Method and Apparatus for Controlling an Operation of Plant

The present invention relates to a method and an apparatus for controlling an operation of turbine plant, and more particularly for preventing a flashing when the load on the turbine is decreased abruptly.

The turbine plant is used widely for the purpose of electric power generation. In connection with the electric power demand, the turbine is not always required to operate with full power, but required to operate with full power in the daytime to meet a large demand for electric power and to stop or operate with partial load in the night time in which the demand for electric power is rather small. Such alternation of start and stop of operation in one day or partial load operation imposes a problem that the flashing occurs in the deaerator or in the boiler feedwater pump when the power is decreased in conformity with a reduction in the load level. Such flashing adversely affects the control of operation of the plant.

The reason why the flashing occurs is as follows. When the load of the turbine is decreased abruptly, the interior pressure in the deaerator, to which the heated steam is supplied from the turbine, is also decreased. On the other hand, when the load of the plant is decreased below a predetermined level, the feedwater pump is stopped and the hot water in the downcomer pipe remains high temperature. Consequently, the interior pressure in the downcomer pipe becomes lower than the saturated vapour pressure corresponding to an inlet temperature, thus the flashing is occurred in the deaerator and the downcomer pipe. It is also experienced that re-starting of the feedwater pump is often failed because the pump suction head is lowered as a result of the flashing.

Although various proposals have been made to overcome the above-described problems, these proposals are confined to control the plant partially, and no attempt has been made to control the whole plant. For instance, Japanese Patent Laid-Open Publication No. 143103/1976 discloses one proposal to prevent an occurrence of flashing in the downcomer pipe connecting a deaerator to the feedwater pump.

When a main turbine is tripped from 100% load, the downcomer pipe is filled with hot water of the same temperature as the hot water in the deaerator on 100% load, so that flashing occurs in the downcomer pipe. According to the proposal, in order to prevent the occurrence of flashing, the hot water in the downcomer pipe is fed to the boiler through a branch pipe upon such turbine trip so as to remove the hot water remaining at the inlet side of the feedwater pump. Accordingly the occurrence of flashing is prevented even when the condensate in the deaerator, the temperature of which has been lowered due to the turbine trip, reaches the inlet side of the feedwater pump.

According to this arrangement, however, the hot water cannot be sufficiently removed from the downcomer pipe through the branch pipe in response to a reduction in the turbine load and, therefore, the temperature in the downcomer pipe cannot be lowered in response to the turbine load reduction. With this countermeasure, it is not possible to perfectly avoid the occurrence of flashing.

Accordingly, an object of the invention is to provide a method and an apparatus for controlling an operation of a turbine plant having a deaerator, a feedwater pump and a downcomer pipe connecting them, which is capable of eliminating flashing and other related troubles which may occur when the load level on the turbine is changed, and of ensuring a high efficiency of the operation.

To this end, according to the invention, an automatic computing means receives data such as the measured turbine load and the measured pressure and temperature in the downcomer pipe, as well as the demands such as the level to which the load is to be lowered and the time duration in which the lowering of the load is to be completed, and computes the desirable load reduction manner which will not cause any flashing. Then, the load on the turbine is reduced in accordance with the computed manner. In other words, the turbine is so controlled that the turbine load is reduced while remaining the pressure in the downcomer pipe higher than the saturation vapor pressure corresponding to the temperature of the hot water in the downcomer pipe, such as to avoid occurrence of flashing due to the reduction in the pressure in the deaerator and high temperature of the hot water in the downcomer pipe.

FIG. 1 is a system diagram of a turbine plant to which an embodiment of the invention is applied;

FIG. 2 illustrates a process for determining the load on the turbine;

FIG. 3 is an illustration of the principle of the controlling method in accordance with the invention; and

FIGS. 4 and 5 are diagrams showing changes in the temperature and pressure in relation to time, as observed in an embodiment of the invention.

Referring to FIG. 1, the condensate is delivered from a condenser 10 to a deaerator 21 through a condensate pipe 12. The condensate is temporarily stored in a tank 22 and then is forwarded to a feedwater pump system. In the illustrated case, the feedwater pump system has three subsystems which are suffixed by a, b and c, respectively. These three sub-systems will be referred to as groups A, B and C, respectively, hereinunder. These groups A, B and C have feedwater pumps 34a, 34b and 34c, respectively.

The feedwater pumps 34a and 34b of the groups A and B have capacities amounting to 50% of the rated capacity of the respective boilers. On the other hand, the feedwater pump 34c of the group C has a capacity amounting to 25% of the rated capacity of the corresponding boiler. These three groups A, B and C in combination constitute a boiler feedwater system.

During the operation of the plant, the condensate is pumped by a condensate pump 11 from the condenser 10 to the deaerator 21 through the condensate pipe 12, feedwater heater 13 and a check valve 14.

The condensate in the deaerator 21 is heated and deaerated by a heated steam from a steam pipe 24, and is temporarily stored in the tank 22. The condensate is then supplied to the boiler feedwater system through downcomer pipes 23a, 23b and 23c. The group A in the boiler feedwater system has a series connection of a booster pump inlet valve 31a, a booster pump 32a, feedwater pump suction pipe 33a, a feedwater pump 34a, a feedwater pump discharge pipe 35a, a check valve 36a and a feedwater pump outlet valve 37a. The feedwater pump outlet valve 37a is connected at outlet side thereof to a header 38 which is common to three groups A, B and C. A line having a series connection of a warming pipe 41a, a warming valve 42a and an orifice 43a is disposed between the header 38 and the feedwater pump 34a. Other groups B and C are constructed substantially in the same forms as the group A.

When the load on the plant is greater than 50% of the rated load thereof, the feedwater pumps 34a and 34b operate while the feedwater pump 34c does not operate. However, when the load on the plant is below 50% of the rated load thereof, either one of the feedwater pumps 34a and 34b operates, while the other is used as a back-up. In this system, the pressure and the temperature of the water at the inlet of the feedwater pump are measured as the pressure and the temperature in the down comperpipe.

The controlling apparatus according to the invention applied to this steam turbine plant has a load detecting means for detecting the data I which represent the level of the load on the turbine. In this case, the load detecting means includes a load signal transmitter 6 which is provided on the generator 5 to detect a load on the generator 5, i.e. a load rate on the turbine 4.

The apparatus also has a pressure detecting means for detecting the data II which represent the pressures at the inlets of the feedwater pumps 34a, 34b and 34c. In this case, the pressure detecting means includes pressure transmitters 2a, 2b and 2c which are provided on the suction pipes 33a, 33b and 33c, respectively to detect the pressure at the inlets of the feedwater pumps.

The apparatus further has a temperature detecting means for detecting the data III representing the water temperatures at the inlet side of the feedwater pumps 34a, 34b and 34c. The temperature detecting means includes feedwater temperature detectors 3a, 3b and 3c which are disposed at the downstream sides of the pressure transmitters 2a, 2b and 2c to detect the feedwater temperatures in the respective suction pipes of the feedwater pumps.

An example of the process for determining the load on the turbine will be explained hereinunder with reference to FIG. 2.

The reduction rate L.sub.X in the turbine load is computed by a load reduction rate computing section 1.2 in the computing means 1 on the basis of the detected turbine load Lo, the demand load L which represents the level to which the turbine load is to be reduced, and the time t during which the turbine load has to be reduced, in accordance with the following formula. ##EQU1##

The computing means 1 further includes a saturation pressure computing section 1.1 which computes the saturation pressure P.sub.Tn on the basis of the temperature Tn (n=1, 2, 3) of the water in the feedwater pump suction pipes, detected by the feedwater pump inlet temperature transmitter 3 (see FIG. 2). This computation is done with reference to the Enthalpy-Entropy chart (Mollier chart) which is stored in the section 1.1. The saturation pressure P.sub.Tn is determined as the point at which the detected feedwater temperature Tn crosses the saturation limit line Z in the Mollier chart. In some cases, a certain margin is assumed on the saturation limit line Z. In such a case, a certain area is assumed as denoted by broken lines Z' in the chart. The region above the line Z is the region where the flashing occurs, whereas the region below the line Z is the region in which the flashing cannot occur. Therefore, the flashing can be avoided safely if the saturation pressure computing section determines a value below the point of crossing with the line Z as a saturation pressure.

The computing means further has a saturation time computing section 1.3 which determines the time duration Y until the saturation pressure is reached, through computation of the pressure difference .DELTA.Pn (n=1, 2, 3). The pressure difference .DELTA.Pn is computed on the basis of the load reduction rate L.sub.X and the saturation pressure P.sub.Tn computed as above, as well as the feedwater pump inlet pressure Pn (n=1, 2, 3) from the feedwater pump inlet pressure transmitter 2 (see FIG. 2), in accordance with the following formula.

The computing means also has a function to determine the smallest .DELTA.Pn (MIN) among three pressure differences .DELTA.Pn's. This means to select a feedwater suction pipe 33a, 33b or 33c which has the greatest possibility of the occurrence of flashing (see FIG. 1). The selection of the smallest pressure difference, however, is not always necessary. Namely, if no problem is expected in the feedwater pump operation, the smaller one among the pressure difference except the pressure difference not to be considered is used for the determination of the feedwater suction pipe in which the flashing is most likely to occur.

The time Y is computed using the selected smallest pressure difference .DELTA.Pn(MIN), feedwater pump suction pressure Pn (n=1 or 2 or 3) and the load reduction rate L.sub.X. Since the pressure in the deaerator and the feedwater pump suction pressure are reduced at the rate substantially equal to the turbine load reduction rate, the pressure reduction rate can be expressed as (L.sub.X .times.Pn). ##EQU2##

The determined saturating time Y is the time duration in which the flashing does not occur when the turbine load is reduced at the load reduction rate computed by the load reduction rate computing section 1.2. The turbine load L.sub.Y at such time is expressed as follows. ##EQU3##

After the computation, the command load L.sub.Y is inputted to a plant operation load pattern judging section 1.5, in which a manner of reduction of the turbine load is determined on the basis of the command load, i.e., the optimum desired load, L.sub.Y and the load reduction rate L.sub.X.

If the obtained command load L.sub.Y is below the demand load L, the turbine load is reduced at the load reduction rate L.sub.X computed in the section 1.2 down to the demand load L. Conversely, when the command load L.sub.Y is greater than the demand load L, the turbine load is not reduced to the demand load L, but to the command load L.sub.Y. If the load is born by only one plant, the load is reduced once down to the command load and then the load is further reduced again after the temperature in the downcomer pipe comes down, or the hot water in the downcomer pipe is displaced to avoid any possibility of flashing. When the load is born by a plurality of plants, some of the plants are stopped safely while other plants continue to operate to bear the load. For instance, assuming here that the total load which has been born by two plants has to be reduced from 100% to 50%, the control is conducted not in a manner to reduce the load level down to 50% in each plant but in such a manner as to stop one of the plants safely and to operate the other plant at 100% load to meet the demand for 50% reduction of the total load. This control is conducted by a plant controlling section 60 either manually by an operator in accordance with the result of the judgement in the plant load judging section displayed on the display 8 or automatically.

The described control can be applied directly to the case where there is only one downcomer pipe. In the case where the pumps 34a, 34b and 34c are connected directly to the deaerator 21 unlike the arrangement shown in FIG. 1, the group including the stopped pump is omitted from the consideration in some cases.

As has been described, the plant operation controlling method in accordance with the invention can be carried out fully automatically by arranging such that the plant load is controlled in accordance with a plant starting or stopping instruction which is produced on the basis of the result of computation by the computing means 1.

The function and the storage memory required for the computing means 1 are rather small, so that a small-capacity computer which is rather inexpensive can be used only for this purpose. Alternatively, since the required capacity is rather small, suitable vacancy or surplus capacity of the large-capacity computer used for the control and observation of the whole plant may be used for the construction of the computing means 1.

FIG. 3 is an illustration of the principle of the controlling method of the invention, which is conducted fully automatically. The data I, II and III derived respectively from the generator load transmitter 6, feedwater inlet pressure transmitter 2 and the feedwater pump inlet temperature transmitter 3 are delivered to the automatic computing means 1 which performs the above-mentioned computation such as to determine the command load L.sub.F and the load reduction rate L.sub.X. The determined command load L.sub.F and the load reduction rate L.sub.X are inputted to an APC (Automatic Plant Control) 50 which controls the operations of the turbine 4, the boiler 7' and the generator 5 in accordance with the inputted values.

The states of operation of the plant, i.e., of the boiler, the turbine and the generator which are varied by the APC 50 are fed back to the APC 50. On the other hand, the load on the generator, i.e., the load on the turbine plant, after being changed by the operation of the APC 50, are fed back to the generator load transmitter 6 again. This feedback is materially equivalent to the feedback to the computing means 1. Then, the computing means 1 again computes a command load L.sub.Y, and the process explained above is conducted again to reduce the turbine load in accordance with the newly computed command load L.sub.Y and the load reduction rate L.sub.X.

Thus, the initially judged command load L.sub.Y and the load reduction rate L.sub.X are fed back and judged and determined as being adequate values. Therefore, as this process is repeated, the optimum values are determined. Although various patterns determined by the command load level and the load reduction rate are available, the above-described feedback method offers the optimum pattern. In general, where a temperature is given, there is a certain relationship between the load and the pressure for avoiding occurrence of flashing. In other words, the level of pressure required at a certain level of load in order to avoid the flashing may be determinable. This relationship, however, may vary depending on the command load L.sub.Y and the load reduction rate L.sub.X. In addition, the temperature is not fixed but is variable. Therefore, it is the most reasonable way to determine the optimum value by the feedback method explained hereinbefore.

Referring now to FIG. 4, assuming here that the feedwater pump inlet temperatures L(a)(b) start to come down with a time lag t.sub.4, the saturation pressure N(a)(b) of water corresponding to the feedwater pump inlet temperature starts to come down. Then, as the plant load J is decreased below 50%, the feedwater pump 34b is stopped as explained before. The moment at which this pump is stopped is represented by t.sub.2. If the turbine load J is further reduced from the moment t.sub.2 to the moment t.sub.3, the booster pump inlet pressure O(a)(b) also goes on to be reduced till the moment t.sub.3. On the other hand, the inlet pressure P(a) of the feedwater pump 34a which is still operating is reduced along a line substantially parallel to the line M representing the pressure in the deaerator. Since the booster pump 32b (see FIG. 1) is stopped simultaneously with the stopping of the feedwater pump 34b, the pressure difference between the outlet and the inlet of the booster pump 32b is nullified, so that the pressure P(b) of the inlet of the feedwater pump 34b is lowered drastically and laps the inlet pressure O(a)(b) of the booster pump 32b after the moment t.sub.2. Thus, the inlet pressure P(b) of the feedwater pump 34b is abruptly lowered but the inlet temperature L(b) of this pump is maintained substantially constant after the moment t.sub.2 as a result of stopping of this pump. Consequently, the saturation pressure N(b) corresponding to the feedwater pump inlet temperature also is maintained substantially constant after the moment t.sub.2. In consequence, the inlet pressure P(b) of the feedwater pump 34b comes equal to the saturation pressure N(b) corresponding to the inlet temperature of this pump at a point A and, thereafter, comes down below the saturation pressure N(b), so that the feedwater in the suction side of the feedwater pump 34b flashes undesirably. It will be understood how the flashing takes place when one pump 34b of two feedwater pumps is stopped in response to a reduction in the plant load J.

Referring now to FIG. 5, a line L(c) represents the temperature at the inlet side of the feedwater pump 34c which is stopped, while a line N(c) represents the saturation pressure of water corresponding to the temperature at the inlet side of the feedwater pump 34c. In this case, since the feedwater pump 34c has been stopped, the feedwater stagnates in the downcomer pipe 23c and the suction pipe 33c of the feedwater pump 34c and the temperature thereof is maintained at a substantially constant level below the temperature of the water stored in the deaerator, even though the plant load J is changed from the moment t.sub.1 to t.sub.2.

In consequence, at a point B, the inlet pressure P(c) of the feedwater pump 34c and the booster pump inlet pressure O(c) become equal to the saturation pressure corresponding to the temperature at the inlet side of the feedwater pump 34c and, thereafter, comes down below the saturation pressure N(c), thus allowing the flashing of the feedwater in the suction pipe of the feedwater pump 34c.

The reason why the flashing takes place has been described. It will be understood from the foregoing explanation that the greater the absolute value of the load reduction and the rate of load reduction become, the larger the possibility of flashing is.

In order to avoid the occurrence of flashing, according to the invention, the computing means 1 produces, upon receipt of the detected values corresponding to the pressures and temperatures in the downcomer pipes, an output which serves to maintain, in the period after the point A, the plant load at the same level as the load attained at the point A.

As a result of such a control, referring to FIG. 4, the inlet pressure P(b) of the feedwater pump 34b becomes equal to the saturation pressure N(b) corresponding to the inlet temperature of this pump and is maintained at the same level in the period after the point A. In the case of FIG. 5, the inlet pressure P(c) of the feedwater pump 34c becomes equal to the saturation pressure N(c) corresponding to the inlet temperature of this pump, and this pressure is maintained in the period after the point B.

It will be seen that the occurrence of flashing is avoided insofar as the saturation pressure corresponding to the inlet temperature and the inlet pressure of the feedwater pump, in accordance with the controlling method of the invention described hereinbefore.

As has been described, according to the invention, it is possible to prevent the occurrence of flashing in the deaerator and downcomer pipes at the time of reduction in the load on the turbine of a steam turbine plant. Although the invention has been described with reference to the case where only one steam turbine plant is used for bearing the load, it will be clear to those skilled in the earth that the invention is applicable to the case where two or more plants are used to bear the electric power generating load.