With PLC and inverter control of the gas Compressor

Publish：2010-08-03

Sinopec Group has two sets of a gas compressor, single rated power 75kW, opened a way prepared to run, but only in the actual production output of about 45kW. Compressor operation below rated conditions, the load is low, and its size depends on pressure and flow regulating valves manually, operational difficulties, but also wasted much electricity. To this end, consider the transformation of frequency conversion technology, with PLC automatic adjustment and various control functions. Operation proved that the program is stable and reliable economic effect is obvious. A technical requirement
(1) the normal production process, the two compressors should have at least one computer running, even when switching between, the two machines are not allowed to stop the phenomenon of all.
(2) maintain the compressor outlet pressure at a predetermined value.
(3) to achieve the analysis of the compressor running, in order to achieve predictive maintenance.
Second, the system control theory
(1) process specialties normal export pipeline compressor pressure P1, while the actual measured pressure field P2, according to ΔP (= P2-P1) values within the PID function of the size of the PLC module PID computation, control inverter to change motor speed to reach the required pressure. When ΔP> 0, the on-site high pressure, then increase the output frequency to accelerate the motor speed and improve the actual wind pressure; when ΔP <0, the field pressure is low, so speed is reduced, ΔP decreased. This continuously adjusted, so that ΔP tends to 0, the scene near the actual pressure fluctuations in the set pressure, the pressure to ensure stability. System architecture shown in Figure 1.
 

(2) long-running compressor, resulting in various parts of larger space, so the vibration will cause more and more, likely to cause damage to various components of the compressor. By the PLC to judge the situation on the spot analysis of vibration can be planned in advance of the compressor maintenance, this would greatly extend the service life of equipment, improve equipment reliability, reduce equipment failure caused by unplanned stop.
Three design
The program mainly by a Taiwan Siemens ECO1-7500 / 3 converter, 1 S7-200 type PLC (CPU215/216, supporting EM235 expansion module) and the contactors, buttons, an on-site pressure transmitter and 2 sets of vibration measuring device (vibration transmitter), with the compressor outlet pressure with the PLC to achieve single-loop closed-loop PID control, and compressor start, stop, switch, fault handling and other electrical control functions, by the vibration of the compressor state transmitter monitoring and analysis, in order to achieve predictability dimension
Roadway Maintenance. Main circuit in Figure 2.

(1) PID operation function of the realization of
S7-200 series CPU215/216 with 32-bit floating-point operation instructions and built-in PID instruction and other special features adjustment. Use, simply fill in the PLC memory, a PID control parameters table (see table below), then the implementation of directives: PID TABLE, LOOP, to complete PID operation. Which use a variable memory operand TABLE VBx, used to specify the starting address of the control loop; operand LOOP is to control the ring number (constant, 0 ~ 7). Numbered 2,4,5,6,7 parameters fixed, the main program can be set in the PLC; numbered 1,3,8,9 parameters with real-time, to fill in the call when the PID instruction.

As the S7-200 input and output switching capacity, while the transducer, pressure transmitter and vibration signals for the analog transmitter, the EM235 module to achieve the D / A converter. An EM235 module to extend three analog input channels (pick one way pressure signal, two vibration signals) and an analog output channels (connected to the inverter).
(2) start
M1 and M2 have two starting mode, can switch select frequency / frequency start method.
(3) run
Normally, the motor M1 in the frequency control state, the motor M2 is in shutdown standby. Pressure transmitter site pressure testing the pipe network (4 ~ 20mA analog signal), and compared with the predetermined value, exclusively for internal use by the PLC PID instruction for operation, get the desired frequency signal frequency converter, automatically adjust motor speed to reach the required pipeline pressure.
(4) Stop
Press the "stop button", PLC control of all contactors disconnect, inverter stopped working.
(5) Switch
When the need to switch from the motor M1 to M2, the contactor KM2 disconnect, KM1 closed, this time working in the frequency motor under the M1, the inverter completely stopped, KM4 closed, inverter to re-start, the motor M2, inverter drive Next start; completely after starting, KM1 disconnect, motor M1 stops, switching operation is complete. M2 M1 motor switch to a similar process.
(6) alarm and fault diagnosis
Through the PLC internal procedures set alarm and interlock protection, a breakdown of the corresponding action to immediately stop and alarm. For the fault diagnosis function, taking into account the cost, not design PC, only to set the appropriate fault code, by four digital display, so that maintenance personnel can easily find fault information to the point of failure. Such as: (a) compressor oil pressure, water pressure and other fault signals, explosion-proof electrical contacts can be on-site measured directly sent to the PLC, the PLC controls achieving sound and light alarm and delay stop; (b) additional on-site vibration sensors, the signal sent to the PLC, the status display on the compressor and diagnosis.
4 Experience and design issues to be aware of
(1) variable frequency control, the realization of a compressor soft start, reducing the starting current of the power of the impact; energy-saving effect is obvious, one year, the investment will be.
(2) by PLC, the composition of closed-loop automatic control system for automatic adjustment, more stable and reliable operation.
(3) inverter, PLC, contactors, etc. can be installed on a control cabinet, or in respect of remote control operation, simple and flexible.
(4) a better system scalability. If more than one compressor in the frequency / frequency power supply to run, simply lead to increased information or signal to the PLC, you can control the entire system; if production needs, the system can also facilitate access to DCS or host computer, the establishment of human-machine interface control system.
(Five) predictive maintenance inspection can be greatly extended compressor life, improve reliability, reduce parking losses, lower operating costs.
(6) PLC control motor frequency / frequency switch mode power supply, to ensure close and disconnect the contactor and sufficient delay in order to prevent the motor winding voltage induced load to the inverter output inverse change on the bridge, causing damage.
(7) PLC to achieve interlocking between KM2 and KM4, to prevent the two motors while starting frequency, frequency converter damage due to overload.
(8) by two motors for a short time at lower frequency and frequency to run, so the total frequency conversion control cabinet power switch Xuan 2 sets of motor load to consider.