JPS6024362B2 - Water supply flow control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a feedwater flow rate control system for controlling the feedwater flow rate of a turbine-driven feedwater pump by controlling the rotational speed of the feedwater pump.

Water is supplied to the boiler of a steam turbine plant by a feed water pump that is generally a combination of a feed water pump driven by a plurality of steam turbines and a feed water pump driven by an electric motor.

In the case of a turbine-driven water pump, this control of the feed water flow rate is usually done by controlling the rotation speed of the turbine, or by installing a regulating valve in the case of an electric motor-driven water pump. This is done by controlling the opening degree of the valve. Recently, in order to increase the efficiency of steam turbine plant operation under partial load, the steam control valve of the steam turbine is kept fully open to reduce the steam pressure in the boiler, thereby reducing the amount of steam entering the turbine and increasing the plant load. A variable voltage operation system is used to lower the load.

The relationship between load and steam pressure during this variable pressure operation is, for example, a graph as shown in FIG. In addition, Figure 2 shows the feedwater pump characteristics and system head curves of a steam turbine plant. Compare with flow rate control. The system head curve during constant pressure operation is indicated by the symbol A in FIG. 2, and the system head curve during variable pressure operation is indicated by the symbol B if the load pressure characteristics are as shown in FIG. 1. Also, from code No. to N? 1 and 2 respectively show flow rate-pressure curves of the water supply pump when the rotational speed of the water supply pump is from No to N7.

The pressure and flow rate during actual operation are the water pump flow rate -
It is shown by the intersection of the pressure curve and the system head curve, and the water supply flow rate can be controlled by changing the pump rotation speed, but as is clear from the figure, during variable pressure operation,
When the water supply flow rate is reduced, the pump rotation speed must be considerably lower than during constant pressure operation.

In this way, during variable pressure operation, the rotation speed of the water supply pump and the rotation speed of the turbine that drives it must be kept considerably low. will occur.
Water supply pumps and turbines as rotating bodies usually have a minimum rotational speed limit due to critical speeds and other reasons, and it is not preferable to operate them at rotational speeds below this limit. Therefore, in conventional water supply flow control during variable pressure operation, if the rotation speed of the water pump falls below the minimum rotation speed, the pump rotation speed is maintained at this limit rotation speed, and the water supply flow rate below this limit is controlled. This was done using an adjustment type installed on the discharge port side of the pump. However, controlling the water supply flow rate using this regulating valve has drawbacks such as a complicated operation control method due to the installation of a main line and a bypass line with the regulating valve, line switching, etc., and an increase in cost. The present invention has been made in view of these points, and by using the recirculation valve of the water pump to control the water supply flow rate, it is possible to control the water supply of the turbine-driven water pump when the flow rate is relatively low during variable pressure operation. It is an object of the present invention to provide a control device that can suitably control the flow rate without lowering the rotational speed of a turbine and a water supply pump below the minimum rotational speed.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 3 is a piping system diagram near the feed water pump of a steam turbine plant in which the control device according to the present invention is used.

In this piping system, a recirculation valve 1 is disposed in the middle of a recirculation line 4 that runs from the discharge side of a feedwater pump 2 to a deaerator 3, and the feedwater pump 2 is driven by a steam turbine 5. has been done. Usually, a plurality of water supply pumps are used, but here, for simplicity, only one water supply pump will be explained. The recirculation valve 1 is generally provided to prevent the temperature of the pump rotating parts from rising when the pump is started or when the flow rate is low. In other words, when the pump suction flow rate is large, such as during high load, the recirculation valve 1 is fully closed, but when the water supply flow rate becomes less than the pump suction minimum flow rate, the recirculation valve 1 is opened, and the water supply flow rate and the pump suction minimum flow rate are closed. The difference between the flow rate and the flow rate is passed through the recirculation valve 1 to ensure a constant minimum pump suction flow rate and prevent accidents such as seizure of the pump rotating part. Opening/closing control of the recirculation valve 1 is normally performed by a flow rate controller (not shown) that detects the pump suction flow rate using a transmic system. The present invention makes more active use of such a function of the recirculation valve 1 to control the water supply flow rate of the pump.

First, the basic concept of controlling the water supply flow rate by opening and closing the recirculation valve will be explained using FIG. 4. Fourth
The figure shows the feedwater pump characteristics of a steam turbine plant and the system head curve when a remarriage ring valve is used. The recirculation valve transfers the discharge flow rate from the water supply pump to the deaerator 3.
This resistance curve becomes a quadratic curve. This curve differs depending on the degree of recirculation valve, and when the opening is 50%, the curve is indicated by the symbol L, and when the opening is 100%, it is the curve indicated by the symbol M, which is to the right of the opening when the opening is 50%. It becomes a curved line. The system head curve when the recirculation valve 1 opens during water supply operation is a combination of the system head curve of the line and the recirculation valve resistance curve as shown in Figure 2, and the opening degree of the recirculation valve is The composite system head curves when the relationship is 50% and 100% are as indicated by the symbols x and Y, respectively. Therefore, for example, if the recirculation valve's degree of flow is 50% when the pump rotation speed is N5, the operating point is the intersection of the feed water pump's flow rate-pressure curve and the system head curve, and the pump's discharge flow rate is From the figure (
1-lo). Of these, the flow rate flowing into the recirculation valve becomes from 1, to (1, 1 L) at this pressure, according to the recirculation valve resistance curve (sign L because the relationship is 50%). Therefore, the water supply flow rate to the boiler is the value obtained by subtracting the recirculation flow rate (1,1 lo) from the pump discharge flow rate (1-ra), that is, (1-1.). Similarly, at the same rotation speed N5, and when the recirculation valve's relationship is 100%, find the respective flow rates.Since the operating point is point K in the diagram, the pump discharge flow rate is (K-Ko) , the recirculation flow rate is (K, -Ko) and the water supply flow rate is (K-K,). Further, when the recirculation valve is fully closed at the same rotation speed N5, the water supply flow rate is (F-Fo). As is clear from Figure 4, from the properties of the recirculation valve resistance curve and the composite system head curve when the recirculation valve relationship is different, the magnitude of the water supply flow rate depending on the opening degree of each recirculation valve is (F-F.)>
There is a relationship of (1-1,)>(K-K,), and the flow rate of water supply decreases as the recirculation valve is increased. By controlling the flow rate of the recirculation valve 1 in this manner, the pump water supply flow rate can be controlled. FIG. 5 shows a diagram of the water supply control device of the present invention, which uses this concept to control the pump water supply flow rate.

This is a diagram. Reference numeral 6 is a command value for the water supply flow rate to the boiler, and this command value 6 is compared at reference numeral 8 with a feedback value 7 of the total water supply flow rate actually supplied to the boiler, and the signal as a result of this comparison is Enter PI controller 9. The PI controller 9 operates on the above-mentioned input signals and distributes control signals to a plurality of turbine-driven water supply pumps and electric pumps. Therefore, the output signal from the PI controller 9 is sent to the control device for each water pump driving turbine and electric motor for each river, but for simplicity, in FIG. Only the figure is shown, and other control devices are omitted. P
The I controller 9 is connected to 11, where the output signal 10 of the PI controller 9 and the pump suction flow signal 12 are compared. Reference numeral 11 is further connected to a PI control 14. This PI controller 14 performs the function of calculating and determining the rotation speed of the water supply pump necessary to obtain the required water supply flow rate, and the rotation speed of the water supply pump is usually calculated as the rotation speed of the turbine that drives it. , is output as the turbine rotation speed setting signal 13. (The output side of this PI controller 14 is connected to a high value priority circuit 15 and a comparator 16, and the turbine rotation speed setting signal 13 calculated and determined by the PI controller 14 is connected to a high value priority circuit 15 and a comparator 16, respectively. This high value priority circuit 15 is connected to a minimum limit rotation speed setter 17 determined from the critical speed of the turbine or water pump, and further connected to the turbine rotation speed control device 1.
8 and the comparator 16 described above. This high value priority circuit 15 compares the turbine rotation speed setting signal 13 from the PI controller 14 with the lowest limit rotation speed signal 19 from the minimum limit rotation speed setter 17 of the water supply pump or the driving turbine. The signal with the higher value is preferentially selected as the turbine rotation speed signal 20. Further, the comparator 16 compares the turbine rotation speed setting signal 13 from the PI controller 14 with the turbine rotation speed signal 20 from the high value priority circuit 15, and determines that the turbine rotation speed indicated by the turbine rotation speed signal 20 is higher than the turbine rotation speed. Only when the turbine rotation speed is higher than the turbine rotation speed indicated by the setting signal 13, the control signal 21 for the water supply flow rate corresponding to the difference in rotation speed is extracted. The output of the comparator 16 is followed by a high value priority indicator circuit 22 of the recirculation control system. The input side of this high value priority indication circuit 22 is connected to the output side of the PI controller 24 that issues the recirculation flow rate control signal 23, and the output side of the high value priority indication circuit 22 is the input side of the recirculation valve 1 having a built-in valve opening/closing mechanism. connected to the side. PI controller 24
The set flow rate signal 26 from the recirculation flow rate setting device 25, which is determined empirically in advance using FIG. 4, and the pump suction flow rate signal 27 (
For convenience, this signal is the pump suction flow signal 12 described above.
(Although the signals are shown with different symbols, the sources of the signals are substantially the same), the resultant signal is entered at 28. Further, the aforementioned turbine rotation speed control layer 18 is connected to the water supply pump driving turbine 5, and this turbine 5 is further connected to the water supply pump 2. The water supply flow rate from each water pump is 29
are summed up at , and entered into code 8 as feedback value 7 of the water supply flow rate. Next, the operation of the water supply flow rate control device configured as described above will be explained.
First, a command value 6 for the flow rate of water to be supplied to the boiler is issued in accordance with the operating conditions of the steam turbine plant, and this is compared with the feedback value 7 of the total water supply flow rate by the feed water pump at 8, and the comparison value is further sent to the PI controller 9. The calculated output signal 10 is distributed to the control system of each water pump. The distributed output signal 1 is compared with the water pump suction flow rate signal 12 at the signal 11 and input to the PI controller 14 . The PI controller 14 calculates the rotational speed of the water supply pump necessary to obtain the required water supply flow rate, that is, the rotational speed of the water pump drive bin, and outputs a turbine rotational speed setting signal 13. This turbine rotational speed setting signal 13 enters a high value priority circuit 15 and is compared with a minimum limit rotational speed signal 19 of the turbine determined from the critical speed of the turbine or water pump. In this case, the signal with higher turbine rotation speed is selected preferentially, so if the turbine rotation speed setting signal 13 from the PI controller 14 is smaller than the minimum limit rotation speed signal 19, the lowest limit The rotational speed signal 19 is adopted as the output of the high value priority circuit 15, and the turbine rotational speed signal 20 is sent to the turbine rotational speed control device 18 that maintains the lowest limit rotational speed, and the turbine rotational speed signal 20 is sent to the turbine rotational speed control device 18 that maintains the lowest limit rotational speed. is driven to rotate at the lowest limit rotation speed. Therefore, the flow rate of the water supply pump 2 becomes larger than the flow rate that should be supplied to the boiler. Therefore, the flow rate corresponding to the difference is controlled by opening the recirculation valve 1 and allowing the flow to flow into the recirculation system. Specifically, this is done as follows. That is, the turbine rotation speed setting signal 13 from the PI controller 14,
The turbine speed signal 20 from the high priority circuit 15 enters a comparator 16 where the magnitudes of the speeds are compared. As a result, if the turbine rotation speed signal 20 is larger,
An output signal 21 corresponding to the difference is sent to a high value priority indicating circuit 22 of the recirculation control system. Turbine rotation speed setting signal 1
3 is larger than the signal 19 from the minimum limit rotation speed setter 17, the signal sent to the high value priority instruction circuit 22 becomes 0 since the signals 13 and 20 are the same. In this case, the recirculation valve 1 receives a signal 26 from the recirculation flow rate setting verifier 25, which is determined to prevent the temperature rise of the water supply pump, etc.
and the pump suction flow rate signal 27 from the flow meter, P
It is controlled only by the recirculation flow rate control signal 23 calculated and output by the I controller 24. In this case, therefore, the recirculation valve 1 is not directly involved in controlling the pump water flow rate. On the other hand, as described above, when the turbine rotation speed setting signal 13 becomes smaller than the turbine rotation speed signal 20, the water supply flow rate control signal 21 corresponding to the difference is sent to the high value priority instruction circuit 22. This signal 21 is compared with the recirculation flow control signal 23 from the PI controller 24. The larger one of these signals is preferentially adopted as an output signal to open and close the recirculation valve 1. That is, even if the suction flow rate of the water supply pump does not require opening the recirculation valve to perform recirculation for purposes such as preventing the temperature rise of the water supply pump, the recirculation valve is opened by the water supply flow rate control signal 21 from the comparator 16. 1 to the required degree. As a result, a flow rate corresponding to the difference between the flow rate to be supplied and the water supply pump discharge device will flow into the recirculation system,
The water supply flow rate is controlled. In this case, the valve opening degree of the recirculation valve 1 can be easily determined if a composite system head curve for each valve opening degree is obtained as shown in FIG. For example, if the minimum rotation speed of the water supply pump is N5, the minimum rotation speed of the water supply pump is maintained by the high value priority circuit 15 so as not to fall below this value, and the water supply flow rate control signal 21 from the comparator 16 is It is transmitted to the recirculation system and the recirculation valve 1 is opened. That is, as the valve opening increases,
The rotation speed of the water supply pump remains N5, and the water supply flow rate is (F-
Fo) decreases from (1-1,) to (K-K,), and the water supply flow rate is controlled. As described above, the control device according to the present invention can maintain the minimum rotational speed of the feed water pump and drive turbine by using an existing recirculation valve and simply adding a simple circuit to the existing recirculation valve control system. It is possible to control a relatively small flow rate of water supply during variable pressure operation while keeping the pressure constant.

Furthermore, compared to the conventional method in which a regulating valve is arranged in a bypass circuit to control the water supply flow rate, complicated operations such as switching operations are not required, and continuous and good control can be performed at low cost. Note that for a water supply pump driven by an electric motor, the water supply flow rate is usually controlled by providing a regulating valve.

In addition, the present invention has a particularly great effect when used to control the flow rate of water supplied to the boiler of a steam turbine plant during variable pressure operation; In the case where the water supply flow rate of a water supply pump driven by a prime mover is controlled by controlling the rotation speed of the water supply pump, similar effects can be obtained.

[Brief explanation of drawings]

Figure 1 shows the relationship between load and steam pressure during variable pressure operation of a steam turbine plant, Figure 2 shows the feedwater pump characteristics and system head curve, and Figure 3 shows the water supply with a recirculation system. A piping system diagram near the pump, FIG. 4 is a diagram showing water supply pump characteristics and a system head curve when a recirculation valve is used, and FIG. 5 is a block diagram of the water supply control device of the present invention. 1...Recirculation valve, 2...Water pump, 4...
... Recirculation line, 5 ... Turbine, 1
4...PI controller, 15...High value priority circuit, 16...Comparator, 17...Minimum limit rotation speed setter, 18...Turbine rotation Numerical control device, 22... High value priority instruction circuit. Younger brother J figure Leopard 2 figure 3 Younger brother 4 figure Younger brother ○ figure

Claims (1)

[Claims] 1. In a control device that controls the feed water flow rate of a turbine-driven feed water pump equipped with a recirculation system with a recirculation valve by controlling the pump rotation speed, a high value priority circuit that compares the allowable minimum rotational speed of a water supply pump, selects the higher rotational speed among the rotational speeds as the rotational speed of the water supply pump, and sends a turbine rotational speed signal for driving the water supply pump to the turbine; Compare the number of revolutions of the water supply pump necessary to obtain the required water supply flow rate with the number of revolutions of the water supply pump selected by the high value priority circuit, and if the number of revolutions selected by the high value priority circuit is larger; A comparator that sends a feed water flow rate control signal corresponding to the difference in rotation speed to the recirculation system, and a comparator that is arranged in the recirculation system and that is necessary to maintain the feed water flow rate control signal and the minimum limit flow rate of the water pump. Compare the recirculation flow rate signal with
Water supply flow rate control characterized by having a high value priority indication circuit which selects the signal corresponding to the higher flow rate as the signal for determining the opening degree of the recirculation valve and sends a valve opening/closing operation control signal to the recirculation valve. Device.