Compressor flexible control system and control method thereof

Abstract

The invention discloses a compressor flexible control system and method, and the system comprises a flexible control unit which is used for collecting the operation parameters of a compressor and generating a flexible control instruction; the flexible control execution mechanism is used for receiving the control instruction and adjusting the opening degree of an air inlet valve of the compressor so as to realize flexible control of the load; the hydraulic power unit provides power for the flexible control executing mechanism; the flexible control unit comprises a control box, a flexible controller FCU and an actuating mechanism controller ECU which are connected in sequence; the control box sends a control signal to the flexible controller FCU, the flexible controller FCU generates a control instruction, and the execution mechanism controller ECU serves as an intermediate control unit, sends the control instruction of the flexible controller FCU to the flexible control execution mechanism and receives an operation state feedback signal. According to the method, the problems of system pressure fluctuation, pressure ratio change of each stage and exhaust temperature rise under the condition of variable working conditions of the BOG compressor can be solved, and the operation reliability of the compressor is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of compressor control, in particular to a compressor flexible control system for BOG compressor of LNG receiving station and a control method thereof. BACKGROUND

[0002] BOG (Boil Off Gas) compressor is a key equipment in LNG (Liquefied Natural Gas) receiving station, mainly responsible for handling the gas generated by natural evaporation in the LNG storage tank, maintaining the stability of the storage tank pressure, and ensuring the safe operation of the receiving station, so it is figuratively called the "heart" of the LNG receiving station. According to the working principle and structural characteristics of the compressor, BOG compressor can be mainly divided into vertical and horizontal reciprocating compressors. In actual engineering application, it can be divided into high-pressure compressor and low-pressure compressor according to the pressure grade.

[0003] Reciprocating BOG compressor is the mainstream model currently used in many LNG receiving stations, and its working principle is based on the reciprocating motion of the piston in the cylinder to compress and recover the BOG evaporation gas, and realize pressure boosting and delivery. Taking a typical double-acting reciprocating compressor as an example, when the piston starts to move from the outside starting point position to the inside of the cylinder, the volume of the outside cylinder increases, and the expansion of the residual gas in the cavity causes the pressure to drop, resulting in a pressure difference with the intake cavity. When this pressure difference is greater than the intake valve spring force, the intake valve opens, and as the piston continues to move, the gas is sucked into the cylinder. The suction process is completed when the piston reaches the inner dead point, and then the return motion begins, at which time the suction valve is closed and the cylinder volume is continuously reduced, and the gas is compressed and the pressure gradually rises. When the pressure difference in the cylinder is greater than the exhaust valve spring force, the exhaust valve opens, and the compressed gas begins to be discharged. In order to ensure the stable operation of the compressor, the compressor usually has the characteristics of multi-stage compression, which can avoid excessive pressure ratio or high compression temperature of single-stage cylinder.

[0004] In actual operation, BOG compressor faces the challenge of complex variable operating conditions. For example, abnormal fluctuation of inlet pressure, zero output condition and unloading operation, etc. These special conditions put higher requirements on the performance and control system of the compressor. Fluctuation of inlet pressure will cause the compressor to shut down frequently, the system is unstable, and the efficiency of the compressor is reduced, and the energy consumption of the compressor is increased. The performance of BOG compressor under special conditions is directly related to the overall safety and operating flexibility of LNG receiving station. Therefore, in order to ensure the safe and stable operation of BOG compressor under complex and variable conditions, it is necessary to study the behavior of the compressor under the scenes of shutting down low-pressure pump, sudden change of external delivery, and fluctuation of BOG treatment capacity, and to propose an optimized control strategy and control method. SUMMARY

[0005] The application aims to: invent and disclose a compressor flexible control system for a BOG compressor of an LNG receiving station and a control method thereof, and solve the problem that the pressure and flow fluctuation, the pressure ratio and the exhaust temperature of the compressor exceed the standard under the variable working condition operation of the compressor adopting the prior art, which is harmful to the safe operation of the compressor.

[0006] The technical scheme of the application is: A compressor flexible control system comprises: A flexible control unit is configured to collect compressor operation parameters and generate flexible control instructions. A flexible control execution mechanism is installed on a compressor body and configured to receive the control instructions and adjust the opening degree of a compressor inlet valve to realize flexible control of a load. A hydraulic power unit provides power for the flexible control execution mechanism. The flexible control unit comprises a control box, a flexible controller FCU and an execution mechanism controller ECU connected in sequence, the control box is installed in a cabinet of a compressor site or a remote control room, sends control signals to the flexible controller FCU, the flexible controller FCU generates control instructions, the execution mechanism controller ECU serves as an intermediate control unit, sends the control instructions of the flexible controller FCU to the flexible control execution mechanism, and receives an operation state feedback signal.

[0007] Preferably, the flexible controller FCU adopts PLC and has an independent touch control screen, multiple input ports and output ports, the input quantities are compressor inlet pressure, inter-stage pressure of each stage and outlet pressure, discharge tank pressure and storage tank pressure, and the output quantities are compressor load control signals of each stage.

[0008] Preferably, the signal type of the compressor load control signals of each stage is a 4-20 mA control current signal.

[0009] Preferably, the flexible controller FCU is internally provided with flexible control algorithm software, the software contains a compressor load flexible control method realized by a PI proportional-integral regulator. The proportional gain Kc and integral time Ti parameters of the PI regulator can be automatically optimized and assigned intelligently without manual intervention according to the adjustment stability and adjustment time requirement of the compressor system.

[0010] Preferably, the flexible control algorithm software is configured as follows: the PI regulator of each stage of the compressor compares the current inter-stage pressure with specified set values, the set values are calculated according to the current suction pressure and exhaust pressure, balanced compression of two stages of the compressor is realized, inter-stage pressure flexible control is realized, and the change of the inter-stage pressure is prevented from being too fast and too large, so that the pressure ratio and the exhaust temperature of the compressor do not exceed the standard.

[0011] Preferably, the PI regulators at each stage include a primary inlet PI regulator, an inter-stage PI regulator, wherein: The primary inlet PI regulator is used to steplessly adjust the load of the primary cylinder by taking the primary inlet pressure Ps1 as the main control variable; The inter-stage PI regulator is used to steplessly adjust the load of the secondary cylinder by taking the inter-stage pressure Ps2 as the main control variable; The set value Ps2_set of the inter-stage pressure Ps2 is dynamically calculated according to the current primary inlet pressure Ps1 and the secondary outlet pressure Pd, and the calculation formula is: .

[0012] Preferably, the correction coefficient Ks2 ranges from 1.00 to 1.05.

[0013] Preferably, the flexible control actuator is installed on the compressor body and realizes the flexible control of the compressor load by controlling the opening and closing of the compressor inlet valve.

[0014] A flexible control method of a compressor, which adopts the flexible control system of the compressor, comprises the following steps: S1: Real-time acquisition of the primary inlet pressure, the inter-stage pressure and the secondary outlet pressure of the compressor through a sensor; S2: Generation of a load control instruction by the flexible controller based on the built-in flexible control algorithm according to the acquired pressure data; S3: Sending of the control instruction to the flexible control actuator installed on the compressor body by the actuator controller; S4: Adjustment of the opening degree of the compressor inlet valve by the flexible control actuator according to the instruction to realize the flexible control of the compressor load, so as to maintain the stability of the system pressure and the balance of the pressure ratio at each stage.

[0015] According to the present application, the BOG compressor has the design of flexible control when operating in a variable condition, which can solve the problems of system pressure fluctuation, change of pressure ratio at each stage and increase of exhaust temperature of the BOG compressor in a variable condition, reduces the operation difficulty of the operator and improves the operation reliability of the compressor. BRIEF DESCRIPTION OF DRAWINGS

[0016] The present application will be further described below in combination with the drawings and embodiments: Figure 1 The control system layout with the function of flexible control; The components include: 1. Compressor; 2. Flexible control unit (FCU); 3. Actuator control unit (ECU); 4. Hydraulic power unit (HPU); 5. Primary inlet pressure sensor (PTs); 6. Interstage pressure sensor (PT2); 7. Secondary outlet pressure controller (PTd); 8. Control box. Figure 2 This is a schematic diagram of the compressor flexible control method. Among them: 9. First-stage inlet PI controller; 10. Interstage PI controller. Detailed Implementation

[0017] like Figure 1 As shown, the present invention proposes a flexible compressor control system, comprising: The flexible control unit is used to collect the operating parameters of compressor 1 and generate flexible control commands; A flexible control actuator, installed on the compressor 1 body, is used to receive the control command and adjust the opening of the compressor 1 intake valve to achieve flexible load control. The hydraulic power unit HPU 4 provides power to the flexible control actuator; The flexible control unit includes a control box 8, a flexible controller FCU 2, and an actuator controller ECU 3 connected in sequence. The control box is installed in the cabinet of the compressor 1 on-site or in the remote control room, and sends control signals to the flexible controller FCU. The flexible controller FCU generates control commands. The actuator controller ECU, as an intermediate control unit, sends the control commands of the flexible controller FCU to the flexible control actuator and receives operating status feedback signals.

[0018] The compressor is also equipped with a primary inlet pressure sensor PTs 5; an interstage pressure sensor PT2 6; and a secondary outlet pressure controller PTd 7.

[0019] The flexible controller FCU adopts a PLC and has an independent touch control screen with multiple input and output ports; the inputs are compressor inlet pressure, interstage pressure and outlet pressure, unloading trough pressure and storage tank pressure; the outputs are compressor load control signals for each stage.

[0020] The signal type of the compressor load control signals at each stage is a 4-20mA control current signal.

[0021] like Figure 2 As shown, the flexible controller FCU has built-in flexible control algorithm software, which includes a flexible control method for each stage of compressor load implemented by a PI proportional-integral regulator; the proportional gain Kc and integral time Ti parameters of the PI regulator can be intelligently and automatically optimized and assigned values ​​without manual intervention according to the regulation stability and regulation time requirements of the compressor system.

[0022] The flexible control algorithm software is configured as: a primary inlet PI regulator 9 of the compressor, an inter-stage PI regulator 10, comparing the current inter-stage pressure with the specified set value, which will be calculated according to the current suction pressure and discharge pressure, achieving balanced compression of the two stages of the compressor, realizing flexible control of the inter-stage pressure, and avoiding too fast and too large changes in the inter-stage pressure, which will lead to excessive compressor pressure ratio and excessive discharge temperature.

[0023] The preferred embodiment of the present application.

[0024] For a BOG reciprocating compressor, the speed is constant at 495 rpm, the shaft power is 1564 KW, the primary inlet pressure is 0.13 Mpa (a), and the primary inlet temperature is -139℃; the secondary inlet pressure is 0.35 Mpa (a), and the secondary inlet temperature is -76℃. The secondary outlet pressure is 1.26 Mpa (a), and the outlet temperature is 25℃. According to the requirements of variable operating conditions, the compressor load is divided into five working conditions of 0, 25%, 50%, 75% and 100%, and the load adjustment is realized through the suction valve unloader of the compressor. However, the above five kinds of adjustment conditions cannot match the actual working conditions, and there is an adverse effect of increasing the compressor pressure ratio and increasing the discharge temperature, so an optimized control strategy and control method are needed.

[0025] The compressor of the embodiment is a two-stage compression, and each stage of the cylinder is provided with a flexible control actuator. The actuator can realize stepless adjustment of the cylinder displacement by controlling the opening and closing time of the suction valve. The actuator receives a control signal from a control unit ECU, and the control unit ECU receives a control instruction from a flexible control unit FCU to perform flexible control on the compressor load adjustment. The flexible control unit FCU collects the primary inlet pressure Ps, the inter-stage pressure Ps2 and the secondary outlet pressure Pd as control variables. The flexible control unit is provided with a primary inlet pressure PI regulator, which takes the primary inlet pressure as the main control variable and steplessly adjusts and controls the load of the primary cylinder according to the set value of the primary inlet pressure. The flexible control unit is also provided with a secondary inlet pressure PI regulator, which takes the secondary inlet pressure (inter-stage pressure) as the main control variable and steplessly adjusts and controls the load of the secondary cylinder according to the set value of the inter-stage pressure.

[0026] In the embodiment, flexible control system and control method are adopted to reduce the adverse effects of variable working conditions on the compressor as much as possible. The flexible control algorithm is set in the flexible controller FCU through control method software, the control method software includes the flexible control method software of the compressor load of each stage composed of PI proportional-integral regulator, and the software calculation has an intelligent optimization selection of the control parameters of the PI regulator without artificial intervention. In order to realize flexible control, the Kc proportional gain and integral time Ti of the PI regulator are automatically assigned according to the regulation stability and regulation time of the compressor system.

[0027] In the embodiment, the PI controller can automatically control the compressor load through the selection and setting of its controller parameters, integral time Ti and proportional gain Kc. The parameters are fine-tuned according to the system control requirements in actual operation after initial setting. The control strategy of the PI controller is defined as follows: , Wherein, k c is the proportional gain, Ti is the integral time, e is the control error, and y is the control signal.

[0028] In the embodiment, the current inter-stage pressure is compared with the specified set value by the first-stage inlet PI regulator and the second-stage inlet PI regulator. The set value is calculated according to the current first-stage inlet pressure and second-stage exhaust pressure to realize balanced compression of the two stages and avoid excessive pressure ratio or exhaust temperature of a stage during variable working condition operation of the compressor. In the control unit FCU, the calculation method of the set point is as follows: ,

[0029] Wherein, P s2 is the inter-stage pressure set calculation value, P s1 is the first-stage inlet pressure, P d is the second-stage exhaust pressure, and K s2 is the correction coefficient, which is 1.02 in the embodiment.

[0030] The above embodiments only exemplarily illustrate the principles and effects of the present application, and are not used to limit the present application. Any person skilled in the art can modify or change the above embodiments without departing from the spirit and scope of the present application. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the disclosed spirit and technical thought of the present application should be covered by the claims of the present application.

Claims

1. A compressor flexible control system, characterized by, The flexible control system comprises: a flexible control unit for collecting compressor operating parameters and generating flexible control instructions; a flexible control execution mechanism installed on the compressor body for receiving the control instructions and adjusting the opening degree of the compressor inlet valve to achieve flexible control of the load; a hydraulic power unit for providing power for the flexible control execution mechanism; wherein the flexible control unit comprises a control box, a flexible controller FCU and an execution mechanism controller ECU connected in sequence; the control box is installed in a cabinet in the compressor site or a remote control room, sends control signals to the flexible controller FCU, the flexible controller FCU generates control instructions, and the execution mechanism controller ECU, as an intermediate control unit, sends the control instructions of the flexible controller FCU to the flexible control execution mechanism and receives operating state feedback signals.

2. The compressor flexible control system of claim 1, wherein: The flexible controller FCU adopts PLC and has an independent touch control screen, multiple input ports and output ports; the input quantities are the compressor inlet pressure, inter-stage pressures of each stage and outlet pressure, discharge tank pressure and storage tank pressure; the output quantities are compressor load control signals of each stage.

3. The compressor flexible control system of claim 2, wherein: The signal type of the compressor load control signals of each stage is a 4-20 mA control current signal.

4. The compressor flexible control system of claim 2, wherein: The flexible controller FCU is internally provided with a flexible control algorithm software, which contains a compressor load flexible control method realized by a PI proportional-integral regulator; the proportional gain Kc and integral time Ti parameters of the PI regulator can be automatically optimized and assigned intelligently without manual intervention according to the adjustment stability and adjustment time requirement of the compressor system.

5. The compressor flexible control system of claim 4, wherein: The flexible control algorithm software is configured in such a manner that the PI regulators of each stage of the compressor compare the current inter-stage pressure with specified set values, which are calculated according to the current suction pressure and discharge pressure, to realize balanced compression of two stages of the compressor, realize flexible control of the inter-stage pressure and avoid too fast and large changes in the inter-stage pressure, which may cause the compression ratio and discharge temperature of the compressor to exceed the specified values.

6. The compressor flexible control system of claim 5, wherein: The PI regulators of each stage include a primary inlet PI regulator and an inter-stage PI regulator, wherein: the primary inlet PI regulator uses the primary inlet pressure Ps1 as the main control variable to steplessly adjust the load of the primary cylinder; the inter-stage PI regulator uses the inter-stage pressure Ps2 as the main control variable to steplessly adjust the load of the secondary cylinder; wherein the set value Ps2_set of the inter-stage pressure Ps2 is dynamically calculated according to the current primary inlet pressure Ps1 and secondary outlet pressure Pd, and the calculation formula is: 。 7. The compressor flexible control system of claim 6, wherein: the correction coefficient Ks2 has a value range of 1.00-1.

05.

8. The compressor flexible control system of claim 1, wherein: The flexible control execution mechanism is installed on the compressor body and realizes flexible control of the compressor load by controlling the opening and closing of the compressor inlet valve.

9. A flexible control method for a compressor, characterized by, The compressor flexible control system according to any one of claims 1-8 comprises the following steps: S1: collecting the primary inlet pressure, inter-stage pressure and secondary outlet pressure of the compressor in real time through a sensor; S2: the flexible controller generates load control instructions based on the built-in flexible control algorithm according to the collected pressure data; S3: the execution mechanism controller sends control instructions to the flexible control execution mechanism installed on the compressor body; S4: the flexible control execution mechanism adjusts the opening degree of the compressor inlet valve according to the instructions to realize flexible control of the compressor load, so as to maintain the stability of the system pressure and the balance of the pressure ratio of each stage.

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