US3247671A

US3247671A - Boiler-turbine control system

Info

Publication number: US3247671A
Application number: US303773A
Authority: US
Inventors: James H Daniels
Original assignee: Leeds and Northrup Co
Current assignee: Leeds and Northrup Co; Phillips Petroleum Co
Priority date: 1963-08-22
Filing date: 1963-08-22
Publication date: 1966-04-26
Anticipated expiration: 1983-04-26

Description

April 26, 1966 J. H. DANIELS BOILER-TURBINE CONTROL SYSTEM 2 Sheets-Sheet 1 Filed Aug. 22, 1963 I'll FEED WATER F'Low April 26, 1966 J. H. DANIELS 3,247,671

BOILER-TURBINE CONTROL SYSTEM Filed Aug. 22, 1963 2 Sheets-Sheet 2 flfigfi 306 HULTIPLIER E DIVIDER (Pf-Ps) Pf l I I I i l 1 i 3020 i l l i United States Patent 3,247,671 BOILER-TURBINE CONTRGL SYSTEM James H. Daniels, North Wales, Pa., assignor to Leeds and Northrup Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Aug. 22, 1963, Ser. No. 303,773 8 Claims. (U. 6li-67) This invention relates to a turbine control system and, I

more particularly, to a control system achieving improved results by utilizing a signal indicative of the position of the turbine control valves.

In certain turbine control systems it is desirable to generate a signal indicative of the power input to the steam turbine. This signal may commonly be fed back to the control system which regulates the network frequency and the power output of the electrical generator driven by the turbine. The signal may also be used as a demand signal for the control system regulating the fuel, air and feedwater inputs to a boiler supplying the steam turbine.

In a steam turbine, the input is controlled by a turbine control valve and a signal indicative of the setting of this turbine control valve is desirable for results such as here indicated, In practice, however, this turbine control valve is commonly a plurality of valves arranged to introduce steam at various points along a circular arc of the turbine housing. Heretofore, it has been quite difficult to establish a signal which will represent the effective total area of the valve opening for this group of valves. Such a signal would closely represent the power input to the turbine at a specific steam supply pressure and temperature.

However, the measurement of the throttle pressure at the input to the turbine control valve is relatively simple. Similarly, the pressure in any of the stages of the turbine may be easily measured. Measurements of these pressures by themselves are not indicative of the setting of the turbine control valves. These pressures are dependent upon the turbine control valve settings and on the conditions of the boiler which supplies the steam input to the turbine.

In accordance with one aspect of 'my invention, a signal indicative of the effective setting of the turbine control valves is obtained by developing a signal proportional-to the ratio of the throttle pressure to the pressure in one stage of the turbine. This ratio is not dependent on boiler conditions or load conditions on the turbine but is directly related to the turbine control valve settings. A signal proportional to this ratio can be used in control systems regulating the operation of the turbine generator and its associated boiler or boilers. The ratio measurement is independent of steam temperature and is thus a correct indication regardless of steam pressure and temperature.

vAccordingly, it is an object of this invention to provide means for readily establishing a signal which is indicative of the input to a steam turbine at specific steam supply conditions so that the signal is independent of the variations in steam conditions upstream of the turbine control valve as occur upon readjustment of that valve or upon variation of boiler operating conditions. A signal of this nature is useful in a steam-electric power control system by representing, during a period of deviating steam supply conditions, the turbine steam flow or power output which will prevail upon restoration of standard conditions.

The foregoing and other objects, features and advantages of this invention will be better understood from the following more detailed description, appended claims and drawings in which:

FIG. 1 shows a turbine control system employing my invention; and

FIG. 2 shows a modification of the invention.

This invention is shown in one preferred form in FIG. 1 wherein it is utilized both in a load frequency control system as a source for feedback signals for that system ice and in a boiler control system wherein the invention is utilized to provide a demand signal for the control of the boiler. FIG. 1 shows a system for controlling the output of electrical generator 10 which comprises the source of power in station 1 of three stations which make up the generating capacity of area A which area may be interconnected with other areas for interchange of power therebetween.

The control system of FIG. 1 also shows the means for controlling the various inputs to boiler 16 as may be required to generate the desired steam output for operating the turbine 18 as may be required to produce the desired generation from generator 10.

As shown in FIG. 1, the firing rate of the boiler 16 is controlled by an adjustment of valve 20 which serves to regulate the flow of fuel oil in line 24. This fuel oil is supplied to the burner 28 and sufiicient air for the combustion of the oil is supplied by way of the air flow 30; the flow in the air flow line 30 being regulated by the adjustment of damper 34. 1

The water flow to the boiler is provided by pump 40 which serves to pump water through the water flow regulating valve 42 and through the coils- 48 to the boiler output line 50. By establishing the proper firing rate by the control of valve 20 and damper 34 and by simultaneously establishing a proper water flow by the adjustment of valve 42, standard steam conditions, namely, standard conditions of temperature and pressure, may be established in the output line 50.

The rate at which the steam generated by boiler 16 is to be utilized by turbine 18 is adjusted by the turbine control valves 60. The effective opening of the control valves till is thus indicative of the demand being placed on boiler 16. This opening is also indicative of the input supplied to turbine 18 by way of the connecting steam lines 62 and it, therefore, establishes the amount of power which will be generated by generator 10 in response to its rotation by turbine 18 as by the mechanical linkage shown as 68. A standard type of speed governor 70 is utilized to adjust by way of the mechanical coupling 72. the opening of the turbine control valves 60 in response to variations in the speed of the turbine 18 and, therefore, the frequency of the output from generator 10. The governor 70, as shown in FIG. 1, would normally comprise a centrifugal clement rotated by a mechanical coupling 74 connected to the shaft 68 of the turbine 18.

Four of the valves 60 have been shown in FIG. 1 for illustration but there may be more or less of such valves arranged to introduce steam at various points along a circular arc of the turbine housing. These valves being thus oriented are usually operated in sequence by a cam or other arrangement. It is thus difiicult to establish a signal which will represent the effective valve opening for this group of valves.

As has been the practice, the governor 70 is adjusted by the governor motor in a manner to establish by its adjustment a position of valves 60 to establish the desired output from generator 10. The control of the governor motor 80 should desirably be responsive to the electrical conditions in area A because it is usually desirable that the outputs of stations 1, 2 and 3 in area A be sufiicient to supply the total load in that: particular area while maintaining a predetermined frequency. In order to establish the particular value for the desired generation of generator 10, there is utilized, as shown in FIG. 1 as block 90, a load dispatchers office which would contain the computing circuitry for establishing that desired generation signal for generator 10, Gdl, which is shown as being the output on line 94. The load dispatchers office will commonly make these calculations based upon measurements of the interchange of power between area A and the other areas interconnected to area A by tie-lines 98. The net flow of power over these tie-lines is meas ured by totalizing wattmeter 100 and the frequency in the system is measured by equipment shown as 102.

By utilizing the frequency measurement and the power interchange measurement in any of a number of known computing circuit arrangements, the desired generation value Gd1 may be established on line 94. These computing arrangements may be, for example, of the type shown and described in US. Patents 2,692,342 Nichols et 211.; 2,773,994 Cohn; 2,754,429 Phillips; 2,836,731 Miller, Jr., or other similar arrangements.

To establish a suitable control for the governor motor 80, the signal Gdl supplied on line 94 is utilized as a primary control signal for generator 10. This signal is compared with the signal supplied on line 110, the comparison being made by the operational amplifier 114. The signal on line 110 is a signal representing the effective opening of turbine control valves 60 as established by the novel circuit arrangement to be later described.

Any difference between the signal on line 94 and the signal on line 110 is effective to produce an output from operational amplifier 114 which represents an error signal Grl which may be considered as the generation change requirement for generator 10. This error signal is supplied to a controller 118, the output of which is effective to operate governor motor 80 in response to the error signal with the position of the governor motor 80 being fed back to the controller 118 by way of the mechanical coupling 120. The controller 118 may desirably be of the type shown in US. Patent 2,666,170 issued to E. T. Davis.

In order to insure that the adjustment of governor motor 80 will be effective to establish an output from generator equal to the desired generation Ga'l as calculated by the computing circuitry in the load dispatchers office 90, the signal Gdl as supplied on line 94 is compared with the signal Gal representing the actual generation which is measured by the power measuring instrument 120 and supplied over line 122 as an input to operational amplifier 124. The other input to operational amplifier 124 is the signal Gdl which is supplied by way of line 94 and line 128. Any difference between the signals Gdl and Gal produces an output from operational amplifier 124 which represents an error signal. This error signal is supplied to a controller 130 which may desirably be of the type which produces on its output line 132 a current flow related to the time integral of the signal from amplifier 124. Inother words, the'controller 130, which in this case is a current adjusting type of controller, may be adjusted to have a reset function as its primary function. A suitable controller is shown in US. Patent 3,092,321. In this patent the leads 42 and 43 (which is ground) are the inputs to the controller and the lead 57 is the output.

The output of the controller 130 which appears on line 132 is then provided as another input to operational amplifier 114 and, therefore, contributes to the control of governor motor 80 to establish a positioning of valves 60 by Way of adjustment of governor 70 which will provide the desired output from generator 10.

The signal supplied on line 110, as previously described, is representative of the effective opening of the turbine control valves 60. This signal is established in a novel 7 manner as will now be set forth.

A signal indicative of the flow of steam in the turbine 18 is established by a pressure measurement, Pf, which is desirably made in the first stage of the turbine 18. It can also be made in any of the subsequent stages of the turbine. A pressure measuring instrument 140 is connected by line 142 to the first stage of turbine 18. This pressure measuring instrument may desirably be of the type which utilizes a Bourdon tube connected with its input line 142 in order to establish a motion of its measuring elements indicative of the pressure being measured. This motion may in any of a number of Ways be converted into an electrical signal on the output line 144 such that the signal current is directly related to the pressure being measured. A suitable transducer which may be used as the pressure measuring element is shown in US. Patent 3,076,137 to Moore. The transducer is denoted 10 in that patent. As shown in FIG. 1, this output signal is supplied as an input to operational amplifier 148 whose output is in turn supplied as one input of a divider circuit 150. A suitable divider is shown in S.N. 227,900, Magnetic Multiplier System, James C. White, filed October 2, 1962, now Patent No. 3,165,650. In that divider, the input denoted V and V correspond to Pf and Pt. Under standard steam conditions in the boiler output line 50, the pressure measurements made in the first stage of turbine 18 would be indicative of the effective opening of valves 60 and hence the demand on boiler 16. However, the adjustment of valves 60 by governor 70 is effective to change the demand on the boiler 16 and this, as Well as other conditions related to the boiler, will cause variations in the steam pressure in boiler output line 50 which pressure may be referred to as the throttle pressure of the turbine. Variations in this pressure cause the pressure measurement in the first stage of turbine 18 to deviate from its normal relationship with the effective opening of valves 60. In order to correct this deviation and to thereby maintain the normal relationship with the effective opening of valves 60 and the signal produced on line 110, there is introduced into divider 150 another input Pt which represents the throttle pressure measured in line 50. This pressure is measured by pressure measuring instrument 156 which is similar to that shown as 140.

The output of the instrument 156 is fed as an electrical input over line 158 to operational amplifier 160 Whose output is the other input to divider 150. The divider 150 is operative to produce a signal on line 152 and hence on line 110 which is related to the quantity Pf/Pt. This signal has been found to be representative of the effective opening of valves 60 even under conditions of varying throttle pressure in line 50. It is thus indicative of not only the demand on boiler 16 but also the input to turbine 18 under standard steam conditions and hence the output from generator 10 which will result when the standard steam conditions have been established by elements of the boiler control to be hereinafter explained.

The signal on line 152 representing the quantity Pf/Pt is not only a useful feedback signal in the load frequency control as described above, but is also useful as a demand signal for the control of boiler 16 as will now be described. The signal P Pt is supplied over line as one of the inputs to operational amplifier 172. The other input to operational amplifier 172 is a signal which is utilized to assure that the throttle pressure in line 50 is always returned to its desired set point as established by the manually positioned contact a of slidewire 180 across which there is connected a supply source E which may, for example, be a DC. source of supply. The contact 180a thus establishes on line 182 a potential representative of the set point for the pressure in line 50 while the actual pressure is established from the output of pressure measuring instrument 156 to line 158. The actual pressure is then compared with the pressure set point Ps by the operational amplifier 186. There is thus produced as an output from operational amplifier 186 an input signal for controller 190. The controller 190 may be of the same type as controller 130, namely, the current adjusting type such that there is produced on its output line 192 a signal which is related to the time integral of the difference between Pt and Ps. There is thus supplied to operational amplifier 172 not only the primary control signal Pf/Pz by way of line 170 but also a reset signal by way of line 192. It may be desirable under certain conditions to also produce by controller 190 other control functions such as proportional control.

The output signal of operational amplifier 172, which is indicative of the demand on the boiler 16 is utilized to control both the firing rate of the boiler and the rate of feedwater fiow.

In order to establish a signal indicative of the desired firing rate for boiler 16, the output from operational amplifier 172 is supplied to operational amplifier 174 over line 176 as one of the inputs to operational amplifier so 'as to establish as an output from operational amplifier 174 a signal on line 178 indicative of the desired firing rate for boiler 16. Simultaneously, there is established as an output of operational amplifier 181 on line 183 a signal indicative of the difference between the desired rate of feedwatcr flow and the actual rate as will be later described.

It order that the proper relationship may be established between the firing rate signal on line 178 and the feed-water flow rate error signal on line 183, there is established on

lines

190 and 191 potentials of opposite polarity from opposite ends of slidewire 194 across which is connected a suitable D.C. potential E. In order to establish the desired relationship between the signals on

lines

190 and 191, the temperature of the steam in output steam line 50 is measured as by thermocouple 2G0 and in response to the measurement established by thermocouple 200, the steam temperature controller 204 operates control motor 206 to adjust by way of the mechanical coupling 208 the position of the slidewire tap 194a so as to properly pro: portion the firing rate and feedwater flow rate. The steam temperature measuring and controlling instrument 204 I thus establishing signals on

lines

178 and 183 which will also change in opposite sense in response to the steam temperature deviations.

The signal established on line 183, which is indicative of the desired feedwater flow rate error, is provided as an input to controller 210 whereby it establishes by operation of the control motor 212 from the output of controller 210 a position for valve 42 as necessary to maintain the steam temperature in line 50 for the load conditions represented by the signal on line 170, namely, Pf/Pt. The valve position is provided as a feedback to controller 210 by way of the mechanical coupling 214.

The controller 210 may be of the well known position adjusting type which is capable of incorporating in the operation of its control motor 212 proportional as well is reset and rate actions as may be required.

In order to establish a signal on line 183 as an error signal that is a deviation of the actual feedwater fiow rate from a desired value, there is established as an input to operational amplifier 181 by way of line 220 a signal representative of the measured feedwater fiow rate. This signal is established by a standard flow measurement which incorporates orifice 224 to establish a pressure drop which is measured by the differential pressure measuring device 226. This device may be any of a number of standard differential pressure measuring instruments. In order to establish a linear relationship between the signal on line 220 and the feedw-ater flow as measured by the differential pressure measuring instrument 226, the output of that instrument is supplied to a square root extractor 228 which is effective to take the square root of the ou-t put signal of the differential pressure transmitter 226 and thus establish on line 220 a signal having a linear relationship to the feedwater flow. The signal supplied to operational amplifier 181 on line 220 is in opposite sense to that supplied on line 184, thus the two signals are compared and the error signal as biased by the signal on line 191 is established on line 183.

The signal on line 178 which is related to the desired firing rate for boiler 16 is utilized as an input signal to the

operational amplifiers

230 and 232. This signal is compared by operational amplifier 230 with the signal supplied to its input on line 234 indicative of the actual fuel flow. The difference between the actual fuel flow and the desired firing rate thus provides an output from operational amplifier 230 which provides an error signal into controller 240 effective to operate controller motor 242 and adjust valve 20 to thereby control the rate of fuel supply through the line-24. The signal on line 234 representative of the actual rate of flow of the fuel in line 24 is established by the differential pressure transmitter 243 in conjunction with the square root extractor 245 in a similar fashion as described for the feedwater flow measurement. Likewise, the air flow in the air flow line 30 is controlled by damper 34 which is under the control of controller 250 in response to the output from operational amplifier 232. The controller 250 is effective to position damper 34 by its control motor 252. The necessary actual airflow signal is fed back as an input to operational amplifier 232 by way of line 256. This latter signal is established by a differential pressure transmitter 258 in conjunction with the square root extractor 260 in a similar fashion to that previously described.

In order to establish the desired air-to-fuel ratio for proper combustion conditions in boiler 16 an excess oxy-.

gen measurement is made by drawing a sample of the gases through sampling line 280 into an oxygen analyzer and controller 282 which is operative to position the controller motor 284 and change the air-fuel ratio as necessary to return the excess oxygen in the flue gases to a value necessary for efficiency of the combustion in boiler 16.

The oxygen analyzer may be of the type shown and described in US. Patent 2,422,129 and the associated controller, which in conjunction with that analyzer makes up the unit 282, may be of the type disclosed in US. Patent 2,666,170.

The positioning of the control motor 284 is effective through the mechanical coupling 286 to control the position of slidewire contact 288a on slidewire 288 so that any changes in the position of slidewire contact 288a will be effective to establish as inputs to the

operational amplifiers

230 and 232 oppositely varying signals as by way of

lines

290 and 292. These signals will be effective to establish the relationship between air flow and the fuel flow which is necessary for optimum combustion conditions in boiler 16.

In order that there may be a flexibility in the adjustment of the output signals of

operational amplifiers

230 and 232, there are provided for each of them a variable feedback resistor 300 as Well as a potentiometer 382 which is connected by way of line 304 to provide a manually adjustable biasing input which may be set as desired to establish the optimum relationship between the fuel and air control for the boiler.

Since the throttle pressure normally varies only slightly from its set point under normal circumstances, the ratio Pf/Pt on line 152 may be replaced by a signal which is approximately equal to this ratio. It has been found that a good approximation to this ratio may be computed as the product of stage pressure times unity minus the per unit error or deviation of throttle pressure from its set point. This may be expressed as P l-E) where:

Pf=the stage pressure, and E=the deviation of throttle pressure from its set point expressed as a fraction of the set point value.

This expression may be expanded to (PtPs) Ps where Pt=throttle pressure,

7 Ps=throttle pressure set point, and

' Pt P s E Ps FIG. 2 shows a circuit which can be substituted for the block 300a in FIG. 1 in order to use this method of approximation. The operational amplifier 302a receives inputs representing Pt and Ps from

lines

158 and 182 respectively and produces an output which represents their difference. This quantity, which is Pt-Ps, is multiplied by P which is supplied as an output from operational amplifier 304a in response to its input Pf from line 144 and divided by Ps which is obtained with its proper sign by means of a connection to line 182 by way of operational amplifier 305. The output of amplifier 304a is applied to the multiplier-divider 306 which may be the same unit as divider 150 in FIG. 1. The output of th multiplier-divider is thus which is subtracted from Pf by operational amplifier 310 to produce the desired signal The inputs to amplifier 310 are obtained from both the output of multiplier-divider 306 on line 307 and from the output of amplifier 304a after its sign has been changed by operational amplifier 303 which is connected to amplifier 310 by line 308.

In order to maintain the throttle pressure at the desired set point C.A.T. controller 190 is used in the same manner as in FIG. 1. I

While a particular embodiment of the invention has been shown and described, it will, of course, be understood that various m'odifications may be made. The appended claims are, therefore, intended to cover any such modifications within the true spirit and scope of the invention.

What is'claimed is:

1. In a system including a turbine driven by fluid under pressure, the combination with adjustable valve means for varying the supply of said fluid to said turbine of means for developing from throttle pressure upstream of said valve means and the pressure in at least one stage of the turbine an output signal indicative of the setting of said valve means, comprising means for producing a first signal indicative of the throttle pressure upstream of said valve means, means for producing a second signal indicative of the pressure in one stage of said turbine, and

means for producing a third signal which is a function of the ratio of said first and said second signals, said third signal being indicative of the setting of said valve means.

2. A system for producing a signal indicative of the efiective opening of control valve means for a turbine, comprising means for producing a first signal indicative of the throttle pressure at the input to said control valve, means for producing a second signal indicative of the pressure in one stage of said turbine, and

means for producing a third signal which is a function of at least one of the products and the ratio of said first and said second signals, said third signal being indicative of the effective opening of said control valve.

3. A system for producing a signal indicative of the effective opening of control valve means for a turbine, comprising means for producing a first signal indicative of the throttle pressure at the input to said control valve,

means for producing a second signal indicative of the pressure in one stage of said turbine, and

means for producing a third signal which is the ratio of said second signal divided by said first signal, said third signal being indicative of the eiTective opening.

of said control valve.

4. A system for producing a signal indicative of the eifective opening of control valve means for a steam turbine, comprising means for producing a first signal, Pf, proportional to the pressure in one stage of said turbine,

means for producing a second signal, Pt, proportional to the pressure upstream of said turbine control valve means, and

means responsive to a function of the product Pf Pt of said first and second signals for producing said resultant signal.

5. The system recited in claim 4 in which said function of the product of said first and second signals is equal to the first signal divided by the second signal.

6. A system for controlling the inputs to a steam boiler supplying steam through a turbine control valve to a turbine under varying load conditions for said turbine, comprising means for developing a signal related to the ratio of the pressure at a particular stage in said turbine and the pressure upstream of said valve, and

means operable in response to said signal for varying the inputs to said boiler in accordance therewith.

7. A system for controlling the inputs to a steam boiler to provide the desired output from a turbine driven generating unit coupled for operation in accordance with the setting of a turbine control valve which controls the steam flow from said boiler to said turbine, comprising means'for producing a signal representative of the desired output of said generating unit,

means for producing a signal proportional to the ratio between the pressure at a particular stage in said turbine and the pressure upstream ofsaid turbine control valve,

means for comparing said desired output signal and said ratio signal, means to position said turbine control valve in response to. said comparison to substantially equalize said ratio signal and said desired output signal, and

means responsive to said ratio signal to change the inputs to said boiler in direct relationship with said ratio signal. 8. A system for controlling the output of a generator driven by a turbine connected to a source of steam by an adjustable turbine control valve, comprising means for producing a first signal proportional to the stage pressure at a particular stage in said turbine,

means for producing a second signal proportional to the throttle pressure upstream of said turbine control valve,

means for producing the ratio between said first and said second signals,

means responsive to said ratio between said stage pressure and said throttle pressure for providing a feedback signal indicative of the output from said generator corresponding to steady state operating conditions for the existing source of steam,

means providing a desired generation signal in accordance with the desired output for said generator, and

means operative in accordance with the difference between said desired generation signal and said feedback signal for positioning said turbine control valve to substantially reduce said difference to zero.

No references cited.

SAMUEL LEVINE, Primary Examiner.

Claims

6. A SYSTEM FOR CONTROLLING THE INPUTS TO A STEAM BOILER SUPPLYING STEAM THROUGH A TURBINE CONTROL VALVE TO A TURBINE UNDER VARYING LOAD CONDITIONS FOR SAID TURBINE, COMPRISING MEANS FOR DEVELOPING A SIGNAL RELATED TO THE RATIO OF THE PRESSURE AT A PARTICULAR STAGE IN SAID TURBINE AND THE PRESSURE UPSTREAM OF SAID VALVE, AND MEANS OPERABLE IN RESPONSE TO SAID SIGNAL FOR VARYING THE INPUTS TO SAID BOILER IN ACCORDANCE THEREWITH.