JPH0248802B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a liquid level control device for an evaporative concentrator in a nuclear power plant or an evaporative generator suitable for concentrating products (such as sugar), and in particular, the present invention relates to a liquid level control device for an evaporative generator suitable for concentrating products (such as sugar). This invention relates to a liquid level control device for an evaporation generator that is resistant to

Evaporative concentrators (hereinafter referred to as concentrators) are used not only in chemical plants but also in nuclear plants, etc.
is important as a water treatment device that provides high purification and decontamination efficiency. In particular, in the waste treatment system of nuclear power plants, it occupies a major position as a key point in the treatment of various waste liquids. However, at present, stable operation cannot be achieved because the system relies on manual operation based on the operator's experience. In particular, manual operation has become a problem in terms of operating efficiency of waste treatment equipment in nuclear power plants where the output capacity per unit has been increased recently.

Therefore, it is necessary to make the concentrator fully automatic. In this case, fully automatic operation means everything from starting operations such as filling with water and warming up, to evaporation concentration, high temperature standby, and
The boiling liquid level must be controlled at a constant level throughout the series of treatment steps from discharge to shutdown to maintain stable operating conditions. Here, constant control of the evaporated liquid level is important for maintaining the condensation efficiency of the concentrator (ratio of contamination level between treated waste liquid and distilled water) and for preventing abnormalities such as overflow and dry firing. However, the boiling liquid level in the concentrator is affected by the large amount of voids that occur in the heating section, and the dynamic characteristics of these voids are opposite to changes in the amount of water, so control tends to become unstable and operation becomes difficult. It is also the reason why it becomes difficult. This tendency is particularly remarkable in the case of two-barrel circulation type concentrators, which have increased processing efficiency.

Hereinafter, in explaining the conventional apparatus, first, the characteristics of the boiling surface liquid level of the concentrator will be explained with reference to FIG. The boiling surface liquid level of the condenser must be kept constant by balancing the heating steam flow rate in order to obtain an evaporation flow rate commensurate with the feed liquid flow rate (processing amount). As shown in FIG. 1 a and b, it is assumed that the heating steam flow rate a and the liquid supply flow rate b are balanced, and the liquid level d is kept stable. If the set value of the liquid supply flow rate b is changed to b' in the state of The boiling surface liquid level curve 1 shown below should be obtained. However, as the feed liquid flow rate b' increases, the amount of voids generated within the concentrator also changes. In other words, the amount of voids decreases as the amount of liquid supplied increases. Therefore, as shown in the liquid level curve 2, the actual liquid level drops once in period f due to the disappearance of voids at the same time as the liquid supply change.
It starts to rise when the amount of void generation stops changing. This characteristic also holds true for changes in the heating steam flow rate, just with the opposite polarity.
In this way, the boiling liquid level in the concentrator is proportional to the sum of the liquid amount in the concentrator + the amount of bubbles (voids), and has nonlinear characteristics as shown in FIG. In a process like this that shows changes in both positive and negative directions in response to input changes, if feedback control is applied only in one direction, a positive feedback cycle will be applied to the control rate, resulting in an unstable and runaway state. present.

Next, a conventional device will be explained with reference to FIG. That is, in this device, the processing liquid passing through the supply path is detected by the liquid supply flow rate detector 3, this is calculated as a set value S2 by the liquid supply flow rate controller 4, and the liquid supply flow rate is adjusted based on this. The processing liquid is supplied to the evaporation generator 6 while controlling the control valve 5. On the other hand, the liquid level controller 7
is calculated based on the set value S1 and the liquid level detected by the liquid level detector 8, and controls the heated steam flow rate control valve 9 based on this calculated value while supplying the heated steam through the heating pipe 11 of the heater 10. As a result, the processing liquid in the evaporation generator 6 is evaporated, and the liquid level in the evaporation generator 6 is controlled at a fixed value to a set value. 12
is a circulation pump.

However, in this device, the set value S of the liquid supply flow rate
2, the boiling surface liquid level of the evaporation generator 6 takes on a liquid level change characteristic as shown in FIG. 3c. Now, in FIG. 3, the heating steam flow rate a (the third
It is assumed that the liquid supply flow rate b (see Figure 3b) is balanced and the boiling surface liquid level d is kept constant. Here, when the supply liquid flow rate b is increased by c in the positive direction to become b', the boiling surface liquid level d decreases due to the influence of the void, causing a negative deviation from the set value S1. For this reason, the liquid level d apparently decreases, and the liquid level controller 7 attempts to throttle the heated steam flow rate control valve 9 in order to increase the liquid amount. Then, since the amount of voids in the evaporation generator 6 decreases more and more, the liquid level d decreases and reaches the lower limit value. When the liquid level d rises due to an increase in the supplied liquid, the output of the controller 7 increases the opening degree of the heated steam flow rate control valve 9 to 100%.
As a result, the heating steam flow rate reaches the upper limit value. In this way, if a unidirectional control loop is constructed by considering the change in the boiling surface liquid level S1 as only a change in the liquid volume, a positive feedbark will be applied in the liquid level state with the opposite characteristics due to the change in bubbles, and the control rate will increase. becomes unstable, and in extreme cases, the heating steam flow rate 21 and the liquid level 22 exhibit a divergent state as shown in FIG.

FIG. 4 shows a conventional device as well, but this device shows the flow rate detection value of the heated steam flow rate detector 15 and the set value S.
3, the heating steam flow rate controller 16 adjusts the heating steam flow rate, and the liquid level controller 7 adjusts the supply liquid flow rate control valve 5 based on the liquid level of the steam generator 6 and the set value S1. be.

However, even in this device, since the control system is only unidirectional, the liquid level characteristics are similar to those shown in FIG. 2. Therefore, since the conventional device exhibits a divergent state as the set values are changed, it is necessary to change the settings of the supply liquid flow rate and the heated steam flow rate slowly.
Controllability will deteriorate accordingly. In addition, the drawing speed of the concentrated liquid must be reduced in order to suppress liquid level fluctuations. Further, there is a drawback that the liquid level diverges due to a change in the set value, which is unsuitable for automatic operation.

The present invention was made in view of the above circumstances, and its purpose is to improve the stability of the control system by suppressing the influence of the amount of void generation, which is a dominant factor regarding the behavior of the boiling surface liquid level. The present invention also provides a liquid level control device for an evaporation generator that enables fully automatic operation of the plant by utilizing computers and increases the operational efficiency of the treatment system.

An embodiment of the present invention will be described below with reference to FIG. In addition, in this figure, the same parts as in FIG. 1 are given the same reference numerals, and detailed explanation thereof will be omitted. The main differences between this device and conventional devices include the provision of a compensation element calculation unit 21 that calculates a compensation element from the deviation between the liquid level set value S1 and the detected liquid level, and the provision of a compensation element calculation unit 21 that calculates a compensation element from the deviation between the liquid level set value S1 and the detected liquid level, and that the compensation element and the liquid supply flow rate controller are 4 is provided with an adder 22 for obtaining a control signal for the liquid supply flow rate control valve 5 by adding the operation outputs of 4 and 4. In addition, 3 is a liquid supply flow rate detector, 4 is a liquid supply flow rate controller, 5 is a liquid supply flow rate control valve, 6 is an evaporation generator, 7 is a liquid level controller, and 8
9 is a liquid level detector that detects the boiling surface liquid level of the evaporation generator 6; 9 is a heated steam flow rate control valve whose opening degree is controlled by the operation output of the liquid level controller 7; 10 is a heating pipe 11 for supplying heated steam; This is a heater that evaporates the processing liquid in the evaporation generator 6 by circulating it through the evaporation generator 6.

Next, the operation of the apparatus configured as described above will be explained. Now, the heating steam flow rate a (see Fig. 6 a) and the supply liquid flow rate b (see Fig. 6 b) are balanced, and the liquid level d in the evaporation generator 6 becomes a constant value d as shown in Fig. 6 e. Retained. Here, the liquid supply flow rate setting value S2
When the setting is changed to b' by increasing C by C, the boiling surface liquid level decreases due to the effect of voids and falls below the set value S1. As a result, a deviation as shown in FIG. 6f occurs between the liquid level detected by the detector 8 and the set value S1. computation of the compensation element.
This compensation element calculating section 21 is composed of a proportional-advanced element, and performs calculations based on the following calculation formula.

G=Kp(1+Td・S)・Δe However, in the above equation, Kp is a proportional coefficient, Td is a differential coefficient, Δe is a deviation, and S is a Laplace transform prime number.
Here, between each calculation element, Kp＞0.Td＜0.Δe
=S1-L relationship exists. L is the detected liquid level.
Therefore, after obtaining the deviation Δe, the compensation element calculation unit 21 calculates a negative differential coefficient as shown in the above equation to obtain a compensation output as shown in FIG.
supply to. Here, the adder 22 adds the compensation output and the operation output of the liquid supply flow rate controller 4 to obtain a signal as shown in FIG. 6d, and controls the liquid supply flow rate control valve 5 based on this signal. Here, since the compensation output has a negative waveform, the valve control output after the addition by the adder 22 has a characteristic that it temporarily decreases despite the increase in the set value S2. Note that the reason why a negative differential coefficient is used in the above equation is as follows. Originally, it is better to increase the opening degree of the control valve 5 to increase the liquid supply flow rate in response to a steady deviation due to a negative liquid level change. However, when a transient deviation occurs due to a change in the liquid level, if only the proportional element is used for control, the deviation will further increase due to a reverse response. Therefore, the meaning of applying the negative differential coefficient in the above equation is to divide the change component of the liquid level into the steady deviation component and the rate of change, and to perform proportional control in the positive direction for the former, and to perform proportional control in the positive direction for the latter. The solution is to use the inverse response to control the rate of change at a negative rate, and to combine the two. In other words, an element with a negative differential coefficient increases the deviation in a steady state, but gives an opposite response in a transient manner, so this point is used inversely to control the deviation in a direction that reduces it.

Note that the present invention is not limited to the above embodiments. For example, as shown in FIG.
A compensation element may be obtained based on the deviation Δe between the detected liquid level and the detected liquid level, and this may be added to the operation output of the heated steam flow rate controller 16 to control the steam flow rate. In addition, the present invention can be implemented with various modifications without departing from the gist thereof.

As described in detail above, according to the present invention, it is possible to perform stable control with quick convergence without causing overshoot or hunting phenomena not only for steady-state deviations but also for transient changes. Strong control can be achieved. Therefore, it becomes easy to change the settings of the supply liquid flow rate and the heated steam flow rate, and the change time can also be shortened, which is very advantageous for automatic operation of the concentrator. Furthermore, since the compensation output does not include an integral element, the control signal after addition does not leave an offset with respect to the initial setting change value C. Furthermore, if the control system can be stabilized, it is possible to provide a liquid level control device for an evaporation generator that also contributes to plant safety.

[Brief explanation of the drawing]

Figures 1 a to c are diagrams showing general characteristics of liquid level control, Figure 2 is a configuration diagram of a conventional device, Figures 3 a to c are liquid level control characteristics of the device shown in Figure 2, and Figure 3 is a diagram showing general characteristics of liquid level control. 4 is a block diagram showing another example of the conventional device, FIG. 5 is a block diagram showing one embodiment of the device of the present invention, and FIGS.
A liquid level control characteristic diagram of the device shown in the figure, and FIG. 7 is a configuration diagram showing another example of the device of the present invention. 3... Liquid supply flow rate detector, 4... Liquid supply flow rate controller, 5... Liquid supply flow rate control valve, 6... Evaporation generator,
7...Liquid level controller, 8...Liquid level detector, 9...Heating steam flow rate controller, 10...Heater, 12...Circulation pump, 15...Heating steam flow rate detector, 21...
...Compensation element calculation unit, 22...Adder.

Claims (1)

[Claims] 1. Maintain one of the flow rate of liquid supplied to the evaporation generator and the flow rate of heated steam to heat and evaporate the feed liquid in the evaporation generator at a constant value, and change the setting of the other flow rate. In a liquid level control device for an evaporation generator that obtains a valve operation output, there is an excessive difference between the set value of one flow rate and the detected liquid level value of the evaporation generator, which changes as the set value of the other flow rate changes. Compensation element calculation means for calculating a compensation element output by calculating a negative differential coefficient with respect to the deviation; A liquid level control device for an evaporation generator, characterized in that the device includes: an addition means that is used as an operation output, and suppresses a decrease in the liquid level of the evaporation generator that changes as the other flow rate setting value is increased.