JPS6024885B2 - Control method for once-through boiler with circulation device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of controlling a once-through boiler with a circulation device.

In a once-through boiler evaporator, when the load is low and the amount of water passing through the evaporator is low, flow imbalance and flow oscillation occur.

This is because when the flow rate in one tube of an evaporator made up of parallel tubes decreases, evaporation becomes more active, which increases the flow resistance in that tube and further reduces the flow rate. This phenomenon usually occurs at a low load of 1/3 load or less, so when the boiler is used at this low load, a circulation device is installed to increase the amount of water flowing through the evaporator to a flow rate equivalent to 1/3 load or more. .
The circulation system consists of a steam/moisture mixer, a separation tank, a circulation pump, etc. At low loads of about 1/3 load or less, a water level appears in the separation tank and the circulation pump works, and at high loads exceeding 1/3 load, the circulation pump works. The water level in the separation tank disappears and the circulation pump stops. However, the characteristics of the water level in the separation tank are static characteristics when the load on the boiler does not change, and when the load changes rapidly, the water level in the separator may not appear or disappear in response to the load. .

For example, if the load suddenly changes from a high load exceeding 1/3 load to a low load below 1'3 load, the amount of fuel and water supply will decrease while being adjusted to the ratio commensurate with each load, but the water level in the separation tank will decrease. Due to the heat capacity of the evaporator tube, the effect of a decrease in fuel does not appear immediately, and the amount of evaporation increases temporarily due to a decrease in water supply, and then slowly decreases. Therefore, the water level in the separation tank does not appear easily,
It may appear only after the load changes have stopped and a new lower load has been reached. Conversely, if the load suddenly changes from a low load of 1/3 load or less to a high load of more than 1/3 load, the amount of fuel and water supply will increase while being adjusted to a proportion commensurate with each load, but Since the amount of heat transferred to the fluid does not increase immediately, the amount of evaporation does not increase in proportion to the fuel. Therefore, the water level in the separation tank does not disappear easily, and the water level in the steam/water separator may rise, causing water to flow into the superheater. As mentioned above, if the water level does not appear in the separator under low load, the circulation pump cannot be activated to increase the amount of circulating water in the evaporator, which may cause flow imbalance or vibration. The load cannot be lowered until the water level appears.

Also, if the water level in the separator rises under high load and water flows into the superheater, the steam temperature will drop in various parts of the boiler, causing various problems, so the load must be increased until the water level in the separator disappears. I can't. The present invention has been made to solve this problem, and aims to provide a control method that can quickly follow load changes and cause the water level in the separation tank to appear and disappear.

A method for controlling a once-through boiler with a circulation device according to the present invention will be described below.

First, a once-through boiler with a circulation device for carrying out this method will be explained with reference to FIG. 1. The boiler includes a feed water pump 1, an evaporator 2, a steam separator 3, superheaters 4 and 4', and a superheater connected in series. 4 and 4', a separation tank 6 provided in a path from the steam separator 3 to the inlet of the evaporator 2, and a circulation pump 7. This once-through boiler with a circulation device is equipped with a control device, and the control device includes a water feed control valve 8 provided at the outlet of the feed water pump 1, a water feed controller 9 for controlling it, and a fuel control device provided in a fuel supply path 101. Valve 1
1, a fuel controller 12 that controls it, a water injection control valve h4 provided in the injection channel 13 extending from the outlet side of the feed water control valve 8 to the superheat reducer 5, and a fuel control valve h4 that controls the water injection control valve h4 so that the boiler outlet steam temperature is constant. The system is comprised of a water injection controller 15 that controls water supply, and a load controller 16 that controls the water supply controller 9 and fuel controller 12. Now, FIGS. 2 and 3 show changes in the water supply amount W, the fuel amount F, and the dryness x at the outlet of the evaporator 2 over time in the once-through boiler with a circulation device. The time integral of the shaded area below the dryness x = 1 at the outlet of the evaporator 2 in Figure 2 corresponds to the water level in the separation tank 6 (when the circulation pump 7 is not activated), and the evaporation rate in Figure 3 The shaded area where the dryness X at the outlet of the vessel 2 is equal to or higher than X = 1 is steam, and therefore water does not flow into the separation tank 6. On the contrary, the water in the separation tank 6 decreases due to self-evaporation, and the water level decreases. It turns out.

Figure 2 shows the case where the load is lowered from a high load exceeding 1/3 load to a load lower than 1/3 load, and Figure A shows the ratio of water supply amount W and fuel amount F to each load (for simplicity Make it constant.

), the dryness at the outlet of the evaporator 2 is
It increases once and then decreases, and it takes a very long time to reach x=1 or less, and water is stored in the separation tank 6 only when x=1 or less. However, in the control method of the present invention, the load controller 16 and the water supply controller 9 in FIG. 1 control the water supply control valve 8 so that the water supply amount W decreases with the load as shown in FIG. The load controller 16, fuel controller 12, and fuel control valve 11 are configured so that the water supply amount W decreases at a higher rate than the water supply amount W, and when the load change ends, the load controller 16, fuel controller 12, and fuel control valve 11 rapidly increase to a ratio commensurate with the water supply amount W and the final load. It's about controlling.

Thus, the dryness X at the outlet of the evaporator 2 quickly reaches x=1 or less, and the amount of water stored in the separation tank 6 rapidly increases as shown by the shaded area.

Next, using Figure 3 to explain the case where the load is increased from a load lower than 1'3 load to a high load exceeding 1/3 load, Figure A shows that the water supply amount W and fuel amount F are changed at a ratio commensurate with each load. In the secondary method, the dryness X at the outlet of the evaporator 2 decreases once and then increases, and it takes a very long time to reach X = 1 or more, and only when X = 1 or more does the separation tank 6 water will be gone.

However, the control method of the present invention controls the load controller 16, water supply controller 9, and water supply control valve 8 shown in FIG. 1 so that the water supply amount W increases with the load as shown in FIG. B, and the fuel amount F increases as shown in FIG. The load controller 16, fuel controller 12, and fuel control valve 11 are controlled so that the water supply amount W increases at a higher rate than the water supply amount W, and when the load change ends, the load controller 16, fuel controller 12, and fuel control valve 11 rapidly decrease to a ratio commensurate with the water supply amount W and the final load. It is something to do.

In this way, the dryness x at the outlet of the evaporator 2 quickly reaches X;1 or more, and the water in the separation tank 6 disappears far away.

In the control method of the present invention described above, it is also effective to delay the timing of the change in the water supply amount W compared to the fuel amount F, and when the load change speed is slow, the ratio between the water supply amount W and the fuel amount F is changed from the conventional one. Therefore, the control device shown in FIG. 1 is provided with a selection circuit based on the load change rate, and the above control of the present invention is performed only when the load change rate is equal to or higher than a predetermined value. The method may be performed.

In addition, in the control method of the present invention when the load changes from low load to high load as shown in FIG. Yes, but
If X is larger than 1, it means that the steam at the outlet of the evaporator 2 will be overheated, so it is better that x is not much larger than 1. Therefore, the ratio between the fuel amount F and the water supply amount W during the load increase should be determined with this point in mind. As detailed above, the control method of the present invention, in a once-through boiler with a circulation device, controls the water supply amount to increase or decrease with the load when the load changes, increases or decreases the fuel amount at a higher rate than the water supply amount, and By rapidly decreasing the amount of fuel to a ratio commensurate with the amount of water supplied and the final load when this is completed, the water level required for the circulation system can quickly follow the load change and appear and disappear. Therefore, the circulation pump can be operated during low loads to increase the amount of water being circulated in the evaporator, and water will not flow into the superheater during high loads.

Therefore, there is no flow imbalance or vibration in the evaporator, no drop in steam temperature at any part of the boiler, and no troubles.

[Brief explanation of the drawing]

Figure 1 is a system diagram of a once-through boiler with a circulation device equipped with a control device, and Figure 2 is a graph showing the control method when the load changes from high load to low load, where A is the conventional control method and B is the original control method. This is a control method of the invention, and FIG. 3 is a graph showing the control method when the load changes from low load to high load, where A is the conventional control method and B is the control method of the present invention. 1... Water supply pump, 2... Evaporator, 3
...Sea water separator, 4,4'...superheater, 5...
...Superheat reducer, 6... Separation tank, 7...
... Circulation pump, 8 ... Water supply control valve, 9 ... Water supply controller, 10 ... Fuel supply path, 11 ... Fuel control valve, 12 Fuel controller, 13. ...Injection channel,
14...Water injection control valve, 15...Water injection controller, 16...Load controller, W...Water supply amount, F
・・・・・・Fuel amount, X・Skin・Dryness at the outlet of evaporator 2. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1 In a once-through boiler equipped with a circulation device, the amount of water supplied when the load changes is controlled so that it increases or decreases with the load, the amount of fuel is increased or decreased at a higher rate than the amount of water supplied, and when the load change ends, the amount of fuel is changed to the amount of water supplied. A method for controlling a once-through boiler with a circulation device, characterized in that the water level is rapidly decreased and increased to a ratio commensurate with the final load, and the water level appears and disappears as necessary in the circulation device to quickly follow the load change. .