JPH0566481B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a method for operating a once-through boiler, and particularly to a method for operating a superheater portion under variable voltage.

[Background of the invention]

Originally, a once-through boiler is a constant pressure boiler, but in recent years, it has come to be used as a boiler for intermediate loads, and in order to improve thermal efficiency, once-through boilers are designed to be operated at variable pressure.

FIG. 8 is a schematic system diagram of an existing constant pressure once-through boiler modified for variable pressure operation.

In the figure, 1 is the boiler feed water pump, 2 is the high pressure water heater, 3 is the economizer, 4 is the furnace, 5 is the primary superheater,
6 is a superheater pressure reducing valve, 7 is a superheater stop valve, 8 is a secondary superheater, 9 is a turbine bypass valve, 10 is a turbine control valve, 11 is a primary superheater bypass valve, 12 is a secondary superheater bypass valve, 13 is a superheater vent valve, 14
15 is a flush tank, 15 is a deaerator, 16 is a high pressure turbine, 17 is a reheater, 18 is a medium and low pressure turbine, 19 is a condenser, and 20 is a condensate pump.

To perform variable pressure operation in this once-through boiler,
Since the furnace is designed for low pressure, a pressure reducing valve 6 is installed between the primary superheater 5 and the secondary superheater 8 as shown in the figure, and the inlet side of the secondary superheater 8 and high pressure turbine 16 is It is necessary to depressurize.

FIG. 4 is a diagram of a pressure transformation pattern in the pressure variation operation of a once-through boiler. Conventionally, as shown by the straight line in the figure, the turbine inlet steam pressure (main steam pressure) was increased by operating the superheater pressure reducing valve 6, which is called a ramping operation, from a load of 7% to a load of 25%.
In other words, the pressure reducing valve 6 is throttled only during startup bypass operation when the primary superheater bypass valve 11 is open, and when the startup bypass operation is switched to once-through operation at 25% load, the superheater pressure reducing valve 6 is fully opened and constant pressure operation is achieved. The large capacity superheater stop valve 7 is also fully opened.

The problem with conventional superheater transformer operation is that, especially at low load, since the pressure of the fluid is reduced by the superheater pressure reducing valve 6, the pressure reducing valve outlet fluid (superheater inlet fluid) approaches the saturation region. An example of this is shown in the enthalpy pressure diagram of FIG.

The curve X in the figure is a saturation curve, and the broken line Y is a line connecting the enthalpy pressure characteristic values at various locations at a low load of 25%. Point A on broken line Y is the entrance of the economizer, and point B is the primary superheating Point C at the outlet of the superheater indicates a characteristic point at the inlet of the secondary superheater, and the pressure reduction from point B to point C indicates the pressure reduction by the superheater pressure reducing valve 6.

As shown in this figure, at 25% load, the inlet steam temperature of the secondary superheater has almost no margin with respect to the saturation temperature (curve X), and operation is restricted if load changes are taken into consideration. That is, there is a high possibility that the gas-liquid mixed fluid will flow into the secondary superheater, and there is a concern that the temperature of the pipe metal will rise abnormally due to the gas-liquid mixed fluid, especially near the tube bend portion of the secondary superheater.

In addition, when pressure is reduced using one superheater pressure reducing valve 6, especially at low load, this superheater pressure reducing valve 6
A high differential pressure is applied to the valve, and valve erosion becomes a problem.

[Purpose of the invention]

An object of the present invention is to eliminate such drawbacks of the prior art and to provide a method for stable variable pressure operation of a once-through boiler without problems such as abnormal rise of the superheater tube metal and erosion of the pressure reducing valve.

[Summary of the invention]

In order to achieve the above-mentioned object, the present invention provides that at least a first superheater and a second superheater are arranged in series along the flow direction of steam, and on the upstream side of the first superheater. A first pressure reducing valve is provided, and the second pressure reducing valve is provided.
A second pressure reducing valve is provided on the upstream side of the superheater,
After the first pressure reducing valve performs isenthalpic pressure reduction to a predetermined steam pressure according to the load, the steam is superheated in the first superheater, and then the second pressure reducing valve performs isenthalpic pressure reduction to a predetermined steam pressure according to the load. It is characterized in that after isenthalpic reduction is carried out so that the vapor pressure reaches a vapor pressure of

[Embodiments of the invention]

Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic system diagram for explaining variable pressure operation of a once-through boiler according to a first embodiment of the present invention. In the diagram, 1 is the boiler feed water pump, 2 is the high pressure water heater,
3 is an economizer, 4 is a furnace, 5 is a primary superheater, 6a is a first superheater pressure reducing valve, 6b is a second superheater pressure reducing valve, 7 is a superheater stop valve, 8a is a first secondary superheater superheater,
8b is a second secondary superheater, 9 is a turbine bypass valve, 10 is a turbine control valve, 12 is a secondary superheater bypass valve, 13 is a superheater vent valve, 14 is a flash tank, 15 is a deaerator, 16 is a high pressure turbine;
19 is a condenser.

As shown in this figure, the secondary superheater 8 is divided into a first secondary superheater 8a and a second secondary superheater 8b, which are connected in series, and the first secondary superheater 8a A first pressure reducing valve 6a is arranged upstream of the second secondary superheater 8b, and a second pressure reducing valve 6b is arranged upstream of the second secondary superheater 8b.

First, the operation at the time of startup of the once-through boiler with this configuration will be explained. After securing the minimum water supply flow rate with the boiler feed water pump 1, ignition is performed. At that time, since the fluid temperature is low, the superheater stop valve 7 and the first superheater pressure reducing valve 6a are fully closed, and the fluid bypasses the secondary superheater and is dumped into the flash tank 14. As the bypass fluid temperature increases, the flash tank pressure increases and reaches approximately 85 kg/cm ² g. At this time, the second superheater pressure reducing valve 6b is fully opened, and the saturated steam in the flash tank 14 is passed through the superheater ventilation valve 13, the second superheater pressure reducing valve 6b, and the turbine bypass valve 9 to the secondary superheater 8a. , 8b and warming the main steam pipe. When the steam conditions for turbine ventilation are established, the high pressure turbine 16 is vented and the turbine bypass valve 9 is closed. When the turbine speed increases to a specified speed, the generator is synchronized with the grid and the load is taken off. The turbine control valve 10 is gradually opened to increase the load, and when the load reaches about 25%, the steam passing through the flash tank 14 becomes superheated steam due to the characteristics of the boiler.

When the load reaches 25%, the first superheater pressure reducing valve 6a is fully closed, and the pressure on the inlet side of this valve 6a is approximately 250 kg/cm ² g, and the pressure on the outlet side is approximately 85 kg/cm ² g.
The valve differential pressure is approximately 165 kg/cm ² g. On the other hand, since the second superheater pressure reducing valve 6b is fully open, the valve differential pressure is approximately zero, but as mentioned above, the first superheater pressure reducing valve 6a is fully closed, so the valve differential pressure is approximately zero. There is no need to worry about eroticism.

When increasing the load from 25% load, opening the first superheater pressure reducing valve 6a will increase the pressure according to the main steam pressure curve shown in Figure 4. At the same time as opening the valve 6a, the second superheater pressure reducing valve 6b is operated in the closing direction, and the load is increased while controlling the differential pressures applied to the respective valves 6a and 6b to be approximately equal.

When the load decreases, the operation is reversed, but the differential pressure of the valve is halved compared to when only one unit is used to decrease the pressure, which greatly improves erosion resistance and improves the reliability of the valve. It leads to improved sexual performance.

In the main steam pressure curve shown in FIG. 4, the curve is the pressure transformation curve due to the conventional ramping operation.
is the transformation curve considering the boiler efficiency, and the curve is 40
% transformation curve, curve is 75% transformation curve, curve is 95
% transformation curves are shown respectively. In the case of the present invention, for example, variable pressure operation as shown in curves 1 to 3 is possible, thereby increasing the efficiency of the boiler.

Figure 2 shows the enthalpy pressure diagram of each part at 25% load. In the figure, X is the saturation curve, point A is the inlet of the economizer, point B is the outlet of the primary superheater (the inlet of the first superheater pressure reducing valve), and point C is the inlet of the first secondary superheater (the inlet of the first superheater pressure reducing valve).
point D is the first secondary superheater outlet (the inlet of the second superheater pressure reducing valve), point E is the second secondary superheater inlet (the second superheater pressure reducing valve point F is the characteristic value of the second secondary superheater outlet.

The main steam pressure is isenthalpically reduced from the inlet of the first superheater pressure reducing valve (point B) at the first superheater pressure reducing valve, and the main steam pressure at the inlet of the first secondary superheater (point C) is approximately
The weight is 165Kg/cm ² g, which provides a larger margin than the saturation point. The enthalpy of the inlet steam of the second secondary superheater increases due to heat absorption in the first secondary superheater (from point C to point D), so it is isenthalpy reduced in pressure by the second superheater pressure reducing valve. There is plenty of leeway without worrying about entering the saturation range.

Figure 5 is an explanatory diagram showing the pressure setting values of each superheater pressure reducing valve.
is the secondary superheater outlet side pressure setting value, is the burden differential pressure value at the first superheater pressure reducing valve, and is the burden differential pressure value at the second superheater pressure reducing valve. In this embodiment, the differential pressure borne by the first superheater pressure reducing valve and the differential pressure borne by the second superheater pressure reducing valve are divided equally; however, this is not necessarily limited to this, and there is a certain predetermined value. It is also possible to share in any ratio depending on the ratio or the surrounding boiler conditions.

Next, a method of controlling the first and second superheater pressure reducing valves will be explained using the control block diagram shown in FIG.

21 in the figure is the first secondary superheater inlet side pressure detector, 22 is the first secondary superheater inlet side temperature detector,
23 is a first secondary superheater outlet side pressure detector; 24
is the main steam pressure detector, 25 is the load request controller, 2
6 is a second secondary superheater inlet side temperature detector, 27 is a second secondary superheater inlet side pressure detector, 28 is a function generator, 29 and 30 are subtractors, 31 is PI
regulator, 32, high signal selector, 33 and 34;
is a function generator, 35 is a subtractor, 36 is a PI adjuster,
37 is a high signal selector, 38 is a subtracter, 39 is a function generator, 40 is a first secondary superheater inlet side saturation temperature deviation value signal, and 41 is a second secondary superheater inlet side saturation temperature deviation value 42 is the first superheater pressure reducing valve pressure set value signal, 43 is the second superheater pressure reducing valve pressure set value signal, 44 is the main steam pressure control signal, 45 is the second
This is the saturation temperature deviation signal on the inlet side of the secondary superheater.

The first secondary superheater outlet side pressure setting value signal 42 obtained by the function generator 23 in response to the load signal from the load request command device 25 and the first secondary superheater outlet side pressure detector 23 Therefore, the deviation from the detected actual pressure signal is calculated by a subtracter 30 and passed through a PI regulator 31 as a pressure control signal.

On the other hand, the first secondary superheater inlet side pressure detector 21
The first secondary superheater inlet side saturation temperature deviation value signal 40 calculated by the function generator 28 based on the inlet side pressure detected by the first secondary superheater inlet side temperature A subtractor 29 calculates the deviation from the actual inlet temperature signal detected by the detector 22.

This deviation signal and the pressure control signal are input to the high signal selector 32, and the opening degree of the first superheater pressure reducing valve 6a is adjusted based on the control signal outputted from the high signal selector 32, and the inlet side of the first secondary superheater is adjusted. The pressure of the secondary superheater can be controlled while ensuring the degree of superheating of the fluid.

Similarly, the second superheater pressure reducing valve 6b also outputs the main steam pressure control signal 44 and the second secondary superheater inlet side saturation temperature deviation signal 45 to the high signal selector 3.
The opening degree is adjusted by a control signal output through 7.

Here, the saturation temperature control circuit may be omitted if the inlet side temperature of each secondary superheater ensures the degree of superheating in all load zones. Further, the set values of the function generator 33 for the first secondary superheater outlet side pressure and the function generator 34 for the main steam pressure are set to the first
In addition, the load pressure difference between the second superheater pressure reducing valves 6a and 6b is set to be approximately equal (see FIG. 5).

FIG. 6 is a system diagram for explaining a second embodiment of the present invention. In the case of this embodiment, a third superheater pressure reducing valve 8c is installed on the upstream side of the primary superheater 5 in addition to that of the first embodiment. Since the system is such that multi-stage pressure reduction is performed using three pressure reducing valves, the third superheater pressure reducing valve 8c, the first superheater pressure reducing valve 8a, and the second superheater pressure reducing valve 3b, the pressure is reduced. The differential pressure burden for one valve can be further reduced.

FIG. 7 is a system diagram for explaining a third embodiment of the present invention. This embodiment differs from the second embodiment in that the second superheater pressure reducing valve 6b and the second secondary superheater 8b are omitted.

〔Effect of the invention〕

Since the present invention is configured as described above,
It is possible to improve the erosion resistance of multiple installed pressure reducing valves, and as shown in Figure 2, the steam on the inlet side of the secondary superheater does not enter the saturated region, which allows boiler transformer operation with margin. is possible.

[Brief explanation of the drawing]

FIG. 1 is a system diagram for explaining the variable pressure operation of the once-through boiler according to the first embodiment of the present invention, FIG. 2 is a characteristic diagram showing the relationship between steam pressure and enthalpy in the variable pressure operation, and FIG. A block diagram showing an example of the control method for the variable pressure operation, FIG. 4 is a characteristic diagram showing the relationship between load and main steam pressure during variable pressure operation, and FIG. 5 is an explanation showing the pressure setting values of each superheater pressure reducing valve. Figures 6 and 7 are system diagrams for explaining the second and third embodiments, Figure 8 is a system diagram for explaining variable pressure operation of a conventional once-through boiler, and Figure 9 is a system diagram for explaining the variable pressure operation of a conventional once-through boiler. It is a characteristic diagram showing the relationship between steam pressure and enthalpy in the variable pressure operation. 5... Primary superheater, 6a... First superheater pressure reducing valve, 6b... Second superheater pressure reducing valve, 6c... Third
superheater pressure reducing valve, 8a...first secondary superheater, 8
b...Second secondary superheater.

Claims (1)

[Claims] 1 At least a first superheater and a second superheater are arranged in series along the flow direction of steam, and a first pressure reducing valve is provided on the upstream side of the first superheater,
A second pressure reducing valve is provided on the upstream side of the second superheater, and after the first pressure reducing valve performs isenthalpic pressure reduction so that the steam pressure reaches a predetermined steam pressure depending on the load, the first pressure reducing valve A once-through boiler characterized in that the boiler is heated in the superheater, isenthalpic reduced by the second pressure reducing valve so that the steam pressure reaches the required steam pressure on the turbine side, and then superheated in the second superheater. Transforming voltage operation method.