In recent years, due to the country’s development and utilization of natural gas fields, natural gas has played a huge role in promoting my country’s economic development and has promoted the development of related industries. Because natural gas has high calorific value, clean, pollution-free, non-toxic, and convenient transportation, it has been Called green energy, environmentally friendly energy.

With the increase of fuel costs and the enhancement of human environmental awareness, compressed natural gas (CNG) for vehicles is widely used in urban taxis, buses and private vehicles. At present, a standard CNG filling station is mainly composed of pressure regulating and metering, purification system, compression system, gas storage system, and CNG gas sales system.

The natural gas compressor unit composed of the compressor and its driving motor and related equipment is the core part of the entire process system of the natural gas filling station, and it is also one of the important equipment. The control system of the compressor unit is the central pivot for the safe operation of the compressor, which controls the normal operation, automatic protection, and sequential charging of the compressor unit.

The superiority of the compressor unit control system directly affects the safe operation of the compressor unit, and directly affects the output and benefits of compressed natural gas production. The main factors that affect its safe production are compressor over-temperature, over-pressure, water cut, oil cut and other factors.

With the rapid development of the gas filling station network, the requirements of gas filling stations for equipment are getting higher and higher, and the construction of intrinsically safe filling stations has become the core of future gas filling station production and construction. Equipment is constantly updated and improved. Reliability, ease of operation, monitorability, and ease of maintenance are the most basic requirements. The control circuit built with relays has poor reliability, is not easy to maintain, and is not easy to monitor, and can no longer meet the current requirements. There are also some hidden dangers to safe production. With the rapid development of electronic technology, software technology, and control technology, programmable controllers (PLCs) have developed rapidly, with high performance and relatively reasonable prices. The control circuit built with relays has a very large advantage and has been widely used in natural gas compression. On the unit, the safety and stability of the equipment are greatly improved.

(2) The role of PLC in natural gas compressors

Programmable controller (PLC) is a new type of general control device, which integrates traditional relay control technology, computer control technology and communication technology. It is specially designed for industrial control. It has strong functions, general flexibility, and reliability. Strong, good environmental adaptability, simple weaving, easy to use, small size, light weight, low power consumption and a series of advantages.

1. Natural gas filling station process flow

After the raw material gas is pressure-regulated and dried, the natural gas enters the compressor, which is compressed and pressurized to 20MPa, and then inflated into the high, medium, and low-pressure underground gas storage wells through the sequence control panel. After the high-pressure underground gas storage wells are fully filled, the compressor Automatically stop running.

When CNG vehicles come to the station for filling, the high-pressure natural gas in the high-pressure underground gas storage well is used to fill the CNG vehicles through the gas vending machine. When the natural gas in the high-pressure underground gas storage well is too little to satisfy the filling of CNG vehicles, the compressor starts, and the high-pressure underground gas storage well is filled while satisfying the CNG vehicle filling, until the high-pressure underground gas storage well is fully filled. The compressor automatically stops running.

2. PLC control function and control principle in natural gas compressor

Using a programmable controller (PLC) as the core controller, through the detection instrument to provide the PLC with the signal parameters needed in the control to perform automatic circuit detection and control of the compressor. The main parameters to be monitored are compressor intake pressure, compression cylinder temperature, lubricating oil pressure, temperature, motor temperature, circulating water pressure, temperature; compressor discharge pressure, gas storage well high, medium, and low pressure detection parameters.

(1) Protection control function: ①Detection of motor current and voltage; ②Detection of oil temperature and oil pressure of one, two, three and four-stage cylinder; ③Detection of exhaust temperature, pressure and motor temperature of one, two, three and four-stage; ④Inlet pressure detection ⑤Instrument wind pressure detection; ⑥Circulating water temperature and pressure detection; ⑦Gas storage well pressure detection;

(2) Protection control principle Before starting the main engine, open the intake valve and control the pressure to the normal starting pressure. First, open the oil pump to lubricate the compressor cylinder. After 10s, the water pump starts. When the starting conditions are met, there is no fault. Signal, the indicator light on the control panel that allows the start of the machine will be on, and the compressor can be started. After the start button is pressed, the PLC receives the instruction and starts to execute the running program. The motor starts with no load. When the motor runs normally for 10 seconds, the intake solenoid valve opens, the first, second, third, and fourth stage valves of the compressor are energized to open, and the compressor runs normally. PLC analyzes and processes all on-site collected data, and monitors whether each data is abnormal.

When the water temperature exceeds the set value, the fan starts to cool the circulating water. When the compressor is running and one of the monitored parameters fails within the set range, the PLC will issue an alarm command to remind the staff to pay attention to abnormal parameter changes. When the monitoring process data reaches the stop condition, the PLC will immediately issue a stop command. After the compressor receives the PLC stop command, the compressor's main motor will stop working, and the feed gas inlet valve will be closed at the same time. Compressors one, two, three, The valve between the four stages is de-energized and closed. At the same time, the compressor shuts down and the pressure relief valve is energized to open, and the pressure between the stages of the compressor is discharged from the compressor. At this time, the compressor oil pump, circulating water pump, and fan will work intermittently. After the oil temperature and water temperature of the compressor are reduced to normal values, they will automatically stop working.

Compressor operation instructions are all issued by the controller PLC, which essentially guarantees the safe and stable operation of the compressor.

(3) Summary

The successful application of PLC technology on natural gas compressors not only solves many problems existing in the natural gas compressor system, but also makes the entire system configuration of natural gas compressors complete and reasonable, safe and reliable in operation, simple and convenient to maintain, and will be further extensive in the future. The application has accumulated successful experience.