For example, in the light ash section, the condensate of the washing tower is pumped to the steaming section of the steaming section; in this process, the material balance of the two sections is related, in order to ensure the washing tower of the light gray section The production process is carried out steadily, that is, it is hoped that the liquid level at the bottom of the condensation tower is relatively stable (the liquid level control system was originally set)
I. INTRODUCTION In the chemical industry, the production equipment is closely linked. Often, the discharge of the former equipment is the feed of the latter equipment; the equipments are related to each other and affect each other. For example, in the light ash section, the condensate of the washing tower is pumped to the steaming section of the steaming section; in this process, the material balance of the two sections is related, in order to ensure the washing tower of the light gray section The production process is carried out steadily, that is, it is hoped that the liquid level at the bottom of the condensation tower is relatively stable (the liquid level control system was originally set up); at the same time, it is hoped that the feed volume of the fresh liquid column storage tank in the steaming section can be relatively stable, for a single tower. It is okay to consider, but there will be contradictions in the towers that have material links before and after. See Figure 1 (curve and liquid level separately controlled curves):

Figure 1 (single channel control)

This is a plot of the condensate flow and liquid level history of a certain day; blue is the flow curve, black is the liquid level curve; visible, the flow and liquid level fluctuate greatly, generally the manual operation by the craftsman, liquid The position is not self-adjusting, and the labor intensity is large; in particular: the flow changes from several squares to hundreds of squares, the flow and liquid level fluctuate greatly, and the recording curve is like the Great Wall. It can be seen that if this system is used in this system The control method is very difficult for the system's on-site operation and data recording. Therefore, it is necessary to find a suitable control method to solve the current process.


Second, the implementation of cascade uniform control system due to the uncertainty of liquid level changes, if you want to completely balance the liquid level and flow, it is almost impossible, if you use single-channel control often take care of this, can not solve the fundamental problem, This specifically introduces cascade uniform control. The following describes the process of the system:


1. Process status:
The stability of the liquid level of the light ash section washing condensing tower is achieved by controlling the amount of liquid discharged from the bottom of the tower; therefore, its discharge amount is inevitably unstable. The discharge amount of the washing and condensing tower is exactly the feeding amount of the liquid liquid column of the steaming section; therefore, when the liquid level of the front tower is stabilized, the feed amount of the rear tower cannot be stabilized. On the contrary, if the feed amount of the rear tower is stabilized, it will inevitably cause the liquid level of the front tower to be unstable. That is to say, the requirements for the stability of the front tower liquid level and the rear tower feed amount are contradictory and cannot be guaranteed at the same time.


2. Exploration of instrument control scheme:
(1) One of the ways to solve this contradiction is to add a buffer tank with a certain capacity between the front and rear towers. However, this requires an additional container equipment, resulting in increased investment, increased floor space, and increased energy consumption for liquid transport. Especially serious is that the condensate residence time is long, it will crystallize in the pipeline and the buffer tank, and the material is interrupted and other adverse consequences, obviously taking into account the overall quality of the system can not use this method!
(2) The second method to solve this contradiction is to implement a cascade-level uniform control system. The output of the liquid level controller is used as the set value of the flow controller, and the output of the flow control is used to operate the actuator. Since the secondary circuit is added to the system, the pressure change in the tower or the discharge end can be overcome in time. The flow changes. This forms a uniform control system with the characteristics of a cascade control system. For this system, the AI-808A2X3L2L2S4 instrument of Xiamen Yudian Automation Technology Co., Ltd. is used to form a cascade-level uniform control system.

3. Principle of cascade-level uniform control system:
Cascade uniform control is a structure that utilizes cascade control. The core of the ratio control introduces a flow signal to form uniform control while controlling the liquid level. The control flow chart is as follows:

Figure 2: Block diagram of cascade-level uniform control system

4. Determination of the positive and negative effects of the main control and sub-control instruments of the cascade-level uniform control system In this system, the instrument models used by the main control and the sub-control are all AI-808AX3L2L2S4; in the whole control system, the positive and negative effects of the control instrument are determined. It is also very important; for the main control AI-808A instrument, the rise and decrease of the liquid level directly affects the increase and decrease of the flow rate, that is, the flow rate controlled when the liquid level rises needs to be increased, and the flow rate when the liquid level is lowered Need to reduce, so the main control instrument should be positive action control, set the instrument when the CF=1 meter will be positive action control. For the sub-control instrument, when the output of the main control increases, the set value SV of the sub-control increases, and in order to increase the flow rate, the output of the meter must also increase, so the reaction adjustment adopted by the meter, the CF parameter is set to 0.


5, the debugging process of cascade-level uniform control is introduced in many books. In the debugging of cascade-level uniform control, the master control and the secondary control are first debugged in the manual state, and then debugged one by one from the master to the slave. Although the method can get satisfactory results, it takes more time to debug; according to my experience in on-site debugging, the sub-control can be manually operated in the whole process, which is also convenient for obtaining the control parameters of the main control instrument. The reason why the cascade-level uniform control system can coordinate the relationship between the two variables is achieved by setting the control parameters. In the cascade-level uniform control, the parameter setting is not for the variable to return to the given value as soon as possible. Instead, the variables are required to make slow changes within the allowable range. The testing of the parameters is also different from the general system. The general system is to adjust the parameters from large to small, the cascade control system is just the opposite, there is a small to major. Moreover, it is necessary to pay special attention to the fact that in the cascade-level uniform control system, the controller generally adopts pure proportional control. When the requirement is relatively high, an appropriate integral action is introduced to prevent the deviation value from being too large and exceeding the allowable range. After the sub-control instrument is manually operated, the output flow can be controlled according to the measured value of the main control instrument, and the relationship between the main control liquid level and the auxiliary control flow, the relationship between the flow change and the manual output of the sub-control can be recorded to set the main control. The ratio of the value to the sub-control instrument.


6. Parameter setting of AI-808AX3L2L2S4 Main control instrument parameter setting: HIAL=600 LOAL=200 CtrL=4 M5=0 P=8 T=0 CTL=4 Sn=33 DIP=1 dIL=0 dIH=800 CF=1 Addr=1 bAUd=9600
Parameter setting of the sub-control instrument: HIAL=700 LOAL=100 CtrL=4 M5=0 P=4 T=0 CTL=2 Sn=32 DIP=1 dIL=0 dIH=900 CF=8 addr=2 bAUd=9600
The above parameters are obtained after the simulation of the scene, after the actual use of the scene observation, fine-tuning results. After a period of trial use by light ash and steaming process personnel, the effect is good, see Figure 3 (after cascading uniform control):

Figure 3: (Cascade uniform)

This is a plot of condensate flow and liquid level history operation taken from one day; blue is the flow curve and black is the liquid level curve; it can be seen that the flow and liquid level are both gentle and stable, and the light ash and steaming sections are two sections. The material process is smooth! In particular: the light ash process personnel used a double uniform adjustment system to reasonably control the condensate material balance, save energy, lay a foundation for high-soda production, and reduce labor intensity!

III. Conclusion After the introduction of cascade-level uniform control in the whole system, the stability of the system has been greatly improved, effectively solving the unnecessary losses caused by excessive system fluctuations, and better saving human resources.

Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn

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