inventors: Justus C Barber; Theron W Jenkins Jr
Original assignee: Leeds and Northrup Co
Current assignee: Leeds and Northrup Co
Priority date: 1969-07-23
Filing date: 1969-07-23
Publication date: 1970-12-08
Anticipated expiration: 1987-12-08

Description

J. c. BARBER E'rAL- 3,545,207 .BOILER CONTROL SYSTEM 9` 2 snees-sheef 1` INVENTUM Jusrus c. BARBER `THERON w. JENKINSJR.

r AGENT Dec. `8, 197

Dec. `1970 J. c. BARBER ET AL4 `BOILER CONTROL SYSTEM A Filed July 23, 1969 United States Patent O 3,545,207 BOILER CONTROL SYSTEM v Justus C. Barber, King of Prussia, and Theron W. Jenkins,

Jr., Ambler, Pa., assignors to Leeds & Northrup Company, Philadelphia, Pa., a corporation of Pennsylvania Filed July 23, 1969, Ser. No. 843,946

Int. Cl. F23n 1/10 U.S. Cl. 60--106 10 Claims ABSTRACT OF THE DISCLOSURE A control system responding to the energy demand of the turbine to provide separate demand signals for each of the boiler inputs as required to meet the turbine demand and modifying the boiler inputs when one of them deviates from its demand value by a predetermined allowable limit so as to maintain all inputs in the proper relationship and magnitude. Also, when the deviations occur, the turbine input is modified to keep the turbine input in balance with the boiler input.

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 yenergy 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 simplied 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 predeter mined 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 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. 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 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 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. l.

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 bined at the burner 14 with the air flowing through line 16 so as to provide the necessary heat input to the boiler while the feedwater flow is provided through line 18. The air ow 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 notationm"`PI in block 22. The air flow through line 16 is measured by the ilowrneter 24 which by means of the associated pressure taps responds to the pressure differential established across the flow restriction 26 inline 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 owmeter 34 in response to the pressure drop across the orice plate 36, the assumption being that the fuel supplied is either a liquid orl 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 those utilized to control both the fuel and the air flow, namely

controllers

22 and 32, respectively. Similarly, the feedwater ow is measured by the owmeter 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 I52 responds to the temperature detected by thermocouparticular control system shown in FIG. l is a measurement of the pressure in the iirst 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 iirst stage or impulse chamber of turbine 68. The pressure measured by the device `62 is noted as Pt while the pressure measured by the device 70 is noted as P1.

As shown in FIG. l, the turbine 68 which is supplied with steam from boiler 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 Ga.

For the purpose of establishing the desired output of the generator 74 there is provided at terminal 82 a signal Gd representing the desired generation for generator 74. The signal Gd 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 I86 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 ilow to the turbine 66.

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 Gd 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 modied so as to change the electrical .power output in response to the changes in the signal Gd 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 Gd and Ga 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 PID 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 Gd which is supplied on line 106 and a signal Pl/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 s 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 Pl/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 rst stage or impulse chamber pressure Ineasuring 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 signiilcance and the manner for producing the signal Pl/Pt on 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 supplies pulses to relay operator 122 by way of relay contact 124a 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 122a is closed so that the potential supplied at the terminal 126 can be connected through relay contact 128a and relay contact 130a to line 91 and hence to the governor motor in governor 86. The relay contacts 128a and 130:1 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 134a, which is subject to the actuation by the relay actuator 134 so that the pulses supplied on line 121 when the relay contact 134a is closed causes an energization of the relay actuator 132 to close the relay contact 132a and thereby connect the potential supplied at terminal 126 through the closed contact 132a, and the contact 130b as well as contact 128b to line 92. In FIG. 1, the contacts 130g and 130b are shown in the state which is normal when their actuator 130 is de-energized. Also, the

contacts

128a and 128b 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 Pt in line 60 as well as the desired steam temperature Ts 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 Pl/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 Gd. The turbine energy demand signal supplied on line 116 is subject to modification in accordance with the deviation of the throttle pressure Pt measured in line 60 from its desired value or set point. This modication 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. l 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 owmeter 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 eW 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 eW 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 t attempt to control the feedwater ow 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 ow, the signal RBD `on line 154 is modified by the multiplier 166 in response to the deviation of the steam temperature TS 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 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 ef 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 3,0 to maintain the fuel ow 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 ow 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 de- `sired 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 j 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 How 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,L which provdestan input to the controller 22 so that the controller can adjust the valve to modify the air ow 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 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 isI 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 ew, ef and ea 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 RBD1 provided as an output on block 220 whenever the respective error signals ew, ef and e,L 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 134g and 124a. Opening the contact 134e, for example, would prevent the controller 112 from causing an increase in the output of generator 74 while the opening of the contact 124a Would prevent the controller 112 from causing a decrease in the output of the generator.

Other outputs of the block 220 are the signals Obd supplied on line 228 and the signal Obu supplied on line 230, which respectively energizes the relay actuators l and 128 so as to provide for the decrease of the opening of valve 90 when the signal Obd appears on line 228 and `so as to increase the valve opening when the signal Obu appears on line 230. It will be evident that if a signal appears on line 228 to energize relay 130, the contact 130b will be opened and the contact 130:1 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 Obu on line 230 to energize relay actuator 128, the contact 128b will be connected to the contact 128e and the contact 1280 will be opened. The closing of the contact 12811 on contact 128e 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 128a will prevent any pulses generated by the controller 112 from causing a signal on line 91 just as the opening of the contact 130b in response to the appearance of the signal Obd prevented any signal from appearing on line 92 asa 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 1152, 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 amplier 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 theinverting 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

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 fiow through the resistor 316 will be in the direction shown by the arrow, likewise current fiow through the diode 320 will be in the forward direction, namely that shown vby the arrow. The source of the current through resistor 316 and the current through diode 320 is the potential lE 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 ew, ef or ea exceeds a predetermined value, it is desirable to cause the signal RBD on line 154 to represent a modified value of the sig nal DBD on line 152 rather than the same value and as previously mentioned. If, for example, the error signal ew which appears on line 162 can vary from -10 to +10 volts, the network which includes operational amplifier 338W can be so arranged that when the signal on line 1 62 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 whlch the error signal eW 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 342W 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 338W 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 t-l-E. The input from the contact 342W to the summing junction 348W is by way of input resistor 352W. Effectively, the potential y-l-E supplied by way of line 351 and input resistor 350W represents the predetermined allowable limit which should not be exceeded by the error signal ew, and the input signal from line 152 through input resistor 346W to the summing junction 348W modifies the predetermined limit which the error signal ew 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 ew 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 348W at the non-inverting input of amplifier 338W is effective to compare the inputs through the three input resistors 346W, 350W 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 352W, namely by the adjustment of contact 342W 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 340W will be conductive to cause the amplifier 338W to determine the potential on line 310 by back-biasing diode 308. When the error signal ew causes the amplifier 338W 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 fiow 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 ew 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 358W to a ground connection.

The above operation of the circuit including amplifier 338W would occur if the signals ef and ea are either zero or are within the predetermined limits established for them.

Each of the other error signals ef and ea, is connected to a similar network as is error signal ew; thus the error signal ef is connected to a network which includes arnplifier 338)c while the error signal ea is connected to a network which includes amplifier 338a. The other elements of the network have reference characters comparable to those shown in the network including amplifier 338W with the exception that the suffix letter f or "a is used depending on whether it is associated with the network for the error signal ef or the network for the error signal ea.

The networks which include the

amplifiers

300, 338W, 338f and 338a and their associated diodes 308, 340W, 340)c and 340a not only are effective to compare the error signals ew, ef and ea with the predetermined values representing their allowable limits but also by virtue of the diodes 308, 340W, 340]c and 340a 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]c and 338:1.

If', for example, the feedwater iiow to the boiler exceeds the feedwater demand as established by the signal on line 156 of FIG. l, 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 342W 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 eW not exceeded its predetermined allowable limit. Hence, the required boiler demand is increased if the feedwater fiow exceeds the demand previously established, as for example by the signal from line 152I by way of amplifier 300 and amplifier 312. By increasing the required boiler demand signal on line 154 it will be evident from FIG. l that there will be a comparable increase in the separate demand signals produced on lines 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 re- 9 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 ew, ef and ea 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]'c and 340a 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 ew, 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 33'8w.

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 361W 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 364W 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 eW 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 eW 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 ew exceeds its allowable limit and hence the feedwater demand signal will constantly decrease until the deviation of the feedwater l flowfrom the feedwater demand is at or within the allowable limit.

If' any of the other error signals ef or ea exceeds its predetermined allowable limit by an amount such that the potential on the corresponding lines 3611 or 361a exceeds the signal on line 361W, then one of the other amplifiers 360]c or 360a will be operable to determine the potential on line 322 since it will establish on its output line 376]c or 376a a potential of lower positive value than that previously established, thus making the associated diodes 370]c or 370a conductive and causing the other diodes connected to line 322 to be back-biased. Thus,

vthe amplifiers

312, 360W, 3601 and 360a with their associated diodes 320, 370W, 37'0f and 370a 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 ew, ef or ea, Whichever is the greatest positive value. Since the error signals ew, ef and el 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 ew, ef and e, are at or within the predetermined allowable limits. In other words, the appropriate one of the amplifiers 338W, 338]l and 338a or 360W, 360i and 360o 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 ew, ef or ea 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. l, 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 ew, ef or esu is sufliciently negative to cause the potential on line 310 to be determined by an amplifier other than amplifier 300 and hence it is neces sary 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 ew, ef or e8L is sufiiciently 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 9o.

Whenever the signal DBD on line 152 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.

l l 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 316 40K 326 20K 358W, 358]c and 358a 100K 356W, 356f and 356a 200K 346W, 346]c and

346a M

350W, 350f and 350a 15M 352W, 352]c and

352a 100K

368W, 368f and 368a M 366W, 366)c and

366a 10M

362W, 362f and

362a 100K

365W, 365f and 365a 100K 374W, 374]c and

374a 300K

354W, 354f and 354a 4/rf. 372W, 372f and 372a 4,af. 344W, 344]", 344:1 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, lluidic 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 1116 and 144 may be replaced by an addition of those signals or a subtraction depending on the desired mode of modilication. Likewise, the

multipliers

166 and 190 may be omitted if desired thus making the temperature measurement Ts 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 ow ratio and the air-fuel ratio, respectively.

What is claimed is:

1. In a control system which produces a signal representing the desired boiler demand to supply the boiler load and derives from the desired boiler demand signal separate demand signals associated with the separate inputs to the boiler for controlling those inputs so that they tend to have a predetermined relationship and a magnitude required to meet the boiler load, the improvement which comprises means for modifying the desired boiler demand signal so that it produces modified separate demand signals in response to an error signal varying in accordance with the difference between the actual value of a boiler input and the corresponding separate demand signal when that error signal exceeds a predetermined allowable limit, said modification being such that the modified demand signal and the separate demand signals derived therefrom tend to reduce said error signal to said predetermined limit While maintaining the inputs in said predetermined relationship.

2. In a control system which produces from a measure of the energy demand for a turbine a signal representing the desired boiler demand to supply the turbine demand and derives from the desired boiler demand signal separate demand signals asociated with the separate inputs to the boiler for controlling those inputs so that they tend to have a predetermined relationship and a magnitude required to meet the turbine demand, the improvement which comprises means for modifying the desired boiler demand signal so that it represents the required boiler demand and produces modified separate demand signals in response to an error signal varying in accordance with the dierence between the actual value of a boiler input and the corresponding separate demand signal when that error signal exceeds a predetermined allowable limit, said modiication being such that the modified demand signal and the separate demand signals derived therefrom tend to reduce said error signal to said predetermined limit While maintaining the inputs in said predetermined relationship.

3. A control system as set forth in claim 2 which includes means responsive to a predetermined difference between the desired boiler demand signal and the required boiler demand signal and operable to vary the steam flow to the turbine so as to tend to maintain said difference between the desired and required boiler demand signals within the predetermined magnitude.

4. A control system as set forth in claim 2 in which the means for modifying the desired boiler demand signal includes auctioneering circuits operable to modify said desired boiler demand signal to produce the required boiler demand signal so that it varies in response to that error signal which exceeds its predetermined allowable limit by the greatest amount.

5. A control system as set forth in claim 4 in which the auctioneering circuits include a rst auctioneering circuit which modies the desired boiler demand signal in response .to error signals 0f one polarity and a second auctioneering circuit which modies the desired boiler demand signal in response to error signals of an opposite polarity.

6. A control system for controlling the inputs to a boiler and to a connected turbine so that the inputs to the boiler have a predetermined relationship to each other and to the turbine input comprising means for measuring the turbine demand,

means for producing a signal indicative of said turbine demand,

means for modifying said turbine demand signal in response to the deviation of the steam pressure in the boiler output line from its desired value to produce a desired boiler demand signal indicative of the input required to the boiler to provide the measured turbine demand and the desired steam pressure,

means for measuring the rate of supply of each of said boiler inputs,

means for producing from said rate of supply measurements separate signals each indicative of the flow rate of one of the boiler inputs,

means for producing from said desired boiler demand signal separate demand signals each indicative of the value of a separate one of said boiler inputs required to meet the desired boiler demand,

means for producing an error signal for each of said boiler inputs in accordance With the difference between the flow rate signals representing the measured value of each of said inputs and the corresponding separate demand signals,

means for controlling said boiler inputs in direction and extent to reduce said error signals toward zero, and

means for modifying said desired boiler demand signal to produce a signal indicative of the required boiler demand said required boiler demand signal being different from said desired boiler demand signal and producing correspondingly modied separate demand signals when one of said error signals exceeds a predetermined allowable limit.

7. The method of controlling a boiler which produces a signal representing the desired boiled demand to supply the boiler load and derives from the desired boiler demand signal separate demand signals associated with the separate inputs to the boiler for controlling those inputs so that they tend to have a predetermined relationship and 13 a magnitude required to meet the boiler load, including the step of modifying the desired boiler demand signal so that it represents the required boiler demand and produces modified separate demand signals in response to an error signal varying in accordance with the difference between the actual value of a boiler input and the corresponding separate demand signal when that error signal exceeds a predetermined allowable limit, said modification being such that the required boiler demand signal and the separate demand signals derived therefrom tend to maintain the inputs in said predetermined relationship and to maintain said error signal below said predetermined allowable limit.

8. The method for controlling the inputs to a boiler and to a connected turbine so that the inputs to the boiler have a predetermined relationship to each other and to the turbine input comprising the steps of measuring the turbine demand,

producing a signal indicative of said turbine demand,

modifying said turbine demand signal in response to the deviation of the steam pressure in the boiler output line from its desired value to produce a desired boiler demand signal indicative of the input required to the boiler to provide the measured turbine demand and the desired steam pressure,

measuring the rate of supply of each of said boiler inputs,

producing from said rate of supply measurements separate signals each indicative of the How rate of one of the boiler inputs,

producing from said desired boiler demand signal separate demand signals each indicative of the value of a separate one of said boiler inputs required to meet the desired boiler demand,

producing an error signal for each of said boiler inputs in accordance with the difference between the ow rate signals representing the measured value of each of said inputs and the corresponding separate demand signals,

controlling said boiler inputs in direction and extent to reduce said error signals toward zero, and

modifying said desired boiler demand signal to produce a signal indicative of the required boiler demand said required boiler demand signal being different from said desired boiler demand signal and producing correspondingly modified separate demand signals when one of said error signals exceeds a predetermined allowable limit until said one error signal does not exceed its allowable limit.

9. The method of claim 8 which includes the steps of modifying the steam input to the turbine from the boiler when said required boiler demand signal differs from said desired boiler demand signal by an amount in excess of a q predetermined allowable limit, said modification being in direction and magnitude to maintain the diierence between the desired boiler demand signal and required boiler demand signal within said allowable limit.

10. The method of claim 8 which includes the steps of producing the separate demand signal associated with the feedwater input to the boiler directly in accordance with said required boiler demand signal, producing the separate demand signal associated with the `fuel ow input to the boiler by modifying the required boiler demand signal in response to the deviation of the steam temperature in the boiler output line from its desired value, and producing the separate demand signal associated with the air flow input to the boiler by modifying the fuel flow demand signal in response to the deviation of the oxygen concentration in the boiler stack from its desired value.

References Cited UNITED STATES PATENTS 3,388,553 6/1968 Anderson 60-106X 3,417,737 12/ 1968 lShinskey et al. 122-448 MARTIN P. SCHWADRON, Primary Examiner A. M. OSTRAGER, Assistant Examiner U.S. Cl. X..R. 122-448