Journal of Beijing Institute of Petrochemical Technology Natural Gas Compressor Automatic Control Technology Gao Wei, Liu Dejun, Wang Fu, Ma Wei, Li Xiaoyue, Gao Jiqing (Liaoning University of Petroleum and Chemical Technology, Fushun 113001, Liaoning) lost to the designated location. Since the production of associated gas in the Tanhai oilfield is relatively small, the natural gas compressor needs to start and stop frequently during the compression process.

In order to save energy, reduce costs, and improve the stability of machine operation, natural gas compressors can be automatically controlled. The author introduces a method for automatic control of natural gas compressors. This method can compress natural gas into high-pressure gas cylinders without manual control.

By describing the design principle, design scheme and some problems that should be paid attention to during the design, the automatic control research of high-pressure compressor is provided.

With the increasing energy shortage, the position of natural gas in energy is becoming more and more important. As a kind of natural gas, the associated gas in the beach oil field has gradually been valued by people. At present, because the economic benefits brought by its recycling are not obvious, most of them at home and abroad adopt the method of venting or burning to deal with associated gas. In order to increase the efficiency of the oilfield, an economical and rational solution should be designed to recover the associated gas. 1. 1NG recovery process Compressed natural gas (CNG) is to pressurize natural gas to about 25 MPa to reduce its volume. When the natural gas is at 25 MPa, the volume is 1/250 of that before compression. This process effectively utilizes the compression characteristics of natural gas and improves the storage and transportation efficiency. The pretreatment and compression process of CNG is relatively simple, the processing technology and equipment are mature, and the investment cost is moderate.

The process of recovering associated gas in Tanhai Oilfield by CNG technology: After the pre-treatment (desulfurization, decarburization, dehydration, dust removal, etc.), the associated gas is pressurized and then stored in a buffer tank (about 100m3), when the buffer tank pressure is reached. After a certain value, the compressor is started, and the gas is compressed by high pressure to about 25 MPa. The standard high pressure gas cylinder of 630 mm X 11580 mm is installed, the volume is about 2.82 m3, and the high pressure gas cylinder is transported by ship: 2013-03-20 to the destination.

After the recovery process is determined, considering that the output of the associated gas is small, the compressor cannot work continuously. A buffer tank can be set at the inlet of the compressor to store the gas. According to the gas pressure value in the buffer tank, the compressor can be automatically controlled. This not only saves human resources, but also improves the stability of the machine. The principle of the automation system is to automatically control the compressor and electric valve using the PLC control cabinet according to the change of pressure.

2 Automatic Control System With the development of technology, PLC technology is more and more mature, its flexibility and expandability are greatly improved compared with relays, and it is not easily affected by the environment. It has been widely used in the field of automation. Therefore, the author uses PLC and relay joint control, not only has the characteristics of compact structure, less electrical appliances, small size, easy maintenance, etc. It can also adapt the system equipment to the harsh working environment and reduce the cost.

2.1 Process requirements for system operation To ensure normal operation, the system should meet the following requirements: normal start and stop of the compressor and normal opening and closing of the electric valve; during normal production, ensure that at least 2 high-pressure gas cylinders can work normally; Check the health of the machine at any time.

The working principle of the control system of the Journal of Beijing Institute of Petrochemical Technology 2.2 The treated natural gas enters the buffer tank. When the pressure in the buffer tank reaches a certain value, the compressor starts, and the natural gas is compressed into a high-pressure gas cylinder for storage and transportation. When the high-pressure gas cylinder storing natural gas is full, in order not to affect the normal production, another high-pressure gas cylinder needs to be replaced for storage and transportation, and the compressor continues to work normally. When the buffer tank gas pressure drops to a certain value, the compressor stops working. After the pressure in the buffer tank reaches a fixed value, the compressor restarts and the above process is cycled. According to the above process, the author designs a set of schemes to make the whole system automatically cycle production without manual operation. As long as the timing is maintained, the equipment can be normally produced. The control principle of the system is as shown.

The operation principle of the control unit of the control tank is as follows: 2.3 The design scheme of the control system consists of electric contact pressure gauge, electric valve and PLC. The pressure change in the electric contact pressure gauge causes the control circuit in the PLC to automatically control the system to ensure the start and stop of the natural gas compressor and the switching of the electric valve. The engineering drawings of the scheme are as shown.

An electric contact pressure gauge capable of detecting the air pressure value in the buffer tank is disposed at the outlet end of the buffer tank, and one electric valve is disposed at the intake end of the compressor. Both the electric valve and the electric contact pressure gauge are connected to the control circuit in the PLC. When the electric contact pressure gauge detects that the natural gas pressure in the buffer tank reaches the preset pressure value, the control circuit controls the electric valve to open while the compressor is started. When the electric contact pressure gauge detects that the natural gas pressure in the buffer tank is lower than the second preset pressure value, the electric valve is closed and the compressor stops working. A check valve is used between the high pressure gas cylinder and the compressor to prevent backflow when the compressor is stopped. An electric contact pressure gauge is arranged near the intake port of each high-pressure gas cylinder, and the electric contact pressure gauge can detect the pressure value in the high-pressure gas cylinder. An electric valve is installed between the electric contact pressure gauge and the check valve, and the valve is controlled by the PLC control system. When the pressure in the high pressure cylinder reaches a preset value, the control circuit controls the electric valve corresponding to the high pressure cylinder to be closed, so that the other electric valve is opened.

2.3.1 Automatic start and stop of the natural gas compressor The automatic start and stop circuit of the compressor is as shown. When the electric contact pressure gauge connected to the buffer tank detects that the pressure value in the buffer tank reaches the preset pressure value, the upper limit contact P1 of the pressure gauge closes the switch 3, which is equivalent to pressing the start button 1 of the PLC control cabinet. The compressor is started; when the electric contact pressure gauge detects that the pressure value in the buffer tank is lower than the second preset value, the lower limit contact P2 of the electric contact pressure gauge causes the switch 4 to be closed, and at this time, the relay 1 is closed, and the normally open switch of the relay The closing of 1 to 1 is equivalent to pressing the stop button 2 of the PLC control cabinet, and the compressor stops working.

2.3.2 Automatic switching between electric valves The control motor of the electric valve is controlled by the PLC control cabinet.

The circuit for automatic switching between electric valves is as shown. KM is the contactor in the PLC control cabinet. When the compressor is started, the contactor KM normally closed contact 5 is closed, the relay 2 is energized, the normally closed switch 2-1 is disconnected, the normally open switch 2-2 is closed, electric The control switch K2 of the valve is opened and the natural gas enters the corresponding pressure vessel through the valve. When the electric contact pressure gauge connected to the pressure vessel detects that the pressure value in the pressure vessel is higher than a preset value, the electrical contact pressure gauge contact P4 closes the contactor normally open contact 6 and the corresponding normally closed contact 5 open. The relay 3 is energized, the normally closed switch 3-1 is opened, the normally open switch 3-2 is closed, and the other electric valve K3 is opened; at this time, the normally open switch 2-2 is disconnected, and the electric valve K2 is closed. This process loops during normal operation.

Point; a contactor KM normally closed contact; a contactor KM normally open contact; 2 - 1 a relay normally closed switch; 2 2, 3 2 - relay normally open switch between the electric valve automatic switching circuit 2.3.3 Automatic control of electric valve The control motor of the electric valve is a three-phase asynchronous motor, which is connected with the contactors KM2 and KM3. By controlling the contactors KM2 and KM3, the three-phase asynchronous motor three-phase input power is commutated, and then the positive control of the motor is controlled. Reverse, to achieve the purpose of automatic opening and closing of the electric valve. Its circuit structure diagram is as shown.

KM1 is the contactor in the PLC control cabinet. When KM1 is turned on, the normally open contact 1 of KM1 is turned on, the normally closed contact 2 is opened, the relay 4 is turned on, and then the contactor KM2 is turned on, and the normally open touch is closed. When the motor is de-energized, the normally open contact 1 is opened, the normally closed contact 2 is closed, the relay 5 is energized, the normally open switch 5-2 is closed, the KM3 is energized, and the normally closed contact KM3― 2 closed, the motor reverses and the electric valve closes.

3 The actual basis of the system design According to the actual gas production volume of the oil field and the local environment and other specific conditions, the compressor, buffer tank and pressure vessel should be selected. After the actual production requirements are met, the automation system design is carried out according to the parameters of each equipment; During the design process, the compressor should be prevented from starting and stopping several times in a continuous manner. If the number of times exceeds the specified number, the compressor cannot be produced normally. In addition, when the automatic control system is implemented, it is necessary to select appropriate hardware equipment according to the actual situation and working environment of the oil field, and then put into production after equipment installation, commissioning and setting.

4 Conclusions This paper mainly introduces a natural gas compressor automatic control system, which realizes natural gas compression and canning through the control of the compressor and electric valve through the PLC control cabinet. The compressor is prone to damage during long-term operation. The PLC can judge the operation of the machine and predictive maintenance and maintenance of the compressor before it fails. This can improve the stability of the machine operation, extend its service life, and avoid the normal production of the unit due to equipment failure. The system has the following advantages: reliable operation, quick response, high work efficiency;) can test compressors and other equipment to improve the safety of operation;) can reduce energy consumption and reduce unnecessary losses; (4) Less investment can reduce production costs and improve the economic benefits of oil fields.

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