US3908897A

US3908897A - Off-line integration of bridge and boiler controls

Info

Publication number: US3908897A
Application number: US428286A
Authority: US
Inventors: Holtz Leopold C Van; Adrianus G Hop
Original assignee: Shell Oil Co
Current assignee: Shell USA Inc
Priority date: 1972-12-21
Filing date: 1973-12-26
Publication date: 1975-09-30
Anticipated expiration: 1992-09-30
Also published as: NL7217434A; DE2363270A1; SE384403B; GB1454416A; JPS4989001A; NO137518C; NO137518B

Abstract

An apparatus and method for controlling a steam generator operating under rapidly varying load conditions. The method measures steam flow and pressure and combines the measurements with a signal that is proportional to the rate of change of steam flow and utilizes the resulting signal to control the fuel and air flow to the steam generator.

Description

United States Patent 11 1 van Holtz et a1.

1451 Sept. 30, 1975 1 1 OFF-LINE INTEGRATION OF BRIDGE AND BOILER CONTROLS [75] Inventors: Leopold C. van Holtz; Adrianus G.

Hop, both of Amsterdam,

211 Appl. No.: 428,286

[56] References Cited UNITED STATES PATENTS 3.042.007 7/1962 Chien et a1 122/448 R 3,284,615 11/1966 Yetter 1 1 236/14 X 3.415.232 12/1968 Garrett et al. 122/448 R 3.616.997 11/1971 Oldenburg 236/14 Primary E.\'aminer--Edward G. Favors [57] ABSTRACT 7 Claims, 6 Drawing Figures [301 Foreign Application Priority Data Dec. 21 1972 Netherlands 7217434 52 us. c1 1. 236/14; 122/448 511 lm. c1.-' F2311 1/08 [58] Field of Search 236/14, 15 E; 122/449, 122/448 IB 1 ADOER l 1 14 l I l 1 suPER I7 I 5 HEATER 6 1 I 1 1 1 c0/vr TROLLER SET I I3 I I P0\lqNT I CON- I 792. FLOW I I 6* I w HEATER I2 FUEL am I a FLOW METER i I CON- 1 TROLLER US Patent Sept. 30,1975 Sheet 1 of 2 3,908,897

18 F IG. 7 r l T -AOOER l P: T I SUPER I i i 77 i 5 HEATER 6 i 1 T l CON- l TROLLER 1 SE/ 73 I I POINT I I l CON- 79- FLOW 1 TROLLER METER I FLOW- SUPER METER A/R HEATER 7 72 FUEL /V\ 8 FLOW l 'METER I I CON- l TROLLER FIG. 3 23 ,L

SIGNAL GENERATOR B FIG 2 1 1 SIGNAL l I GENERATOR '6 1 "w 1 T 22 AOOER ADDER Q l l 5 i PI A P 73 l i cOMPuTER CON- T" CON l TROLLER TROLLER l l 20 1 FLOW FLOW FIG. 4

US. Patent J W 9 m M 7 1-1 EOE h m M 7 P23 2/ m u ll l A A0, 9 /|l| 3 W R 4 u R r ME 7 wm m R \9 w 0 R U N WR E 11|||||||1|J01 l L 0 FE w m M FIG. 6

FIG. 5

66 SDP bar SOP bar SOT C 507 C 535 DL mm OFF-LINE INTEGRATION OF BRIDGE AND BOILER CONTROLS BACKGROUND OF THE INVENTION The invention relates to a process for the automatic control of a steam generator with varying load and an apparatus for carrying out the process,

A steam generator with varying load requires special measures to be taken in order to rapidly adapt the steam production to the varying demand. Under varying load there is a chance that the heat production and, hence, the steam production, may lag very much behind variations in the demand for steam. Lagging will result in large variations in steam pressure and when the pressure rises excessively, losses may occur. When the pressure is too low, it may be impossible to supply the quantity of steam required. Problems of this kind are encountered particularly with a steam generator installed on board a ship during maneuvering the load changes. During maneuvering variations from no load to full load and vice versa occur in rapid succession.

A known process is to control the combustion process in the steam generator by using as the desired value the sum of the output signal of the steam pressure controller and a signal that is proportional to the rate of steam offtake. It has been found, however, that this control configuration leads to the above-mentioned disadvantages, and the invention provides the means for an improved control configuration.

BRIEF SUMMARY OF THE INVENTION The invention therefore relates to a process for the automatic control of a steam generator with varying load, in particular of a steam generator installed on board a ship. The combustion process in the steam generator is controlled by adjusting the supply of fuel and air, the desired value consisting of the sum of the output signal P of the steam pressure controller and a signal A that is proportional to the rate of steam offtake. A signal B proportional to the rate of change of the steam offtake is added to the sum of signal A.

Addition here is in the algebraical sense, that is to say the signal B can be either positive or negative in sign.

This control process uses a combination of proportional and derivative action, derived from the load of the steam generator, and of steam pressure control. These signals together adjust the heat production, This results in a considerable improvement of the speed of response of the steam generator. Naturally, this control configuration is complemented by standard control systems belonging to a steam generator, such as a level control system, a system for the control of the fuel/air .ratio, a system for the control of. the boiler feedwater supply, a steam temperature control system, etc,

Furthermore, a considerable improvement is achieved if the signal B is proportional to the expected rate of change of the steam offtake, the signal B being derived from an independent signal generator. Thus, as far as the adjustment of the heat production is concerned, one anticipates the new situation to be expected. As soon as one wishes to change the demand for steam, then to the signal from the pressure controller and the signal A, which signals are always present, the signal B is added. Thus the quantity of steam supplied starts to change when the signal B already exerts its full influence on the control of the combustion process. The heat production already adjusts itself to the new condition, as a result of which the aforementioned lagging is reduced. The magnitude of the signal B can be adapted to the expected rate of change of the steam offtake by manually adjusting the independent signal generator or by coupling this signal generator with the signal generator for the steam demand.

The signal B now is not therefore obtained by deriving it from the measured steam offtake. This has the advantage that the signal B now is free from noise and, consequently, the fire in the combustion chamber becomes more quiet.

Configurations for the control of the steam generator are known in which the rate of change of the steam offtake is confined to extreme limits. Thus it is ensured that a suddenly required change in the steam offtake does not result in damage to the steam turbine and/or the boiler, as may happen in marine propulsion plant, particularly when this plant is operated from the bridge without technical personnel being engaged in the operation. To ensure that the extreme limits referred to above are not exceeded, the rate of change of the valve position in the steam line to the turbine can be confined to a maximum positive or a maximum negative value. The absolute values of these limits may be equal, but need not be. For example, the rate of decrease of the load may be higher. According to another characteristic of the invention, in a control configuration of this type the signal B can have a fixed positive, respectively a fixed negative value, which sign is adapted to the sign of the change of the quantity of steam, which fixed value is automatically derived from an independent signal generator and added to the signal A as soon as the steam demand is changed. The explanation given earlier applies here as well. However, here a fixed value of the signal B may suffice, because for instance during maneuveringof a ship changes in the steam demand occur of such an extent that changing the position of the steam valve takes place in accordance with the permitted extreme limits.

The combustion process will follow the required changes even more closely if the said sum of the signals not only is passed as the desired value to the system for control of the fuel supply and to the system for control of the air supply but is also added to the output signal of the air controller and passed to the air flow correcting unit. When this control configuration is applied the rate of the air supply and, hence, of the fuel supply changes very rapidly, which reduces the lag of the steam production behind changes in the demand. Also, it has been found that with the control process according to the invention the speed of response of the steam generator can be increased by a factor of at least 1.5 relative to conventional control processes if the variations in boiler pressure and water level are maintained the same in the two cases.

An apparatus for the automatic control of a steam generator with varying load, in particular of a steam generator belonging to a steam turbine plant on board a ship, is characterized by an adder of which one input is connected to the output of a steam pressure controller, another input to the signal output of a flowmeter measuring the quantity of steam supplied and still another input to a signal generator supplyinga signal that is proportional to the rate of change of the quantity of steam supplied. The adder output is connected to the input(s) for the set value of the system for the control of the supply of fuel and air to the combustion of the steam generator.

The signal generator may consist of a computing element of which the input is connected to the signal output of the flowmeter measuring the quantity of steam supplied. The computing element output is connected to that input of the adder which receives the signal proportional to therate of change of the quantity of steam supplied, which signal can be made by the computing element.

The signal generator may be an independent signal generator which is capable of supplying a signal that is proportional to the expected rate of change of the quantity of steam supplied. The signal generator output is connected to the appropriate input of the adder.

The apparatus may be equipped with a controller by which the rate of change of the steam offtake can be confined to extreme limits. The said signal generator is an independent signal generator capable of supplying a signal of a fixed positive or negative value, which sign correspond to the sign of the change in the quantity of steam supplied. The signal generator is connected, by a switch element for effecting the connection to the signal of the desired sign, to the appropriate input of the adder. The switch element may be coupled to a correcting unit for the quantity of steam supplied.

The apparatus may be further equipped with an adder of which one input is connected to the input of a controller for the air supply and another input is connected to the output of the first adder. The second adder is capable of supplying the sum of the signals P, A and B. The output of the first adder being connected to the correcting unit for the air supply, while the output of the second adder capable of supplying the sum of the signals P, A and B also receives the desired value of. the fuel supply controller and the air supply controller.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more easily understood from the following detailed description of a preferred embodiment when taken in conjunction with the attached drawings in which:

FIG. 1 is a block diagram of one form of the invention;

FIG. 2 is a block diagram of one circuit for obtaining a control signed;

FIG. 3 is a block diagram of a modified form of the circuit shown in FIG. 2;

FIG. 4 is a block diagram of the control system of FIG. 1 applied to a boiler; and

FIGS. 5 and 6 are response curves of the prior art control systems and the system of this invention.

According to FIG. 1 a boiler 1 for the production of steam is heated by a flame 2. The fuel supply is adjusted by a valve 3, the air flow by a device 4. The steam leaves the boiler via a line 5 and subsequently passes a superheater 6. This may be heated by, for instance, flue gases from the boiler. There may also be present a superheater 7, after which the superheated steam can be passed via discharge line 8 to its destination, for instance a marine turbine. In the boiler 1 there is also present a heat exchanger 9. Through this heat exchanger superheated steam from 6 can be passed to the discharge line 8. As the temperature of the water and of the steam in the boiler is lower than the temperature of the steam leaving the superheater 6, that steam is being cooled when it passes the heat exchanger 9. The quantity of steam passing through the heat exchanger 9 can be adjusted by

valves

10 and 11.

A pressure gauge 12 measures the pressure in the line 8. The signal from this pressure gauge goes to the controller 13 and is there compared with a set value of the pressure. The output signal P of controller 13 goes to an adder 14. A flowmeter 15 measures the rate at which steam is supplied via line 8 and the signal from this meter also goes to the adder 14. This signal is the signal A. The adder 14 also received the signal B.

The sum signal thus obtained goes as desired value to a controller 16 for the fuel supply and to a controller 17 for the air supply. These controllers receive, respectively, signals from the flowmeter l8 measuring the fuel flow and the flowmeter l9 measuring the air flow. The valves or correcting units 3 and 4 are adjusted by the corresponding output signals.

The output signals of the pressure controller 13 can also be used for adjusting valve 10. Valve 1 1 can be adjusted with a steam temperature controller.

In this way the ratio between fuel flow and air flow remains constant. This can also be achieved in other ways, which in themselves are known, for instance by means of a ratio controller. However, the output signal of the adder 14 will always determine the magnitude of the fuel flow and the air flow. As far as the conventional part of the control configuration is concerned, many variations can be applied which will not be further described can be combined with the control process according to the invention.

The signal B can be passed to the adder 14 by the action of the signal generator supplying the signal representing the steam demand, but it also is possible to use other methods.

In FIG. 2 the signal B is obtained by passing the signal from flowmeter 15 to a computing element 20, from which is obtained a signal corresponding to the derivative of the steam flow through line 8 with respect to time.

In FIG. 3 the signal B is obtained by connecting a switch element 21 to one of the

signal generators

22 or 23, which are capable of supplying signals of opposite sign, possibly of the same value. Switch element 21 is operated simultaneously with the required change in the quantity of steam from line 8, which is symbolized by the dotted line 24.

In the more detailed diagram shown in FIG. 4 relating to the fuel/air control, there is present in the air line a vane pump 25 which can be driven by a motor 26. This motor 26 can be a steam turbine. It is also possible for an electric motor to be applied. The supply of air to the pump 25 can be controlled not only by controlling the power of the motor 26 but also by an air register 27. In the case of the pump 25 being a steam turbine, the supply of steam is adjusted by a valve 28. The number of revolutions per minute of the turbine is measured by meter 29 and controlled by controller 30, which adjusts the valve 28.

The quantity of air supplied is measured with flowmeter 19. The signal from this meter goes to controller 17, which as desired value receives the output signal of the adder 14. The output signal of controller 17 goes to an adder 31 as also does the output signal of 14. The sum of these signals adjusts the air register 27. In this way it is ensured that the air register acts immediately in response to the signal B, which, as outlined hereinbefore, corresponds to the rate of change or the expected rate of change of the quantity of steam supplied. The controller 17 rather has the function of a trim controller. The output signal of the adder 31 also goes to the input of controller 30 receiving the set value for that contorller, so that the motor 26 also acts rapidly in response to the signal B.

The output signal of the adder 14 also passes to a low signal value selector 32, as also does the signal of the flowmeter 19. In this way it is ensured that when the load of the steam generator increases, first the supply of air to the combustion chamber is increased and after that the supply of fuel, whichis desirable from the point of view of safety and atmospheric pollution.

In FIGS. 5 and 6 results are shown. The horizontal axes represent the same time scale. The series of curves in FIG. 5 have been obtained with a conventional control configuration, the series in FIG. 6 with a control configuration according to the invention.

The curves with indication BL show how the steam demand varies with time. Large changes in the upward and in the downward direction are represented, in FIG. 5 at a rate of l.3%/s and in FIG. 6 even at a rate of 4%/s. The other curves relate to the responses to these changes, SDP representing the pressure in the steam drum, SOP the steam pressure at the superheater outlet, SOT the steam temperature at the same location (line 8 in FIG. 1), DL the water level in the steam drum and RF the heat generated under the drum.

From a comparison of the results obtained with conventional control configuration (FIG. 5) with those obtained with a control configuration according to the invention (FIG. 6) it follows that at the same variations in drum pressure and water level the control configuration according to the invention permits operation at a higher rate of change of the quantity of steam supplied.

We claim as our invention:

1. A process for the automatic control of a steam generator operating under varying load conditions, said process comprising: measuring the exit steam pressure from the generator; comparing the measured steam pressure with the desired steam pressure and generating a correcting signal P; measuring the steam flow from the generator and generating a signal A related thereto; measuring the change in steam flow and generating a signal B related to the rate of chang in the steam flow; algebraically adding the signals P, A and B to obtain a control signal; and using the control signal to control the fuel and air flow to the combustion process of said generator.

2. A process according to claim 1, in which process the signal B is proportional to the expected rate of change of the steam flow, the signal B being derived from an independent signal source.

3. A process according to claim 1 where the rate of change of the steam flow is confined to extreme limits, in which the signal B has a fixed positive and a fixed negative value, with the signal being the same as the sign of the change of the quantity of steam.

4. An apparatus for the control of a steam generator operating under varying load condition, in particular a steam generator on board a ship, said apparatus comprising:

a flow meter disposed to measure steam flow from the generator and provide a flow signal related to the measured flow;

a pressure measuring means disposed to measure the pressure of the steam flow from the generator and supply a pressure signal related to the measured pressure;

a controller having a set point, said pressure measuring means being coupled to said controller said controller supplying a control signal in response to the difference between the pressure signal and said set point;

a signal generator responsive to the flow signal to supply a rate of change signal related to the rate of change in said flow;

an adding circuit, said flow, control and rate of change signals being supplied to said adding circuit, said adding circuit algebraically adding said flow, control and rate of change signals and supplying a fuel-air control signal and;

control means disposed to separately control the fuel and air supplied to the combustion chamber of said generator, said adder being coupled to said control means whereby said control means controls the air and fuel supplied to the combustion chamber.

5. An apparatus according to claim 4, in which apparatus the signal generator consists of a computing element having an input connected to the signal output of the flow meter measuring the quantity of steam supplied and an output connected to that input of the adding circuit.

6. An apparatus according to claim 4, in which apparatus the signal generator is an independent circuit capable of supplying a signal that is proportional to the expected rate of change of the quantity of steam supplied, said signal generator having an output connected to the input of the adding circuit.

7. An apparatus according to claim 6 in which said signal generator comprises an independent circuit capable of supplying a signal of a fixed positive and fixed negative value the sign of the signal being adapted to the sign of the change in the quantity of steam supplied, said signal generator being connected to a switch element for connecting the signal of the desired sign to the appropriate input of the adding circuit.

Claims

4. An apparatus for the control of a steam generator operating under varying load condition, in particular a steam generator on board a ship, said apparatus comprising: a flow meter disposed to measure steam flow from the generator and provide a flow signal related to the measured flow; a pressure measuring means disposed to measure the pressure of the steam flow from the generator and supply a pressure signal related to the measured pressure; a controller having a set point, said pressure measuring means being coupled to said controller said controller supplying a control signal in response to the difference between the pressure signal and said set point; a signal generator responsive to the flow signal to supply a rate of change signal related to the rate of change in said flow; an adding circuit, said flow, control and rate of change signals being supplied to said adding circuit, said adding circuit algebraically adding said flow, control and rate of change signals and supplying a fuel-air control signal and; control means disposed to separately control the fuel and air supplied to the combustion chamber of said generator, said adder being coupled to said control means whereby said control means controls the air and fuel supplied to the combustion chamber.