JPS6045321B2 - 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 the evaporator of a once-through boiler, when the load is low and the amount of water passing through it is reduced, 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 113 load or less, so when the boiler is used at this low load, a circulation device is provided to set the amount of water flowing through the evaporator to a flow rate corresponding to 113 load or more.
The circulation system consists of an anhydrous separator, a separation tank, a circulation pump, etc. At a low load of about 113 loads or less, the water level appears in the separation tank and the circulation pump works, and at a high load exceeding 113 loads, the water level in the separation tank decreases. It disappears and the circulation pump stops. However, the nature of the water level in the separation tank is a static characteristic when the load on the foiler 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 113 load to a low load below 113 load, the amount of fuel and water supply will decrease while being adjusted to a ratio commensurate with each load, but the water level in the separation tank will change due to the heat capacity of the evaporator tube. Because of this, the effects of the reduction in fuel do not appear immediately, and the amount of evaporation increases temporarily due to the reduction in water supply, and then slowly decreases. Therefore, the water level in the separation tank does not appear for a long time, and may appear only after the load change has stopped and the water level has settled down to a new low load. Conversely, if the load suddenly changes from a low load of 113 loads or less to a high load of more than 113 loads,
The amount of fuel and the amount of water supplied increase while being adjusted in proportion to each load, but the amount of heat transferred to the fluid in the evaporator tubes does not increase immediately, so the amount of fuel in the evaporator does not increase in proportion to the amount of 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, operate the circulation pump to increase the amount of water circulating in the evaporator. This can lead to flow imbalances and oscillations, and it is therefore not possible to reduce the load until a water level appears in the separator.

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 these problems, and it is an object of the present invention 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 of 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. and a desuperheater 5 between the superheaters 4 and 4'', and a separation tank 6 provided in the path from the steam separator 3 to the inlet of the evaporator 2.
, and a circulation pump 7. This once-through boiler with a 2j circulation device is provided with a control device, and the control device includes a feed water control valve 8 provided on the outlet side of the feed water pump 1, a water feed controller 9 that controls it, and a feed water control valve 9 provided in the fuel supply path 10. It is composed of a fuel control valve 11 that controls the fuel control valve 11, a fuel controller 12 that controls the same, and a load controller 16 that controls the water supply control valve 3 (controller 9 and the fuel controller 12).
A water injection control valve 14 and a water injection controller 15 for controlling the water injection control valve 14 are installed in the injection channel 13 provided in the desuperheater 5 from the outlet side of the boiler. It is controlled by receiving a signal from the temperature. Also, as shown in the figure, the water injection controller 15 receives a signal from the load controller 16, but conventionally, this signal is used to statically change the amount of water injection depending on the load, or when the load changes. The purpose of this method was to change the amount of water injected in advance before the temperature changed, but in the present invention, it is used for control as described below. As can be seen from the figure, the total water supply amount (water injection amount + water supply amount through the boiler evaporator) is controlled by the load, so the water supply amount through the boiler evaporator is the difference between the total water supply amount and the water injection amount. . Now, Figure 2 is
FIG. 3 shows changes in the water supply amount W1 through the boiler evaporator, the fuel amount F1, and the dryness x at the outlet of the evaporator 2 over time according to the conventional control method in the once-through boiler with a circulation device. Figures 2a and 3a show changes in the amount of water supplied through the boiler evaporator W1, the amount of fuel F1, the amount of water injected S1, and the dryness x at the outlet of the evaporator 2 over time according to the control method of the invention. The shaded area where x=1 or more of the dryness X at the outlet of the evaporator 2 is steam, so water does not flow into the separation tank 6, and on the contrary, the water in the separation tank 6 decreases due to self-evaporation. The water level will continue to fall, and the water level in the separation tank 6 ( This corresponds to the case where the circulation pump 7 is not operated).

Figure 2 a shows the ratio of the water supply amount W and fuel amount F passing through the boiler evaporator to each load (simplified in the figure) in order to increase the load from a load lower than 113 load to a high load exceeding 1 no 3 load. (keep it constant) to change it? In the conventional control method, the dryness X at the outlet of the evaporator 2 decreases once and then increases until x=1.
The cow always stays for a long time until it reaches x=
The water in the separation tank 6 will run out only when it reaches 1 or more. However, in the control method of the present invention, the load control shown in FIG. 1 is applied so that the fuel amount F increases with the load as shown in FIG.
Controls the f unit 16, the fuel controller 12, and the fuel control valve 11,
The amount of water supplied through the boiler evaporator W is expressed as the amount of fuel F in Figure 3a]
When the load change is over, the amount of water increases rapidly to a ratio (F) commensurate with the amount of fuel F equivalent to the final load. This is done by controlling the rate of increase and the difference in the amount of water injected.

In other words, the temperature is normally controlled by increasing the water injection flow rate and increasing the amount of water supplied to the evaporator so that the superheated air temperature reaches a predetermined value using a control system not shown in Figure 1.) The water injection amount S is controlled by its temperature. The load controller 16, the water injection controller 1, and the water injection control valve 14 are controlled so that the water injection amount returns to the original temperature control amount after the increase and load change are completed regardless of the change in the load. Due to the control system described above, the total water supply amount increases at the same rate as the fuel amount as the load increases, so the water injection amount temporarily increases (1) Therefore, the water supply amount W fuel passing through the boiler evaporator increases. Since the amount increases by a small amount as shown in the figure, the dryness x at the outlet of the evaporator 2 quickly reaches 1 or more, and the water in the separation tank 6 quickly disappears. Figure 2b shows that in order to reduce the load from a high load exceeding 113 load to a load lower than 113 load, the water supply amount W and fuel amount F passing through the boiler evaporator are set at a ratio (
In the conventional control method, the dryness x at the outlet of the evaporator 2 increases once and then decreases, and it takes a very long time to reach x = 1 or less. Water is stored in the separation tank 6 only when x=1 or less.

However, in the control method of the present invention, as shown in FIG. 3b, the fuel amount F
The load controller 16, fuel controller 12, and fuel control valve 11 shown in Fig. 1 are controlled so that the amount decreases with the load, and the water supply amount W passing through the boiler evaporator is controlled at a rate smaller than the fuel amount F as shown in Fig. 3b. When the load change ends, the fuel amount F corresponding to the final load decreases.
The difference between the rate of decrease in the amount of water supplied W and the rate of decrease in the amount of fuel is controlled by controlling the amount of water injected.

That is, the superheated steam temperature is normally controlled by the control system (not shown) in FIG. 1 by reducing the water injection flow rate and reducing the water injection amount entering the evaporator, but the temperature control is reduced regardless of the temperature control. When the load change is finished, the load controller 16, water injection controller 15, and water injection control valve 14 are controlled so that the water injection amount is returned to the original temperature control amount. In such a case, the total water supply amount is reduced by the above control system at the same rate as the fuel amount as the load increases, so as the water injection amount temporarily decreases, the water supply amount W passing through the boiler evaporator becomes smaller than the fuel amount. , 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. In the above control method of the present invention, it is also effective to delay the timing of change in the amount of water supplied through the boiler evaporator w relative to the amount of fuel F, and when the load change rate is slow, the amount of water supplied through the boiler evaporator W and Fuel amount F
Situations where it is okay to change the rate of return as before.
Since there is a load on the control device shown in Figure 1? A selection circuit based on the change speed may be provided so that the above control method of the present invention is performed only when the load change speed is equal to or higher than a predetermined value F.

As explained above, if the water injection amount is changed, even temporarily, regardless of the boiler outlet steam temperature, the boiler outlet steam temperature cannot be maintained constant.

However, in reality, water injection into a once-through boiler is normally performed in two stages, so the water injection amount control described above is performed in one of the stages.
It is possible to maintain the boiler outlet steam temperature constant by controlling the remaining amount of water injection in a corrective manner either by the boiler outlet steam temperature signal or by a signal that controls one of the water injection amounts. As described in detail above, the control method of the present invention, in a once-through boiler with a circulation device, controls the amount of fuel to increase or decrease with the load when the load changes, and increases the amount of water supplied through the boiler evaporator by a proportion smaller than the amount of fuel. When the load change ends, the water injection amount is increased or decreased independently of temperature control so that the water supply amount rapidly increases or decreases to a ratio commensurate with the fuel amount equivalent to the final load, and when the load change ends, By returning the water injection amount to the original amount for temperature control, the water level required for the circulation system can quickly follow the load change and appear and disappear, thereby preventing water from flowing into the superheater during high loads. In addition, when the load is low, the circulation pump can be operated to increase the amount of water circulating in the evaporator.

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

[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 conventional control method, where a is the control method when changing from low load to high load, and b is the control method when changing from low load to high load. This is a control method when the load changes from low load to low load. FIG. This is a control method when the load changes from low to low. 1... Water supply pump, 2... Evaporator, 3
...Steam water separator, 4,4'' ...Superheater, 5 ...Superheat reduction O device, 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 passing through the boiler evaporator, F ... Fuel amount, water injection amount, x ...
...Dryness at the outlet of evaporator 2.

Claims (1)

[Claims] 1 Equipped with a circulation device, which controls the amount of fuel and the total amount of water supplied, which is the sum of the amount of water supplied through the boiler evaporator and the amount of water injected for temperature control, according to the load, and the main steam temperature is adjusted to the total amount of water supplied. In once-through boilers, which are controlled by the amount of water injected into the boiler, when the load changes, the amount of fuel is controlled to increase or decrease with the load, and the amount of water supplied through the boiler evaporator, which is the difference between the total amount of water supplied and the amount of water injected, is The water injection flow rate is increased or decreased so that the amount of water supplied through the boiler evaporator is increased or decreased at a rate smaller than the amount of fuel corresponding to the final load, and when the load change ends, the amount of water supplied through the boiler evaporator is rapidly increased or decreased to a rate commensurate with the amount of fuel equivalent to the final load. By changing the amount of water supplied to the evaporator to increase or decrease the amount of water injected independently of temperature control, and then returning it to the original amount of water for temperature control when the load change ends, the water level required for the circulation system can be adjusted. A method for controlling a once-through boiler with a circulation device, characterized in that the appearance and disappearance are made to quickly follow the load change.