Method Of Controlling The Feed Of Forced Circulation Steam Generators

Abstract

The rate of feedwater flow is controlled in a first load range by the level of water in the separator while in a higher load range the rate is controlled by the steam temperature. Signals are produced which are compared with preset values and are integrated so as to produce a negative control signal in one case and a positive control signal in the other case. Only one of these control signals is superimposed on a base signal only when the generator is in a respective load range so as to produce a command signal for adjusting the feed controller of the feed valve.

Description

This invention relates to a method of controlling the feed of forced circulation steam generators.

Briefly, the invention provides a method of controlling the rate of feed of a forced circulation steam generator wherein a frist controlled condition acts predominantly on a feedwater flow in a first load range and a second controlled condition acts predominantly on the feedwater flow in a second load range. The feedwater flow is controlled by a command signal produced by a predetermined or preset base signal having superimposed thereon in the first load range only the first control signal and only when the first control signal is of the sense to reduce the command signal and having superimposed thereon in the second load range only the second control signal and only when the second control signal is of the sense to increase the command signal.

The method includes the steps of feeding a flow of feedwater to the steam generator, producing a first control signal in response to the first controlled condition, producing a second control signal in response to the second controlled condition, producing the base signal and of generating the command signal in the first load range by superimposing only the first control signal on the base signal and only when the first control signal is of a sense to reduce the base signal and in the second load range by superimposing only the second control signal on the base signal and only when the second control signal is of a sense to increase the base signal.

In one application of the present invention, the steam generator is of the kind operated with sliding pressure and the frist controlled condition is used in the load range in which steam delivery pressure is subcritical and the second controlled condition is used when the steam delivery pressure is supercritical.

The invention provides a method in which the change-over between control by the first controlled condition and control by the second controlled condition occurs smoothly and without the two controlled conditions acting simultaneously and in conflict with one another (one demanding more and the other demanding less feed water) and without sudden changes in the control signal which might lead to severe damage due to thermal shock, for example by sudden displacement of the location of the end of the zone in which evaporation takes place or by the ejection of water from the separator into the super-heater.

The invention allows construction of a control system by which a steam generator can be controlled which is simple and which uses comparatively inexpensive control equipment.

The invention is particularly suitable for application to forced through-flow steam generators with forced circulation in a limited load range. In this case, the base signal preferably has a value which is sufficiently high to produce a feed water supply which is sufficient to cool the evaporator of the steam generator at the top of the limited load range. When the method is applied to a forced through-flow steam generator with superimposed forced circulation, a surge-free change-over is permitted from operation with forced circulation to operation without forced circulation when the separator has run dry even under conditions in which there is no return of water from the separator to the inlet of the feed pump or in which steam is returned from the separator to the circulating pump.

These and other objects and advantages of the invention will become more apparent from the following detailed description and appended claims taken in conjunction with the accompanying drawings in which:

FIG. 1 diagrammatically illustrates a forced through-flow steam generator with superimposed circulation controlled in accordance with the invention;

FIG. 2 graphically illustrates the change in the amount of working medium as a function of the load for the steam generator shown in FIG. 1; and

FIG. 3 illustrates a modification of the control system of the steam generator shown in FIG. 1 which is suitable for handling particularly rapid and large disturbances.

Referring to FIG. 1, a steam generator 6 has a feed pump 1 which supplies working medium through a feed line 2, a feed valve 3 and a flow-rate metering position 4 to an economiser 5. The water which is preheated in the economiser 5 flows from the economiser 5 through a connecting line 7 and a circulating pump 8 into an evaporator 9 and then enters a water separator 10 as a vapor after being completely evaporated or as a vapor-water mixture which is separated in the separator. The separated steam then flows through three superheaters 11, 12 and 13 and a live steam valve 14 to the load (not shown) while the water is returned to the connecting line 7 through a line 20 having a valve 21 therein which is controlled by the water level in the water separator 10 in a manner which is not shown.

The separator 10 is provided with a level-measuring apparatus 25 of suitable construction. The water level measured by the apparatus 25 represents a first controlled condition. A temperature measuring means 26 of suitable construction is provided on the connecting line between the first superheater 1 and the second superheater 2. The temperature measured at this position represents a second controlled condition.

The output of the level-measuring apparatus 25 is connected to a comparator 27 to deliver a signal thereto representative of the water level in the separator 10. The comparator 27 is supplied through a line 28 with a water level set value which is preferably constant. An output line from the comparator 27 branches at 29 so that an output signal representative of the difference between the actual water level signal and the set value signal is supplied to a function generator 30 having integral action (hereafter referred to as an I-element) and to a function generator 31 having proportional action (hereafter referred to as a P-element). The outputs of the two elements 30 and 31 are combined at position 32 to form a first control signal which is supplied to a limiter 33 which limits the control signal to positive values.

An analogous procedure takes place on the output of the temperature-measuring means 26. The temperature actual value signal from the means 26 is compared at a comparator 37 with temperature set value which is supplied through a line 38 and whose magnitude is dependent upon the load or steam delivery pressure. A signal representative of the deviation of the actual value signal from the set value signal is supplied via an output line to a branch point 39 and is supplied to an I-element 40 and to a function generator 41 having proportional-plus-derivative action (hereafter referred to as a PD-element). The outputs of the two elements 40 and 41 are added at position 42 and are supplied to a limiter 43.

At a point 45, the inverse of the output of the limiter 33, the uninverted output of the limiter 43 and the output of a base signal transmitter 46 are added together and the sum thus formed is supplied in the form of a command signal along a line 47 to a feed controller 48 which actuates the feed valve 3. A working medium flow rate signal formed by a flow-rate-measuring device 49 connected to the position 4 is supplied as a measured value to the feed controller 48.

The base signal transmitter 46 delivers an output signal which is constant in the lower and middle load ranges and which increases approximately linearly relative to the firing rate in the upper load range in which the temperature effect predominates. In an alternative arrangement, the base signal is constant in the upper load range also.

The first control signal formed at position 32 and the second control signal formed at position 42 are supplied through lines 50 and 51 to analog-digital converters 52 and 53, respectively. These converters 52, 53, respectively. These converters 52, 53 energize separate relays 54, 55, respectively, if the respective control signals become negative so that the connections between the branching points 29, 39 and the I-elements 30, 40, respectivley, are interrupted. This avoids a run-away increase in the output signal from either of the I-elements when it is not in control.

To explain the operation of the system it will be assumed that initially the boiler is operating in the lower load range, the upper limit of which is at a load of 55 percent of full load. The base signal transmitter 46 provides a working medium flow rate set value of 60 percent of the amount required at full load. If the steam generator is operated at, for example, a load of 40 percent, excess water will collect in the separator 10 until the first control signal formed at position 32 is sufficiently large to reduce the command signal in line 47 to 40 percent. The second control signal derived from the temperature is negative in the aforementioned load range and thus is ineffective. If the load is gradually increased, the level in the separator 10 will drop and as a result the magnitude of the first control signal, fed in negatively, gradually diminishes while the command signal in line 47 increases. This continues until the first control signal supplied through point 32 becomes negative and the command signal in line 47 becomes equal to the base signal. The first control signal will then no longer be conducted to the addition point 45. The feed rate therefore remains constant although the firing rate has been further increased. Accordingly, the temperature measured by the measuring means 26 increases beyond the set value 38 which is set at a higher value than the temperature which occurs at that position in wet operation and as a consequence the second control signal will come into action at point 45 in the additive sense. Thus, the command signal in the linr 47 and, therefore, the amount of working medium fed into the steam generator 6 are both increased.

The water content of the separator 10 is subsequently gradually lost, partly through the circulating pump 8 and partly by evaporation after the valve 21 is closed.

If the load is reduced, the cycle will be performed in the opposite sense. The command signal in line 47 diminishes with a diminishing second control signal until the latter becomes zero. The command signal then stops at the residual value of the base signal while the separator 10 is full, and thereafter diminishes due to the action of the first control signal.

Referring to FIG. 2, the kinked line f shows the characteristic of the feed rate and the curve v indicates the rate of flow of working medium through the evaporator as a function of the load. The broken line g relates to the change of base signal while the dash-dot straight line d represents the amount of generated steam. The difference in the vertical direction between d and f corresponds to the amount of water which is injected through a duct 15 (FIG. 1) to the working medium between the superheaters 12 and 13 and which is adjusted in dependence upon the steam delivery temperature while the difference R between v and f relates to the circulated amount of water.

At the height of the line g, the line f has a distinct stopping point extending over the load interval L.sub.1 to L.sub.2. In the load range below L.sub.1 the command signal is formed by subtraction from the base signal (g) of the first control signal corresponding to the difference (g-f) while in the load range above L.sub.2 the command variable is performed by addition to the base signal (g) of the second control signal which corresponds to the difference (f-g).

While the system shown in FIG. 1 has an excellent control characteristic at medium rates of load change and in conditions of normal disturbances, it is possible for the dynamic characteristics of the system to be improved by refining the control method for conditions of high rates of load change and severe disturbances. Such a refinement is obtained if integration by the I-element 30 associated with the first controlled condition is interrupted when both deviations, i.e. the results of the comparisons made by the comparators 27, 37, are negative and if integration by the I-element 40 associated with the second controlled condition is interrupted when both deviations are positive. The width of the interval L.sub.1 to L.sub.2, which may be adjusted by variation of the set value 38, may thus be reduced.

FIG. 3 shows a part of the control system shown in FIG. 1 with elaborations by means of which the refined method may be performed. As shown, analog-digital converters 60 and 61 are connected to the output lines from the comparators to form separate digital signals from the deviations formed by the comparators 27 and 37, each digital signal changing from 0 to 1 if the respective deviation changes its sign from negative to positive and vice versa. The outputs of the two converters act on an OR element 64 and on an AND element 65, respectively. The inverted output of the OR element 64 acts on a relay 66, the contact set of which is connected in series with the contact set of the relay 54 in the feed of the I-element 30. The output of the AND element 65 acts uninverted on a relay 67, the contact set of which is connected in series with the contact set of the relay 55 in the feed of the I-element 40.

In either of the arrangements described, the circulating pump 8 may be arranged to operate only when the load is below a predetermined proportion of full load. In this case, the base signal delivered by the transmitter 46 should be sufficiently high to insure that immediately after the pump 8 shuts off should the load increase above this proportion, the flow of feed-water through the valve 3 will be high enough to adequately cool the evaporator 9.

It is a remarkable property of the method of control described that the provision of two control means 30, 31 and 40, 41 enables the setting values thereof to be matched to the characteristics of the associated control circuits which may vary widely from each other. A very high degree of control quality may be achieved by such optimizing.

Another favorable property of the method described is due to the fact that the range in which circulation occurs may be varied by a simple increase or reduction of the base signal (46). This particularly useful when the firing system of the steam generator is changed from oil to gas and vice versa.

Claims

What is claimed is:

1. A method of controlling a rate of feed of water in a forced circulation steam generator comprising the steps of

feeding a flow of feedwater to the steam generator;

producing a first control signal in response to a first controlled condition acting predominantly on the flow of feedwater in a first load range;

producing a second control signal in response to a second controlled condition acting predominantly on the flow of feedwater in a second load range;

producing a predetermined base signal; and

generating a command signal to control the flow of feedwater in said first load range from a superposition of said base signal with only said first control signal and only when said first control signal is of a sense to reduce said base signal and in said second load range from a superposition of said base signal with only said second control signal and only when said second control signal is of a sense to increase said base signal.

2. A method as set forth in claim 1 wherein said first controlled condition is the water level in a water-steam separator in the steam generator in the flow path of the feedwater and said second controlled condition is the steam temperature of the steam generated from the feedwater downstream of a superheater in the steam generator.

3. A method as set forth in claim 1 wherein said base signal corresponds to a feedwater flow rate prevailing in a transition zone between the predominance of said first controlled condition and the predominance of the second controlled condition.

4. A method as set forth in claim 1 wherein said first control signal is formed by a separate controller having integral action from a signal responsive to said first controlled condition and which further comprises the step of blocking integration by the controller as soon as said first control signal becomes negative.

5. A method as set forth in claim 1 wherein said second control signal is formed by a separate controller having integral action from a signal responsive to said second controlled condition and which further comprises the step of blocking integration by the controller as soon as said second control signal becomes negative.

6. A method as set forth in claim 5 wherein said first control signal is formed by a second separate controller having integral action from a signal responsive to said first controlled condition and which further comprises the steps of blocking integration by the second controller as soon as said first control signal becomes negative, of blocking integration by one of the controllers when the signals supplied to the controllers are both positive and of blocking integration by the other of the controllers when the signals supplied to the controllers are both negative.

7. A method as set forth in claim 3 wherein the steam generator is a forced through-flow steam generator with forced circulation in a limited load range and having an evaporator, and wherein said base signal has a value sufficiently high to produce a feedwater rate sufficient to cool the evaporator at the top of said limited load range.

8. In combination with a steam generator having a feed controller for controlling the rate of feedwater flow to an evaporator, a water separator downstream of said evaporator and at least one superheater downstream of said water separator; means for controlling said feed controller to control the rate of flow of the feedwater, said means including a water-level measuring means connected to said separator to produce a first control signal in response to the level of water in said separator, a steam temperature measuring means downstream of said superheater to produce a second control signal in response to the temperature of the steam thereat, a base signal transmitter for emitting a predetermined base signal, and means for super-imposing said base signal with only said first control signal in a first load range when said first control signal is negative relative to said base signal and with only said second control signal in a second load range when said second control signal is positive relative to said base signal for producing a command signal, said latter means being connected to said feed controller to deliver said command signal thereto for adjustment of the rate of feedwater flow in response thereto.