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The use of mass flow meters and PLC to achieve caustic soda automatic delivery


1 Problems with Manual Shipment

ionized membrane caustic soda with a production quality score of 32% and 48% is sold externally. The original car transporting alkali must first obtain the quality of the skin through the truck scale, and then install the alkali in the tank area, then the truck scale, calculate the net quality of the alkali, and finally leave the pounds to leave the factory. Alkaline delivery is performed entirely by manual operation. The operation method is as follows: The operator manually opens the process alkali valve, then presses the manual button to turn on the pump, and starts alkali generation. The alkali pipeline is equipped with a high-precision mass flowmeter, and the mass flowmeter will The flow signal is sent to the paperless recorder, and the operator judges whether the required delivery volume is achieved based on the accumulated flow value displayed on the paperless recorder. When the accumulated value is close to the delivery volume, the process alkaline valve is quickly closed to complete the alkalinity generation. operating.

This manual operation has the following problems. 1 can not accurately control the amount of alkali. Less equipment is required, and the second base is required to affect the work efficiency; if more equipment is installed, alkali will be spilled, which will not only cause economic losses, but also may threaten the safety of the surrounding workers. 2 The operator has a large workload, not only to operate the valve, the pump, but also to record the data of the paperless recorder, which is used to calculate the next generation of alkali. The three steps are cumbersome. After the alkali is completed, the pounds are also required to obtain accurate quality and cause repeated labor.

2 Implementation of automatic delivery

Based on the problems of manual delivery, Qingdao Haijing adopted the following solution: use the existing mass flow meter as a flow measuring element; increase the flow of alkali through the pneumatic switch valve on the pipeline; add a small PLC control system in the operating room, Connect the signals of various instruments, valves and pumps to the PLC system. The PLC is responsible for centralized processing of various signals sent from the mass flow meter and other fields, and controls the pumps and valves to perform corresponding operations according to the accumulated values to achieve automatic control, thus achieving The purpose of automatic delivery.

2.1 Mass Flow Meters and PLC Systems

The mass flow meter is a more advanced flow measurement instrument and mainly relies on the vibration measurement flow of the U-type flow tube in the sensor. The mass flow meter directly measures the quality without being influenced by the characteristic parameters of the fluid. The on-line density of the measuring fluid does not need to be periodically sampled by other technologies, and has the advantages of high measurement accuracy, high reliability, and low maintenance.

The automatic delivery system is relatively small, the total number of I/Os is about 20, and the amount of data calculation is not large, so the Siemens S7200 small PLC system was selected. The S7200 is a logical control device designed by Siemens for various small control systems. The system has a compact hardware design and many functions. It can handle signals such as current, voltage, pulse, digital, and thermal resistance. It contains bit logic, timers, complex mathematical operations, and various intelligent module communication instructions that can be used for various controls. The application makes the ideal solution. The S7200 supports host computer communication. You can use the WinCC software to display the variables, timers, and I/O status of the PLC on the host computer, or control variables and outputs.

2.2 Automatic Delivery System Design

The design idea of the control system is as follows. A mass flow meter measures the quality of caustic soda flowing through the pipe and converts it into a current signal for transmission to the PLC. 2 Install 3 pneumatic ball valves on the pipeline. Each ball valve is equipped with a solenoid valve on the cylinder. The solenoid valve receives the electrical signal from the PLC and controls the flow of alkali by changing the switch of the pneumatic control valve. Each ball valve has a position feedback remote transmission device, which can send the status signal of the valve to the PLC for logic judgment. 3 in the alkaline pump start circuit in series PLC switch signal, so that the PLC can control the start and stop of the alkaline pump; At the same time, the operation of the alkaline pump signal is fed back to the PLC system for logic judgments. 4 In order to prevent the failure of the PLC system and affect the delivery, a backup manual switch is added to the surface of the cabinet. When the operator confirms the PLC fault, the delivery control can be performed through the manual switch.

The schematic diagram of the system structure is shown in Figure 1.

2.3 Automatic Delivery Process

PLC automatic delivery interface shown in Figure 2.

Taking 32% caustic soda as an example, the operator first puts the “hand/automatic” switch on the cabinet surface into the “automatic” position, and then enters the amount of alkali generated in the “set quality” column of the “shipping parameters” page. After the value is correct, click "Start Shipping" button, PLC automatic delivery program starts to run, first open the return valve, PLC receives the return valve to open the feedback signal, it will issue an open pump instruction, after the pump starts, the alkali in the alkaline storage The circulation between the tank, pump and return valve ensures that the pipeline is filled with lye, which can reduce the measurement error of the mass flowmeter. After circulating for 10s, the PLC issues an instruction to open the charging valve 1, delay open the valve 2, and close the return valve. The flowmeter starts to measure, and the mass flowmeter converts the measured flow into a 4-20mA current signal and sends it to the PLC system. The PLC converts the current signal into a digital signal and accumulates it. At the same time, it compares the accumulated value with the delivery "settings. "Quality", when the "secondary cumulant" and the "set quality" difference between 600kg, open the return valve; when the "secondary cumulant" and "set quality" difference between 300kg, off the valve 2; When the “secondary cumulant” reaches the “set quality”, the feed valve 1 is closed, the pump is stopped, and the return valve is closed for a delay. The shipment is completed.

To prevent violent vibration of the pipeline and affect the accuracy of the mass flowmeter, when the pump is started, the PLC system is interlocked by the program to ensure that the valve is opened in advance; when the pump is stopped, the valve must be closed for a delay; in the event of a fault, to prevent equipment damage, increase Some protection chains, such as all valves without feedback, prohibit the start of pump chaining; the difference between “secondary cumulant” and “set quality” can be modified according to the actual delivery time to reduce delivery time. Reduce energy consumption. All of the above can be applied to the PLC system to modify and set.

2.4 Data Storage and Printing

Prior to the application of automatic delivery system, the tanker had to go to the truck scale twice to obtain the skin quality and hair quality. Then the net weight of the alkali was calculated by the weighing software of the ground balance, and the important data such as the receiving manufacturer and the vehicle number were obtained. Stored together, the clerk prints the stored data into a pound sheet, hands it to the driver, and the driver goes out with a pound single.

The data storage and printing of the automatic delivery system are realized through WinCC's OPC, SQL functions and VB. OPC (OLEfor Process Controll - Data Communication Technology for Process Control) is an industry standard developed jointly by the world's leading automation and software and hardware companies and Microsoft Corporation. Simply put, users who use the Microsoft operating platform can use DCS. The data of the process control system such as PLC and PLC is read out by the third party software and used for statistics, inquiry, and processing.

The storage and printing interface is shown in Figure 3.

First use the VB production program, the program refers to the Siemens plug-in, you can read data from the WinCC OPC interface, in order to ensure the accuracy of the data, all data is set to update once every 0.5s, in the program to join the SQL database insert , modify, read, and other commands, perform various operations.

When creating the documents on the WinCC monitor screen, the quality is automatically accumulated by the PLC according to the signal of the mass flowmeter, and no manual input is required; the data such as the vehicle number, base unit name, and delivery order number require the operator to follow the financial plan. The data on the single is manually entered. In order to facilitate the operation, the base unit name and the vehicle number will be used to encode and store in the table specified by the database. If the operator inputs the code, the VB program will automatically read out the corresponding unit name and car number from the database for storage or printing. .

After each shipment is stopped, the operator must check the entered data and store it without error. At this point, the VB program starts reading the data input by the operator from WinCC and then stores the data in the table specified by the database. After the storage is completed, click the Print button. The VB program reads out the newly stored data from the database and prints it in the specified format.

The operation and printing process is shown in Figure 4.

After many times of debugging and modification, the quality of the automated delivery system calculation is exactly the same as the weight of the weighbridge, and the accuracy is higher. It can be accurate to 1kg (accuracy to 20kg), and the document storage and printing program can also run stably for a long period of time. Now it has completely replaced the manual production of alkali, pounds, with the pound single factory operation process.

After the automatic delivery system is put into operation, the labor intensity of the operator is greatly reduced; the driver no longer has to worry about overdose of alkali causing safety accidents, and it does not need to line up in pounds, saving time. According to the actual use situation and the company's needs, through the network connection to the host computer database and financial database, real-time data transmission, and preparation for Qingdao Haijing ERP network in advance.

3 Conclusion

The automatic delivery system has been operating stably for more than one year, which not only simplifies the process of caustic soda delivery, improves work efficiency, improves the working environment, reduces the labor intensity of operators, and improves the safety of liquid caustic soda filling process. It also eliminates man-made interventions in liquid-alkali ex-factory data, improves metering accuracy, reduces trade frictions, and is worth promoting in the chlor-alkali industry.

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