Method for precooling LNG loading pipeline through BOG
By using BOG pre-cooling in LNG loading pipelines and controlling the cooling rate and temperature gradient, the loss, complexity and thermal stress management problems of LNG loading pipelines are solved, and economical, safe and environmentally friendly pre-cooling effects are achieved.
Patent Information
- Application Number
- CN202510597191.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-05-09
- Publication Date
- 2025-08-15
AI Technical Summary
The existing LNG loading pipeline pre-cooling methods have problems with LNG products with large losses, complex operations, high costs and thermal stress management, especially the stratification problems when nitrogen pre-cooling.
The BOG pre-cooling method is adopted to introduce BOG into the LNG loading pipeline, and use temperature sensors to monitor the pipe wall temperature, calculate the cooling rate and temperature gradient, dynamically adjust the BOG flow rate, control the cooling process, avoid thermal stress, and combine the heating and buffering in the BOG processing system to achieve accurate cooling.
Significantly reduce LNG loss, reduce operational costs, simplify operations, improve safety, extend pipeline life, and reduce environmental emissions.
Smart Images
Figure CN120488118A_ABST
Abstract
Description
Technical Field
[0001] The invention relates to a method for precooling an LNG loading pipeline by utilizing BOG, and belongs to the technical field of LNG. Background Art
[0002] Liquefied natural gas (LNG) is typically stored and transported at extremely low temperatures (approximately -162°C, or 111K). Before large quantities of LNG are transferred from storage tanks to transport vessels via loading pipelines, the pipelines, which are at ambient temperature (e.g., approximately 290-300K), must be precooled to near the LNG's operating temperature. This precooling step is crucial due to the significant temperature difference between ambient and LNG temperatures. LNG pipelines are typically made of materials such as austenitic stainless steel (such as grades 304 or 316), which have significant thermal contraction coefficients. Without sufficient and controlled precooling, rapid cooling of the pipeline wall can lead to large temperature gradients, resulting in excessive thermal stress. This thermal stress can cause material damage such as cracking, plastic deformation, or fatigue failure, as well as thermal buckling of the pipeline, compromising operational safety and the structural integrity of the equipment. Therefore, controlling the cooling rate during precooling (for example, a target rate of 3 to 10 Kelvin per hour) and managing temperature gradients (for example, the temperature difference between the top and bottom of the pipe, and gas stratification) are critical to preventing thermal shock and ensuring safe and reliable pipeline operation. Currently, there are several conventional methods for precooling LNG loading pipelines, but each has its own disadvantages: Direct LNG pre-cooling: A common practice is to use LNG itself for pre-cooling. Figure 1 , typically introduced into the pipeline at a low flow rate. However, the main disadvantage of this method is the significant consumption of valuable LNG product, resulting in economic losses. Furthermore, if the flow rate is not properly controlled, the liquid LNG can cause rapid, uncontrolled cooling of the pipeline, resulting in extremely high thermal stresses. This process also generates a large amount of flash gas (BOG) during the initial phase due to the rapid evaporation of the LNG. Nitrogen pre-cooling: Another method is to use nitrogen (liquid or gaseous) for pre-cooling. This is usually done in two steps: first, the pipeline is cooled to an intermediate temperature (e.g., -120°C) with nitrogen, and then LNG is introduced to complete the final cooling. The disadvantage of this method is that it requires external nitrogen supply, storage, and handling, which increases operational complexity and cost. Gas nitrogen pre-cooling is less efficient because it relies solely on sensible heat transfer from the gas and is prone to severe thermal stratification, which is a large temperature difference between the top and bottom of the pipeline (possibly exceeding 50°C). This can cause high thermal stress in localized areas and may cause problems such as weld tearing. Liquid nitrogen, while more efficient, still involves external supply costs and handling requirements. Problems with Existing Technologies: As discussed above, existing LNG loading line precooling methods have numerous drawbacks, including LNG product loss, operational complexity, additional costs (e.g., nitrogen supply), and challenges in effectively managing thermal stress (particularly stratification during nitrogen precooling). Currently, a more efficient, cost-effective precooling method with better thermal stress management is lacking. Summary of the Invention
[0003] In order to solve the problems of high power consumption and high operating cost of multi-stage refrigerant compressors in the above background technology, the present invention aims to provide a method for pre-cooling an LNG loading pipeline using BOG.
[0004] The present invention provides a method for precooling an LNG loading pipeline using BOG, the method comprising the following steps: a. Establishing fluid communication between the BOG source associated with the LNG storage tank and the inlet portion of the LNG loading line, and between the outlet portion of the LNG loading line and the BOG processing system; b. The BOG from the BOG source is introduced into the inlet portion of the LNG loading line; c. allowing the BOG to flow through the LNG loading line toward the outlet portion; d. The BOG flowing through the LNG loading pipeline is directed from the outlet portion of the LNG loading pipeline to a BOG compressor.
[0005] Further, the BOG source is the vapor space of the LNG storage tank.
[0006] Furthermore, before the BOG compressor is compressed, the method further includes heating the BOG derived from the LNG loading pipeline using a BOG heater.
[0007] Furthermore, in step (d), the BOG discharged from the LNG loading pipeline flows through a buffer tank after being heated and before entering the BOG compressor.
[0008] Furthermore, the BOG import in step b also includes: b1. monitoring the pipe wall temperature measured by at least one temperature sensor arranged along the LNG loading pipeline; b2. calculating an actual cooling rate and / or temperature gradient based on the measured tube wall temperature; b3. comparing the actual cooling rate and / or temperature gradient with a preset maximum cooling rate limit and / or a maximum allowable temperature gradient limit to obtain a comparison result; b4. Based on the comparison result, dynamically adjust the opening of the control valve that controls the BOG flow rate so that the actual cooling rate and / or temperature gradient does not exceed the preset limit.
[0009] Furthermore, the BOG import in step b further includes the following steps: b5. During at least one initial stage of the pre-cooling process, controlling the introduced BOG to have a first temperature; b6. In at least one cooling stage after the initial stage, the introduced BOG is controlled to have a second temperature lower than the first temperature, and a mechanism for controlling the BOG temperature is adjusted based on the measured tube wall temperature and / or the pre-cooling stage.
[0010] The present invention offers the following advantages: Compared to existing technologies, it significantly reduces the loss of valuable product caused by the direct use of LNG in traditional pre-cooling methods, thereby lowering operating costs and improving economic efficiency. Furthermore, it efficiently utilizes the cryogenic cold energy inherent in BOG, converting byproducts that would otherwise require additional processing (such as compression and reliquefaction) or waste (such as flaring) into a valuable cooling medium, aligning with the trend of energy conservation, consumption reduction, and resource optimization. Furthermore, compared to the direct use of liquid LNG or nitrogen, the use of gaseous BOG and controlled flow rate allows for a smoother and more controllable cooling process, effectively managing and reducing temperature gradients and resulting thermal stresses along the pipeline wall, thereby maintaining the pipeline's structural integrity and extending its service life. Furthermore, by avoiding the direct introduction of high LNG flow rates or reliance on external nitrogen supplies during the initial cooling phase and leveraging the facility's existing BOG system, operational safety is improved. Finally, by reducing the potential flaring of BOG, this method also offers environmental benefits, helping to reduce greenhouse gas emissions. BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Figure 1 A system block diagram of the background technology of the present invention; Figure 2 This is a system block diagram of the present invention. DETAILED DESCRIPTION
[0012] In order to better understand the above technical solution, the above technical solution will be described in detail below with reference to the accompanying drawings and specific implementation methods.
[0013] To make the objectives, technical solutions, and advantages of the present invention more clearly understood, the following describes in detail a method for precooling an LNG loading pipeline using BOG, provided by the present invention, in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are intended only to illustrate the present invention and are not intended to limit the present invention.
[0014] Example 1 refer to Figure 2 This embodiment provides a method for precooling an LNG (liquefied natural gas) loading pipeline using BOG. The LNG loading pipeline (2) is typically made of austenitic stainless steel (e.g., 304 or 316 grades) with a significant thermal contraction coefficient. Before loading large quantities of LNG (storage temperature approximately -162°C, or 111K), it needs to be precooled from ambient temperature (e.g., approximately 290-300K) to near operating temperature.
[0015] In this embodiment, the precooling method is carried out according to the following steps: Step 1: Establish fluid connectivity and isolation Before starting the pre-cooling operation, first connect the pipeline and isolate it. The specific operations are: 1. Open the valve connecting the vapor space of the LNG storage tank (serving as the BOG source in this embodiment) and the inlet of the LNG loading pipeline 2 to be pre-cooled to establish a BOG supply path.
[0016] 2. Open the valve connecting the outlet of the LNG loading pipeline 2 and the BOG treatment system (in this embodiment, specifically the suction pipeline of the BOG compressor 5) to establish a BOG outlet passage.
[0017] 3. At the same time, ensure that the isolation valve between LNG loading pipeline 2 and the main LNG supply source (such as the LNG storage tank outlet pipeline or the main loading pump outlet pipeline) is closed to completely isolate the loading pipeline from the liquid LNG during the pre-cooling stage.
[0018] Step 2: Introducing BOG and Controlling Flow After establishing the pathway, low-temperature BOG from the vapor space of the LNG storage tank begins to be introduced into the inlet of the LNG loading pipeline 2. To effectively manage thermal stress and prevent thermal shock, the introduction process is precisely controlled: 1. Temperature Monitoring: Multiple temperature sensors are placed along the LNG loading pipeline 2 (e.g., at the inlet, middle, outlet, and top and bottom of the pipeline). These sensors monitor the temperature of the pipeline wall in real time.
[0019] 2. Cooling rate and temperature gradient calculation: A control system continuously receives readings from temperature sensors and calculates the actual cooling rate of the pipeline (e.g., temperature drop per unit time, K / hour) and temperature gradient (e.g., temperature difference between the top and bottom of the pipe, °C) based on these readings.
[0020] 3. Comparison with limits: The control system compares the calculated actual cooling rate and actual temperature gradient with pre-set safety limits. These limits are determined by the pipeline material, size, and design specifications. For example, the maximum cooling rate limit may be 3 to 10 Kelvin per hour, and the maximum allowable temperature gradient limit (to avoid severe gas stratification) may be set to no more than 50°C.
[0021] 4. Dynamic Flow Control: Based on the comparison results, the control system dynamically adjusts the opening of a control valve installed in the BOG inlet pipeline. If the actual cooling rate or temperature gradient approaches or exceeds a preset limit, the system automatically closes the control valve, reducing the BOG inlet flow rate and slowing the cooling process. Conversely, if the cooling rate is far below the target range and below the limit, the valve is opened wider to improve pre-cooling efficiency, always ensuring that the safety limit is not exceeded.
[0022] Step 3: BOG flows through the pipeline for cooling. The controlled, low-temperature BOG flows from the inlet to the outlet of the loading pipeline. During this flow, the BOG exchanges heat with the hotter pipe wall, absorbing the sensible heat from the pipe wall, gradually increasing its own temperature while simultaneously causing the temperature of the pipeline wall to gradually and controllably decrease.
[0023] Step 4: BOG discharge and subsequent treatment The BOG that has flowed through the entire loading pipeline and completed heat exchange and has a higher temperature is discharged from the outlet of the loading pipeline and guided to the BOG compressor 5.
[0024] 1. Export to compressor: In this embodiment, the BOG is directly introduced into the suction port of the BOG compressor 5 .
[0025] 2. (Optional) Preheating: In a preferred sub-embodiment, to meet the inlet gas temperature requirements of the BOG compressor 5 (to prevent damage to the compressor from excessively low temperatures), a heater is installed before the BOG enters the compressor suction port. This heater can utilize ambient air, low-pressure steam, or other available heat sources to moderately heat the BOG discharged from the loading pipeline using the BOG heater 3 to reach the minimum allowable suction temperature of the compressor.
[0026] 3. (Optional) Buffering / Separation: In another preferred sub-embodiment, particularly after the preheating step, the BOG passes through a buffer tank 4 or separator tank before entering the BOG compressor 5. This helps stabilize the airflow entering the compressor and removes any trace condensate that may be carried over, further ensuring safe operation of the compressor.
[0027] 4. Subsequent processing: After the BOG entering the BOG compressor 5 is compressed, it can be sent to the fuel gas system for use as fuel, or sent to the recondensing unit for liquefaction and returned to the LNG storage tank to achieve resource recycling, depending on the overall design of the LNG plant.
[0028] (Optional) Step 5: Staged Control of BOG Temperature To optimize pre-cooling efficiency and control, this embodiment may also include staged control of the inlet BOG temperature: 1. Initial stage (higher temperature): In the initial stage of the pre-cooling process (step b5), when the pipeline temperature is significantly different from the ambient temperature, the introduced BOG can be controlled to have a relatively high first temperature (for example, saturated BOG directly taken from the top of the storage tank).
[0029] 2. Subsequent Cooling Stage (Lower Temperature): After the pipeline has been initially cooled to a certain temperature (step b6), to accelerate cooling or achieve a lower pre-cooling target temperature, a specific BOG conditioning mechanism (e.g., a BOG subcooler, mixing with other cooler streams, etc.) can be used to control the incoming BOG to a second temperature lower than the first. The timing and method of adjusting the BOG temperature can be automatically or manually determined based on real-time pipe wall temperature data and / or a pre-defined multi-stage pre-cooling program.
[0030] Beneficial Effects of This Embodiment The following effects are achieved by adopting the method described in this embodiment: 1. Economical: It significantly reduces the loss of expensive LNG products caused by the direct use of LNG in traditional pre-cooling methods, thereby reducing operating costs.
[0031] 2. Resource Utilization: Efficiently utilize the cold energy contained in the naturally generated BOG in the LNG storage and transportation system, converting the by-product into a valuable cooling medium, in line with the principles of energy conservation and resource optimization.
[0032] 3. Thermal Stress Management: Compared to directly using liquid LNG or nitrogen (especially nitrogen, which is prone to stratification), using gaseous BOG and precisely controlling its flow rate (with optional staged temperature control) enables a smoother, more uniform, and more controllable cooling process. This effectively manages and reduces temperature gradients within the pipeline wall and across its cross-section, significantly reducing thermal stresses caused by thermal shock and thermal stratification. This protects the structural integrity of the pipeline (especially critical areas such as welds), avoids the risk of cracking, deformation, or fatigue failure, and extends the service life of the equipment.
[0033] 4. Operational simplification and safety: This method utilizes the existing BOG and its processing system within the LNG facility, avoiding the need to introduce external nitrogen supply, storage and processing facilities, simplifying the operating process, and reducing complexity and potential safety risks.
[0034] 5. Environmental benefits: By effectively utilizing BOG for pre-cooling, the amount of BOG that needs to be discharged through flaring or other means is reduced, which helps reduce greenhouse gas emissions and has positive environmental significance.
[0035] In summary, compared with the existing technology, the method provided in this embodiment is a more economical, efficient, safe, controllable and environmentally friendly pre-cooling solution for LNG loading pipeline 2, which effectively solves the pain points mentioned in the background technology, such as LNG loss, nitrogen dependence, high cost and difficulty in thermal stress management (especially stratification problem).
[0036] The above is a further detailed description of the present invention in conjunction with specific preferred embodiments, and the specific implementation of the present invention should not be considered to be limited to these descriptions. For those skilled in the art of the present invention, without departing from the concept of the present invention, several simple deductions or substitutions can be made, which should be considered to fall within the scope of protection of the present invention.
Claims
1. A method for precooling an LNG loading pipeline using BOG, the method comprising the following steps: Establishing fluid communication between a BOG source (1) associated with the LNG storage tank and an inlet portion of the LNG loading line (2), and between an outlet portion of the LNG loading line (2) and the BOG treatment system; introducing BOG from the BOG source (1) into the inlet portion of the LNG loading pipeline (2); Allowing the BOG to flow through the LNG loading pipeline (2) toward the outlet portion; The BOG that has flowed through the LNG loading pipeline (2) is guided from an outlet portion of the LNG loading pipeline (2) to a BOG compressor (5).
2. The method according to claim 1, characterized in that The BOG source (1) is the vapor space of the LNG storage tank.
3. The method according to claim 2, characterized in that The method further includes heating the BOG derived from the LNG loading pipeline (2) using a BOG heater (3) before compression by the BOG compressor (5).
4. The method according to claim 3, characterized in that In step (d), the BOG drawn from the LNG loading pipeline (2) flows through a buffer tank (4) after the BOG is heated and before entering the BOG compressor (5).
5. The method according to claim 1, wherein The BOG import in step b also includes: b1. monitoring the pipe wall temperature measured by at least one temperature sensor arranged along the LNG loading pipeline (2); b2. calculating an actual cooling rate and / or temperature gradient based on the measured tube wall temperature; b3. comparing the actual cooling rate and / or temperature gradient with a preset maximum cooling rate limit and / or a maximum allowable temperature gradient limit to obtain a comparison result; b4. Based on the comparison result, dynamically adjust the opening of the control valve that controls the BOG flow rate so that the actual cooling rate and / or temperature gradient does not exceed the preset limit.
6. The method according to claim 5, characterized in that The BOG import in step b also includes the following steps: b5. During at least one initial stage of the pre-cooling process, controlling the introduced BOG to have a first temperature; b6. In at least one cooling stage after the initial stage, the introduced BOG is controlled to have a second temperature lower than the first temperature, and a mechanism for controlling the BOG temperature is adjusted based on the measured tube wall temperature and / or the pre-cooling stage.