Abstract : The automatic control system for sand mixing trucks, based on Siemens PLC-300 hardware, is designed for oilfield fracturing truck processes and achieves automatic control of parameters such as liquid level, discharge pressure, additives, and sand ratio. Through a user-friendly interface switchable between Chinese and English, users can input the set parameters for each stage according to actual needs. The system then runs automatically after startup, achieving precise control of all parameters.
Keywords : PLC; sand mixing truck; automatic control; system design
The sand mixing truck plays an extremely important role in the oilfield fracturing truck group. The control method and control accuracy of the sand mixing truck seriously affect the cost and efficiency of oilfield operations. At present, some traditional fracturing truck groups in China still use manual control for the sand mixing truck[1]. The liquid level, sand density, liquid additive ratio, dry powder additive ratio and discharge pressure are controlled manually. The manual control method has many drawbacks in actual operation. This control method is inefficient and the accuracy of parameters is difficult to guarantee. Because of the non-repeatability of oilfield fracturing operations, once the mixed fluid is input into the oil well by the sand mixing truck, whether the important parameters such as the sand density, liquid additive ratio and dry powder additive ratio of the mixed fluid meet the design requirements of fracturing operations will directly affect the quality of oilfield operations.
To ensure the quality of fracturing operations, this sand mixing truck's automatic control system, based on the truck's excellent mechanical performance, utilizes advanced automatic control functions to achieve computer-automated tracking of the sand mixing process. Sensors measure data from each module in real time and transmit it to the PLC. The PLC automatically controls each stage of the sand mixing truck according to the software-designed control algorithm, ensuring that parameters such as liquid level, discharge pressure, additives, and sand ratio all meet the set requirements, thus guaranteeing the quality of the sand mixing operation.
1 PLC Automatic Control System
1.1 Automatic control of liquid level in sand mixing tank
The liquid level in the mixing tank of the system is mainly regulated by controlling the speed of the suction pump, auger, and additive pump, as well as the opening degree of the suction valve. The liquid level is monitored in real time by an ultrasonic sensor. The PLC uses a PID algorithm to control the suction pump based on the difference between the measured actual liquid level and the set liquid level, adjusting the amount of base liquid drawn in by the suction pump to stabilize the liquid level. The principle of liquid level PID control is shown in Figure 1.
Figure 1 Schematic diagram of liquid level feedback control principle
In addition, the system has set upper and lower limits for both the maximum and minimum liquid levels. When the liquid level reaches the set limits, the auger, additive pump, and suction valve adjust accordingly to further ensure liquid level stability. This "double-insurance" automatic control method effectively controls the liquid level in the mixing tank within the specified range. A schematic diagram of the overall system level control is shown in Figure 2.
Figure 2 Schematic diagram of liquid level control
1.2 Automatic control of discharge pressure
The discharge pressure control adopts a fast control method with time tolerance. The control software controls the discharge outlet pressure by adjusting the speed of the discharge pump to ensure that the discharge outlet pressure is between the set upper limit and the set lower limit of the discharge outlet pressure [2]. If the discharge outlet pressure is higher than the upper limit or lower than the lower limit of the discharge outlet pressure, the control software adjusts the control voltage of the discharge pump at specified time intervals to ensure that the discharge pressure is within a suitable range.
1.3 Automatic control of additives
The control software determines the set additive addition rate based on the 20 stages of initial and final additive concentrations set in the job settings interface and the real-time suction flow rate measured by the flow meter. It then compares the set addition rate with the measured actual addition rate and uses a PID algorithm to adjust the control voltage of the additive pump. Furthermore, depending on whether the set initial and final additive concentrations are the same, the software can control the additive pump to add additives in stages or in a linear fashion, better meeting the needs of actual operations. A schematic diagram of the job settings interface is shown in Figure 3.
Figure 3. Schematic diagram of the job settings interface
1.4 Automatic control of sand ratio
The sand ratio of the sand mixing truck is calculated after measurement by a density meter on the discharge pipeline. Since the sand ratio and proppant concentration can be converted using a formula, the sand ratio can be controlled by setting and controlling the proppant concentration. The system software obtains the set sand conveying rates for auger 1 and auger 2 based on the proppant initial concentration, final concentration, auger speed ratio, and auger magnification set in the operation settings interface. The control software compares the set sand conveying rate with the actual sand conveying rate obtained from the sensor, and uses an incremental PID algorithm to control the auger's control voltage, thus achieving stable control of the sand ratio.
2 System Overall Design
The control system of this sand mixing truck adopts the Siemens S7-300 model, equipped with digital I/O modules and analog I/O modules, and corresponding sensors are configured in each hardware component. The automatic control system samples the input data in real time and sends it to the computer for software control. The software system calculates the set control parameters of each subsystem from top to bottom. Each subsystem adjusts its output signal through its own control algorithm, thereby controlling the actuators of each component. In automatic mode, after the completion of stage 1, it automatically enters stage 2, and so on. The overall system design structure diagram is shown in Figure 4.
Figure 4 System overall design structure diagram
3. Conclusion
The automatic control system for the sand mixing truck based on PLC-300 is feature-rich and easy to operate. For the first time, the liquid level control employs a "double-insurance" control method, combining coarse adjustment of the suction valve and suction pump with ultrasonic sensor detection and PID fine adjustment. Both coarse and fine adjustments ensure stable liquid level during sand mixing operations. The discharge pressure control method adapts to operational needs based on actual conditions. The incremental PID control method for additives and sand ratio is fast and accurate, ensuring the quality of the mixed sand. Furthermore, the system uses a human-machine interface for setup and monitoring, making operation convenient and safe, and better adapting to the development trend of sand mixing trucks.
References:
[1] Fan Xinqiang, Lou Guoxiang. Research on automatic control system of liquid level of sand mixing truck based on PLC control [J]. Petroleum and Mining Machinery. 2011 , 40(3) : 88~89 .
[2] Deng Binqi. Research on the development trend of sand mixing truck. China Petroleum and Chemical Standards and Quality [J]. 2011 , 31(7) .
