JPS5914241B2 - Evaporator control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a control system for controlling liquid level in an evaporator.

Generally, evaporators are used in various fields such as chemical industry and nuclear power plants.

For example, in the chemical industry, it is necessary to concentrate molasses, desalinate seawater, and in nuclear power plants, to decontaminate liquid waste treatment.
FIG. 1 shows one conventional evaporator control system. This device is used for the liquid to be concentrated in the evaporation tank 1. The liquid level in the evaporation tank 1 is controlled at a constant value by adjusting the flow rate of the liquid to be supplied so that a certain amount of heated steam is added and the liquid is supplied in an amount commensurate with the evaporated amount. First, to concentrate the liquid to be concentrated, heated steam is applied to the evaporation tank 1. Then, an evaporation effect corresponding to the heated steam flow rate will occur,
As a result of this step 5, the liquid to be concentrated in the evaporation tank 1 is concentrated in proportion to the cumulative amount of liquid supplied. This heated steam flow rate is measured by a flow rate detector 2, and the resulting flow rate signal is supplied to a flow rate controller 3. The flow rate controller 3 receives the flow rate signal and obtains an operation signal for adjusting the throughput to a predetermined amount. After converting this operation signal by the electro-pneumatic converter 4, it is applied to the flow rate control valve 5 to perform constant value control of the heated steam flow rate. On the other hand, liquid supply control is performed as follows. First, the differential pressure between the two bubbler tubes 6 and T is detected by the level transmitter 81.
After the signal is converted and detected as a liquid level in step 5, this level detection signal is supplied to a level controller 9. The level controller 9 receives the level detection signal, obtains an operation signal that reaches a predetermined level, and applies this to the liquid supply flow rate control valve 10 to control the liquid supply. Reference numeral 11 denotes an electric pneumatic converter.

FIG. 2 shows another conventional device.

This eliminates the drawback of the apparatus shown in FIG. 1, which is poor controllability with respect to disturbances in the flow rate of liquid supplied. In other words,
In the device shown in Fig. 1, when the liquid supply flow rate changes due to changes in the discharge pressure of the pump, etc., the liquid supply flow rate is not adjusted until a deviation occurs in the liquid level in the evaporation tank 1. Therefore, there is a problem with control accuracy. Therefore, the second
The device shown in the figure performs cascade control so that the target value of the liquid supply flow rate controller 12 is controlled by the output of the level controller 9.

13 is a flow rate detector.

In this device, disturbances in the liquid supply flow rate are corrected by the liquid supply flow rate controller 12, so that the disturbances in the liquid level in the evaporation tank 1 can be controlled without any deviation. However, the devices shown in FIGS. 1 and 2 control the liquid supply flow rate control valve 10 directly or in cascade with the output of the level controller 9, and the control characteristics depend only on the level detection signal. .

It is well known that a reverse level response phenomenon occurs as a process characteristic of the evaporator control device. this is,
This is an apparent level response due to an increase or decrease in the amount of board generated in the boiling liquid. When the flow rate of the liquid supplied to the boiling liquid in the evaporation tank 1 increases, the board contained in the liquid rapidly decreases due to cooling, so the level increases. decreases temporarily. Furthermore, when the heating steam flow rate increases, the boiling of the liquid is accelerated, so the amount of board increases and the level temporarily rises. This phenomenon worsens controllability because it is in the direction of divergence, as opposed to the normal control operation direction in which the flow rate of the liquid supply is throttled when the level rises and the flow rate of the liquid supply is increased when the level falls. A typical example of this level reverse response characteristic is shown in FIG.

FIG. 3A shows a characteristic that when the liquid supply flow rate is changed stepwise, the level of the evaporation tank 1 temporarily decreases as the board decreases as shown in FIG. 3B, but gradually increases again. Conversely, when the heating steam flow rate is changed in steps as shown in Figure 3C, the level of the evaporation tank 1 rises temporarily due to the sudden increase in the number of boards, and then decreases as shown in Figure 3D. There is.
FIG. 4 is a diagram showing control results of the apparatus shown in FIG. 1.
This figure shows the characteristics of the level and supply liquid flow rate when heating steam is controlled at a constant flow rate.As mentioned above, due to the reverse response characteristics of the level, the supply liquid flow rate control is at the H1 limit or at the L limit.
. It diverges to either limit and performs on/off operations. The present invention has been made in view of the above circumstances, and provides an evaporator control device that performs more stable control against nonlinear characteristics such as reverse responsiveness of the evaporator liquid level.

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

FIG. 5 shows an example of a control configuration applied to a single-barrel evaporator, and the same parts as in FIG. 2 are given the same reference numerals, and some explanations thereof are omitted. The difference between this device and the conventional device lies in the control configuration of the liquid supply flow rate. This device corrects the bias and coefficients of the output of the level controller 9 via an adder/subtractor 21 for setting bias coefficients and a scaler 22 for setting correction coefficients, and supplies the output to one input section of an adder/subtractor 23. Further, the detected flow rate signal of the flow rate detector 2 is subjected to balance correction by a balance coefficient setting scaler 24, and is supplied to the other input section of the adder/subtractor 23. The adder/subtractor 23 adds the two input signals and uses the resultant value as a target value of the liquid supply flow rate. Next, the operation of the evaporator control device configured as described above will be explained.

First, in controlling the heating steam flow rate, the flow rate signal P is detected by the flow rate detector 2 as shown in FIG. The flow control valve 5 is operated to perform constant value control of the heating evaporation flow rate. On the other hand, for liquid supply flow rate control, the operation signal MV output from the level controller 9 is input to the subsequent bias coefficient setting adder/subtractor 21, where the bias coefficient K1
Perform subtraction correction.

Further, the signal output from the adder/subtractor 21 is input into a correction coefficient setting scaler 22, multiplied by a correction coefficient K2, and inputted into an adder/subtracter 23. On the other hand, the flow rate signal PV detected by the flow rate detector 2 is input to the balance coefficient setting scaler 24, where it is multiplied by the balance coefficient K3 and then input to the adder/subtractor 23.

The adder/subtracter 23 adds the output signal from the level controller 9 side and the output signal from the heating steam flow rate system, and sets this added value as the control target value SV of the liquid supply flow rate controller 12. Therefore, the setting formula for this control target value SV is:
It becomes S-K3・PV+K2 (M-K1).

Therefore, this device controls the feed liquid flow rate with the ratio of the steam flow rate using the balance coefficient K3, while correcting the balance change between the feed liquid flow rate and the steam flow rate using the output signal M of the level controller 9. .

Further, the output signal M of the level controller 9 is subtracted by the bias coefficient K1 by the adder/subtractor 21, so that the liquid supply flow rate control based on the output of the level controller 9 has a correction effect. According to the device of the present invention as described above, the ratio control of the supply liquid flow rate to the steam flow rate is basically used, and the adjustment operation for level deviation is suppressed by a correction coefficient. The inverse response characteristic of
It does not appear in the control results as a sudden change.

This changes the conventional concept of level control that relies only on the deviation of the evaporation tank level, returns to the principle of the evaporation process, and is based on supplying a feed liquid flow rate commensurate with the heating steam flow rate, which is a parameter proportional to the evaporation amount. This is because there is. The experimental results using this device can be shown in FIG. As is clear from this figure, it can be seen that the adjustment operation of the liquid supply flow rate due to the deviation in the evaporation tank level is performed extremely slowly. Note that the present invention is not limited to the above-mentioned embodiments, and can be implemented in various modifications.

FIG. 7 shows one of its modifications. This device has a configuration in which the liquid supply flow rate is controlled at a fixed value, and conversely, the heated steam flow rate is adjusted as a manipulated variable for level control by a level controller 9.
The advantage of this device is that the throughput in the evaporator can be set directly with the feed flow rate. That is, the steam flow rate is controlled in proportion to the liquid supply flow rate, and at the same time, the output signal of the level controller 9 is added to the set value as a correction amount. Next, FIG. 8 is a diagram showing a modification of the present device, in which a heating tank 31 for heat exchange and an evaporation tank V for evaporation are placed separately and connected via a circulation pump 32. This is applied to a so-called forced circulation type evaporator.

Since the configuration of the control system is the same as that shown in FIG. 3, its explanation will be omitted here. In addition, the present invention can be modified in various ways without departing from the spirit thereof.

As detailed above, the apparatus of the present invention has the following effects.

In other words, since the conventional device uses only a level signal as a control input, the influence of nonlinear characteristics such as a level inverse response phenomenon deteriorates direct controllability. On the other hand, in the case of the device of the present invention, the level signal acts as a correction for the supply liquid flow rate adjustment signal, which is the operational output of level control, and the main element is set as a ratio to the steam flow rate, so fluctuations in the steam flow rate It is possible to reduce the influence of unstable elements in the control system, such as level reverse responsiveness, not only to fluctuations due to disturbances in the supply liquid flow rate and evaporation tank level, but also to stabilize control. be able to. Viewed from a different perspective, this shows that rapid changes in level can be ignored and only level fluctuations that occur gradually can be corrected over a long period of time. In general, waste treatment systems at nuclear power plants are operated in such a way that the waste generated in a day is processed in 8 hours a day, and the amount of waste processed by the evaporator (flow rate of liquid supplied) is calculated as the amount of waste generated on that day. Since it is necessary to change the setting according to the amount of heat generated, the setting of the heating steam flow rate, which sets the throughput, is frequently changed. Furthermore, when the concentrated waste liquid in the evaporation tank reaches a certain concentration, a portion of the concentrated liquid is discharged, causing level fluctuations. As described above, the present invention can provide an evaporator control device that can maintain stable level control without interrupting operation in response to changes in steam amount or level fluctuations, and can improve operational efficiency.

[Brief explanation of drawings]

Figures 1 and 2 are configuration diagrams of conventional equipment, and Figures 3A-D.
4 is a characteristic diagram explaining the operation of the conventional device, and FIG. 5 is a characteristic diagram explaining the operation of the conventional device.
The figure is a block diagram showing one embodiment of the evaporator control device according to the present invention, FIG. 6 is a characteristic diagram obtained by the device of the present invention, and FIG.
8 and 8 are configuration diagrams showing other examples of the device of the present invention. 1...Evaporation tank, 2...Flow rate detector, 3...Flow rate controller, 5...Flow rate control valve, 8...Level Transmitter, 9... Lebel controller, 10... Liquid supply flow rate control valve, 12...
...Liquid supply flow rate controller, 13...Flow rate detector, 2
1... Addition/subtraction device for bias coefficient setting, 22...
...Scaleler 1, 23 for setting correction coefficient...
Adder/subtractor, 24... Scaler for setting balance coefficient.

Claims (1)

[Claims] 1. In the device which evaporates and concentrates the liquid to be concentrated in the steam tank while detecting the heated steam flow rate and performing fixed value control, and also controls the liquid level in the evaporation tank by detecting and controlling the supply liquid flow rate, the above-mentioned Means for correcting a liquid level adjustment signal in a steam tank using a bias coefficient and a correction coefficient and outputting it as a level controller side output signal; and a balance coefficient for correcting the heated steam flow rate with a balance coefficient to obtain a steam flow rate side output signal. An evaporator control device comprising: a setting scaler; and means for controlling the supply liquid flow rate by adding both signals obtained by the scaler and the above-mentioned means.