Long distance cryogenic lng pipeline commissioning system and method based on multi-point pre-cooling

Abstract

The application discloses a long-distance low-temperature LNG pipeline commissioning system and method based on multi-point precooling, relates to the technical field of LNG pipeline transportation, and comprises a long-distance low-temperature LNG pipeline system, a multi-point injection and discharge system in communication with the long-distance low-temperature LNG pipeline system at different positions, a top and bottom temperature difference adjusting system adopting a medium active disturbance mode to reduce the top and bottom temperature difference of the long-distance low-temperature LNG pipeline system, a safety discharge system in communication with the multi-point injection and discharge system, and an overpressure protection system in communication with the long-distance low-temperature LNG pipeline system. The long-distance low-temperature LNG pipeline is subjected to nitrogen primary precooling, secondary precooling coupled nitrogen replacement and LNG filling and the like through the long-distance low-temperature LNG pipeline system, the multi-point injection and discharge system, the top and bottom temperature difference adjusting system, the safety discharge system and the overpressure protection system, so that the long-distance low-temperature LNG pipeline is efficiently and safely commissioned.

Description

Technical Field

[0001] This invention relates to the field of LNG pipeline transportation technology, and more specifically to the field of commissioning systems and methods for long-distance cryogenic LNG pipelines based on multi-point precooling. 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 via pipeline in its gaseous phase to the natural gas network, adhering to natural gas pipeline transportation standards. Within LNG receiving terminals, short-distance liquid-phase pipelines are generally used to transport received LNG to storage tanks, employing cryogenic transportation. However, long-distance cryogenic LNG pipeline transportation (i.e., long-distance transportation of cryogenic LNG to cold energy utilization points or LNG-to-ethane / propane extraction plants, etc.) is rare. Because cryogenic LNG pipelines operate at temperatures of -150℃ and below, pre-cooling of the pipeline system is essential before commissioning. This prevents excessive temperature drops caused by directly introducing cryogenic LNG, which could lead to uncontrollable deformation and material damage. Currently, LNG receiving terminal pipelines generally employ nitrogen pre-cooling followed by LNG filling, or a direct LNG pre-cooling and LNG filling commissioning approach. Because the capacity of LNG pipelines within the terminal is much smaller than that of long-distance LNG pipelines, and the precooling process requires a relatively low cooling rate, directly applying the aforementioned precooling and commissioning methods to long-distance LNG pipelines would consume a significant amount of precooling time, resulting in substantial waste of production windows and pipeline transportation resources. Furthermore, since LNG pipelines within the terminal are relatively short, a single-point injection and single-point discharge precooling strategy is currently employed. This presents a problem for long-distance LNG pipelines, where downstream sections lack precooling input for a period of time. Additionally, the handling of the precooling medium primarily utilizes the existing venting system of the terminal; whether this is suitable for the commissioning of long-distance LNG pipelines requires further investigation.

[0004] However, there are currently few reports on the precooling and commissioning of long-distance LNG pipelines, and the corresponding standards and specifications have not yet been released. Summary of the Invention

[0005] The purpose of this invention is to address the technical problem that the single-point injection and single-point discharge precooling strategy for short-distance LNG pipeline transportation leads to a lack of precooling cold energy input in the downstream pipeline section for a period of time, making it unsuitable for precooling and commissioning of long-distance LNG pipeline transportation. This invention provides a long-distance cryogenic LNG pipeline commissioning system and method based on multi-point precooling.

[0006] To achieve the above objectives, the present invention specifically adopts the following technical solution:

[0007] One aspect of the present invention provides a long-distance cryogenic LNG pipeline commissioning system based on multi-point precooling, including a long-distance cryogenic LNG pipeline system, a multi-point injection and discharge system connected to different locations of the long-distance cryogenic LNG pipeline system, a top and bottom temperature difference adjustment system that uses active medium disturbance to reduce the top and bottom temperature difference of the long-distance cryogenic LNG pipeline system, a safety discharge system connected to the multi-point injection and discharge system, and an overpressure protection system connected to the long-distance cryogenic LNG pipeline system.

[0008] Specifically, by setting up this system, the pre-cooling and commissioning problems faced by long-distance cryogenic LNG pipelines are addressed in a targeted manner. The system adopts the approach of active medium disturbance to reduce the temperature difference between the top and bottom of the pipeline, adopts multi-point simultaneous pre-cooling to effectively reduce pre-cooling time, safely handles the pre-cooling and commissioning medium, and provides thermal overpressure protection for the cryogenic LNG pipeline after filling. This comprehensive approach achieves efficient and safe pre-cooling and commissioning of long-distance cryogenic LNG pipelines.

[0009] In one embodiment, the cryogenic LNG pipeline system includes an upstream LNG pipeline, a long-distance cryogenic LNG pipeline, and a downstream LNG pipeline connected in sequence; the upstream LNG pipeline is equipped with an upstream shut-off valve, the downstream LNG pipeline is equipped with a downstream shut-off valve, and the long-distance cryogenic LNG pipeline has multiple π-shaped bends continuously distributed along it.

[0010] Specifically, the upstream LNG pipeline, the long-distance cryogenic LNG pipeline, and the downstream LNG pipeline are all made of cryogenic austenitic stainless steel and are equipped with insulation layers and fiber optic temperature detection along the pipeline.

[0011] In one embodiment, the multi-point injection and discharge system includes multiple nitrogen injection pipeline systems and at least one safety discharge pipeline system. The multiple nitrogen injection pipeline systems are connected to different locations on the long-distance cryogenic LNG pipeline, and each safety discharge pipeline system is connected to different locations on the long-distance cryogenic LNG pipeline. A control valve is installed on the long-distance cryogenic LNG pipeline between two adjacent safety discharge pipeline systems.

[0012] Specifically, this scheme first injects a pre-cooling medium sequentially from multiple nitrogen injection pipeline systems into the long-distance cryogenic LNG pipeline. Then, LNG is introduced from the upstream LNG pipeline into the long-distance cryogenic LNG pipeline for secondary replacement and pre-cooling, and the pipeline medium is filled to achieve system commissioning.

[0013] In one embodiment, each nitrogen injection pipeline system includes a nitrogen injection pipe communicating with the interior of a long-distance cryogenic LNG pipeline, a movable nitrogen injection skid communicating with the end of the nitrogen injection pipe, and a nitrogen injection shut-off valve installed on the nitrogen injection pipe.

[0014] In one embodiment, each safety discharge pipeline system includes a nitrogen discharge pipe connected to the interior of a long-distance cryogenic LNG pipeline, and a nitrogen discharge shut-off valve installed on the nitrogen discharge pipe, with the end of the nitrogen discharge pipe connected to the safety discharge system.

[0015] In one embodiment, the system further includes an upstream bypass pipeline system, which includes an upstream bypass pipeline. The upstream bypass pipeline is connected at both ends to the upstream LNG pipeline on both sides of the upstream shut-off valve. The upstream bypass pipeline is equipped with an upstream bypass regulating valve, an upstream bypass flow transmitter, and a vaporizer.

[0016] In one embodiment, the top-bottom temperature difference regulation system includes multiple static mixers and multiple distributed flow vanes spaced along the fluid flow direction inside the long-distance cryogenic LNG pipeline. The number of static mixers is the same as the number of nitrogen injection pipeline systems and corresponds one-to-one. Each static mixer is installed at the connection between the nitrogen injection pipe and the long-distance cryogenic LNG pipeline.

[0017] Specifically, when a multi-point injection and discharge system includes two nitrogen injection pipeline systems (upper and lower), the top-bottom temperature difference regulation system includes an upstream injection static mixer, a downstream injection static mixer, and distributed flow vanes along the pipeline. These are used to create forced turbulence during the injection and flow of the precooling medium in the LNG pipeline, effectively improving the uneven precooling problem caused by the uneven temperature distribution between the top and bottom during the precooling process in large-diameter LNG pipelines. Additionally, the top-bottom temperature difference regulation system also includes liquid nitrogen pumps on the upstream and downstream movable nitrogen injection skids, used to moderately increase the flow velocity of nitrogen within the pipeline, also achieving the effect of top-bottom temperature difference regulation.

[0018] In one embodiment, the safety discharge system includes a safety discharge pipe connected to the end of a nitrogen discharge pipe and a vent riser installed at the end of the safety discharge pipe. The safety discharge pipe is also equipped with a discharge pipe pressure transmitter, a discharge pipe flow transmitter, and a discharge regulating valve.

[0019] Specifically, the safety discharge system includes a safety discharge pipeline, a discharge pipeline pressure transmitter, a discharge pipeline flow transmitter, a discharge regulating valve, and a vent riser. The safety discharge system is used in the nitrogen precooling process to safely discharge the injected nitrogen into the atmosphere, preventing low-temperature nitrogen from accumulating near the pipeline discharge port and causing the risk of frostbite and suffocation to personnel.

[0020] In one embodiment, the overpressure protection system includes an overpressure protection inlet pipe, an overpressure protection shut-off valve, an overpressure protection pressure transmitter, a thermal safety valve, an overpressure protection regulating and venting valve, and an overpressure protection venting pipe.

[0021] The overpressure protection inlet pipe is internally connected to the long-distance cryogenic LNG pipeline. The overpressure protection shut-off valve is installed on the overpressure protection inlet pipe. Two pipelines are connected in parallel between the end of the overpressure protection inlet pipe and the overpressure protection relief pipe. The overpressure protection pressure transmitter and thermal safety valve are installed on one pipeline, and the overpressure protection regulating relief valve is installed on the other pipeline.

[0022] Specifically, the overpressure protection system is used to protect against overpressure caused by thermal expansion during the filling process of long-distance cryogenic LNG pipelines, and to discharge the overpressure relief medium into the downstream station BOG venting system. It is also used to provide nitrogen and nitrogen-containing natural gas emission and concentration detection functions when replacing nitrogen in long-distance cryogenic LNG pipelines.

[0023] Another aspect of the present invention provides a method for commissioning long-distance cryogenic LNG pipelines based on multi-point precooling, employing the aforementioned long-distance cryogenic LNG pipeline commissioning system based on multi-point precooling, comprising the following steps:

[0024] S1. First-stage precooling of long-distance cryogenic LNG pipeline: Close the upstream and downstream shut-off valves. Before the long-distance cryogenic LNG pipeline is precooled, the pipeline has completed nitrogen replacement and drying. Open the nitrogen injection and nitrogen discharge shut-off valves to start the first-stage precooling of the long-distance cryogenic LNG pipeline with nitrogen, precooling to -40℃.

[0025] S2. The first two-stage and first three-stage precooling of the long-distance cryogenic LNG pipeline is carried out in accordance with the steps of step S1. The long-distance cryogenic LNG pipeline is precooled to -80℃ in the first two-stage nitrogen precooling and precooled to -120℃ in the first three-stage nitrogen precooling. The nitrogen injection cut-off valve and the nitrogen discharge cut-off valve are then closed.

[0026] S3. Inject vaporized LNG into the long-distance cryogenic LNG pipeline through the upstream bypass pipeline system, and open the overpressure protection shut-off valve and overpressure protection regulating and venting valve of the overpressure protection system to establish the replacement and secondary precooling of the long-distance cryogenic LNG pipeline: open the temperature detection system along the long-distance cryogenic LNG pipeline to continuously detect the temperature at the top and bottom of the long-distance cryogenic LNG pipeline, control the maximum temperature drop rate of the long-distance cryogenic LNG pipeline to 5℃ / h, and gradually cool down to -160℃. Control the nitrogen emission rate through the overpressure protection regulating and venting valve, and sample the components of the discharged gas through the overpressure protection venting pipeline until the nitrogen content of the discharged medium is less than 0.5%, indicating that the replacement of nitrogen with vaporized LNG is qualified.

[0027] S4. Filling long-distance cryogenic LNG pipelines: Filling is carried out by gas-phase pressurization and slow liquefaction.

[0028] The beneficial effects of this invention are as follows:

[0029] 1. This invention, by setting up a long-distance cryogenic LNG pipeline system, a multi-point injection and discharge system, a top-bottom temperature difference regulation system, a safety discharge system, and an overpressure protection system, performs nitrogen pre-cooling, secondary pre-cooling coupled with nitrogen replacement, and LNG filling operations on the long-distance cryogenic LNG pipeline, thereby achieving efficient and safe commissioning of the long-distance cryogenic LNG pipeline and achieving the goal of making a breakthrough in promoting the development of long-distance cryogenic LNG pipeline technology.

[0030] 2. Scientific Design of this Invention: A multi-point injection system simultaneously injects a pre-cooling medium (cryogenic nitrogen) into the main LNG pipeline from both the beginning and end points, increasing the primary pre-cooling speed and dividing the process into three pre-cooling gradients. This ensures pre-cooling safety while improving efficiency. A top-to-bottom temperature difference adjustment system (mainly including an injection static mixer and distributed flow plates along the pipeline) generates active turbulence during the flow of the pre-cooling medium (liquid nitrogen, vaporized LNG, etc.), overcoming the problem of excessive top-to-bottom temperature differences that may occur during conventional LNG pipeline pre-cooling. Furthermore, an injection and static circulation scheme is implemented to address the issue of irregular axial temperature changes. Additionally, the flow rate of the pre-cooling medium is adjusted. This further mitigates the problem of uneven temperature distribution of the medium across the pipeline cross-section; a safety venting system is installed to complement the multi-point injection and venting system, providing a safe venting system in the middle of the pipeline for nitrogen injected at the starting and ending points, avoiding the risk of low-temperature nitrogen vented after pre-cooling posing a low-temperature hazard to adjacent pipelines and personnel; during the commissioning of the cryogenic LNG pipeline, a gas-phase pressurization and gas-phase liquefaction method is adopted for filling the long-distance cryogenic LNG pipeline with liquid LNG, avoiding severe boiling and turbulence phenomena that would occur if LNG with high temperature is directly introduced into the long-distance cryogenic LNG pipeline from upstream; an overpressure protection system is installed to protect against overpressure caused by thermal expansion during the filling process of the long-distance cryogenic LNG pipeline. Overall, given the current lack of experience in long-distance cryogenic LNG commissioning in China, this invention proposes solutions to the above-mentioned key issues.

[0031] 3. This invention boasts strong economic advantages: By employing a multi-point injection and discharge system, this invention improves the pre-cooling and commissioning efficiency of long-distance LNG pipelines. The conventional "one-in-one-out" nitrogen pre-cooling scheme is replaced with a "multi-in-one-out" system through multi-point nitrogen injection. The use of a static mixer and distributed flow vanes along the pipeline mitigates the large temperature difference between the top and bottom of the LNG pipeline, thereby shortening the time to reach the pre-cooling temperature through coupling. Simultaneously, regarding nitrogen replacement within the pipeline, the direct introduction of vaporized LNG for nitrogen replacement and secondary pre-cooling replaces the "nitrogen replacement first, then LNG secondary pre-cooling" approach, further shortening the pipeline pre-cooling and commissioning time. In summary, by adopting a reasonable technical solution, this invention can shorten the pre-cooling and commissioning time of long-distance cryogenic LNG pipelines, increase the actual pipeline operating time, and enhance pipeline transportation efficiency. Furthermore, the nitrogen injection skid in this invention is a movable module that can be reused for other similar cryogenic pipelines.

[0032] 4. This invention promotes technological development: my country has not yet carried out the construction and operation of large-scale, long-distance cryogenic LNG pipelines. The solution proposed in this system has played an important role in engineering guidance and reference. The proposed commissioning system and method for long-distance cryogenic LNG pipelines based on multi-point precooling helps to improve the commissioning efficiency and safety of pipelines, and promotes the development of technical concepts and technological progress in this field. Attached Figure Description

[0033] 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.

[0034] Figure 1 This is a schematic diagram of the structure of a long-distance cryogenic LNG pipeline commissioning system based on multi-point precooling according to the present invention;

[0035] 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;

[0036] 11-Upstream movable nitrogen injection skid, 12-Downstream movable nitrogen injection skid, 13-Upstream injection pipe, 14-Upstream injection shut-off valve, 15-Downstream injection pipe, 16-Downstream injection shut-off valve, 17-Intermediate discharge pipe, 18-Intermediate discharge shut-off valve, 19-Upstream bypass pipe, 20-Upstream bypass regulating valve, 21-Upstream bypass flow transmitter, 22-Vaporizer;

[0037] 31-Upstream injection static mixer, 32-Downstream injection static mixer, 33-Distributed flow around the pipeline; 41-Safety discharge pipeline, 42-Discharge pipeline pressure transmitter, 43-Discharge pipeline flow transmitter, 44-Discharge regulating valve, 45-Vent riser;

[0038] 51-Overpressure protection inlet pipe, 52-Overpressure protection shut-off valve, 53-Overpressure protection pressure transmitter, 54-Thermal safety valve, 55-Overpressure protection regulating and relieving valve, 56-Overpressure protection relieving pipe. Detailed Implementation

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] This invention provides a long-distance cryogenic LNG pipeline commissioning system based on multi-point precooling, including a long-distance cryogenic LNG pipeline system, a multi-point injection and discharge system connected to different locations of the long-distance cryogenic LNG pipeline system, a top and bottom temperature difference adjustment system that uses active medium disturbance to reduce the top and bottom temperature difference of the long-distance cryogenic LNG pipeline system, a safety discharge system connected to the multi-point injection and discharge system, and an overpressure protection system connected to the long-distance cryogenic LNG pipeline system.

[0044] Specifically, by setting up this system, the pre-cooling and commissioning problems faced by long-distance cryogenic LNG pipelines can be addressed in a targeted manner. The system adopts the approach of active medium disturbance to reduce the temperature difference between the top and bottom of the pipeline, adopts multi-point simultaneous pre-cooling to effectively reduce the pre-cooling time, safely handles the pre-cooling and commissioning medium, and provides thermal overpressure protection for the cryogenic LNG pipeline after filling. This comprehensive approach achieves efficient and safe pre-cooling and commissioning of long-distance cryogenic LNG pipelines.

[0045] In one embodiment, the 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; the upstream LNG pipeline 1 is equipped with an upstream shut-off valve 2, the downstream LNG pipeline 5 is equipped with a downstream shut-off valve 4, and the long-distance cryogenic LNG pipeline 3 has multiple π-shaped bends continuously distributed on it.

[0046] Specifically, the upstream LNG pipeline 1, the long-distance cryogenic LNG pipeline 3, and the downstream LNG pipeline 5 are all made of cryogenic austenitic stainless steel and are equipped with insulation layers and fiber optic temperature detection along the pipeline.

[0047] In one embodiment, the multi-point injection and discharge system includes multiple nitrogen injection pipeline systems and at least one safety discharge pipeline system 41. The multiple nitrogen injection pipeline systems are respectively connected to different locations of the long-distance cryogenic LNG pipeline 3, and each safety discharge pipeline system 41 is connected to different locations of the long-distance cryogenic LNG pipeline 3. A control valve is installed on the long-distance cryogenic LNG pipeline 3 between two adjacent safety discharge pipeline systems 41.

[0048] Specifically, this scheme first injects a pre-cooling medium into the long-distance cryogenic LNG pipeline 3 sequentially from multiple nitrogen injection pipeline systems. Then, LNG is introduced into the long-distance cryogenic LNG pipeline 3 from the upstream LNG pipeline 1 for secondary replacement and pre-cooling, and the pipeline medium filling is completed to put the system into operation.

[0049] In one embodiment, each nitrogen injection pipeline system includes a nitrogen injection pipe communicating with the interior of the long-distance cryogenic LNG pipeline 3, a movable nitrogen injection skid communicating with the end of the nitrogen injection pipe, and a nitrogen injection shut-off valve installed on the nitrogen injection pipe.

[0050] In one embodiment, each of the 41 safety discharge pipeline systems includes a nitrogen discharge pipe that communicates with the interior of the long-distance cryogenic LNG pipeline 3, and a nitrogen discharge shut-off valve installed on the nitrogen discharge pipe. The end of the nitrogen discharge pipe is connected to the safety discharge system.

[0051] In one embodiment, the system also includes an upstream bypass pipeline system, which includes an upstream bypass pipeline 19. The upstream bypass pipeline 19 is connected at both ends to the upstream LNG pipeline 1 on both sides of the upstream shut-off valve 2. An upstream bypass regulating valve 20, an upstream bypass flow transmitter 21, and a vaporizer 22 are installed on the upstream bypass pipeline 19.

[0052] In one embodiment, the top-bottom temperature difference regulation system includes multiple static mixers and multiple distributed flow plates 33 distributed along the fluid flow direction inside the long-distance cryogenic LNG pipeline 3. The number of static mixers is the same as the number of nitrogen injection pipeline systems and corresponds one-to-one. Each static mixer is installed at the connection between the nitrogen injection pipe and the long-distance cryogenic LNG pipeline 3.

[0053] Specifically, when the multi-point injection and discharge system includes two nitrogen injection pipeline systems, upstream and downstream, the top-bottom temperature difference regulation system includes an upstream injection static mixer 31, a downstream injection static mixer 32, and 33 distributed bypass blades along the pipeline. These are used to create forced turbulence during the injection and flow of the pre-cooling medium in the LNG pipeline, effectively improving the uneven pre-cooling problem caused by the uneven temperature distribution between the top and bottom during the pre-cooling process in large-diameter LNG pipelines. In addition, the top-bottom temperature difference regulation system also includes liquid nitrogen pumps in the upstream movable nitrogen injection skid 11 and the downstream movable nitrogen injection skid 12, used to moderately increase the flow velocity of nitrogen in the pipeline, which can also achieve the effect of top-bottom temperature difference regulation.

[0054] In one embodiment, the safety discharge system includes a safety discharge pipe 41 connected to the end of a nitrogen discharge pipe and a vent riser 45 disposed at the end of the safety discharge pipe 41. The safety discharge pipe 41 is also equipped with a discharge pipe pressure transmitter 42, a discharge pipe flow transmitter 43 and a discharge regulating valve 44.

[0055] Specifically, the safety discharge system includes a safety discharge pipe 41, a discharge pipe pressure transmitter 42, a discharge pipe flow transmitter 43, a discharge regulating valve 44, and a vent riser 45. The safety discharge system is used in the nitrogen pre-cooling process to safely discharge the injected nitrogen into the atmosphere, preventing low-temperature nitrogen from accumulating near the pipe discharge port and causing the risk of frostbite and suffocation to personnel.

[0056] In one embodiment, the overpressure protection system includes an overpressure protection inlet pipe 51, an overpressure protection shut-off valve 52, an overpressure protection pressure transmitter 53, a thermal safety valve 54, an overpressure protection regulating and venting valve 55, and an overpressure protection venting pipe 56.

[0057] The overpressure protection inlet pipe 51 is internally connected to the long-distance cryogenic LNG pipeline 3. The overpressure protection shut-off valve 52 is installed on the overpressure protection inlet pipe 51. Two pipelines are connected in parallel between the end of the overpressure protection inlet pipe 51 and the overpressure protection relief pipe 56. The overpressure protection pressure transmitter 53 and the thermal safety valve 54 are installed on one pipeline, and the overpressure protection regulating relief valve 55 is installed on the other pipeline.

[0058] Specifically, the overpressure protection system is used to protect the pipeline from thermal expansion overpressure that may occur during the filling process of the long-distance cryogenic LNG pipeline 3, and to discharge the overpressure relief medium into the downstream station BOG venting system. It is also used to provide nitrogen and nitrogen-containing natural gas emission and concentration detection functions when the long-distance cryogenic LNG pipeline 3 is purged with nitrogen.

[0059] Example 1

[0060] Another aspect of the present invention provides a method for commissioning a long-distance cryogenic LNG pipeline based on multi-point precooling, using the aforementioned long-distance cryogenic LNG pipeline commissioning system based on multi-point precooling. The multi-point injection and discharge system, taking as an example a system comprising upstream and downstream nitrogen injection pipeline systems and an intermediate safety discharge pipeline system, has the following detailed structure:

[0061] The upstream nitrogen injection pipeline system includes an upstream injection pipe 13 connected to the front end of the long-distance cryogenic LNG pipeline 3, an upstream injection shut-off valve 14 installed on the upstream injection pipe 13, and an upstream movable nitrogen injection skid 11 installed at the end of the upstream injection pipe 13.

[0062] The downstream nitrogen injection pipeline system includes a downstream injection pipe 15 connected to the rear end of the long-distance cryogenic LNG pipeline 3, a downstream injection shut-off valve 16 installed on the downstream injection pipe 15, and a downstream movable nitrogen injection skid 12 installed at the end of the downstream injection pipe 15.

[0063] The intermediate safety discharge pipeline 41 system includes an intermediate discharge pipe 17 connected to the middle of the long-distance cryogenic LNG pipeline 3, and an intermediate discharge shut-off valve 18 installed on the intermediate discharge pipe 17.

[0064] It also includes an upstream bypass pipeline system, which includes an upstream bypass pipeline 19. The upstream bypass pipeline 19 is connected at both ends to the upstream LNG pipeline 1 on both sides of the upstream shut-off valve 2. An upstream bypass regulating valve 20, an upstream bypass flow transmitter 21 and a vaporizer 22 are installed on the upstream bypass pipeline 19.

[0065] The materials and uses of the above structures are as follows: The upstream movable nitrogen injection skid 11 and the downstream movable nitrogen injection skid 12 are both movable liquid nitrogen storage, pressurization, flow regulation, and vaporization skids, used to provide cryogenic nitrogen at different temperatures and flow rates before the cryogenic LNG pipeline is put into operation for pipeline precooling. The materials used are both cryogenic austenitic stainless steel or 9% nickel steel. The upstream injection pipe 13 is located downstream of the upstream shut-off valve 2 and provides a flow channel for cryogenic nitrogen precooling. The preferred material is cryogenic austenitic stainless steel. The upstream injection shut-off valve 14 is located on the upstream injection pipe 13, a manually operated ball valve, normally closed, and opens under cryogenic nitrogen precooling. The preferred material is cryogenic austenitic stainless steel. The downstream injection pipe 15 is located upstream of the downstream shut-off valve 4 and provides a flow channel for cryogenic nitrogen precooling. The preferred material is cryogenic austenitic stainless steel. The downstream injection shut-off valve 16 is located on the downstream injection pipe 15, a manually operated ball valve, normally closed, and opens under cryogenic nitrogen precooling. The preferred material is cryogenic austenitic stainless steel. Intermediate discharge pipe 17 is located in the middle section of long-distance cryogenic LNG pipeline 3, providing a nitrogen discharge channel under nitrogen precooling operation. The preferred material is cryogenic austenitic stainless steel. Intermediate discharge shut-off valve 18 is located on intermediate discharge pipe 17, a manual ball valve, normally closed, opening under cryogenic nitrogen precooling, and the preferred material is cryogenic austenitic stainless steel. Upstream bypass pipe 19 is located at both ends of upstream shut-off valve 2, providing a flow channel for secondary precooling and LNG filling of downstream long-distance cryogenic LNG pipeline 3, and the preferred material is cryogenic austenitic stainless steel. Upstream bypass regulating valve 20 is located on upstream bypass pipe 19, an automatic regulating valve, used to regulate the replacement and secondary precooling flow of upstream LNG into downstream long-distance cryogenic LNG pipeline 3, and the preferred material is cryogenic austenitic stainless steel. Vaporizer 22 is a component of upstream movable nitrogen injection skid 11, used to vaporize the LNG entering downstream long-distance cryogenic LNG pipeline 3 and control the temperature of the vaporized LNG, and the preferred material is 9% nickel steel.

[0066] The commissioning method for this long-distance cryogenic LNG pipeline system is as follows:

[0067] S1. One-stage precooling: Close the upstream cut-off valve 2 and the downstream cut-off valve 4. Before the long-distance cryogenic LNG pipeline 3 is precooled, the pipeline has completed nitrogen replacement and drying. Open the nitrogen injection cut-off valve and the nitrogen discharge cut-off valve to start the one-stage precooling of the long-distance cryogenic LNG pipeline 3 with nitrogen, and precool to -40℃.

[0068] The specific steps are as follows: Keep the upstream shut-off valve 2, downstream shut-off valve 4, and overpressure protection shut-off valve 52 closed; connect the upstream movable nitrogen injection skid 11 and downstream movable nitrogen injection skid 12 to the upstream injection pipe 13 and downstream injection pipe 15 respectively; connect the safety discharge system to the intermediate discharge pipe 17; open the upstream injection shut-off valve 14, downstream injection shut-off valve 16, intermediate discharge shut-off valve 18, and discharge regulating valve 44; first, start the upstream movable nitrogen injection skid 11 and downstream movable nitrogen injection skid 12, which contain liquid nitrogen storage tanks, liquid nitrogen booster pumps, liquid nitrogen regulating valves, liquid nitrogen vaporizers, etc., to provide nitrogen for precooling the long-distance cryogenic LNG pipeline; control the outlet gas phase nitrogen temperature of the upstream movable nitrogen injection skid 11 and downstream movable nitrogen injection skid 12 to -40℃, and gradually inject nitrogen from both ends of the long-distance cryogenic LNG pipeline 3; start the long-distance cryogenic LNG pipeline 3. The temperature monitoring system along the cryogenic LNG pipeline 3 continuously monitors the temperature at the top and bottom of the pipeline, controlling the maximum temperature drop rate of the pipeline to 10℃ / h, and controlling the nitrogen emission rate through the discharge regulating valve 44. The preferred continuous nitrogen injection time is 30 minutes, followed by closing the upstream injection shut-off valve 14, downstream injection shut-off valve 16, intermediate discharge shut-off valve 18, and discharge regulating valve 44 for a preferred 10 minutes. This static method ensures a continuous temperature field distribution along the pipeline, eliminating potential fluctuations in the axial temperature field distribution caused by turbulent pre-cooling. The upstream injection shut-off valve 14, downstream injection shut-off valve 16, intermediate discharge shut-off valve 18, and discharge regulating valve 44 are then reopened and closed again, with nitrogen continuously injected for another 30 minutes. This cycle continues until the temperature along the long-distance cryogenic LNG pipeline 3 reaches -40℃.

[0069] S2. The first two-stage and first three-stage precooling of the long-distance cryogenic LNG pipeline 3 are carried out according to the steps of step S1. The long-distance cryogenic LNG pipeline 3 is precooled with nitrogen in the first two-stage precooling to -80℃; the long-distance cryogenic LNG pipeline 3 is precooled with nitrogen in the first three-stage precooling to -120℃, and the nitrogen injection cut-off valve and nitrogen discharge cut-off valve are closed. Specifically, according to the operation method of step S1, following the step-by-step temperature reduction strategy, the temperature of the entire long-distance cryogenic LNG pipeline 3 is first reduced to -80℃, and then the temperature of the entire long-distance cryogenic LNG pipeline 3 is reduced to -120℃. The upstream injection cut-off valve 14, the downstream injection cut-off valve 16, the intermediate discharge cut-off valve 18, and the discharge regulating valve 44 are closed, thereby completing the nitrogen precooling of the pipeline.

[0070] S3. Inject vaporized LNG into the long-distance cryogenic LNG pipeline 3 through the upstream bypass pipeline system, and open the overpressure protection shut-off valve 52 and the overpressure protection regulating and releasing valve 55 of the overpressure protection system to establish the replacement and secondary precooling of the long-distance cryogenic LNG pipeline 3: Activate the temperature detection system along the long-distance cryogenic LNG pipeline 3 to continuously detect the temperature at the top and bottom of the long-distance cryogenic LNG pipeline 3, control the maximum temperature drop rate of the long-distance cryogenic LNG pipeline 3 to 5℃ / h, and gradually cool it down to the set temperature. Control the nitrogen emission rate through the overpressure protection regulating and releasing valve 55, and sample the components of the discharged gas through the overpressure protection releasing pipeline 56 until the nitrogen content of the discharged medium is lower than 0.5%, indicating that the replacement of nitrogen with vaporized LNG is qualified. Specifically: Connect the vaporizer 22 of the upstream movable nitrogen injection skid 11 to the downstream of the upstream bypass regulating valve 20, and open the upstream bypass regulating valve 20, the overpressure protection shut-off valve 52, and the overpressure protection regulating and releasing valve. Valve 55 establishes a replacement and secondary precooling channel for the long-distance cryogenic LNG pipeline 3. The outlet temperature of vaporizer 22 is set to -160℃. The temperature monitoring system along the long-distance cryogenic LNG pipeline 3 is activated to continuously monitor the temperature at the top and bottom of the pipeline, controlling the maximum temperature drop rate to 5℃ / h. The nitrogen emission rate is controlled by the overpressure protection regulating relief valve 55. The components of the discharged gas are sampled through the overpressure protection relief pipeline 56 until the nitrogen content of the discharged medium is below 0.5%, indicating that the nitrogen replacement for LNG vaporization is qualified. Subsequently, the secondary precooling rate of the LNG pipeline is increased to 10℃ / h, with a preferred continuous LNG vaporization injection time of 30 minutes. Then, the upstream bypass regulating valve 20 and the overpressure protection regulating relief valve 55 are closed, preferably for 10 minutes. The upstream bypass regulating valve 20 and the overpressure protection regulating relief valve 55 are then reopened. This cycle is repeated until the temperature along the long-distance cryogenic LNG pipeline 3 reaches -160℃. Thus, the pipeline replacement and secondary precooling are completed.

[0071] S4. Filling the long-distance cryogenic LNG pipeline 3: Filling is carried out using a gas-phase pressurization and slow liquefaction method. Specifically, since the internal medium temperature of the long-distance cryogenic LNG pipeline 3 is 160℃ and the pressure is atmospheric pressure after secondary precooling, a gas-phase pressurization and slow liquefaction method is used to avoid violent vaporization of the introduced liquid LNG. Specifically: The upstream bypass regulating valve 20 and vaporizer 22 are opened, and the outlet temperature of vaporizer 22 is set to -160℃. Liquid pressurized LNG output from the submersible pump of the upstream LNG storage tank enters the long-distance cryogenic LNG pipeline 3 after passing through the upstream bypass regulating valve 20 and vaporizer 22, and is continuously filled in gaseous form. The pressure transmitter installed on the pipeline is continuously monitored. When the pressure transmitter pressure stops rising, it indicates that the gaseous LNG in the long-distance cryogenic LNG pipeline 3 is gradually liquefied under pressure. Ideally, the pressure in the long-distance cryogenic LNG pipeline 3 at the end of filling is 0.5 MPa.g. At this point, even if upstream liquid LNG with a lower temperature is directly introduced, violent boiling and turbulence will not occur. Furthermore, the overpressure protection shut-off valve 52 is kept continuously open. If the long-distance cryogenic LNG pipeline 3 stops transporting for an extended period, the thermal safety valve 54 can provide pressure protection against overpressure problems caused by the thermal expansion of LNG in the long-distance cryogenic LNG pipeline 3. Subsequently, the upstream bypass regulating valve 20 and the overpressure protection shut-off valve 52 were closed, and the upstream shut-off valve 2 and the downstream shut-off valve 4 were opened to complete the commissioning of a long-distance cryogenic LNG pipeline based on multi-point precooling.

Claims

1. A long-distance cryogenic LNG pipeline commissioning system based on multi-point precooling, comprising a long-distance cryogenic LNG pipeline system, characterized in that, It also includes a multi-point injection and discharge system connected to different locations of the long-distance cryogenic LNG pipeline system, a top-bottom temperature difference adjustment system that uses active medium disturbance to reduce the top-bottom temperature difference of the long-distance cryogenic LNG pipeline system, a safety discharge system connected to the multi-point injection and discharge system, and an overpressure protection system connected to the long-distance cryogenic LNG pipeline system.

2. The commissioning system for long-distance cryogenic LNG pipelines based on multi-point precooling according to claim 1, characterized in that, The 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; the upstream LNG pipeline (1) is equipped with an upstream shut-off valve (2), the downstream LNG pipeline (5) is equipped with a downstream shut-off valve (4), and the long-distance cryogenic LNG pipeline (3) has multiple π-shaped bends continuously distributed on it.

3. The commissioning system for long-distance cryogenic LNG pipelines based on multi-point precooling according to claim 2, characterized in that, The multi-point injection and discharge system includes multiple nitrogen injection pipeline systems and at least one safety discharge pipeline (41) system. The multiple nitrogen injection pipeline systems are respectively connected to different positions of the long-distance cryogenic LNG pipeline (3). Each safety discharge pipeline (41) system is connected to a different position of the long-distance cryogenic LNG pipeline (3). A control valve is provided on the long-distance cryogenic LNG pipeline (3) between two adjacent safety discharge pipeline (41) systems.

4. A long-distance cryogenic LNG pipeline commissioning system based on multi-point precooling according to claim 3, characterized in that, Each of the nitrogen injection pipeline systems includes a nitrogen injection pipe that communicates with the interior of the long-distance cryogenic LNG pipeline (3), a movable nitrogen injection skid that communicates with the end of the nitrogen injection pipe, and a nitrogen injection shut-off valve installed on the nitrogen injection pipe.

5. A long-distance cryogenic LNG pipeline commissioning system based on multi-point precooling according to claim 3, characterized in that, Each of the aforementioned safety discharge pipeline (41) systems includes a nitrogen discharge pipe that communicates with the interior of the long-distance cryogenic LNG pipeline (3), and a nitrogen discharge shut-off valve installed on the nitrogen discharge pipe, the end of which is connected to the safety discharge system.

6. A long-distance cryogenic LNG pipeline commissioning system based on multi-point precooling according to claim 3, characterized in that, It also includes an upstream bypass pipeline system, which includes an upstream bypass pipeline (19), with both ends of the upstream bypass pipeline (1) connected to the upstream LNG pipeline (1) on both sides of the upstream shut-off valve (2). The upstream bypass pipeline (19) is equipped with an upstream bypass regulating valve (20), an upstream bypass flow transmitter (21), and a vaporizer (22).

7. A long-distance cryogenic LNG pipeline commissioning system based on multi-point precooling according to claim 4, characterized in that, The top and bottom temperature difference regulation system includes multiple static mixers and multiple distributed flow-around (33) plates that are spaced along the fluid flow direction inside the long-distance cryogenic LNG pipeline (3). The number of static mixers is the same as the number of nitrogen injection pipeline systems and corresponds one-to-one. Each static mixer is installed at the connection between the nitrogen injection pipe and the long-distance cryogenic LNG pipeline (3).

8. A long-distance cryogenic LNG pipeline commissioning system based on multi-point precooling according to claim 5, characterized in that, The safety emission system includes a safety emission pipe (41) connected to the end of the nitrogen emission pipe and a vent riser (45) installed at the end of the safety emission pipe (41). The safety emission pipe (41) is also equipped with an emission pipe pressure transmitter (42), an emission pipe flow transmitter (43), and an emission regulating valve (44).

9. A long-distance cryogenic LNG pipeline commissioning system based on multi-point precooling according to claim 1, characterized in that, The overpressure protection system includes an overpressure protection inlet pipe (51), an overpressure protection shut-off valve (52), an overpressure protection pressure transmitter (53), a thermal safety valve (54), an overpressure protection regulating and venting valve (55), and an overpressure protection venting pipe (56). The overpressure protection inlet pipe (51) is internally connected to the long-distance cryogenic LNG pipeline (3). The overpressure protection shut-off valve (52) is installed on the overpressure protection inlet pipe (51). Two pipelines are connected in parallel between the end of the overpressure protection inlet pipe (51) and the overpressure protection relief pipe (56). The overpressure protection pressure transmitter (53) and the thermal safety valve (54) are installed on one pipeline, and the overpressure protection regulating relief valve (55) is installed on the other pipeline.

10. A method for commissioning a long-distance cryogenic LNG pipeline (3) based on multi-point precooling, employing a long-distance cryogenic LNG pipeline commissioning system based on multi-point precooling as described in any one of claims 1 to 9, characterized in that, Includes the following steps: S1. First-stage precooling of long-distance cryogenic LNG pipeline (3): Close the upstream cut-off valve (2) and the downstream cut-off valve (4). Before the precooling of long-distance cryogenic LNG pipeline (3), the pipeline has completed nitrogen replacement and drying. Open the nitrogen injection cut-off valve and the nitrogen discharge cut-off valve to start the first-stage precooling of long-distance cryogenic LNG pipeline (3) with nitrogen, and precool to -40℃. S2. The first two-stage and first three-stage precooling of the long-distance cryogenic LNG pipeline (3) is carried out in accordance with the steps of step S1. The long-distance cryogenic LNG pipeline (3) is precooled to -80℃ in the first two-stage nitrogen precooling; the long-distance cryogenic LNG pipeline (3) is precooled to -120℃ in the first three-stage nitrogen precooling, and the nitrogen injection cut-off valve and nitrogen discharge cut-off valve are closed. S3. Inject the vaporized LNG into the long-distance cryogenic LNG pipeline (3) through the upstream bypass pipeline system, and open the overpressure protection shut-off valve (52) and overpressure protection regulating and venting valve (55) of the overpressure protection system to establish the replacement and secondary precooling of the long-distance cryogenic LNG pipeline (3): open the temperature detection system along the long-distance cryogenic LNG pipeline (3) to continuously detect the temperature at the top and bottom of the long-distance cryogenic LNG pipeline (3), control the maximum temperature drop rate of the long-distance cryogenic LNG pipeline (3) to 5℃ / h, gradually cool down to the set temperature, and control the nitrogen emission rate through the overpressure protection regulating and venting valve (55), and sample the components of the discharged gas through the overpressure protection venting pipeline (56) until the nitrogen content of the discharged medium is less than 0.5%, indicating that the replacement of nitrogen with vaporized LNG is qualified. S4. Filling long-distance cryogenic LNG pipelines (3): Filling is carried out by gas phase pressurization and slow liquefaction.

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