Long distance lng pipe medium handling system and method based on natural gas

Abstract

The application discloses a long-distance LNG pipe storage medium processing system and method based on natural gas, relates to the technical field of LNG transportation, and comprises a long-distance low-temperature LNG pipeline system, a medium recovery system, a medium discharge system, a pipeline heating system and a residual pipe storage gas replacement system. The residual pipe storage gas replacement system is communicated with the pipeline close to the first end of the long-distance low-temperature LNG pipeline system, the medium recovery system and the medium discharge system are both communicated with the pipeline close to the end of the long-distance low-temperature LNG pipeline system, and the pipeline heating system is arranged on the long-distance low-temperature LNG pipeline system at intervals. The application is reasonable in design, can efficiently discharge and recover the pipe storage medium of the long-distance low-temperature LNG pipeline before maintenance, can improve the pipeline emptying speed and reduce the pipeline shutdown time on the one hand, and can reduce the discharge amount of the pipe storage medium and the discharge volume on the other hand. Therefore, the long-distance low-temperature LNG pipeline can be efficiently and safely discharged.

Description

Technical Field

[0001] This invention relates to the field of LNG transportation technology, and more specifically to the field of a long-distance LNG pipeline storage medium processing system and method based on natural gas. Background Technology

[0002] Driven by the goals of "carbon peaking and carbon neutrality," natural gas is gradually becoming one of the main energy sources in the modern clean energy system. As the "fifth strategic energy channel" to ensure national energy security, seaborne LNG will remain the main form of China's natural gas imports for some time to come. With the development of technologies for the deep utilization of natural gas products and the increasing application of LNG cold energy, the demand for comprehensive utilization of imported LNG is gradually increasing. Conventional coastal LNG receiving terminals face challenges such as limited processing and utilization of high-value components in LNG and insufficient utilization of cold energy. Vigorously developing long-distance LNG pipeline transportation technology has a promising future, supporting the transportation of cryogenic LNG to onshore areas for further comprehensive utilization of the medium and energy.

[0003] LNG, with a medium temperature as low as -162℃, is typically vaporized and pressurized before being transported out through pipelines into the natural gas network after unloading at receiving terminals; this is essentially a natural gas pipeline transportation model. For long-distance cryogenic LNG pipelines, the pipeline operating temperature is close to -162℃, and because the pipeline medium is a cryogenic liquid phase, its density can reach 450 kg / m³. 3 The pressure is around 100°C, significantly higher than that of conventional high-pressure natural gas transmission, resulting in higher energy efficiency and a considerable gas storage capacity in pipelines. Long-distance cryogenic LNG pipelines face similar maintenance conditions to long-distance high-pressure natural gas pipelines; therefore, the LNG medium within the pipeline must be discharged before maintenance.

[0004] Conventional natural gas pipelines typically use valve chambers or stations for venting, utilizing the pressure of the pipeline medium as the driving force to provide venting energy. In contrast, cryogenic LNG pipelines have very limited venting pressure available during venting (the pipeline's transport pressure is close to atmospheric pressure), which significantly impacts the venting rate, resulting in a longer time required to empty the LNG (or natural gas) from the pipeline before maintenance. Furthermore, for the relatively valuable LNG resource, the efficient utilization of the stored medium is also a direction for the development of pipeline venting technology.

[0005] The main challenges in handling medium stored in long-distance cryogenic LNG pipelines are: the operating pressure of the medium in the pipeline is relatively low, and the driving force for venting is not as strong as that of conventional natural gas pipelines, which may result in excessively long venting times; there are currently few reports on the destination of the vented medium, and since the natural gas storage per unit volume of LNG pipelines is greater than that of conventional high-pressure natural gas, direct gas venting would lead to a significant waste of natural gas; LNG pipelines use insulation materials with good thermal insulation properties, and it takes a long time to increase the pressure of the stored medium using natural convection; after the medium is vented, the temperature of the pipeline body is still relatively low, and a considerable amount of time is required for rewarming to support subsequent maintenance operations. Summary of the Invention

[0006] The purpose of this invention is to address the technical problems of low discharge rate and low reuse rate of stored media in existing long-distance cryogenic LNG pipelines. This invention provides a long-distance LNG pipeline storage media processing system and method based on natural gas.

[0007] To achieve the above objectives, the present invention specifically adopts the following technical solution:

[0008] One aspect of the present invention provides a long-distance LNG pipeline storage medium processing system based on natural gas, including a long-distance cryogenic LNG pipeline system, a medium recovery system, a medium discharge system, a pipeline heating system, and a residual gas replacement system. The residual gas replacement system is connected to the pipeline near the beginning of the long-distance cryogenic LNG pipeline system, and the medium recovery system and the medium discharge system are both connected to the pipeline near the end of the long-distance cryogenic LNG pipeline system. The pipeline heating system is spaced out on the long-distance cryogenic LNG pipeline system.

[0009] The long-distance cryogenic LNG pipeline system connects to the upstream LNG terminal at the front end and to the downstream LNG terminal at the end. The medium discharge system connects to the BOG venting system of the downstream terminal at the end, and the medium recovery system connects to the downstream recovery gas compressor at the end.

[0010] Specifically, by setting up this system, the problem of pre-maintenance venting faced by long-distance cryogenic LNG pipelines is optimized. The system recovers most of the medium to be vented by using natural gas to enter the downstream recovery gas compressor. It also increases the saturated vapor pressure of the medium stored in the pipeline and improves the venting rate by using auxiliary heating of the pipeline after the pipeline is shut down. Finally, it replaces and discharges the remaining low-pressure gas phase natural gas in the pipeline by using nitrogen replacement. Overall, the system achieves efficient and safe venting.

[0011] In one embodiment, the long-distance cryogenic LNG pipeline system includes an upstream LNG pipeline, a long-distance cryogenic LNG pipeline, and a downstream LNG pipeline connected in sequence. An upstream shut-off valve is provided at the connection between the upstream LNG pipeline and the long-distance cryogenic LNG pipeline, and a downstream shut-off valve is provided at the connection between the long-distance cryogenic LNG pipeline and the downstream LNG pipeline. The system also includes a series of π-shaped bends distributed along the long-distance cryogenic LNG pipeline.

[0012] Specifically, the long-distance cryogenic LNG pipeline system in this solution is used to provide a long-distance transportation channel for cryogenic LNG media.

[0013] In one embodiment, the medium recovery system includes a recovery pipeline connected to a long-distance cryogenic LNG pipeline near the downstream shut-off valve. The recovery pipeline is sequentially equipped with a main line reserved venting recovery shut-off valve, a first pressure transmitter for the recovery pipeline, a regulating valve for the recovery pipeline, a second pressure transmitter for the recovery pipeline, a first temperature transmitter for the recovery pipeline, a gas-liquid buffer tank, a second temperature transmitter for the recovery pipeline, and a downstream recovery gas compressor at the end of the recovery pipeline.

[0014] Specifically, the medium recovery system is used to regulate and vaporize the medium that can be output through pressure difference in long-distance cryogenic LNG pipelines, and then introduce it into the downstream recovery gas compressor to collect and use the natural gas.

[0015] In one embodiment, the pipeline heating system includes multiple heating units, each of which includes a removable insulation layer pipeline, an auxiliary electric heat tracing pipeline, and a pressure detection transmitter installed on the corresponding π-shaped bend.

[0016] Specifically, the pipeline heating system is used to promote the local low-temperature medium in the pipeline to absorb external heat and heat up after the pipeline is shut down, so as to help increase the pressure of the medium in the pipeline (increase the saturated vapor pressure of the medium in the pipeline) and accelerate the recovery of the medium.

[0017] In one embodiment, the residual gas replacement system includes a replacement injection pipeline connected to a long-distance cryogenic LNG pipeline near the upstream shut-off valve for external transmission. The replacement injection pipeline is equipped with a replacement injection shut-off valve, a replacement injection temperature transmitter, and a nitrogen replacement injection skid.

[0018] Specifically, the residual gas replacement system is used to replace the remaining gaseous natural gas in the pipeline after the pipeline pressure approaches atmospheric pressure, and to control the overall pipeline temperature to gradually rise, providing a safety guarantee for subsequent maintenance.

[0019] In one embodiment, the medium discharge system includes a medium discharge pipeline, on which a medium discharge pressure transmitter and a medium discharge shut-off valve are installed. Two pipelines are connected in parallel at the end of the medium discharge pipeline. One pipeline is equipped with an overpressure safety valve, and the other pipeline is equipped with a manual relief valve. The ends of both pipelines are connected to the downstream station BOG venting system.

[0020] Specifically, the media discharge system is used for potential overpressure protection during pipeline heating, and can also be used for safe gas discharge during pipeline nitrogen purging, and provides the function of analyzing the composition of the discharged gas.

[0021] A second aspect of the present invention provides a method for processing medium stored in long-distance LNG pipelines for efficient utilization of natural gas, employing the aforementioned long-distance LNG pipeline medium processing system based on natural gas, comprising the following steps:

[0022] S1. Shut down the main cryogenic LNG pipeline: shut down the submersible pumps at the upstream LNG station, and then shut down the upstream and downstream shut-off valves to complete the shutdown of the long-distance cryogenic LNG pipeline.

[0023] S2. Perform pressurized discharge operation on the main cryogenic LNG pipeline;

[0024] S3. Perform auxiliary heating and pressurization discharge operations on the main cryogenic LNG pipeline.

[0025] S4. Perform external nitrogen purging operation on the main cryogenic LNG pipeline.

[0026] In one embodiment, the specific steps for performing pressurized discharge operation on the main cryogenic LNG pipeline in step S2 are as follows:

[0027] S21. Connect the venting medium recovery skid, which consists of the recovery pipeline, the first pressure transmitter of the recovery pipeline, the regulating valve of the recovery pipeline, the second pressure transmitter of the recovery pipeline, the gas-liquid buffer tank of the first temperature transmitter of the recovery pipeline, the second temperature transmitter of the recovery pipeline, and the flow transmitter, to the main line with a reserved venting recovery shut-off valve, and perform nitrogen purging.

[0028] S22. Open the trunk line reserved discharge and recovery shut-off valve, slowly open the recovery pipeline regulating valve, and control the discharge flow to meet the inlet flow requirements of the downstream recovery gas compressor. The discharge pressure in this process is provided by the medium pressure after the long-distance cryogenic LNG pipeline is shut down.

[0029] S23. Turn on the electric heating of the gas-liquid buffer tank and set the temperature to not exceed 10°C above the temperature detected by the first temperature transmitter of the recovery pipeline, so as to promote the vaporization of liquid LNG in the gas-liquid buffer tank.

[0030] S24. As the upstream pressure decreases, gradually increase the opening of the regulating valve in the recovery pipeline. When the flow rate detected by the flow transmitter cannot meet the minimum flow requirement of the downstream recovery gas compressor, it indicates that the upstream medium pressure is insufficient, and proceed to step S3.

[0031] In one embodiment, the specific steps of performing auxiliary heating, pressurization, and discharge operations on the main cryogenic LNG pipeline in step S3 are as follows:

[0032] S31. Disassemble the insulation layer of the π-shaped bend with the pipeline heating system and turn on the auxiliary electric heat tracing pipeline. Use atmospheric heat transfer, auxiliary electric heating and other methods to increase the temperature of the medium in the corresponding pipe section, promote the increase of the temperature of the gas-liquid two-phase low-temperature natural gas in the pipe section and the nearby pipe section, and increase the saturated vapor pressure of the medium.

[0033] S32. Open the medium discharge shut-off valve to provide pressure protection for the upstream main pipeline from the overpressure safety valve;

[0034] S33. The pipeline temperature rise rate shall be monitored by the temperature detection system installed along the long-distance cryogenic LNG pipeline. It shall not exceed 10℃ / h. Otherwise, the auxiliary electric heat tracing pipeline shall be shut down.

[0035] S34. Continue to control the flow rate of the recovered natural gas using the recovery pipeline regulating valve and flow transmitter;

[0036] S35. Continuously monitor the pressure parameters of the first pressure transmitter, pressure detection transmitter (pipeline heating system at various points along the pipeline), and medium discharge pressure transmitter in the recovery pipeline. When the detected pressure continues to rise and reaches 90% of the pipeline design pressure value, the auxiliary electric heat tracing pipeline should be closed first to control the rate of medium pressure rise in the pipeline. If the pressure continues to rise, the manual relief valve should be opened until the detected pressure is lower than 90% of the pipeline design pressure value, after which the manual relief valve should be closed.

[0037] Specifically, when the temperature detection system along the long-distance cryogenic LNG pipeline detects that the average temperature at various points in the pipeline reaches -50°C, it indicates that the natural gas in the pipeline has been basically vaporized. At this time, the auxiliary electric heat tracing pipeline is shut down. If the regulating valve of the recovery pipeline cannot meet the minimum flow requirement of the downstream recovery gas compressor even at its maximum opening, it indicates that the upstream pressure energy is basically exhausted, and the auxiliary heating pressurization and discharge operation is terminated.

[0038] In step S4, the specific method for performing external nitrogen purging on the main cryogenic LNG pipeline is as follows:

[0039] S41. Connect the nitrogen replacement injection skid to the replacement injection pipeline; continue to open the auxiliary electric heat tracing pipeline to heat up the medium in the pipeline, and at the same time open the manual relief valve. This operation can avoid the waste of cooling capacity caused by the vaporization of liquid nitrogen during nitrogen injection. Stop when the average temperature of each point in the pipeline reaches -20℃.

[0040] S42. Open the replacement injection shut-off valve, open the nitrogen injection regulating valve of the nitrogen replacement injection skid, introduce room temperature nitrogen, and replace the residual natural gas in the pipeline. The replacement flow rate is preferably controlled to be 3m / s.

[0041] S43. By taking samples through the gas component sampling port set on the medium discharge pipeline, when the methane concentration is less than 1%, it indicates that the nitrogen replacement is qualified and the replacement operation is completed.

[0042] Working principle:

[0043] From the perspectives of improving the venting rate, reducing direct discharge of the medium, and ensuring venting safety, this plan includes a long-distance cryogenic LNG pipeline system, a medium recovery system, a pipeline heating system, a residual gas replacement system, and a medium discharge system. These systems efficiently discharge and recover the medium stored in the long-distance cryogenic LNG pipeline before maintenance, increasing the pipeline emptying speed and reducing pipeline shutdown time, while also reducing the amount and volume of discharged medium. The long-distance cryogenic LNG pipeline system is the foundational pipeline system of this plan, providing a flow channel for cryogenic LNG. It mainly includes the LNG pipeline, pipeline supports, and insulation layers. The medium recovery system is located at the end of the main LNG pipeline and mainly includes a recovery regulating valve, buffer tank, electric heating system, and vented medium recovery pipeline. It is used to regulate and vaporize the medium that can be output through pressure difference in the pipeline and recover it, introducing it into the downstream recovery gas compressor for natural gas collection and use. The pipeline heating system is located on the main pipeline and mainly includes a removable insulated pipe section and an electric heat tracing pipe section. It is used to promote localization of the gas flow within the pipeline after shutdown. The cryogenic medium absorbs external heat and heats up to help increase the pressure of the medium inside the pipeline (increasing the saturated vapor pressure of the medium inside the pipeline) and accelerate the recovery of the medium. The residual gas replacement system includes a nitrogen injection pipeline at the beginning of the pipeline, used to replace the remaining gaseous natural gas in the pipeline after the pipeline pressure approaches atmospheric pressure, and to control the overall pipeline temperature to gradually rise, providing a safety guarantee for subsequent maintenance. The medium discharge system includes a natural gas and nitrogen venting system at the end of the main pipeline, used for potential overpressure protection during pipeline heating, and can also be used for safe gas discharge during pipeline nitrogen replacement. It should be noted that the long-distance cryogenic LNG pipeline system is the basis of this invention, but not the innovative content of this invention. Thus, the goal of efficient and safe venting of long-distance cryogenic LNG pipelines is achieved.

[0044] The beneficial effects of this invention are as follows:

[0045] 1. This invention is based on the characteristics of long-distance cryogenic LNG pipeline transportation and the physical properties of LNG. Combined with the need for efficient and safe venting of long-distance cryogenic LNG pipelines, it addresses the issues of increasing the venting rate, reducing direct media discharge, and ensuring venting safety. It establishes a long-distance cryogenic LNG pipeline system, a media recovery system, a pipeline heating system, a residual gas replacement system, and a media discharge system to efficiently discharge and recover the media stored in the pipeline before maintenance. This increases the pipeline emptying speed and reduces pipeline shutdown time, while also reducing the amount and volume of media discharged, thereby achieving the goal of efficient and safe venting of long-distance cryogenic LNG pipelines.

[0046] 2. Scientific Design: This invention addresses the characteristics of long-distance cryogenic LNG pipelines, including low transport temperatures, low pressures, and large volumes of stored media. Combining current technical experience with pre-maintenance venting techniques for long-distance natural gas pipelines, and aiming for efficient and safe venting of long-distance cryogenic LNG pipelines, it develops a targeted technical solution from the perspectives of efficient media recovery, safe pipeline warming, and safe venting of residual gas. The media recovery system considers the phase change issue during venting of low-pressure LNG pipelines. By setting up recovery regulating valves, buffer tanks, and electric heating systems, it provides a relay space for the vented media, ensuring that the gas temperature and composition entering the downstream recovered gas compressor meet the pressurization requirements. Simultaneously, the flow transmitter detection results are used as a reference to monitor the venting process. The system controls the discharge rate and other parameters, fully considering the scenarios of LNG transportation pipelines and downstream LNG secondary applications, achieving efficient recovery of LNG awaiting venting. Furthermore, considering the significant temperature impact on LNG pressure after pipeline shutdown, a pipeline heating system is implemented based on the design principles of LNG pipeline insulation to raise the temperature of the medium inside the pipeline, promoting gas volatilization and increasing pipeline pressure, allowing more LNG to be recovered through the medium recovery system. To protect pipeline pressure safety throughout the recovery process, a medium discharge system is installed to control pipeline pressure. Additionally, through the operation of this system, while recovering and reusing stored LNG, the system also achieves a steady warming of the cryogenic LNG pipeline itself. Thus, in the absence of comparable technologies, it achieves efficient medium recovery and steady pipeline warming in long-distance cryogenic LNG pipelines.

[0047] 3. Excellent economic efficiency: This invention achieves efficient recovery of the medium in long-distance cryogenic LNG pipelines by setting up a medium recovery system, avoiding direct venting and resulting in significant economic benefits. At the same time, the pipeline heating system set up in this invention further promotes the rapid evaporation and venting of the medium after the long-distance cryogenic LNG pipeline is shut down, saving a lot of time for cleaning the medium in the pipeline and promoting the pipeline to resume production, resulting in significant operational benefits. In addition, the pipeline heating system can also simultaneously achieve the rewarming of the pipeline body while improving the efficiency of medium recovery in the pipeline, avoiding the need for additional rewarming processes.

[0048] 4. Promoting Technological Development: my country has not yet carried out the construction and operation of large-scale, long-distance cryogenic LNG pipelines. The solutions proposed in this system have played an important role in engineering guidance and reference. The proposed long-distance LNG pipeline storage medium processing system and method based on natural gas helps to improve the efficiency and safety of pipeline storage medium processing and medium recovery, and promotes the development of technological concepts and technological progress in this field. Attached Figure Description

[0049] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0050] Figure 1 This is a schematic diagram of a long-distance LNG pipeline storage medium processing system based on natural gas according to the present invention.

[0051] Attached label: 1-Upstream LNG pipeline, 2-Upstream shut-off valve for external transmission, 3-Long-distance cryogenic LNG pipeline, 4-Downstream shut-off valve for external transmission, 5-Downstream LNG pipeline;

[0052] 11-Main line reserved discharge and recovery shut-off valve, 12-Recovery pipeline, 13-Recovery pipeline first pressure transmitter, 14-Recovery pipeline regulating valve, 15-Recovery pipeline second pressure transmitter, 16-Recovery pipeline first temperature transmitter, 17-Gas-liquid buffer tank, 18-Recovery pipeline second temperature transmitter, 19-Flow transmitter.

[0053] 21-Removable insulation layer piping, 22-Auxiliary electric heat tracing piping, 23-Pressure detection transmitter;

[0054] 31-Displacement injection pipeline, 32-Displacement injection shut-off valve, 33-Displacement injection temperature transmitter, 34-Nitrogen displacement injection skid;

[0055] 41-Media discharge pipeline, 42-Media discharge pressure transmitter, 43-Media discharge shut-off valve, 44-Overpressure safety valve, 45-Manual relief valve. Detailed Implementation

[0056] To make the technical problems, technical solutions, and technical effects of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0057] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0058] It should be noted that similar reference numerals and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. Furthermore, the terms "first," "second," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0059] In the description of the embodiments of the present invention, it should be noted that the terms "inner", "outer", "upper", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship in which the product of the invention is usually placed when in use. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting the present invention.

[0060] Example 1

[0061] like Figure 1 As shown, this embodiment provides a long-distance LNG pipeline storage medium processing system based on natural gas, including a long-distance cryogenic LNG pipeline system, a medium recovery system, a medium discharge system, a pipeline heating system, and a residual gas replacement system. The residual gas replacement system is connected to the pipeline near the beginning of the long-distance cryogenic LNG pipeline system, and the medium recovery system and the medium discharge system are both connected to the pipeline near the end of the long-distance cryogenic LNG pipeline system. The pipeline heating system is intermittently installed on the long-distance cryogenic LNG pipeline system.

[0062] The long-distance cryogenic LNG pipeline system connects to the upstream LNG terminal at the front end and to the downstream LNG terminal at the end. The medium discharge system connects to the BOG venting system of the downstream terminal at the end, and the medium recovery system connects to the downstream recovery gas compressor at the end.

[0063] Specifically, by setting up this system, the problem of pre-maintenance venting faced by long-distance cryogenic LNG pipelines is optimized. The system recovers most of the medium to be vented by using natural gas to enter the downstream recovery gas compressor. It also increases the saturated vapor pressure of the medium stored in the pipeline and improves the venting rate by using auxiliary heating of the pipeline after the pipeline is shut down. Finally, it replaces and discharges the remaining low-pressure gas phase natural gas in the pipeline by using nitrogen replacement. Overall, the system achieves efficient and safe venting.

[0064] Example 2

[0065] This embodiment is a further optimization based on Embodiment 1, specifically:

[0066] The long-distance cryogenic LNG pipeline system includes an upstream LNG pipeline 1, a long-distance cryogenic LNG pipeline 3, and a downstream LNG pipeline 5 connected in sequence. An upstream shut-off valve 2 is installed at the connection between the upstream LNG pipeline 1 and the long-distance cryogenic LNG pipeline 3, and a downstream shut-off valve 4 is installed at the connection between the long-distance cryogenic LNG pipeline 3 and the downstream LNG pipeline 5. It also includes a series of π-shaped bends installed on the long-distance cryogenic LNG pipeline 3.

[0067] Specifically, the long-distance cryogenic LNG pipeline system in this solution is used to provide a long-distance transportation channel for cryogenic LNG media.

[0068] Example 3

[0069] This embodiment is a further optimization based on embodiment 2, specifically:

[0070] The medium recovery system includes a recovery pipeline 12 connected to a long-distance cryogenic LNG pipeline 3 near the downstream shut-off valve 4. The recovery pipeline 12 is arranged in sequence according to the fluid flow direction, including a main line reserved venting recovery shut-off valve 11, a recovery pipeline first pressure transmitter 13, a recovery pipeline regulating valve 14, a recovery pipeline second pressure transmitter 15, a recovery pipeline first temperature transmitter 16, a gas-liquid buffer tank 17, a recovery pipeline second temperature transmitter 18, and 19. The end of the recovery pipeline 12 is introduced into a downstream recovery gas compressor.

[0071] Specifically, the medium recovery system is used to regulate and vaporize the medium that can be output through pressure difference in the long-distance cryogenic LNG pipeline 3, and then introduce it into the downstream recovery gas compressor to collect and use the natural gas.

[0072] The pipeline heating system includes multiple heating units, each of which includes a detachable insulation layer pipeline 21, an auxiliary electric heat tracing pipeline 22, and a pressure detection transmitter 23 installed on the corresponding π-shaped bend.

[0073] Specifically, the pipeline heating system is used to promote the local low-temperature medium in the pipeline to absorb external heat and heat up after the pipeline is shut down, so as to help increase the pressure of the medium in the pipeline, increase the saturated vapor pressure of the medium in the pipeline, and accelerate the recovery of the medium.

[0074] The residual gas replacement system includes a replacement injection pipeline 31 connected to a long-distance cryogenic LNG pipeline 3 near the upstream shut-off valve 2. The replacement injection pipeline 31 is equipped with a replacement injection shut-off valve 32, a replacement injection temperature transmitter 33, and a nitrogen replacement injection skid 34.

[0075] Specifically, the residual gas replacement system is used to replace the remaining gaseous natural gas in the pipeline after the pipeline pressure approaches atmospheric pressure, and to control the overall pipeline temperature to gradually rise, providing a safety guarantee for subsequent maintenance.

[0076] The medium discharge system includes a medium discharge pipeline 41, on which a medium discharge pressure transmitter 42 and a medium discharge shut-off valve 43 are installed. Two pipelines are connected in parallel at the end of the medium discharge pipeline 41. One pipeline is equipped with an overpressure safety valve 44, and the other pipeline is equipped with a manual relief valve 45. The ends of both pipelines are connected to the downstream station BOG venting system.

[0077] Specifically, the media discharge system is used for potential overpressure protection during pipeline heating, and can also be used for safe gas discharge during pipeline nitrogen purging, and provides the function of analyzing the composition of the discharged gas.

[0078] Example 4

[0079] This embodiment provides a method for handling LNG storage media in long-distance pipelines for efficient utilization of natural gas, including the following steps:

[0080] S1. Shut down the main cryogenic LNG pipeline: shut down the submersible pump at the upstream LNG station, then shut down the upstream shut-off valve 2 and the downstream shut-off valve 4 to complete the shutdown of the long-distance cryogenic LNG pipeline 3.

[0081] S2. Perform pressurized discharge operation on the main cryogenic LNG pipeline. Specifically:

[0082] S21. Connect the venting medium recovery skid, which consists of recovery pipeline 12, recovery pipeline first pressure transmitter 13, recovery pipeline regulating valve 14, recovery pipeline second pressure transmitter 15, recovery pipeline first temperature transmitter 16, gas-liquid buffer tank 17, recovery pipeline second temperature transmitter 18, and flow transmitter 19, to the main line with a reserved venting recovery shut-off valve 11, and perform nitrogen purging.

[0083] S22. Open the trunk line reserved discharge and recovery shut-off valve 11, slowly open the recovery pipeline regulating valve 14, and control the discharge flow to meet the inlet flow requirements of the downstream recovery gas compressor. The discharge pressure in this process is provided by the medium pressure after the long-distance cryogenic LNG pipeline 3 is shut down.

[0084] S23. Turn on the electric heating of the gas-liquid buffer tank 17 and set the temperature to not exceed 10°C above the temperature detected by the first temperature transmitter 16 of the recovery pipeline, so as to promote the vaporization of liquid LNG in the gas-liquid buffer tank 17.

[0085] S24. As the upstream pressure decreases, gradually increase the opening of the regulating valve 14 in the recovery pipeline. When the flow rate detected by the flow transmitter 19 cannot meet the minimum flow rate requirement of the downstream recovery gas compressor, it indicates that the upstream medium pressure is insufficient, and proceed to step S3.

[0086] S3. Perform auxiliary heating and pressurization discharge operations on the main cryogenic LNG pipeline, specifically:

[0087] S31. Disassemble the insulation layer of the π-shaped bend with the pipeline heating system, and open the auxiliary electric heat tracing pipeline 22. Use atmospheric heat transfer, auxiliary electric heating and other methods to increase the temperature of the medium in the corresponding pipe section, promote the increase of the temperature of the gas-liquid two-phase low-temperature natural gas in the pipe section and the nearby pipe section, and increase the saturated vapor pressure of the medium.

[0088] S32. Open the medium discharge shut-off valve 43 and provide pressure protection for the upstream main pipeline to the overpressure safety valve 44.

[0089] S33. The temperature rise rate of the pipeline shall be detected by the temperature detection system installed along the long-distance cryogenic LNG pipeline 3. It shall not exceed 10℃ / h. Otherwise, the auxiliary electric heat tracing pipeline 22 shall be shut down.

[0090] S34. Continue to control the flow rate of the recovered natural gas using the recovery pipeline regulating valve 14 and the flow transmitter 19;

[0091] S35. Continuously monitor and recover the pressure parameters of the pipeline heating system and medium discharge pressure transmitter 42 along the pipeline, including the first pressure transmitter 13, pressure detection transmitter 23, etc. When the detected pressure continues to rise and reaches 90% of the pipeline design pressure value, the auxiliary electric heat tracing pipeline 22 is closed first to control the pressure rise rate of the medium in the pipeline. If the pressure continues to rise, the manual relief valve 45 is opened until the detected pressure is lower than 90% of the pipeline design pressure value and then the manual relief valve 45 is closed.

[0092] Specifically, when the temperature detection system along the long-distance cryogenic LNG pipeline 3 detects that the average temperature at each point in the pipeline reaches -50℃, it indicates that the natural gas in the pipeline has basically been vaporized. At this time, the auxiliary electric heat tracing pipeline 22 is closed. If the regulating valve 14 of the recovery pipeline cannot meet the minimum flow requirement of the downstream recovery gas compressor at its maximum opening, it indicates that the upstream pressure energy is basically exhausted, and the auxiliary heating pressurization and discharge operation is terminated.

[0093] S4. Perform external nitrogen purging operation on the main cryogenic LNG pipeline. Specifically:

[0094] S41. Connect the nitrogen replacement injection skid 34 to the replacement injection pipe 31; continue to open the auxiliary electric heat tracing pipe 22 to heat up the medium in the pipe, and at the same time open the manual relief valve 45. This operation can avoid the waste of cooling capacity caused by the vaporization of liquid nitrogen during nitrogen injection. Stop when the average temperature of each point in the pipe reaches -20℃.

[0095] S42. Open the replacement injection shut-off valve 32, open the nitrogen injection regulating valve of the nitrogen replacement injection skid 34, introduce room temperature nitrogen, and replace the residual natural gas in the pipeline. The replacement flow rate is preferably controlled to be 3m / s.

[0096] S43. By taking a sample through the gas component sampling port set on the medium discharge pipeline 41, when the methane concentration is less than 1%, it indicates that the nitrogen replacement is qualified and the replacement operation is completed.

[0097] In this embodiment, regarding the destination of the released medium, the proposed solution is to recover it by connecting it to a downstream recovery gas compressor (or BOG recovery system). Although the upstream medium pressure is low, it still meets the requirements of the recovery process. Therefore, the medium recovery system designed in this invention considers the phase change problem of low-pressure LNG pipelines during venting. By setting up recovery regulating valves, buffer tanks, electric heating systems, etc., a relay space for the released medium is provided to ensure that the gas temperature and composition entering the downstream recovery gas compressor meet the pressurization requirements. At the same time, the venting rate is controlled by using the detection results of a flow transmitter as a reference.

[0098] To address the issue of insufficient pressure in long-distance cryogenic LNG pipelines, this solution proactively raises the temperature of the medium inside the pipeline by setting up a pipeline heating system. This promotes gas volatilization, increases the saturated vapor pressure of the medium inside the pipeline, and thus increases the pipeline pressure, allowing more LNG to be recovered through the medium recovery system. This avoids the method of directly introducing nitrogen for displacement, as this method may cause nitrogen to directly enter the gas phase space and flow downstream after the LNG forms a gas-liquid two-phase structure inside the pipeline, resulting in a high nitrogen content in the downstream recovery medium.

[0099] For the issue of rewarming long-distance cryogenic LNG pipelines before maintenance, this solution adopts an active auxiliary heating medium method. While promoting the volatilization of the medium, it also gradually increases the temperature of the medium. Then, it uses the LNG (gas-liquid two-phase) inside the pipeline to raise the temperature of the pipeline body. On the one hand, it avoids the problem of continuous rewarming after the pipeline is emptied of LNG, and on the other hand, it reduces the cryogenic nitrogen restriction requirements for subsequent nitrogen replacement (that is, there is no need to introduce cryogenic nitrogen to gradually rewarm the pipeline in stages; nitrogen at near room temperature can be used directly).

[0100] Similar to the pre-cooling of LNG pipelines before commissioning, this scheme proposes a heating rate requirement during the reheating process of the pipeline using a pipeline heating system. The purpose is to avoid irregular deformation of the LNG pipeline cross-section due to excessively rapid local heating.

Claims

1. A long-distance LNG pipeline storage medium processing system based on natural gas, characterized in that, The system includes a long-distance cryogenic LNG pipeline system, a medium recovery system, a medium discharge system, a pipeline heating system, and a residual gas replacement system. The residual gas replacement system is connected to the pipeline near the beginning of the long-distance cryogenic LNG pipeline system. The medium recovery system and the medium discharge system are both connected to the pipeline near the end of the long-distance cryogenic LNG pipeline system. The pipeline heating system is spaced out on the long-distance cryogenic LNG pipeline system. The long-distance cryogenic LNG pipeline system is connected to the upstream LNG terminal at its front end and to the downstream LNG terminal at its rear end. The medium discharge system is connected to the downstream terminal's BOG venting system at its rear end, and the medium recovery system is connected to the downstream recovery gas compressor at its rear end.

2. The long-distance LNG pipeline storage medium processing system based on natural gas according to claim 1, characterized in that, The long-distance cryogenic LNG pipeline system includes an upstream LNG pipeline (1), a long-distance cryogenic LNG pipeline (3), and a downstream LNG pipeline (5) connected in sequence. An upstream shut-off valve (2) is provided at the connection between the upstream LNG pipeline (1) and the long-distance cryogenic LNG pipeline (3), and a downstream shut-off valve (4) is provided at the connection between the long-distance cryogenic LNG pipeline (3) and the downstream LNG pipeline (5). The system also includes a series of π-shaped bends distributed on the long-distance cryogenic LNG pipeline (3).

3. The long-distance LNG pipeline storage medium processing system based on natural gas according to claim 2, characterized in that, The medium recovery system includes a recovery pipeline (12) connected to the long-distance cryogenic LNG pipeline (3) near the downstream shut-off valve (4). The recovery pipeline (12) is provided with a trunk line reserved discharge recovery shut-off valve (11), a recovery pipeline first pressure transmitter (13), a recovery pipeline regulating valve (14), a recovery pipeline second pressure transmitter (15), a recovery pipeline first temperature transmitter (16), a gas-liquid buffer tank (17), a recovery pipeline second temperature transmitter (18), and (19) in sequence according to the fluid flow direction. The end of the recovery pipeline (12) is introduced into a downstream recovery gas compressor.

4. A long-distance LNG pipeline storage medium processing system based on natural gas according to claim 2, characterized in that, The pipeline heating system includes multiple heating units, each of which includes a detachable insulation layer pipeline (21), an auxiliary electric heat tracing pipeline (22), and a pressure detection transmitter (23) installed on the corresponding π-shaped bend.

5. A long-distance LNG pipeline storage medium processing system based on natural gas according to claim 2, characterized in that, The residual gas replacement system includes a replacement injection pipeline (31) connected to the long-distance cryogenic LNG pipeline (3) near the upstream shut-off valve (2). The replacement injection pipeline (31) is equipped with a replacement injection shut-off valve (32), a replacement injection temperature transmitter (33), and a nitrogen replacement injection skid (34).

6. A long-distance LNG pipeline storage medium processing system based on natural gas according to claim 2, characterized in that, The medium discharge system includes a medium discharge pipeline (41), on which a medium discharge pressure transmitter (42) and a medium discharge shut-off valve (43) are installed. Two pipelines are connected in parallel at the end of the medium discharge pipeline (41). One pipeline is equipped with an overpressure safety valve (44), and the other pipeline is equipped with a manual relief valve (45). The ends of both pipelines are connected to the downstream station BOG venting system.

7. A method for handling the medium in long-distance LNG pipelines for efficient utilization of natural gas, employing the long-distance LNG pipeline medium handling system based on natural gas as described in claims 1 to 6, characterized in that, Includes the following steps: S1. Shut down the main cryogenic LNG pipeline: shut down the submersible pump at the upstream LNG station, and then shut down the upstream cut-off valve (2) and the downstream cut-off valve (4) to complete the shutdown of the long-distance cryogenic LNG pipeline (3). S2. Perform pressurized discharge operation on the main cryogenic LNG pipeline; S3. Perform auxiliary heating and pressurization discharge operations on the main cryogenic LNG pipeline. S4. Perform external nitrogen purging operation on the main cryogenic LNG pipeline.

8. A method for handling medium in long-distance LNG pipelines for efficient utilization of natural gas, as described in claim 7, is characterized in that... In step S2, the specific steps for performing pressurized discharge operation on the main cryogenic LNG pipeline are as follows: S21. Connect the venting medium recovery skid, which consists of the recovery pipeline (12), the first pressure transmitter (13) of the recovery pipeline, the regulating valve (14) of the recovery pipeline, the second pressure transmitter (15) of the recovery pipeline, the first temperature transmitter (16) of the recovery pipeline, the gas-liquid buffer tank (17), the second temperature transmitter (18) of the recovery pipeline, and the flow transmitter (19), to the main line with the reserved venting recovery shut-off valve (11), and perform nitrogen purging. S22. Open the trunk line reserved discharge and recovery shut-off valve (11), slowly open the recovery pipeline regulating valve (14), and control the discharge flow to meet the inlet flow requirements of the downstream recovery gas compressor. The discharge pressure in this process is provided by the medium pressure after the long-distance cryogenic LNG pipeline (3) stops transporting. S23. Turn on the electric heating of the gas-liquid buffer tank (17) and set the temperature to not exceed 10°C above the temperature detected by the first temperature transmitter (16) of the recovery pipeline, so as to promote the vaporization of liquid LNG in the gas-liquid buffer tank (17). S24. As the upstream pressure decreases, gradually increase the opening of the regulating valve (14) in the recovery pipeline. When the flow rate detected by the flow transmitter (19) cannot meet the minimum flow rate requirement of the downstream recovery gas compressor, it indicates that the upstream medium pressure is insufficient, and proceed to step S3.

9. A method for handling medium stored in long-distance LNG pipelines for efficient utilization of natural gas, as described in claim 7, is characterized in that... In step S3, the specific steps for the auxiliary heating, pressurization, and discharge operation of the main cryogenic LNG pipeline are as follows: S31. Disassemble the insulation layer of the π-shaped bend with the pipeline heating system (21) and open the auxiliary electric heat tracing pipeline (22). Use atmospheric heat transfer and auxiliary electric heating to increase the temperature of the medium in the corresponding pipe section, promote the increase of the temperature of the gas-liquid two-phase low-temperature natural gas in the pipe section and the nearby pipe section, and increase the saturated vapor pressure of the medium. S32, Open the medium discharge shut-off valve (43) and provide pressure protection for the upstream main pipeline by the overpressure safety valve (44); S33. The temperature rise rate of the pipeline shall be detected by the temperature detection system along the long-distance cryogenic LNG pipeline (3) and shall not exceed 10℃ / h; otherwise, the auxiliary electric heat tracing pipeline (22) shall be shut down. S34. Continue to control the flow rate of the recovered natural gas using the recovery pipeline regulating valve (14) and flow transmitter (19); S35. Continuously monitor the pressure parameters of the first pressure transmitter (13), pressure detection transmitter (23), and medium discharge pressure transmitter (42) in the recovery pipeline. When the detected pressure continues to rise and reaches 90% of the pipeline design pressure value, close the auxiliary electric heat tracing pipeline (22) and control the pressure rise rate of the medium in the pipeline. If the pressure continues to rise, open the manual relief valve (45) until the detected pressure is lower than 90% of the pipeline design pressure value and then close the manual relief valve (45).

10. A method for handling medium in long-distance LNG pipelines for efficient utilization of natural gas, as described in claim 7, characterized in that, In step S4, the specific method for performing external nitrogen purging on the main cryogenic LNG pipeline is as follows: S41. Connect the nitrogen replacement injection skid (34) to the replacement injection pipeline (31); continue to open the auxiliary electric heat tracing pipeline (22) to heat up the medium in the pipeline, and at the same time open the manual relief valve (45) until the average temperature of each point in the pipeline reaches -20℃ and then stop. S42. Open the replacement injection shut-off valve (32), open the nitrogen injection regulating valve of the nitrogen replacement injection skid (34), introduce room temperature nitrogen, perform replacement operation on the residual natural gas in the pipeline, and control the replacement flow rate. S43. By taking a sample through the gas component sampling port set on the medium discharge pipeline (41), when the methane concentration is less than 1%, it indicates that the nitrogen replacement is qualified and the replacement operation is completed.

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