The measurement and control system of the methane gas expansion compressor unit adopts the intelligent control instrument, PLC and unit condition monitor as the lower control level, and the industrial control computer as the independent distributed control system (DCS) of the upper monitoring operation station. Since the control and interlocking alarm functions of each part of the expansion compressor unit are respectively undertaken by the lower intelligent instruments, each instrument can be operated independently from the upper computer, and the automation and instrument can be highly dispersed. Even if the upper computer fails, the lower instrument can still guarantee the system. Normal operation, from this point, the PC/DCS system has better security and redundancy than the traditional DCS. The modules in the system are highly dispersed, using universal instruments, universal buses and protocols, which not only improve maintainability, but also are easy to expand and upgrade, and enhance the openness of the system.
When the inlet pressure of the turboexpander or the load of the compressor changes, the speed control system controls the turbine expander to maintain the set speed at the set value by changing the opening of the turbine expander inlet adjustable nozzle. The adiabatic efficiency of the expander remains at its highest value. The realization of the speed control system. The speed regulator adopts SDC-40 type intelligent regulator. The setting value of the speed can be set in the instrument panel. It converts the measured speed to 420mA current signal output, and is sent to the tracometer of the process site through the safety barrier. After entering the speed regulator, the speed regulator performs the PID calculation, controls the angle adjustment mechanism of the inlet of the expander, changes the intake air amount of the expansion end, and controls the rotation speed to stabilize at the set value. Realization of Speed ​​Control Under normal circumstances, the surge of the compressor is mainly caused by the load reduction, and the increase or decrease of the load is determined by the process.
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