Air compressor variable frequency constant pressure gas supply reform based on distributed control system

Innovative little fis high-tech domain water industry based on the distributed control system of air compressor variable frequency constant pressure gas supply transformation Zheng Dejin Sun Jianhua Yang Xu (Hebei Iron and Steel Group Jixian Sijiaying Iron Mine Co., Ltd., Hebei Tangshan 063701 for frequency conversion transformation, realized Air compressor PID automatic constant pressure regulation, energy saving effect is remarkable.

There are two screw air compressors in the Sijiaying Iron Mine Thermal Power Plant, which provide the purge gas source for the ash removal system. It is an important equipment for boiler operation, and its operation status directly affects the normal production of the workshop. In response to the above problems, the Sijiaying Iron Mine Thermal Power Plant reformed the air compressor control system to achieve energy saving and efficiency enhancement.

1 screw air compressor working process and problems 1.1 screw air compressor working process screw air compressor working process as shown: 1 oil cooler I screw air compressor working flow chart as shown, air through the air The filter filters out the groceries, dust, etc., and enters the compression chamber through the suction regulating valve, mixes with the cooling lubricating oil that is invaded at this time, and is compressed by the electric motor into a high-pressure mixed gas, and enters the oil/gas separation through the oil storage tank. After a series of collisions, gravity centrifugation, etc., the air is separated from the oil. The air enters the gas storage tank through the air cooler to supply air to the system, and the oil enters the compressor through the oil filter and the cooler to perform a new round of oil and gas. mixing.

1.2 Problems with air compressors (1) During the loading and unloading process of air compressors, the energy consumption is high, resulting in great waste. (2) The working environment of the air compressor room is harsh and the noise is high. (3) The degree of automation is low, the output pressure is manually adjusted by the regulating valve, the speed is slow, the precision is low, and the supply pressure is unstable.

2 air compressor frequency conversion transformation program 2.1 air compressor economic operation analysis According to air compression theory and asynchronous motor principle, the air compressor shaft power, displacement and shaft speed meet the following formula: According to the above theoretical analysis, in the air compressor Under the premise that the cylinder volume is constant, only the speed of the air compressor can be adjusted to change the displacement. Since the air compressor is a constant torque load, the air compressor shaft power and the speed change are proportional, so adjusting the air supply pressure by adjusting the air compressor speed is an effective method for the economic operation of the air compressor.

2.2 Energy-saving principle of the inverter It is known from the knowledge of the motor that the speed n of the asynchronous motor has the following relationship with the three parameters of the power supply frequency f, the slip ratio S and the motor pole pair p: it can be seen that the pole number of the motor is p- In the case where the slip S changes little, the rotational speed n is substantially proportional to the power supply frequency f, that is, the rotational speed of the motor can be changed by changing the power supply frequency. Therefore, the speed control function of the frequency converter can be used to adjust the speed of the air compressor to realize the economic operation of the air compressor to achieve the purpose of energy saving.

2.3 Distributed Control System Introduces the centralized decentralized control system based on the processor, which can centrally monitor, operate, manage and decentralize the production process, referred to as the DCS system. The system will disperse thousands of microcomputers into process control, and all information is monitored by the upper management computer through the communication network to achieve optimal control. The whole device inherits the advantages of conventional instrument decentralized control and centralized computer control, and overcomes the single function of conventional instruments. The shortcomings of poor human-machine connection and the high concentration of single microcomputer control system not only achieve the three aspects of management, operation and display, but also achieve the dispersion of functions, loads and risks.

2.4 Air compressor frequency conversion transformation scheme The constant pressure air supply control system is mainly composed of DCS, frequency converter, air compressor, pressure transmitter, etc., with the pipe network pressure as the control object. The designed control principle diagram is as shown: constant pressure The schematic system of the gas supply control system adopts negative deviation logic. The deviation e is: the actuator is a frequency converter, and the frequency is adjusted by the V/F control mode. The frequency is controlled according to the set PID control mode, thereby controlling the frequency of the frequency converter. Adjust the air compressor motor speed so that the air supply pressure is maintained at a given pressure to achieve constant pressure control of the air supply.

In order to operate safely, the original power frequency control mode and electrical mechanical interlocking are retained, that is, the power frequency and frequency conversion control modes can be switched. Under normal circumstances, the air compressor operates in the frequency control mode, if the inverter control system If a fault occurs, it will automatically switch to the power frequency operation mode. The electrical schematic diagram of the air compressor power frequency/frequency conversion operation mode is as shown: the electrical schematic diagram of the inverter air compressor is as shown in the figure. When QF1 and QF2 are closed, the air compressor is in the power frequency operation mode; when QF1 and QF3 are closed When the air compressor is in the variable frequency speed control mode.

Among them, QF2 and QF3 have an electrical mechanical blocking relationship, that is, the two cannot be closed at the same time.

3 Air compressor frequency conversion transformation implementation 3.1 Inverter selection selection The inverter capacity is the same as the driven motor capacity.

There are two Fusheng air compressors in the Sijiaying Iron Mine Thermal Power Plant. The model is SA-250A, the motor model is Y2-355M-4, the rated voltage is 380V, the rated current is 433A, and the rated power is 250kW. In order to better achieve the purpose of constant pressure gas supply, we chose two ABB fan pumps for single-drive variable frequency 3.2 constant pressure control system and PID debugging 3.2.1 constant voltage control system logic scheme. The constant voltage control system is based on a DCS control system, and its logic control scheme is as follows.

In this scheme, SELECT2 is the analog two-selection algorithm in the DCS configuration, that is, there are two air supply pressures, and a control object with a pressure of PID needs to be selected. When there is no manual selection signal, the algorithm automatically selects; when both pressure qualities are good, the average value is selected; when one of the pressure qualities is bad, the pressure of good quality is selected; when there is a manual selection signal, If the condition is met (the selected point is of good quality), the selection is made according to the manual requirements, and the point quality of the selected result is also sent.

LAG is the leading/lag link configuration algorithm. Its transfer function is: K gain coefficient T. The lead time (Sec) is mainly used to adjust the gain time of the two input signals of PID through TVT2 setting, and eliminate signal transmission and The delay of the operation process.

Force gas supply IS force (!

The regulator is offset by a reaction.

NORMA/M is a common type of soft hand-operator with limiter and adjustable bias. It is an interface manual operator for frequency converter speed regulation, which can realize automatic/manual switching of speed regulation, which can realize open loop of frequency converter. And closed loop control.

3.2.2 PID debugging of constant voltage control system. After the configuration is completed, the constant adjustment of the supply air pressure can be realized by setting the PID proportional coefficient, the derivative time, and the integration time. Based on the real-time curve, according to the experience of PID adjustment, the size of the three is set, and the amplitude of the curve and the reaction time are repeatedly modified. Finally, when the supply pressure is 0.7 MPa, the PID parameters of the normal operation of the system are : After the inverter is automatically put into operation, the gas supply system runs stably.

4 Conclusion With the continuous innovation and development of DCS system and frequency conversion technology, no matter from the perspective of energy saving or the promotion and application of automation technology, the frequency conversion of air compressor will be an inevitable trend, and the DCS system will be its preferred control system. .