PLC Application In Expander-Compressors For Natural Gas

The Expander Compressor machine is the key equipment of natural gas processing plant. It is used as a source of refrigeration to extract the ethane and natural gas liquids from natural gas. At the same time it produces energy to drive the compressor which compresses the processed sale gas to a higher pressure for transferring to the customer. In normal condition, the yield of propane production is about to 95%. The Expander Compressor machine is a mechanism that always works in the extremity of high rotor speed, pressure and gas flow. The auxiliary system and control device are very complicated. And in process industry, if such a key unit as Expander Compressor emergency shutdown, it will cause a huge lost. So the Expander Compressor machine requests high performance of instrument control system. How to make the instrument control system not only with highly reliability, stability, sensitivity but also with convenience to operation and maintenance is always a difficult question for engineers and technicians.This dissertation paper takes the Expander Compressor machine of Shanghai natural gas processing plant as a study object. According to the control specification of Expander Compressor and serious analyzing of the function and control logic as well the field operating procedures of Expander Compressor machine, SLC Series PLC control system from Allen Bradley company is taken to implement the process control and emergence shutdown system. At the field site, there is a PANEL MATE 1175 touch screen to be the Human Machine Interface (HMI). PLCs are well adapted to a range of automation control tasks and have the facility for extensive input/output arrangement.All control strategy can be expressed to make logic in simple ladder logic which appears similar to electrical schematic diagrams. It is easily accomplished the control function by using ladder logic so that it will reduce training cost of technicians. Because PLCs are designed for immunity to electrical noise, and resistance to vibration and impact, it has a great ability to ensure the safety and stability of mechanism running.