US3802189A

US3802189A - Boiler-turbine control system

Info

Publication number: US3802189A
Application number: US00217484A
Authority: US
Inventors: T Jenkins
Original assignee: Leeds and Northrup Co
Current assignee: Leeds and Northrup Co
Priority date: 1972-01-13
Filing date: 1972-01-13
Publication date: 1974-04-09
Anticipated expiration: 1991-04-09
Also published as: AU464493B2; IT1006563B; JPS4880928A; AU5102173A; JPS605761B2

Abstract

A system for providing a demand signal to control the boiler inputs in a boiler-turbine combination operating in a sliding pressure mode with the throttle valve being controlled to maintain the desired generator output. The demand signal for the boiler inputs is obtained by multiplying the ratio of first stage turbine pressure to throttle pressure by the throttle pressure set point and the throttle pressure set point is controlled so as to maintain the ratio at a preset value. The demand signal is modified to trim the boiler inputs to maintain the throttle pressure at its set point.

Description

United States Patent [191 Jenkins, Jr.

[ BOILER-TURBINE CONTROL SYSTEM Primary Examiner-Edgar W. Geoghegan Assistant ExaminerAllen M. Ostra er ltzTh w. k .Abl,P'. [75] men or eron Jen Jr m er d Attorney, Agent, or Firm-Wllham G. Miller, Jr.; Ray- [73] Assignee: Leeds & Northrup Company, m nd F, MacKay Philadelphia, Pa.

[22] Filed: Jan. 13, 1972 [57] ABSTRACT [21] Appl. No.1 217,484 A system for providing a demand signal to control the boiler inputs in a boiler-turbine combination operating [52] U S Cl /665 in a sliding pressure mode with the throttle valve being [51] In. .0 7/14 controue d to maintain the desired generator Output [58] Fie'ld g The demand signal for the boiler inputs is obtained by 290/4 6 multiplying the ratio of first stage turbine pressure to throttle pressure by the throttle pressure set point and [56] References Cited the throttle pressure set point is controlled so as to maintain the ratio at a preset value. The demand sig- UNITED sTTEs PATENTS ha] is modified to trim the boiler inputs to maintain 3,247,67l 4/1966 Daniels 60/107 X th th ttl pressure t it t i t 3,545,207 12/1970 Barber et al. 60/106 11 Claims, 2 Drawing Figures as 82 W404 9a S i 150 |24 l25 08 I00 PI 144 I 117 X PID 120 122 Ax v v P X PI I s 145 11s 126 LlMl S 141 154 no a! P 91 92 115 x F. 4 X 7 130 192 eov -----1 ilili 20B 1B2 85/ l P1 l Pt 62 184 206 l T 7 l STEAM 40 PI FLOW/ 74 1 90 so BOILER =0 li PI 24 Apr. 9, 1974 BOILER-TURBINE CONTROL SYSTEM BACKGROUND OF THE INVENTION This invention relates to a boiler-turbine control system and more particularly to a control system for controlling the boiler inputs to meet the boiler-turbine system demand while maintaining a predetermined state for the throttle valve. The state of the throttle valve may, for example, be its opening or the pressure difference across it.

In boiler-turbine control systems it is desirable to generate a signal indicative of the power input requirement to the steam turbine from the boiler for the steam conditions. That signal may advantageously be used as a demand signal for controlling the inputs to the boiler.

When the deviation of the power output of the generator from its desired value is controlled by modification of the turbine control valve and the boiler inputs are controlled in response to the demand signal trimmed by a throttle pressure controller with the throttle pressure set point programmed over a limited range, the system is said to be operating in a sliding pressure mode.

During such operation a change in electrical load causes a controller to change the throttle valve opening and hence the power level in the turbine-generator unit. This change then causes the modification of the inputs to the boiler in response to the demand signal developed. As has been set forth by others, the boiler input demand signal can be obtained from the quotient of the first stage pressure in the turbine or any other indication of the steam flow through the turbine divided by the throttle pressure. Multiplying this quotient by the pressure controller set point provides the boiler demand signal for sliding pressure operation. Those past systems, however, have not provided means for effecting a readjustment of the turbine control valve toward a particular preselected state.

Accordingly, it is an object of this invention to provide an improved method and means for establishing a demand signal for controlling the boiler inputs in a boiler-turbine system in which the turbine control valve is adjusted to rapidly provide the desired generator output, and the throttle pressure is allowed to operate over a limited range in a sliding pressure mode, with the throttle pressure set point regulated as required to provide for maintaining the desired generator output, while returning the turbine control valve to a preset state.

SUMMARY OF THE INVENTION In accordance with this invention there is provided a system for controlling the inputs to a steam boiler in response to a demand signal so as to provide a desired output from the associated turbine-generator unit through a turbine control valve which is adjusted to maintain the desired generator output. The boiler demand signal is computed as the ratio of a signal representing the steam flow in the turbine to a signal representing the throttle pressure, all multiplied by the throttle pressure set point, and the demand signal is modified by a control signal responsive to the deviation of the throttle pressure from its set point. The improvement comprises means operable to modify both the throttle pressure set point and the boiler demand signal to reduce the deviation between a signal representing the desired value of a particular state of the turbine control valve and a signal representing the measured value of the particular state of the turbine control valve.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of one form of a boiler control system for carrying out the present invention.

FIG. 2 is a block diagram showing the changes neces sary to provide one modified form of the system of FIG. 1.

DESCRIPTION'OF THE PREFERRED EMBODIMENT In FIG. 1 the boiler 10 is a once through boiler provided with the usual inputs such as fuel flow through line 12 and air flow through line 16. The fuel flow and air flow are combined at the burner 14 so as to provide a necessary heat input to the boiler. 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 controller 22 which is shown as a controller providing both proportional and integral action as noted by the block labeled PI. 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 restriction 26.

Similarly, the fuel flow in line 12 is subject to control by the adjustment of valve 30 by controller 32 which is similar to controller 22. Also the fuel flow is measured by flowmeter 34 in accordance with the pressure drop across the orifice plate 36.

The feedwater flow through line 18 is subject to control by valve 40 in response to the action of a controller (not shown) which can operate to control the feedwater in any one of a number of ways known in the art.

The steam pressure P, produced by the boiler in the output steam line 60, referred to as the throttle pressure, is measured by the pressure measuring device 62 through the tap 64 connected to the output steam line 60.

Another measurement which is necessary for the control system shown in FIG. 1 is a measurement representative of the steam flow to the turbine. That measurement may be a direct measurement of steam flow or may be made by measurement of the pressure in the first stage of the turbine or another appropriate stage, which measurement is related to the flow of steam through the turbine. Thus, in FIG. 1, the first stage pressure P, of the turbine is measured by the pressure measuring device which is connected by tap 72 to the first stage of turbine 68.

As shown in the figure, steam from the boiler 10 is supplied through a turbine control valve 90 to turbine 68. The turbine 68 is mechanically coupled to the generator 74 to produce an electrical output on lines 76. The power output on lines 76 is shown as being measured by a wattmeter 80 to provide on line 81 a signal representative of the actual output G of the generator G.

For the purpose of establishing the desired output of the generator 74 there is provided at terminal 82 a signal G representing the desired generation for generator 74.

The governor mechanism represented by block 86 is mechanically coupled to the turbine-generator by coupling 84. The mechanical outputs of the governor 86 through coupling 88 controls the position of the turbine control valve 90 which in turn controls the steam flow to the turbine 68. Governor 86 is controlled by signals on

lines

91 and 92 to cause the governor to increase or decrease the opening of valve 90 to change steam flow as required to maintain generation measured by signal G on line 81 at its desired value G through the controller 100.

As shown in FIG. 1 provision is made to supply governor control signals on

lines

91 and 92 in response to comparison of the signals G and G, in the comparator 96. The difference between those two signals then appears on line 98 as an input to the controller 100. The controller 100 has both proportional and integral response. That particular controller may be of the type disclosed in US. Pat. No. 3,008,072 and may be designed so as to produce on its

output lines

91 and 92 electrical pulse signals whose duration depends upon the extent of the incremental control action called for by the controller 100.

Having described the manner in which the electrical output of the generator 74 is controlled in response to the signal supplied to terminal 82 there will now be described the manner in which the energy input to the boiler is controlled so as to provide the desired steam pressure P,. As will be explained subsequently, the signal P, is subject to modification by a control system so as to maintain a particular predetermined state for the control valve 90. That state may involve the position of valve 90 or it may involve the pressure drop across valve 90 depending upon the condition which it is desirable to maintain for the type of operation required.

The signal provided on line 108 is representative of the power demand established for the turbine 68. That turbine power demand signal is established by dividing the measured first stage pressure P in the turbine 68 by the measured throttle pressure P, as in the dividing network shown as block 114. There is thus obtained at line 116 a signal representative of the quantity P /P,. That quantity is then multiplied in multiplier 121 by signal P, on llne 117, which signal is representative of the throttle pressure set point.

The signal on line 117 is established, as shown, by a control system. In FIG. 1 the control system for establishing the signal on line 1 17 includes a source providing on line 120 a preset signal representing the desired opening for the valve 90 which signal may be considered as a preset value for the ratio P /P to which the actual ratio signal is compared in the comparator 115. There is then provided on the output line of the comparator 115 a signal indicative of the deviation of the actual valve opening from the desired valve opening, since P /P, is indicative of valve opening. it is that deviation which it is desired to reduce to zero by means of the controller 122 which is shown as being a controller having both proportional and integral action. Thus, it is primarily the object of the control system for producing the signal on line 117 to respond to a deviation between the signal P1/P, supplied from the divider 114 and the signal on line 120 so as to maintain those signals as nearly equal as possible by control action of controller 122 adjusting the signal P, on line 1 17, which through the boiler input control system produces a change in generation so that controller 100 will in turn adjust valve 90 thus changing the P IP, ratio.

The control system for adjusting the value of P, may include the use of a signal for attenuating the deviation signal by the introduction of the signal G into multiplier 123 by way of line 124. By introducing the multiplication in multiplier 123 the deviation signal derived from the comparator 115 can be modified in accordance with the desired generation G so as to thereby determine the relative effect that the deviation signal is going to have on the control of the signal P,.

A signal from line 98 indicative of the deviation of the actual generation from the desired generation is added to the output of multiplier 123 by way of line 125.

It can be seen from the above that the input to the controller 122 comprises an attenuated deviation signal indicative of the deviation of valve from the desired opening as modified by the deviation of G, from G Since the generator output deviation is controlled to zero by controller 100 the controller 122 operates to reduce to zero the deviation of the valve 90 from its desired position.

The output of the controller 122 is modified by the addition of a signal from line 124 into summer 126 so as to provide a control effect on the signal P, which causes the control of P, to anticipate changes in the desired generation G The output of the summing junction 126 then provides an input to the limit circuits shown as block 130. Those limit circuits will include means for limiting the output, namely, the signal P, representative of the pressure set point on line 117 to a maximum and a minimum value as well as limiting the rate at which that value changes. The signal P, thus tends to vary as a result of the operation of controller 122 within a limited range and at a limited rate so as to attempt to maintain an equality between the preset desired opening for valve 90 as established by the set point signal on line and the actual opening of the valve 90 as represented by the ratio 1 /P Thus, while the load control system which operates on the governor 86 to cause changes in generation by modifying the valve 90, the controller 122 serves to cause the valve 90 to return to its desired opening by modifying the signal P, which establishes a modified value of the demand signal on line 108 so that the boiler inputs will be adjusted to effect a change in generation which in turn causes valve 90 hence P /P, to change.

The boiler demand signal on line 108 is subject to modification in accordance with the deviation of the throttle pressure P, from its desired value, set point P This modification is accomplished by utilizing the output signal of the pressure measuring device 62 which is supplied on line as an input to a comparator 141 which has as its other input the signal P, shown as being supplied on line 143. The results of the comparison in the comparator 141 is a signal on line representative of the deviation of the throttle pressure P, from its set point P,. The signal on line 145 is an input to the controller 142. The controller 142 has proportional, integral and derivative action so as to provide an output on line 144 which is added to the signal on line 108 by the summing unit so as to provide an output on line 154 which is a modified demand signal representative of the required boiler inputs. As shown in FIG. 1, the signal on line 154 is used for controlling the energy input to the boiler 10.

The signal on line 154 is representative of the fuel flow to the boiler It) needed to maintain the desired pressure in the steam line 60. The signal on line 154 is introduced by way of line 180 to the comparator 182 where it is compared with the actual fuel flow signal on line 184 as derived from flowmeter 34. The result of the comparison provides an error signal on line 186 which serves as an input to controller 32 so that the controller is effective to adjust the position of valve 30 to maintain the fuel flow in line 12 at a value corresponding with the fuel requirement established by the signal on line 180.

The desired relationship between the fuel flow and the air flow is normally controlled in response to the desired fuel-air ratio signal introduced on line 192 to a multiplier 190.

The output from 190 on line 204 is a signal representative of the required air flow through line 16 needed to maintain the desired air-fuel ratio when the fuel flow in line 12 is in accordance with the signal on line 180.

The signal on line 204 is compared with the signal on line 206 as derived from the flowmeter 24 representing the 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 and provides an output on line 210 which represents an error signal input to 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 corresponding to the signal on line 204.

The opening of the valve 90 when the system is in a quiescent state will normally be set, in accordance with the signal on line 120, very near the wide open position. If, for example, a change in G; should then occur the controller 100 would cause a resetting of the governor 86 which would in turn change the opening in valve 90 in a direction to change the steam flow to the turbine 68 so as to change the output G,, of the generator so that it will equal the new value G The change in the opening of valve 90 will cause a comparable change in the ratio P /P, and consequently a similar change in the demand signal P P /P, on line 108 resulting in immediate changes in boiler inputs. The inputs to the boiler will then be further changed as required by controller 122 until P,/P, is at its preset value with G equal to G Thus, transient increases in load are taken care of by operating the turbine valve 90 to use the storage in the boiler, and the throttle valve is gradually repositioned as the level of input to the boiler replenishes the stored energy in the boiler and increases its output to meet the increased load.

When actual valve position is an accurate measure of valve opening the ratio P,/P, can be replaced with a direct measure of valve position.

In some installations it may be desirable to maintain the pressure difference across the throttle valve 90 constant instead of maintaining the valve opening. For that mode of operation the changes in the system of FIG. 1 which are shown in FIG. 2 would be necessary. As shown in FIG. 2, the signal on line 120 would represent the pressure drop desired across valve 90 and the signal supplied to comparator 115 on line 118 would be a measure of the actual drop AP across valve 90 instead of the ratio P,/P,. In FIG. 2 AP is measured by the differential pressure instrument 97 which is connected to pipe 60 by the

taps

93 and 95.

The function generator 127 is also added in line 129 from line 124 to characterize the set point signal on line to the comparator l 15 as a function of desired generation as may be desired.

What is claimed is:

1. In a control system for a turbine-generator unit which adjusts a control valve controlling the steam flow to the turbine to tend to maintain a desired power level in the turbine-generator unit, and adjusts the inputs to the boiler in accordance with a demand signal computer as the ratio of a signal representing said steam flow to a signal representing boiler output steam pressure all multiplied by a set point for said pressure with means for modifying the demand signal until the throttle pressure returns to a predetermined set point, the combination therewith of:

means operable to modify the throttle pressure set point until the measured state of the control valve corresponds to the desired state.

2. In a system for controlling the inputs to a boiler of a boiler-turbine system in response to a demand signal calculated as the product of a signal representing a set point for the pressure of steam supplied by the boiler and a signal representing the ratio of a signal representing the steam flow in the turbine to a signal representing the measured valve of said pressure with means for modifying said demand signal until said measured pressure returns to said set point, and with a control valve controlling the steam flow to the turbine being controlled to maintain a desired power level in the turbinegenerator unit, the combination therewith of:

means for producing a signal representative of a desired state of said control valve,

means for comparing said last named signal with said ratio, and

means for controlling said set point signal so as to modify said demand signal in a direction and extent to tend to maintain said control valve in said desired state.

3. The method for automatically controlling inputs to a boiler-turbine system so as to tend to maintain a throttle valve controlling steam flow to the turbine in a predetermined state when said valve is subject to control to maintain a desired power output from a generator driven by said turbine comprising the steps of:

producing a demand signal in accordance with the ratio of a signal representing steam flow to the turbine with respect to a signal representing the measured throttle pressure at the boiler output all multiplied by a signal representing throttle pressure set point,

producing a signal representative of the desired state of the throttle valve,

producing a signal representative of the actual state of the throttle valve,

comparing said signals representative of the desired and actual throttle valve states,

modifying the signal representative of said throttle pressure set point so as to tend to reduce the deviation between said compared signals,

modifying the demand signal so as to tend to maintain the said throttle pressure at its modified set point, and

controlling the boiler inputs in accordance with said modified demand signal.

4. The method of claim 3 in which the signal representative of the actual state of said throttle valve is produced in accordance with the pressure difference across it.

5. The method of claim 3 in which the signal repre sentative of the actual state of said throttle valve is representative of the opening in said valve and is established by the ratio of said signal representing steam flow and said signal representing the actual throttle pressure.

6. The method of claim 3 in which the modification of the signal representative of the throttle pressure set point is produced by control action having a proportional and integral function which control action is responsive to the deviation of said compared signals.

7. The method of claim 6 in which the control action is responsive to said deviation times a signal representative of the desired output of said generator.

8. The method of claim 6 in which the control action is responsive to said deviation plus a signal representative of the difference between the desired output of said generator and its measured output.

9. The method of claim 6 in which the control action is responsive to said deviation times a signal representative of the desired output of said generator with the addition to the resulting product of a signal representative of the difference between the desired and measured outputs of said generator.

10. The method of claim 6 in which the modification of the signal representative of the throttle pressure set point by said control action includes the addition to said control action of a signal representative of the desired generation.

11. The method of claim 6 in which the control action is responsive to said deviation times a signal representative of the desired output of said generator with the addition to the resulting product of a signal representative of the difference between the desired and measured outputs of said generator and the control action is modified by the addition of said signal representative of the desired output.

a P0405) UNITED STATES PATENT OFFICE Y o 5 CERTIFICATE OF CORRECTION Patent No. 3,802,189 Dated April 3, 197A Inventofls) THERON w. JENKINS, JR.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, Claim l, lines 8-9, "computer" should read --co mputed-- Column 6, claim 2, line 2M, "valve" should read --va1ue-'- Signed and sealed this 21st day of January 1975.

(SEAL)- Attest:

MCCOY GIBSON JR. C. MARSHALL DANN Attestlng Officer Commissioner of Patents

Claims

1. In a control system for a turbine-generator unit which adjusts a control valve controlling the steam flow to the turbine to tend to maintain a desired power level in the turbinegenerator unit, and adjusts the inputs to the boiler in accordance with a demand signal computer as the ratio of a signal representing said steam flow to a signal representing boiler output steam pressure all multiplied by a set point for said pressure with means for modifying the demand signal until the throttle pressure returns to a predetermined set point, the combination therewith of: means operable to modify the throttle pressure set point until the measured state of the control valve corresponds to the desired state.

2. In a system for controlling the inputs to a boiler of a boiler-turbine system in response to a demand signal calculated as the product of a signal representing a set point for the pressure of steam supplied by the boiler and a signal representing the ratio of a signal representing the steam flow in the turbine to a signal representing the measured value of said pressure with means for modifying said demand signal until said measured pressure returns to said set point, and with a control valve controlling the steam flow to the turbine being controlled to maintain a desired power level in the turbine-generator unit, the combination therewith of: means for producing a signal representative of a desired state of said control valve, means for comparinG said last named signal with said ratio, and means for controlling said set point signal so as to modify said demand signal in a direction and extent to tend to maintain said control valve in said desired state.

3. The method for automatically controlling inputs to a boiler-turbine system so as to tend to maintain a throttle valve controlling steam flow to the turbine in a predetermined state when said valve is subject to control to maintain a desired power output from a generator driven by said turbine comprising the steps of: producing a demand signal in accordance with the ratio of a signal representing steam flow to the turbine with respect to a signal representing the measured throttle pressure at the boiler output all multiplied by a signal representing throttle pressure set point, producing a signal representative of the desired state of the throttle valve, producing a signal representative of the actual state of the throttle valve, comparing said signals representative of the desired and actual throttle valve states, modifying the signal representative of said throttle pressure set point so as to tend to reduce the deviation between said compared signals, modifying the demand signal so as to tend to maintain the said throttle pressure at its modified set point, and controlling the boiler inputs in accordance with said modified demand signal.

5. The method of claim 3 in which the signal representative of the actual state of said throttle valve is representative of the opening in said valve and is established by the ratio of said signal representing steam flow and said signal representing the actual throttle pressure.