LNG reliquefaction control system and method

Abstract

The present invention provides an LNG reliquefaction control system and method. The system includes an LNG skid, a signal acquisition module, a main control module, at least one motor power module, at least one magnetic levitation control module, and a valve control module. The LNG skid is equipped with at least one companding expansion unit. The main control module is configured to execute a system control program and, based on sensor data from the signal acquisition module, control the corresponding companding expansion unit to operate according to preset values, thereby reliquefying BOG in a ship's cargo tank into LNG. This system avoids the waste caused by direct discharge and combustion of BOG while ensuring that the temperature and pressure within the cargo tank remain within appropriate ranges.

Description

Technical Field

[0001] The present invention relates to the technical field of gas reliquefaction, and in particular to an LNG reliquefaction control system and method. Background Art

[0002] Liquefied natural gas (LNG), primarily composed of methane, is produced by cooling natural gas at atmospheric pressure to approximately -162 degrees Celsius, condensing it into a liquid. Liquefied natural gas significantly saves storage and transportation space and boasts high calorific value and superior performance. However, the low-temperature LNG stored in LNG tanks generates large amounts of flash gas (BOG) during storage and transportation due to heat exchange with the surrounding environment. To ensure equipment safety, the resulting BOG is typically discharged directly into the atmosphere, resulting in wasteful use of resources, environmental pollution, and potential safety hazards.

[0003] Therefore, there is an urgent need for an LNG reliquefaction control system that can effectively recover BOG in the LNG storage tank area, reduce BOG emissions, and avoid resource waste and environmental pollution. Summary of the Invention

[0004] The present invention provides an LNG reliquefaction control system and method, which are used to solve the problem in the prior art that BOG is directly discharged into the atmosphere, causing resource waste, environmental pollution, and potential safety hazards.

[0005] In a first aspect, the present invention provides an LNG reliquefaction control system, the system comprising:

[0006] An LNG skid having at least one compander installed thereon, each compander including a motor and a magnetic bearing for controlling the motor;

[0007] A signal acquisition module, which is installed on the LNG skid and is used to collect sensor data at each monitoring point;

[0008] a main control module, connected to the signal acquisition module, configured to execute a system control program and control the corresponding compression expansion unit to operate according to preset values ​​based on the sensor data so as to reliquefy the BOG in the ship's cargo tank into LNG;

[0009] at least one motor power module connected to the main control module, each motor power module including a controller and a frequency converter connected to the controller, the frequency converter controlling the motor of the corresponding compression expander unit according to a control signal sent by the main control module;

[0010] At least one magnetic levitation control module, each magnetic levitation control module is connected to the controller of its corresponding motor power module or directly connected to the main control module, and is used to control the magnetic bearing of the corresponding compression and expansion unit according to the control signal issued by the main control module;

[0011] A valve control module is installed on the LNG skid and connected to the main control module, and is used to control the opening and closing of each valve according to the control signal sent by the main control module.

[0012] In one embodiment of the present invention, the signal acquisition module and the valve control module are located in a first area of ​​the ship's liquid cargo tank, and the main control module, the motor power module and the magnetic levitation control module are located in a second area of ​​the ship's liquid cargo tank, and the danger level of the first area is greater than the danger level of the second area.

[0013] In one embodiment of the present invention, the LNG reliquefaction control system further configures a redundant power supply module for the main control module and a UPS module for each motor power module.

[0014] In one embodiment of the present invention, each magnetic levitation control module is installed inside its corresponding motor power module and is respectively connected to the controller and UPS module of the motor power module.

[0015] In one embodiment of the present invention, each motor power module further includes a switch, and the switch is connected to the inverter and the controller respectively.

[0016] In one embodiment of the present invention, the main control module further includes a touch screen for real-time interaction with an operator.

[0017] In a second aspect, the present invention provides a method of controlling an LNG reliquefaction system according to any one of the first aspects, the method comprising:

[0018] Pre-cold check steps: The main control module checks the power status of the motor power module and the cooling water supply status of the LNG skid according to the inquiry signal sent by the system. If the check passes, it returns a start-up permission signal to the system;

[0019] Precooling step: The main control module continuously calculates the speed of the motor of each companding expansion unit according to the precooling target temperature, and controls the motor through the inverter according to the calculated speed so that the temperature of the heat exchanger of the companding expansion unit approaches the precooling target temperature;

[0020] Refrigeration step: The main control module calculates the required motor speed of each compression expansion unit according to the preset function relationship, and adjusts the real-time speed of the motor through the inverter to re-liquefy the BOG in the ship's cargo tank into LNG;

[0021] Stop cooling step: after the cooling step is completed, the main control module adjusts the motor speed of each compression expansion unit to a first preset value;

[0022] Warming-up step: After the stopping and cooling step is completed, the main control module adjusts the motor speed of each compression expansion unit to a second preset value so that the LNG reliquefaction control system does not generate cooling capacity.

[0023] The second preset value is smaller than the first preset value.

[0024] In one embodiment of the present invention, the pre-cooling step further comprises:

[0025] When the difference between the temperature value of the heat exchanger of the compression expansion unit and the pre-cooling target temperature is within a third preset value and lasts for a first preset time period, it is determined that the pre-cooling step is completed.

[0026] In one embodiment of the present invention, the cooling step further comprises:

[0027] The main control module establishes a functional relationship between the LNG temperature and the motor speed, the cooling water temperature and the cooling capacity, and continuously records the cooling water temperature and LNG temperature of the preset group within the third preset time period every second preset time period and calculates the average value of each group respectively;

[0028] Based on the functional relationship, the required motor speed is calculated and the calculation result is sent to the frequency converter corresponding to the motor through an instruction to adjust the real-time speed of the motor.

[0029] In one embodiment of the present invention, the step of stopping cooling further comprises:

[0030] After the main control module adjusts the motor of each compression expansion unit to a first preset value, the main control module opens the corresponding valve through the valve control module at a first preset speed;

[0031] When the valve is fully opened, the main control module closes the corresponding valve at a second preset speed. If the LNG flow rate drops to zero at this time, the cooling stop step is completed. The LNG reliquefaction control system and method provided by the present invention are implemented. The main control module regulates the frequency converter to control the operation of the compression expansion unit within the LNG reliquefaction control system. Simultaneously, the operating parameters of the motor within the LNG reliquefaction control system and data such as the temperature, pressure, and flow rate at various monitoring points within the LNG skid acquired by the signal acquisition module are monitored in real time. Based on this data, the operating parameters of the frequency converter can be precisely controlled to ensure that the compression expansion unit operates according to preset values, so that the BOG in the cargo tank is recondensed and liquefied into LNG and returned to the cargo tank, avoiding the waste caused by direct discharge and combustion of the BOG while ensuring that the temperature and pressure within the cargo tank are within an appropriate range. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the present invention or the prior art, a brief introduction is given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0033] Figure 1 This is a schematic diagram of the application of the LNG reliquefaction control system provided by the present invention;

[0034] Figure 2 This is a structural block diagram of an LNG reliquefaction control system provided by one embodiment of the present invention;

[0035] Figure 3 is a structural block diagram of an LNG reliquefaction control system provided by another embodiment of the present invention;

[0036] Figure 4 It is a flow chart of the LNG reliquefaction control system method provided by the present invention. DETAILED DESCRIPTION

[0037] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0038] The terms "first," "second," and the like in the description and claims of the present invention and the accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a particular order or precedence. It should be understood that the terms used in this manner are interchangeable where appropriate, such that the embodiments described herein can be practiced in an order other than that shown or described herein.

[0039] To solve the problems in the prior art that BOG is directly discharged into the atmosphere, causing waste of resources, environmental pollution, and potential safety hazards, the present invention provides an LNG re-liquefaction control system and method. The main control module is used to adjust the frequency converter, thereby controlling the operation of the compression-expansion unit in the LNG re-liquefaction control system. At the same time, the operating parameters of the motor in the LNG re-liquefaction control system and data such as the temperature, pressure, and flow rate of each monitoring point in the LNG skid obtained by the signal acquisition module are monitored in real time. Based on these data, the operating parameters of the frequency converter can be accurately controlled to ensure that the compression-expansion unit operates according to the preset value, enabling the BOG in the liquid cargo tank to be re-condensed into LNG and returned to the liquid cargo tank, avoiding the waste caused by direct combustion of BOG and ensuring that the temperature and pressure in the liquid cargo tank are within a suitable range.

[0040] The following combines Figure 1-Figure 4 to describe the LNG re-liquefaction control system and method of the present invention.

[0041] Please refer to Figure 1 , Figure 1 which is a schematic diagram of the application of the LNG re-liquefaction control system provided by the present invention. Figure 1 It shows a schematic connection diagram of the LNG re-liquefaction control system and the LNG cargo tank provided by the present invention. Exemplarily, the connection components between the LNG re-liquefaction control system and the LNG cargo tank include an LNG inlet valve, an LNG outlet valve, an LNG bypass valve, a temperature sensor (TT710), a submersible pump, and a spraying device.

[0042] Among them, the LNG inlet valve can be a pneumatic control valve, which is used to connect the LNG re-liquefaction control system (LSC) to the ship integrated automation system (ISA). To avoid the plate-fin heat exchanger from being subjected to thermal stress, this valve should be opened slowly.

[0043] Among them, the LNG outlet valve can be a hand valve, which is normally open except during maintenance.

[0044] Among them, the LNG bypass valve can be a pneumatic control valve, which is used to cool the LNG pipeline. When |TT710 – TTLSC| < P1 °C (this threshold will be determined during commissioning), it indicates that the refrigeration step is completed. To avoid the plate-fin heat exchanger from being subjected to thermal stress, this valve should be opened slowly.

[0045] Among them, the measurement range of the temperature sensor (TT710) is -196 °C to 55 °C, and this sensor is used to measure the LNG inlet temperature, that is, the temperature of the LNG delivered to the LNG re-liquefaction control system.

[0046] Among them, the submersible pump can be a centrifugal pump, and its working range can be 25m 3 / h, and the submersible pump is used to transport LNG to the LNG re-liquefaction control system.

[0047] Among them, the goal of the spraying device is to spray LNG into the LNG storage tank.

[0048] The LNG reliquefaction control system according to the present invention is described below.

[0049] Please refer to Figure 2 , Figure 2 This is a block diagram of the LNG reliquefaction control system provided by one embodiment of the present invention. This LNG reliquefaction control system can be applied to existing integrated ship automation systems (ISAs). For example, through the additional RS485 module, I / O hardpoint module, and Profibus-DP module, the present invention provides the integrated ship automation system (IAS) with multiple current mainstream communication methods, meeting the communication requirements of various vessels, including retrofitted and newbuilding vessels.

[0050] The LNG reliquefaction control system includes a main control module, an LNG skid, at least one motor voltage module, at least one magnetic levitation control module, a valve control module, and a signal acquisition module.

[0051] Exemplarily, at least one compression-expansion unit is installed on the LNG skid, and each compression-expansion unit includes a motor and a magnetic bearing for controlling the motor.

[0052] It should be noted that the compression expansion unit described in the present invention adopts an integrated expansion unit and compressor unit, but the present invention can also adopt a non-integrated expansion unit and compressor unit, depending on actual needs.

[0053] A skid is a device that is fixed within a structural or containerized frame, and houses the piping, valves, pumps, containers, and instrumentation as a whole. Based on the process flow, spatial orientation, and modular spatial layout, the process equipment, process piping, high structures, or containerized frames within the skid are rationally planned and arranged according to relevant standards, creating a functional, modular, and fixed overall system.

[0054] Exemplarily, the signal acquisition module is installed on the LNG skid, and the signal acquisition module is used to collect sensor data of various monitoring points, such as temperature data, pressure data, and flow data.

[0055] For example, the components supporting the signal acquisition module may include 46 temperature sensors, 4 orifice flow meters, 9 differential pressure transmitters, 12 pressure transmitters and 4 speed sensors, which are used to monitor the physical quantities of each monitoring point of the LNG skid.

[0056] For example, the main control module is connected to the signal acquisition module, the valve control module, the motor voltage module, and the magnetic levitation control module. For example, the main control module can communicate with these modules via Ethernet. The main control module is configured to execute a system control program and, based on sensor data from the signal acquisition module, control the corresponding compression expansion unit to operate according to preset parameters, thereby re-liquefying the BOG in the vessel's cargo tank into LNG.

[0057] It should be noted that the main control module includes a programmable controller, such as an S7-300 series programmable controller. It can also include a touch screen for real-time operator interaction. The touch screen communicates with the main control module via Ethernet and displays all operating parameters of the LNG reliquefaction system.

[0058] Exemplarily, each motor power module is connected to the main control module, and each motor power module includes a controller and a frequency converter connected to the controller, and the frequency converter controls the motor of the corresponding compression and expansion unit according to the control signal sent by the main control module.

[0059] For example, the controller of the motor power module can adopt a programmable controller of the S7-1200 series, which can maintain real-time communication with the main control module and control a compression expansion unit with a rated power of 250kW. The control signal is sent by the main control module.

[0060] The purpose of configuring a controller within each motor power module is to ensure that, even if the main control module unexpectedly fails, the motor power module's controller can continue to control the inverter and magnetic levitation control module to safely shut down. Furthermore, since different ships use different communication protocols, the configuration of the controller can meet the requirements of various communication protocols.

[0061] It should be noted that one motor power module can control one compander. Each motor power module's supporting components, including a high-frequency output reactor, an EMC filter, an HF harmonic filter, a capacitor bank, and a brake resistor, provide power for the compander. The present invention does not limit the number of motor power modules; for example, two, four, or even more motor power modules may be used.

[0062] Furthermore, the LNG reliquefaction control system of the present invention can be simultaneously connected to three-phase AC440V, 60Hz or three-phase AC380V, 50Hz and single-phase AC220V, 60 / 50Hz power supplies to ensure that the primary circuit does not interfere with the secondary control circuit during high-frequency control.

[0063] In some embodiments of the present invention, Figure 3As shown, each motor power module may further include a switch, which is connected to the frequency converter and the controller respectively.

[0064] For example, each magnetic suspension control module is connected to the controller of its corresponding motor power module (e.g. Figure 3 as shown) or directly connected to the main control module (as shown Figure 2 (As shown). The magnetic levitation control module controls the magnetic bearings of the corresponding compander according to control signals from the main control module. The magnetic levitation control module and the inverter also communicate all their parameters to the controller, which then sends them to the main control module. The magnetic levitation control module is a critical component for ensuring the safe operation of the unit.

[0065] In other words, the magnetic levitation control module is installed inside the motor power module. It can collect the power signal on the motor and the signals of 10 displacement sensors, and ensure that the magnetic bearings in the motor operate within a safe distance according to the instructions issued by the main control module.

[0066] For example, the valve control module is also installed on the LNG skid and controls the opening and closing of each valve based on control signals from the main control module. For example, to ensure that inspectors can immediately stop the LNG reliquefaction system if they detect an abnormal malfunction, several emergency stop buttons can be installed at various locations around the LNG skid. These buttons are connected to the valve control module, ensuring that the emergency stop signal is promptly transmitted to the main control module.

[0067] For example, the components of the valve control module may include 9 control proportional valves and 4 emergency stop buttons to control the opening of the valves and meet the safety requirements of system operation.

[0068] In some embodiments of the present invention, the signal acquisition module and valve control module are located in a first area of ​​a vessel's cargo tank; the main control module, motor power module, and magnetic levitation control module are located in a second area of ​​the vessel's cargo tank. The hazard level of the first area is greater than that of the second area.

[0069] For example, signal acquisition modules and valve control modules, where explosion-proof measures are easily implemented, can be placed in the cargo tanks, which are located in the hazardous area of ​​the ship, while the main control modules, motor power modules, and magnetic levitation control modules, where explosion-proof measures are not suitable, can be placed in the safe area. This balances safety requirements while reducing construction costs.

[0070] It should be noted that the motor power module of the present invention reserves sufficient installation space for the magnetic levitation control module to ensure that some magnetic levitation control modules can be placed in the motor power module when they cannot meet the explosion-proof requirements and cannot be installed in dangerous areas. This can reduce the installation volume requirements of the present invention and is more conducive to layout in a small space in the cabin.

[0071] In some embodiments of the present invention, a redundant power supply module is configured for the main control module, and a UPS module is configured for each motor power module. The redundant power supply module configured for the main control module and the UPS system configured for the motor power modules ensure that the system can continue to operate for at least a preset duration (e.g., 30 minutes) in the event of an unexpected power outage, preventing hardware damage within the LNG reliquefaction module caused by the unexpected power outage.

[0072] Based on the above, the present invention can also be improved as follows:

[0073] For example, the signal acquisition module can be replaced with an intrinsically safe module, all signal cables can be replaced with intrinsically safe cables, and all sensors can be replaced with intrinsically safe sensors. This eliminates the need for a flameproof cavity, further reducing the size of the signal acquisition unit and lowering its manufacturing complexity and production costs.

[0074] The beneficial effect of adopting the above solution is that, by realizing an intrinsically safe circuit, the signal acquisition unit does not need to adopt a flameproof cavity, thereby reducing the volume of the signal acquisition unit and reducing the processing difficulty and production cost of the signal acquisition unit.

[0075] For another example, the present invention may further include a wireless data transmission module, which is connected to the controller of the motor voltage module.

[0076] The beneficial effect of adopting the above solution is that the data wireless transmission module can send all real-time data and historical data of the LNG reliquefaction control system of the present invention to the cloud server, realizing offshore data collection and offshore remote maintenance.

[0077] In summary, the present invention provides an LNG reliquefaction control system based on a programmable logic controller. This system interacts with operators in real time through a main control module, which, after calculation, sends instructions to other modules to ensure compliance. The motor power module and magnetic levitation control module ensure safe and stable operation of the compression-expansion unit. The signal acquisition module and valve control module collect various data from the LNG reliquefaction control system, control valve opening status, and ensure emergency shutdown. Furthermore, the LNG reliquefaction control system of the present invention features a modular structure, resulting in low cost and easy maintenance.

[0078] The following describes a method of an LNG reliquefaction control system provided by the present invention. The method of the LNG reliquefaction control system described below and the LNG reliquefaction control system described above can refer to each other.

[0079] Please refer to Figure 4 , Figure 4 The figure is a flow chart of a method for controlling an LNG reliquefaction system provided by the present invention. A method for controlling an LNG reliquefaction system is applied to the above-mentioned LNG reliquefaction control system, and the method comprises:

[0080] Step 410, pre-cold inspection step.

[0081] For example, the main control module checks the power status of the motor power module and the cooling water supply status of the LNG skid according to the inquiry signal sent by the system, and returns a start-up permission signal to the system (i.e., the ship integrated automation system) after the inspection is passed.

[0082] Specifically, the total rated power of the LNG reliquefaction control system (hereinafter referred to as the present system) of the present invention can be set to 1000kW. To ensure the safety of the equipment and the power grid, the distribution board of the ship's control system (hereinafter referred to as the upper system) first sends a heavy load inquiry signal to the present system. After receiving the signal, the main control module with a programmable logic controller automatically checks the power status of the motor power cabinet and the cooling water supply status of the LNG skid. If there is no alarm, it sends a start-up permission signal to the control console of the upper system. The control console of the upper system can only send a start-up command to the present system after receiving the start-up permission signal. When the present system receives the start-up command sent by the upper system, the main control module determines that the pre-cold check step has been completed.

[0083] Step 420, pre-cooling step.

[0084] Exemplarily, the main control module continuously calculates the speed of the motor of each compression expansion unit according to the pre-cooling target temperature, and controls the motor through the inverter according to the calculated speed so that the temperature of the heat exchanger of the compression expansion unit is close to the pre-cooling target temperature.

[0085] Specifically, after the pre-cooling check step is completed, the system automatically enters the pre-cooling stage. The pre-cooling target temperature can be set to a preset value (e.g., -155°C). The main control module constantly calculates the speed of the control motor and sends the data to the inverter to gradually decrease the temperature of the heat exchanger in the compression and expansion unit to the pre-cooling target temperature at a preset rate (e.g., 2°C / min). The pre-cooling step is determined to be complete when the difference between the temperature of the heat exchanger in the compression and expansion unit and the pre-cooling target temperature is within a third preset value (e.g., 10°C) and lasts for a first preset period of time (e.g., 10 minutes).

[0086] Before LNG enters this system, the LNG pipeline must be pre-cooled. Figure 1 As shown, close the LNG inlet valve, open the LNG bypass valve, and circulate LNG through the pipeline via the spray pump. When the temperature sensor (TT710) reaches the low-temperature threshold, the LNG pipeline precooling ends, and the system will send the signal "SIG_LNG_R". Click the "COOLDOWN" button to enter the precooling mode of the system. After the precooling mode ends, the system sends the signal "SIG_LSC_R".

[0087] For example, after the LNG pipeline pre-cooling is completed, the system sends the signal "SIG_LNG_R"; at the same time, when the system pre-cooling is completed, the system sends the signal "SIG_LSC_R". At this time, LNG can be introduced into the system.

[0088] Step 430, cooling step.

[0089] For example, the main control module calculates the motor speed required for each compression expansion unit according to a preset functional relationship, and adjusts the real-time speed of the motor through the inverter based on the calculation result so that the BOG in the ship's liquid cargo tank is re-liquefied into LNG.

[0090] Specifically, after the pre-cooling step is completed, the LNG inlet valve (PGV700) is fully closed and the LNG bypass valve (PGV702) is fully opened, and the LNG supply pump is fed at a rate not less than a preset value (e.g., 30 m 3 LNG is supplied to the system at a steady flow rate of 100 liters per hour. The main control module establishes functional relationships between LNG temperature and motor speed, and between cooling water temperature and cooling capacity. It continuously records cooling water and LNG temperatures for preset groups (e.g., 10 groups) at intervals of a second preset time period (e.g., 30 minutes) and a third preset time period (e.g., 2 seconds), calculates the average value for each, and calculates the required motor speed based on the functional relationship. This command is sent to the inverter to adjust the real-time motor speed so that the system generates sufficient cooling capacity to reliquefy excess natural gas into LNG for delivery back to the cargo tank.

[0091] Step 440, stop the cooling step.

[0092] Exemplarily, after the refrigeration step is completed, the main control module adjusts the motor of each compression expansion unit to a first preset value.

[0093] Specifically, when the system completes its cooling operation and receives a stop command, the main control module adjusts the motor speed to a first preset value (e.g., 40%). The main control module then opens the valve (PGV702) at a uniform speed of a first preset rate (e.g., 0.5% / 30s). Once fully opened, the main control module then uniformly closes the valve (PGV700) at a second preset rate (e.g., 0.5% / 30s, which may be equal to the first preset rate) until it is fully closed. At this point, if the LNG flow rate drops to zero, the cooling stop step is complete.

[0094] Step 450, warm-up step.

[0095] Exemplarily, after the cooling stop step is completed, the main control module adjusts the motor of each compression expansion unit to a second preset value so that the LNG reliquefaction control system does not generate cooling capacity, wherein the second preset value is smaller than the first preset value.

[0096] For example, after the system completes the cooling stop step, the warm-up button illuminates, and the main control module adjusts the motor speed to a second preset value (e.g., 30%), at which the system generates no cooling. When the temperature of the heat exchanger (TI302) in the compression / expansion unit exceeds a set value, which in this embodiment is 10°C, or when the accumulated warm-up time reaches a set value, which in this embodiment is 8 hours, or when the system receives a forced warm-up stop command, the main control module reduces the motor speed to 0 and powers off.

[0097] Furthermore, in order to prevent the main control module from controlling the inverter to adjust the motor speed too fast and causing drastic fluctuations in the cooling capacity, the present invention is equipped with a speed limiter and a cooling capacity limiter. When the two reach the maximum limit value, the motor will be controlled to run at a fixed speed.

[0098] It should be noted here that the method of the above-mentioned LNG reliquefaction control system provided in the embodiment of the present invention can realize the functions realized by the above-mentioned system embodiment and can achieve the same technical effects. The parts and beneficial effects of this embodiment that are the same as those of the system embodiment will not be described in detail here.

[0099] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A method for controlling an LNG reliquefaction system, characterized in that: The LNG reliquefaction control system includes an LNG skid, a signal acquisition module, a main control module, and at least one motor power module; wherein the LNG skid is equipped with at least one compression-expansion unit; the signal acquisition module is mounted on the LNG skid; the main control module is connected to the signal acquisition module; the at least one motor power module is connected to the main control module, and each motor power module includes a controller and a frequency converter connected to the controller; The method comprises: Pre-cold check steps: The main control module checks the power status of the motor power module and the cooling water supply status of the LNG skid according to the inquiry signal sent by the system. If the check passes, it returns a start-up permission signal to the system; Precooling step: The main control module continuously calculates the speed of the motor of each companding expansion unit according to the precooling target temperature, and controls the motor through the inverter according to the calculated speed so that the temperature of the heat exchanger of the companding expansion unit approaches the precooling target temperature; Refrigeration step: The main control module calculates the required motor speed of each compression expansion unit according to the preset function relationship, and adjusts the real-time speed of the motor through the inverter to re-liquefy the BOG in the ship's cargo tank into LNG; Stop cooling step: after the cooling step is completed, the main control module adjusts the motor speed of each compression expansion unit to a first preset value; Warming-up step: After the stopping and cooling step is completed, the main control module adjusts the motor speed of each compression expansion unit to a second preset value so that the LNG reliquefaction control system does not generate cooling capacity. The second preset value is smaller than the first preset value.

2. The LNG reliquefaction control system method according to claim 1, characterized in that: The precooling step further comprises: When the difference between the temperature value of the heat exchanger of the compression expansion unit and the pre-cooling target temperature is within a third preset value and lasts for a first preset time period, it is determined that the pre-cooling step is completed.

3. The LNG reliquefaction control system method according to claim 1, characterized in that: The refrigeration step further comprises: The main control module establishes a functional relationship between the LNG temperature and the motor speed, the cooling water temperature and the cooling capacity, and continuously records the cooling water temperature and LNG temperature of the preset group within the third preset time period every second preset time period and calculates the average value of each group respectively; Based on the functional relationship, the required motor speed is calculated and the calculation result is sent to the frequency converter corresponding to the motor through an instruction to adjust the real-time speed of the motor.

4. The LNG reliquefaction control system method according to claim 1, characterized in that: The stopping cooling step further comprises: After the main control module adjusts the motor of each compression expansion unit to a first preset value, the main control module opens the corresponding valve through the valve control module at a first preset speed; When the valve is fully opened, the main control module closes the corresponding valve at a second preset speed. If the LNG flow rate drops to 0 at this time, it means that the cooling stop step is completed.

5. An LNG reliquefaction control system, characterized in that: A method for implementing the LNG reliquefaction control system according to any one of claims 1 to 4, the system comprising: An LNG skid having at least one compander installed thereon, each compander including a motor and a magnetic bearing for controlling the motor; A signal acquisition module, which is installed on the LNG skid and is used to collect sensor data at each monitoring point; a main control module, connected to the signal acquisition module, configured to execute a system control program and control the corresponding compression expansion unit to operate according to preset values ​​based on the sensor data so as to reliquefy the BOG in the ship's cargo tank into LNG; at least one motor power module connected to the main control module, each motor power module including a controller and a frequency converter connected to the controller, the frequency converter controlling the motor of the corresponding compression expander unit according to a control signal sent by the main control module; At least one magnetic levitation control module, each magnetic levitation control module is connected to the controller of its corresponding motor power module or directly connected to the main control module, and is used to control the magnetic bearing of the corresponding compression and expansion unit according to the control signal issued by the main control module; A valve control module is installed on the LNG skid and connected to the main control module, and is used to control the opening and closing of each valve according to the control signal sent by the main control module.

6. The LNG reliquefaction control system according to claim 5, characterized in that: The signal acquisition module and the valve control module are located in a first area of ​​the ship's liquid cargo tank, and the main control module, the motor power module and the magnetic levitation control module are located in a second area of ​​the ship's liquid cargo tank. The danger level of the first area is greater than the danger level of the second area.

7. The LNG reliquefaction control system according to claim 5, characterized in that: The LNG reliquefaction control system further configures a redundant power supply module for the main control module and a UPS module for each motor power module.

8. The LNG reliquefaction control system according to claim 7, characterized in that: Each magnetic levitation control module is installed inside its corresponding motor power module and is connected to the controller and UPS module of the motor power module respectively.

9. The LNG reliquefaction control system according to claim 5, characterized in that: Each motor power supply module further includes a switch, which is connected to the frequency converter and the controller respectively.

10. The LNG reliquefaction control system according to claim 5, characterized in that: The main control module also includes a touch screen for real-time interaction with the operator.

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