I. Overview
At present, most of the equipment used in the domestic feed industry is the complete sets of equipment and complete solutions provided by Jiangsu Muyang Group. The Huanghai Grain and Oil Feed Factory also uses the products of the group. The overall use effect is good, but the shrimp stabilizer will have frequent pastes. Generally, the paste batch accounts for 1% of the total batch, which affects the yield and product quality. This problem can be effectively solved by adding an automatic temperature control system.
Second process introduction
Brief description of the station process: the semi-finished material of the shrimp material enters the stabilizer through the slip tube together with the humid air, and the steam enters the steam coil in the stabilizer through the pressure reducing valve, and is fully contacted with the wet material to achieve the drying purpose. The production process requires high quality of the product after drying, the moisture content cannot fluctuate too much, and the paste cannot be formed, so the drying temperature is strictly controlled. Practice has proved that the air temperature in the stabilizer fluctuates less than 2 °C up and down at 98 °C, and the control effect is ideal. Through our analysis and research, the main reason for the occurrence of paste is that the baking temperature fluctuates greatly and the steam flow is unstable. This is mainly because the control precision of the domestic pressure reducing valve is not very high, and the steam pressure after decompression always fluctuates. The original design was made according to the ideal conditions of constant pressure. A manual regulating valve was added to the main steam line, and the steam flow was controlled by manual adjustment, so that the temperature could not be controlled properly.
Three control object analysis
An automatic temperature control system was designed for this situation. Figure 1 is the schematic diagram of the control system. The system is a single-loop control system. The controlled parameter is the temperature of the stabilizer, and the control parameter is the amount of heating steam.
Figure 1 adjustment process is as follows:
Under normal circumstances, the steam enters the stabilizer through the manual valve and the regulating valve to heat the raw material. When the temperature in the stabilizer exceeds the set temperature, the regulating valve automatically reduces the opening degree under the control of the meter to reduce the heating into the stabilizer. The amount of steam causes the temperature to drop to the set value. Conversely, when the temperature inside the stabilizer is lower than the set temperature, the regulating valve automatically increases the opening degree under the control of the meter, increasing the amount of heating steam entering the stabilizer, and increasing the temperature. To the set value, this ensures that the temperature of the stabilizer remains within the set value range. The principle of the control function of the instrument is shown in the following instrument.
Four control system
The control system of the automatic control system can be divided into two parts: data acquisition and control. The controlled parameters are detected and transmitted to the control instrument. Then the control instrument analyzes the collected controlled parameters and determines the control according to the determined control law. The process, in time, sends a control signal to the actuator to complete the control. The basic block diagram of this control system is shown in Figure 2.
Figure 2 specific transformation situation:
1. Actuator: On the manual steam valve, a set of automatic regulating valve is added, and the original manual valve line is used as a bypass. Because the design requires the temperature to be controlled at 96--100 °C, the control accuracy of the control valve is very high. The temperature control valve we use is the Fischer valve produced by the German Fullermen Company. The valve has an electric-gas valve. The positioner can utilize the negative feedback principle to improve the positioning accuracy and sensitivity of the regulating valve, so that the regulating valve can be accurately positioned according to the control signal from the controller, and the control precision can reach 0.2%. The valve is in the industries of grain, oil, food, petrochemical, etc. The application is more extensive.
According to the process safety principle and the characteristics of the controlled medium, the air-opening and logarithmic flow characteristics are selected. In this way, the KV (regulating valve amplification factor) of the regulating valve is small at a small opening degree, the control is moderate and smooth, and the KV is large when the regulating valve is at a large opening degree, the control is timely and effective, and the valve is automatically closed when the power is cut off.
2, temperature measuring components and transmitters: because the controlled temperature is below 200 ° C, select the temperature and temperature components and transmitters produced by Germany E + H company, which is an integrated product, using two-wire signal output, 24V DC power supply, very convenient to use, the price is higher than the domestic similar products, but the price is higher. The temperature measuring component of the product is a platinum resistance thermometer, which has high processing precision, mature process, stable physical and chemical performance, high sensitivity and small zero drift of cold junction temperature in the temperature range.
In order to improve the detection accuracy, a three-wire connection method is adopted between the temperature measuring component and the transmitter, so that the connecting wires of the thermal resistance are respectively applied to the adjacent two bridge arms of the unbalanced bridge measuring line, so that the resistance change of the wire When it does not change the output of the bridge, thus reducing its impact on the meter reading, with a standard temperature transmitter, its output signal is 4-20mADC standard signal, support the international common HART protocol, you can use HART hands It is very convenient to set and correct the temperature measurement range and zero point. Because of its high precision, high reliability and easy maintenance, this series of products are widely used in domestic instrument control systems.
3. Controller and its parameter setting: The temperature controller adopts the WT-80 product produced by Hong Kong Shangrun Instrument Co., Ltd., which is a general-purpose instrument, which adopts advanced microprocessor for intelligent control and can automatically calculate according to the controlled object. The best adjustment parameters, optical isolation between each input / output loop, with good anti-interference and stability, accuracy of 1%, with a variety of products. This transformation uses products with input and output of 4-20mADC standard signals, and negative-action control. The controller parameter setting refers to the specific value of the controller's proportionality P, integration time TI, and derivative time TD. The essence of the tuning is to change the parameters of the controller, match its characteristics and process characteristics, to improve the dynamic and static indicators of the system, and to obtain the best control effect. According to the flow characteristics of steam, the system selects the PID control law, and the parameter setting adopts the 4:1 attenuation curve method in the engineering setting method. Firstly, the controller parameters in the process control system are set to pure proportional action (TI=∞, TD=0) to put the system into operation. Then, the proportionality P is gradually reduced from large until the equal amplitude oscillation response occurs. The proportionality at this time is 4:1 attenuation proportionality PS=35%; the time interval between two adjacent peaks is 4:1 attenuation oscillation period TS=10MIN; according to the 4:1 attenuation curve method formula: P= 80%*35%=28%, I=30%*10=3MIN, D=10%*10=1MIN.
Specific steps:
1. Check that the control valve circuit and the pneumatic system are connected correctly and operate reliably.
2. Control parameter setting: In the meter measurement value (PV) display state, press the SET button, and the meter will enter the control parameter setting state. Each time the SET button is pressed, the first level parameter is displayed in order.
(1) Parameter setting status The parameters are listed as follows: Symbol name setting range (word) Description Setting value AL1 First control value -1999—9999 Display first control setting value 100AL2 Second control value-1999-9999 Display second control value 97.5AH1 First alarm back difference value 0--255 Display the backlash value of the first alarm 1.0AH2 Second alarm back difference value 0-255 Display the backlash value of the second alarm 1.0CON Internal parameter CON= 0 Control output is PID control 0P Proportion with full scale display program Proportional band setting value 28I Integration time 0-1999 seconds Display program integration time setting value 600D Derivative time 0-1999 seconds Display program differentiation time setting value 180 in parameter In the setting mode, press the reset button, and the meter will enter the measured value display state after self-test again.
(2) Setting of control target value (SV): When the PV displays the measured value and the SV displays the target value, press and hold the SET button. After 4 seconds, the control target value SV is set. Press and hold the program increase button, the program will automatically increase by 1 until the display parameter target value is 98.0, press the SET button again, and the parameter will latch and exit the parameter setting after 5 seconds.
(3) Two-level parameter setting: In the parameter setting state of the instrument level, after modifying the setting parameter forbidden lock CLK=132, in the PV display CLK, SV display 132 state, press and hold the SET button and the program accumulation button at the same time 30 seconds, the meter enters the secondary parameter setting. In the state of secondary parameter modification, each time the SET button is pressed, a secondary engineering parameter is changed. In the parameter setting mode, without pressing any key, after 30 seconds, the meter automatically saves the parameter and returns to the measured value display state.
The secondary parameters of the instrument are listed as follows:
Symbol name setting range Parameter setting and description SL0 Input index number 0——20 13 (Note: PT100) SL1 Decimal point SL1=1 Decimal point in ten-digit SL2 First control mode SL2=2 Upper limit control SL3 Second control mode SL3 =1 Lower limit control F1 PID action mode F1=1 Reaction F2 PID output type F2=0 Current control output F3 SV (PV2) display mode F3=0 SV display control target value parameter setting is completed, the system can be put into operation. After the system is rebuilt, the operation is stable and reliable, and the comprehensive error is less than 1.5%, which fully meets the requirements for use.
Five conclusions
After more than half a year of use, the control effect is good, fully meet the requirements of the production process, the working temperature is always controlled between 98-100 ° C, the yield of shrimp material reaches 100%, according to the annual output of 10,000 tons of shrimp material, (per The loss of 4,500 yuan of ton paste can save the company an annual savings of 450,000 yuan, reaching the advanced level of the industry.
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