System and method for planned venting of supercritical carbon dioxide pipeline based on low-point release

Abstract

The present invention discloses a system and method for planned venting of a supercritical carbon dioxide pipeline based on low-point discharge, comprising a valve chamber shutoff system, a low-point venting system, a low-point displacement system, and a low-point temperature detection system; the valve chamber shutoff system is arranged in the pipeline valve chamber, comprising a shutoff valve arranged on the pipeline trunk line; the low-point venting system is arranged at or near the lowest point of the pipeline trunk line between the shutoff valves; the low-point displacement system is placed at or near the lowest point of the pipeline trunk line between the shutoff valves; and the low-point temperature detection system is arranged at a low point along the pipeline. Based on the basic physical properties, phase change laws, and planned venting characteristics of supercritical carbon dioxide, the present invention can significantly improve the safe discharge rate compared to conventional means from the perspectives of reducing emissions, efficient venting, and safe operation, thereby achieving efficient and safe operation of planned venting of supercritical carbon dioxide pipelines.

Description

Technical Field

[0001] The present invention belongs to the technical field of carbon dioxide transportation, and in particular relates to a system and method for planned venting of a supercritical carbon dioxide pipeline based on low-point release. Background Art

[0002] Against the backdrop of growing demands for energy conservation, emission reduction, and environmental protection in the energy and chemical industry, large-scale capture and utilization of carbon dioxide is the preferred way to effectively reduce carbon emissions. Long-distance pipeline transportation is a key approach to supporting carbon dioxide capture and utilization. The critical temperature of carbon dioxide is approximately 31.1°C, and the critical pressure is approximately 7.38 MPa. When transporting carbon dioxide via long-distance pipelines, it is generally accepted that a supercritical transport phase mode with a pressure higher than the critical pressure be used to improve the economic efficiency of transportation. Furthermore, carbon dioxide solidifies at low temperatures. The triple point of pure carbon dioxide is -56.6°C and 0.518 MPa, which is a significant characteristic that distinguishes carbon dioxide from conventional hydrocarbon media.

[0003] Similar to conventional hydrocarbon pipeline transportation, periodic internal inspections of CO2 pipelines are key to effectively diagnosing the pipeline's internal condition, and external maintenance also requires regular performance. When an abnormality occurs in the pipeline itself, the target section must be maintained and replaced, requiring the pipeline to be shut down and the medium within that section to be purged. Compared to conventional natural gas pipelines, supercritical CO2 pipelines have significant characteristics such as high unit volume density and phase transitions during the release of the medium. This places specific demands on controlling the release time and temperature changes caused by the vaporization of the medium during the release process. For example, when supercritical carbon dioxide is released between two shut-off valves on a planned basis, its release rate is affected not only by the medium's pressure but also by its phase. During the release process, supercritical carbon dioxide undergoes three typical phases: supercritical phase (dense phase), gas-liquid phase, and gas phase. Simultaneously, during phase transitions, the medium in the pipeline experiences varying degrees of significant temperature drops. Furthermore, during the release process, due to the influence of pipeline fluctuations, the incompletely vaporized carbon dioxide tends to accumulate at low points in the pipeline due to its density, causing even more severe temperature drops in the sections near these low points. This can create blind spots in distributed temperature monitoring and lead to misjudgments of temperature control during the release. Therefore, relevant technical measures are needed to overcome this issue.

[0004] However, there are few publicly available reports on the progress of planned venting technology for supercritical CO2 pipelines. Therefore, it is necessary to conduct relevant research to achieve the efficient and safe release of CO2 from the planned release section after the shutdown of the pipeline, and further provide reference and reference for subsequent large-scale CO2 pipeline transportation projects. Summary of the Invention

[0005] The purpose of the present invention is to provide a system and method for efficiently, economically and safely achieving planned venting of supercritical carbon dioxide pipelines in response to the above-mentioned problems.

[0006] Based on the basic physical properties, phase change laws and planned pipeline venting characteristics of supercritical carbon dioxide, the present invention takes into account the characteristics of large pipe capacity (at least 8 km of pipe length) between the shut-off valves of supercritical carbon dioxide pipelines, high medium inventory, and uneven liquid accumulation in the undulating pipelines. From the perspectives of reducing emissions, efficient venting and safe operation, the present invention respectively sets a valve chamber shut-off system, a low-point venting system, a low-point displacement system, a low-point temperature and pressure detection system, etc., to achieve efficient and safe operation of planned venting of supercritical carbon dioxide pipelines. The valve chamber shutoff system, installed in the pipeline valve chamber, is used to shut off the pipeline during a planned shutdown and support staged discharge. The low-point venting system, installed at the lowest point in the pipeline, is used to connect to a movable discharge system during planned venting, allowing for efficient discharge from the low-point venting section of the pipeline. The low-point displacement system, installed at a potentially low-elevation point in the pipeline, is used to connect to a movable heat medium injection system during planned venting to purge and displace heat medium in the potential liquid accumulation section, raising the temperature of that section. The low-point temperature and pressure detection system is used to monitor the real-time changes in temperature and pressure at the corresponding location during the planned venting process. This enables efficient, economical, and safe planned venting of the supercritical carbon dioxide pipeline.

[0007] The technical solution adopted by the present invention is: a supercritical carbon dioxide pipeline planned venting system based on low-point discharge, characterized by: comprising a valve chamber cutoff system, a low-point venting system, a low-point displacement system and a low-point temperature detection system;

[0008] The valve chamber shutoff system is arranged in the pipeline valve chamber, and includes a shutoff valve arranged in the pipeline trunk line, which is used to shut off the pipeline trunk line before the planned maintenance of the pipeline or after the pipeline leaks;

[0009] The low-point venting system is set at the lowest point of the pipeline trunk line between the shut-off valves or near the lowest point of the pipeline trunk line, and is used to release the supercritical carbon dioxide in the pipeline from the low point of the pipeline elevation after the pipeline is stopped.

[0010] The low-point displacement system is placed at or near the lowest point in the pipeline trunk line between the intercepting valves, and is used to inject carbon dioxide gas into the pipeline at a low altitude in the later stage of pipeline emptying to displace the low-temperature carbon dioxide in the pipeline section at that location;

[0011] The low-point temperature detection system is arranged at a low-lying location along the pipeline and is used to perform real-time temperature and pressure detection on the low-lying location in the pipeline.

[0012] The supercritical carbon dioxide pipeline planned venting system based on low-point discharge described in the present invention, wherein the valve chamber shut-off system includes an upstream shut-off valve group and a downstream shut-off valve group, the upstream shut-off valve group includes a trunk upstream shut-off valve and a corresponding bypass pipe group, the downstream shut-off valve group includes a trunk downstream shut-off valve and a corresponding bypass pipe group, and the upstream and downstream bypass pipe groups have the same structure.

[0013] The supercritical carbon dioxide pipeline planned venting system based on low-point discharge described in the present invention comprises a bypass pipe group including a first bypass shut-off valve, a bypass regulating valve and a second bypass shut-off valve arranged on the bypass, a first bypass temperature transmitter and a first bypass pressure transmitter arranged on the bypass upstream of the main line shut-off valve, and a second bypass temperature transmitter and a second bypass pressure transmitter arranged on the bypass downstream of the main line shut-off valve.

[0014] The supercritical carbon dioxide pipeline planned venting system based on low-point discharge described in the present invention includes a branch line shut-off valve, a vent regulating valve, an air bath heat exchanger and a vent riser connected in sequence on the vent branch line pipeline. The vent regulating valve is provided with a bypass, and a bypass vent regulating valve is provided on the bypass. The bypass vent regulating valve is opened in the initial stage and the later stage of discharge. A branch line pressure transmitter and a branch line temperature transmitter are provided on the vent branch line pipeline.

[0015] The supercritical carbon dioxide pipeline planned venting system based on low-point discharge described in the present invention comprises a venting regulating valve, a bypass venting regulating valve, an air bath heat exchanger and a venting riser skid that form a movable venting system.

[0016] The supercritical carbon dioxide pipeline planned venting system based on low-point discharge described in the present invention comprises a displacement branch line shut-off valve, a regulating valve, a gas heater, a small compressor and a displacement gas storage tank connected in sequence on the displacement branch line pipeline, a displacement branch line pressure transmitter and a displacement branch line temperature transmitter are provided on the displacement branch line pipeline, and an injection pressure transmitter is provided at the outlet end of the gas heater.

[0017] The supercritical carbon dioxide pipeline planned venting system based on low-point discharge described in the present invention comprises a displacement gas storage tank, a small compressor, a gas heater, a regulating valve and an injection pressure transmitter, which are skid-mounted to form a movable displacement system.

[0018] The low-point temperature detection system of the supercritical carbon dioxide pipeline planned venting system based on low-point discharge described in the present invention includes a detection branch line pressure transmitter, a detection branch line temperature transmitter and a detection branch line shut-off valve.

[0019] A method for planned venting of a supercritical carbon dioxide pipeline based on low-point release, characterized by:

[0020] During normal transportation, keep the upstream block valve and the downstream block valve of the trunk line open, the trunk line normally transports supercritical carbon dioxide, close the valves of each branch line, and the movable venting system of the low-point venting system and the movable displacement system of the low-point displacement system are not connected to the trunk system;

[0021] Before a planned pipeline maintenance, reduce the pipeline flow rate, monitor the parameters of temperature transmitters and pressure transmitters at various locations along the pipeline, and maintain the pipeline pressure only 0.5 MPa higher than the saturated vapor pressure of the medium at the corresponding lowest temperature. Reduce the medium inventory in the pipeline during planned release by reducing the medium density. Subsequently, slowly close the upstream and downstream block valves of the main line to stop the pipeline flow;

[0022] Next, connect the movable venting system to the low-point venting system, connect the movable displacement system to the low-point displacement system, open the branch line shut-off valve, vent regulating valve, bypass vent regulating valve and air bath heat exchanger, and vent the trunk pipeline from the supercritical state to the gas-liquid two-phase state; continuously monitor the indication values of the temperature transmitter and pressure transmitter at each low-altitude point; when the temperature of the temperature transmitter is lower than -15°C, immediately adjust the vent regulating valve and bypass vent regulating valve to reduce the discharge volume; when the low-point pressure approaches 3MPa, close the bypass vent regulating valve;

[0023] Then, start the low-point displacement system, turn on the small compressor, gas heater, regulating valve and displacement branch shut-off valve, and inject gaseous carbon dioxide at a temperature of 45-50°C into the main pipeline. Keep the vent regulating valve open. Through low-point displacement, adjust the medium temperature at other low-point locations, reduce the liquid accumulation at the low point, and increase the medium temperature at the low-point location during the discharge process. When the pipeline medium pressure drops to around 1.3MPa, shut down the low-point displacement system. During the discharge process, keep an eye on the temperature transmitter values at relevant low-altitude points to ensure that the values are above -15°C. Otherwise, lower or close the vent regulating valve.

[0024] Finally, open the bypass vent control valve again and cooperate with the vent control valve to release the remaining medium in the pipeline. During the release process, continue to pay attention to the temperature transmitter value at the relevant low-altitude points to ensure that the value is higher than -15°C, otherwise lower or close the vent control valve.

[0025] Compared with the prior art, the present invention has the following positive effects: based on the basic physical properties, phase transition laws, and planned pipeline venting characteristics of supercritical carbon dioxide, and taking into account the large pipe volume between the shut-off valves of supercritical carbon dioxide pipelines, high medium inventory, and uneven liquid accumulation in the fluctuating pipelines, the present invention provides a valve chamber shut-off system, a low-point venting system, a low-point displacement system, and a low-point temperature and pressure detection system from the perspectives of reducing emissions, efficient venting, and safe operation. Compared with conventional methods, the safe discharge rate can be greatly increased, achieving efficient and safe operation of planned supercritical carbon dioxide pipeline venting. As a result, efficient, economical, and safe planned venting of supercritical carbon dioxide pipelines is achieved.

[0026] Specifically:

[0027] (1) Scientific setting

[0028] Based on the basic physical properties, phase change patterns, and planned pipeline venting characteristics, and based on a large amount of simulation experience and demonstration of planned pipeline venting of supercritical carbon dioxide media, this invention proposes the use of efficient venting at the lowest point, thermal medium displacement at other low points, and monitoring of the temperature during the venting process at the pipeline low point, from the perspective of improving the venting rate and ensuring the safety of pipeline materials during the venting process. On the one hand, it greatly improves the venting time affected by the temperature control of the medium in the pipeline. On the other hand, it conducts targeted monitoring of the patterns and areas of the lowest temperature during the pipeline venting process, achieving the comprehensive effect of significantly shortening the venting time and being able to monitor the lowest temperature during the venting process. Furthermore, the use of thermal medium displacement further improves the problem of liquid accumulation in low-lying sections of the pipeline during the venting process, promotes the rapid volatilization of liquid carbon dioxide, and contributes to improving the venting rate. In addition, the discharge system creatively proposed a dual-valve discharge method of a vent control valve and a bypass vent control valve, which more accurately realized the segmented discharge rate control from supercritical state to gas-liquid two-phase state, gas-liquid two-phase state to gas state, and gas state to normal pressure state. The discharge volume is reduced in the stage most susceptible to low temperature effects, and the discharge volume is increased in the stage less susceptible to low temperature effects, thereby supporting efficient, safe and economical discharge.

[0029] (2) Good economic efficiency

[0030] By providing a movable relief system and a movable displacement system, the present invention significantly reduces the investment required to install a fixed relief system and displacement system. By adopting this method, optimizing the relief points and installing a displacement system, the low-temperature problem caused by the pressure drop of the medium in the pipe during the relief process is significantly eliminated. Compared with the conventional valve chamber relief model, the relief time for the same pipeline length can be shortened by 60%, providing support for the rapid resumption of pipeline production. In addition, the present invention proposes a model for reducing the operating pressure of the pipeline before a planned shutdown, thereby reducing the discharge volume of the planned pipeline venting and also supporting rapid resumption of production to a certain extent.

[0031] (3) Promoting technological development

[0032] The solution proposed in this invention plays an important role in engineering guidance and reference. The proposed key process system configurations such as the low-point venting system, low-point displacement system, and low-point temperature and pressure detection system can effectively promote the development of technical concepts and technological progress in this field, and ensure efficient and safe production. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will be described by way of specific embodiments with reference to the accompanying drawings, in which

[0034] Figure 1 It is a structural schematic diagram of the present invention.

[0035] Markings in the figure: 1 is the upstream shut-off valve of the trunk line, 2 is the first shut-off valve of the bypass line, 3 is the bypass regulating valve, 4 is the second shut-off valve of the bypass line, 5 is the first temperature transmitter of the bypass line, 6 is the first pressure transmitter of the bypass line, 7 is the second temperature transmitter of the bypass line, 8 is the second pressure transmitter of the bypass line, 9 is the downstream shut-off valve of the trunk line, 11 is the pressure transmitter of the branch line, 12 is the temperature transmitter of the branch line, 13 is the shut-off valve of the branch line, 14 is the vent regulating valve, 15 is the bypass 16 is an air bath heat exchanger, 17 is a vent riser, 21 is a displacement branch line pressure transmitter, 22 is a displacement branch line temperature transmitter, 23 is a displacement branch line shut-off valve, 24 is a displacement gas storage tank, 25 is a small compressor, 26 is a gas heater, 27 is a regulating valve, 28 is an injection pressure transmitter, 31 is a detection branch line pressure transmitter, 32 is a detection branch line temperature transmitter, and 33 is a detection branch line shut-off valve. DETAILED DESCRIPTION

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings herein can be arranged and designed in various different configurations.

[0037] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but rather merely represents selected embodiments of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. It should be noted that the embodiments of the present invention and the features therein may be combined with each other unless there is a conflict.

[0038] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined or explained in subsequent drawings.

[0039] In the description of the embodiments of the present invention, it should be noted that the indicated orientations or positional relationships are based on the orientations or positional relationships shown in the accompanying drawings, or the orientations or positional relationships in which the inventive product is typically placed when in use, or the orientations or positional relationships commonly understood by those skilled in the art, or the orientations or positional relationships in which the inventive product is typically placed when in use. These are merely 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, be constructed and operate in a specific orientation, and therefore should not be understood as limiting the present invention. In addition, the terms "first" and "second" are used only to distinguish descriptions and should not be understood as indicating or implying relative importance.

[0040] In describing the embodiments of the present invention, it should be noted that, unless otherwise specified or limited, the terms "disposed" and "connected" should be understood broadly. For example, they may refer to fixed connections, detachable connections, or integral connections; they may refer to direct connections or indirect connections through an intermediary. Those skilled in the art will understand the specific meanings of these terms in the present invention based on the specific circumstances.

[0041] like Figure 1 As shown, a supercritical carbon dioxide pipeline planned venting system based on low-point discharge includes a valve chamber cutoff system, a low-point venting system, a low-point displacement system and a low-point temperature detection system.

[0042] Specifically, the valve chamber shutoff system is arranged in the pipeline valve chamber, including a shutoff valve arranged on the pipeline trunk line, which is used to shut off the pipeline trunk line before the planned maintenance of the pipeline or after the pipeline leaks, thereby reducing the pipeline discharge volume.

[0043] Among them, the valve chamber shut-off system includes an upstream shut-off valve group and a downstream shut-off valve group. The upstream shut-off valve group includes a main upstream shut-off valve 1 and a corresponding bypass pipe group. The downstream shut-off valve group includes a main downstream shut-off valve 9 and a corresponding bypass pipe group. The upstream and downstream bypass pipe groups have the same structure. The bypass pipe group includes a bypass first shut-off valve 2, a bypass regulating valve 3 and a bypass second shut-off valve 4 arranged on the bypass. A bypass first temperature transmitter 5 and a bypass first pressure transmitter 6 are provided on the upstream bypass of the main shut-off valve. A bypass second temperature transmitter 7 and a bypass second pressure transmitter 8 are provided on the downstream bypass of the main shut-off valve. The bypass instrument is used to detect the pressure and temperature of the point. In addition, the bypass regulating valve can be used to safely fill the pipeline with medium after maintenance.

[0044] In this embodiment, the upstream shut-off valve and the downstream shut-off valve of the main line are carbon steel full-bore ball valves, which are remotely or in-situ controlled, preferably gas-liquid linkage valves, which are used to connect or close the supercritical carbon dioxide transmission main line; the first bypass shut-off valve, the bypass regulating valve and the second bypass shut-off valve are all carbon steel valves, which are arranged in the bypass of the main line shut-off valve, and are used to provide bypass pressurization function for the upstream and downstream pipelines of the main line shut-off valve; the first bypass temperature transmitter and the first bypass pressure transmitter are arranged in the upstream bypass of the main line shut-off valve, and are used to detect the medium and pressure parameters of the area in real time; the second bypass temperature transmitter and the second bypass pressure transmitter are arranged in the downstream bypass of the main line shut-off valve, and are used to detect the medium and pressure parameters of the area in real time.

[0045] Specifically, the low-point venting system is arranged at or near the lowest point of the pipeline trunk line between the shut-off valves. It is used to release supercritical carbon dioxide in the pipeline from a low point in the pipeline elevation after the pipeline is stopped, so as to reduce the amount of liquid accumulated at the low point of the pipeline as much as possible and significantly shorten the venting time.

[0046] The low-point venting system includes a branch shutoff valve 13, a venting regulating valve 14, an air-bath heat exchanger 16, and a venting riser 17, all connected sequentially to the venting branch pipeline. The venting regulating valve 14 is provided with a bypass, and a bypass venting regulating valve 15 is installed on the bypass. The bypass venting regulating valve 15 is opened during the initial and later stages of the venting process. A branch pressure transmitter 11 and a branch temperature transmitter 12 are installed on the venting branch pipeline. The venting regulating valve 14, the bypass venting regulating valve 15, the air-bath heat exchanger 16, and the venting riser 17 are skid-mounted to form a movable venting system. It should be noted that a blind flange is provided downstream of the branch shutoff valve, which is connected to the movable venting system before venting. The movable venting system can be transported by truck.

[0047] In this embodiment, the branch line pressure transmitter and the branch line temperature transmitter are arranged on the branch line pipeline for real-time detection of the temperature and pressure of the medium in the pipeline in the area; the branch line shut-off valve is a carbon steel manual ball valve, which is normally closed and opened only during discharge. Its downstream end is connected to a blind flange, which is replaced and connected to a movable venting system during discharge; the vent regulating valve and the bypass vent regulating valve are both manually controlled regulating valves made of stainless steel. The vent regulating valve is a regulating valve that is opened first, and the bypass vent regulating valve is preferably opened in the initial stage of discharge, that is, in the supercritical state to the pressure range close to 3MPa and the discharge In the later stage, the pressure range from 1.5MPa to normal pressure is opened for coordinated operation, and the discharge rate is increased as much as possible under the premise of ensuring that the temperature of the medium in the trunk line is not lower than -15°C; the air bath heat exchanger is made of stainless steel and uses air as the heat exchange medium to increase the temperature of the discharged carbon dioxide; the venting riser is made of carbon steel, and its bottom is equipped with an air blast system and a check valve to lift the discharged carbon dioxide to a safe discharge height, preferably above 15m, and dilute the concentration of the discharged carbon dioxide through air blast to reduce the diffusion impact range after its discharge. Furthermore, the movable venting system adopts a skid mode, which can be transported by special vehicles and can radiate planned venting of different pipe sections.

[0048] Specifically, the low-point displacement system is placed at or near the lowest point in the pipeline trunk line between the shut-off valves. It is used to inject carbon dioxide gas into the pipeline at a low location in the later stage of pipeline emptying, displacing the low-temperature carbon dioxide in the pipeline section at that location, promoting its vaporization, and shortening the discharge time.

[0049] Among them, the low-point displacement system includes a displacement branch shut-off valve 23, a regulating valve 27, a gas heater 26, a small compressor 25 and a displacement gas storage tank 24 connected in sequence on the displacement branch pipeline, a displacement branch pressure transmitter 21 and a displacement branch temperature transmitter 22 are arranged on the displacement branch pipeline, and an injection pressure transmitter 28 is arranged at the outlet end of the gas heater 26; the displacement gas storage tank 24, the small compressor 25, the gas heater 26, the regulating valve 27 and the injection pressure transmitter 28 are skid-type to form a movable displacement system.

[0050] In this embodiment, the displacement branch line pressure transmitter and displacement branch line temperature transmitter are installed on the branch line pipeline and are used to monitor the temperature and pressure of the medium in the pipeline in this area in real time. The displacement branch line shut-off valve is a carbon steel manual ball valve that is normally closed and opened only during displacement. Its downstream end is connected to a blind flange, which is replaced and connected to the mobile displacement system during displacement. The mobile displacement gas storage tank is made of carbon steel and is used to store carbon dioxide consistent with the pipeline medium and provide a displacement gas source during the displacement process. The small compressor is installed downstream of the mobile displacement gas storage tank and is activated when the pressure in the mobile displacement gas storage tank is insufficient for injection into the main pipeline. The gas heater is an electric heater used to heat the medium output from the mobile displacement gas storage tank. When the small compressor is activated, the medium that meets the displacement temperature after pressurization is no longer heated. The regulating valve is a manual control valve used to control the displacement gas flow rate and adjust the injection pressure. The injection pressure transmitter is used to monitor the pressure of the displacement gas to be injected in real time.

[0051] Specifically, the low-point temperature detection system is set at a low-lying location along the pipeline, and includes a detection branch pressure transmitter 31, a detection branch temperature transmitter 32 and a detection branch shut-off valve 33. The setting point is determined according to the specific project and is used to perform real-time temperature and pressure detection on the low-lying location in the pipeline. This is mainly because when supercritical carbon dioxide is released, the liquid medium tends to flow to the low point, which causes uneven temperature in various parts of the pipeline, and the temperature at the low point is significantly lower than the temperature at the high point. By setting up a low-point temperature detection system, the defect of representing the temperature of the entire line medium only by using a temperature transmitter set in the shut-off valve chamber can be overcome.

[0052] in,

[0053] The working principle of the present invention is:

[0054] (1) Under normal operating conditions, in order to improve the transportation efficiency as much as possible, supercritical carbon dioxide is used for transportation. Under this transportation condition, the density of carbon dioxide can reach 850kg / m 3 Before a planned pipeline shutdown, a method is proposed to reduce the delivery pressure to reduce the mass of the medium stored in the pipeline while still ensuring the stability of the delivery phase.

[0055] (2) Because long-distance pipelines are laid along the terrain, there are problems with elevation fluctuations. For conventional natural gas pipelines, after a planned shutdown, the target pipe section, that is, the pipe section between the shut-off valves, is discharged in the pipeline shut-off valve chamber. The discharge time is generally 6-8 hours. For supercritical carbon dioxide media, there are problems such as phase change (supercritical phase, gas-liquid two-phase, gas phase) during the discharge process, the phase change causing the medium temperature in the pipeline to drop, and the gas-liquid two-phase medium separation caused by the terrain fluctuation. In particular, the gas-liquid two-phase separation problem caused by the terrain fluctuation will cause the medium temperature in the lower-lying section of the pipeline to be significantly lower than that in the higher-lying section of the same section of the pipeline. This will result in: 1. The temperature transmitter in the valve chamber alone cannot fully represent the low temperature of the pipeline medium; 2. If the discharge is still carried out in the pipeline valve chamber at a higher terrain, in order to keep the medium temperature in the pipeline always above -15℃, it is necessary to restore the medium temperature through long-term intermittent discharge. Therefore, the present invention does not recommend discharging the carbon dioxide medium in the shut-off valve chamber, but proposes setting up a discharge system near the lowest point of the terrain, with the aim of quickly and directly discharging the liquid phase that may be separated from the gas and liquid, and cooperating with the displacement system to displace the high-temperature medium at other relatively low points, which can greatly shorten the discharge time of the pipeline section.

[0056] (3) For the discharge system, the present invention proposes a mode of adopting a movable discharge system, the purpose of which is to set up a more flexible movable discharge system, which can minimize the construction investment, and this part of the system can radiate multiple pipe sections at the same time; similarly, a movable displacement system is set up.

[0057] (4) In order to further improve the discharge rate, the present invention proposes a mode of using dual regulating valves for joint discharge in the initial stage from supercritical phase to gas-liquid two-phase and in the stage from gas phase to complete discharge. The principle is that the phase change in these two processes is small and the temperature drop of the medium in the pipeline is low; in the gas-liquid two-phase discharge stage, since the discharge process continues to change phases, it is required to open only one discharge valve in this process.

[0058] (5) In order to ensure pipeline safety, the present invention proposes the requirement of monitoring the temperature of the medium in the pipeline during the discharge process to prevent the temperature of the medium in the pipeline from being lower than -15°C; in order to improve the low temperature recognition rate, the present invention proposes to set a temperature transmitter at the low point of the pipeline.

[0059] Thus, a working principle of a supercritical carbon dioxide pipeline planned venting system based on low-point discharge was formed.

[0060] The present invention also relates to a method for planned venting of a supercritical carbon dioxide pipeline based on low-point release, the specific method being:

[0061] During normal transportation, keep the upstream block valve and the downstream block valve of the main line open, the main line normally transports supercritical carbon dioxide, close the valves of each branch, and the movable venting system of the low-point venting system and the movable displacement system of the low-point displacement system are not connected to the main line system.

[0062] Before the planned maintenance of the pipeline, reduce the pipeline flow rate, monitor the parameters of temperature transmitters and pressure transmitters at various locations along the pipeline, maintain the pipeline pressure only 0.5MPa higher than the saturated vapor pressure of the medium at the corresponding lowest temperature, reduce the medium inventory in the pipeline during planned discharge by reducing the medium density, and then slowly close the upstream and downstream block valves of the main line to stop the pipeline.

[0063] Next, connect the movable venting system to the low-point venting system, connect the movable displacement system to the low-point displacement system, open the branch line shut-off valve, vent regulating valve, bypass vent regulating valve and air bath heat exchanger, and vent the trunk pipeline from the supercritical state to the gas-liquid two-phase state. The venting pressure range is from the initial pressure to around 3MPa; continuously monitor the indication values of the temperature transmitter and pressure transmitter at each low-altitude point. When the temperature of the temperature transmitter is lower than -15℃, immediately reduce the discharge volume by adjusting the vent regulating valve and bypass vent regulating valve. When the low-point pressure is close to 3MPa, close the bypass vent regulating valve.

[0064] Then, start the low-point displacement system, turn on the small compressor, gas heater, regulating valve and displacement branch shut-off valve, inject gaseous carbon dioxide with a temperature of 45-50°C into the main pipeline, keep the vent regulating valve open, and adjust the medium temperature at other low-point positions through low-point displacement to reduce low-point liquid accumulation and increase the medium temperature at low-point positions during the discharge process. When the pipeline medium pressure drops to around 1.3MPa, close the low-point displacement system. During the discharge process, continue to pay attention to the temperature transmitter values of relevant low-altitude points to ensure that the values are higher than -15°C. Otherwise, lower or close the vent regulating valve.

[0065] Finally, open the bypass vent control valve again and cooperate with the vent control valve to release the remaining medium in the pipeline. During the release process, continue to pay attention to the temperature transmitter value at the relevant low-altitude points to ensure that the value is higher than -15°C, otherwise lower or close the vent control valve.

[0066] The present invention is not limited to the foregoing specific embodiments, and the present invention extends to any new features or any new combination disclosed in this specification, as well as any new method or process steps or any new combination disclosed.

Claims

1. The supercritical carbon dioxide pipeline planned venting system based on low-point release is characterized by: Including valve room cut-off system, low point venting system, low point displacement system and low point temperature detection system; The valve chamber shutoff system is arranged in the pipeline valve chamber, and includes a shutoff valve arranged in the pipeline trunk line, which is used to shut off the pipeline trunk line before the planned maintenance of the pipeline or after the pipeline leaks; The low-point venting system is set at the lowest point of the pipeline trunk line between the shut-off valves or near the lowest point of the pipeline trunk line, and is used to release the supercritical carbon dioxide in the pipeline from the low point of the pipeline elevation after the pipeline is stopped. The low-point venting system includes a branch shutoff valve (13), a venting regulating valve (14), an air bath heat exchanger (16), and a venting riser (17) connected in sequence on the venting branch pipeline, the venting regulating valve (14) is provided with a bypass, a bypass venting regulating valve (15) is provided on the bypass, the bypass venting regulating valve (15) is opened in the initial stage of venting and the later stage of venting, and a branch pressure transmitter (11) and a branch temperature transmitter (12) are provided on the venting branch pipeline; The low-point displacement system is placed at or near the lowest point in the pipeline trunk line between the intercepting valves, and is used to inject carbon dioxide gas into the pipeline at a low altitude in the later stage of pipeline emptying to displace the low-temperature carbon dioxide in the pipeline section at that location; The low-point displacement system comprises a displacement branch shutoff valve (23), a regulating valve (27), a gas heater (26), a small compressor (25) and a displacement gas storage tank (24) connected in sequence on the displacement branch pipeline; a displacement branch pressure transmitter (21) and a displacement branch temperature transmitter (22) are provided on the displacement branch pipeline; and an injection pressure transmitter (28) is provided at the outlet end of the gas heater (26); the displacement gas storage tank (24), the small compressor (25), the gas heater (26), the regulating valve (27) and the injection pressure transmitter (28) are skid-type to form a movable displacement system; The low-point temperature detection system is arranged at a low-lying location along the pipeline and is used to perform real-time temperature and pressure detection on the low-lying location in the pipeline.

2. The supercritical carbon dioxide pipeline planned venting system based on low-point release according to claim 1 is characterized in that: The valve chamber shutoff system comprises an upstream shutoff valve group and a downstream shutoff valve group, wherein the upstream shutoff valve group comprises a trunk upstream shutoff valve (1) and a corresponding bypass pipe group, and the downstream shutoff valve group comprises a trunk downstream shutoff valve (9) and a corresponding bypass pipe group, and the upstream and downstream bypass pipe groups have the same structure.

3. The supercritical carbon dioxide pipeline planned venting system based on low-point release according to claim 2 is characterized in that: The bypass pipe group comprises a bypass first shutoff valve (2), a bypass regulating valve (3) and a bypass second shutoff valve (4) arranged on the bypass, a bypass first temperature transmitter (5) and a bypass first pressure transmitter (6) arranged on the bypass upstream of the main line shutoff valve, and a bypass second temperature transmitter (7) and a bypass second pressure transmitter (8) arranged on the bypass downstream of the main line shutoff valve.

4. The supercritical carbon dioxide pipeline planned venting system based on low-point release according to claim 2 is characterized in that: The vent regulating valve (14), the bypass vent regulating valve (15), the air bath heat exchanger (16) and the vent riser (17) are skid-mounted to form a movable vent system.

5. The supercritical carbon dioxide pipeline planned venting system based on low-point release according to claim 1 is characterized in that: The low-point temperature detection system comprises a detection branch line pressure transmitter (31), a detection branch line temperature transmitter (32), and a detection branch line shutoff valve (33).

6. A method for venting a supercritical carbon dioxide pipeline planned venting system based on low-point release according to claim 4, characterized in that: During normal transportation, keep the upstream block valve and the downstream block valve of the trunk line open, the trunk line normally transports supercritical carbon dioxide, close the valves of each branch line, and the movable venting system of the low-point venting system and the movable displacement system of the low-point displacement system are not connected to the trunk system; Before a planned pipeline maintenance, reduce the pipeline flow rate, monitor the parameters of temperature transmitters and pressure transmitters at various locations along the pipeline, and maintain the pipeline pressure only 0.5 MPa higher than the saturated vapor pressure of the medium at the corresponding lowest temperature. Reduce the medium inventory in the pipeline during planned release by reducing the medium density. Subsequently, slowly close the upstream and downstream block valves of the main line to stop the pipeline flow; Next, connect the movable venting system to the low-point venting system, connect the movable displacement system to the low-point displacement system, open the branch line shut-off valve, vent regulating valve, bypass vent regulating valve and air bath heat exchanger, and vent the trunk pipeline from the supercritical state to the gas-liquid two-phase state; continuously monitor the indication values of the temperature transmitter and pressure transmitter at each low-altitude point; when the temperature of the temperature transmitter is lower than -15°C, immediately adjust the vent regulating valve and bypass vent regulating valve to reduce the discharge volume; when the low-point pressure approaches 3MPa, close the bypass vent regulating valve; Then, start the low-point displacement system, turn on the small compressor, gas heater, regulating valve and displacement branch shut-off valve, and inject gaseous carbon dioxide at a temperature of 45-50°C into the main pipeline. Keep the vent regulating valve open. Through low-point displacement, adjust the medium temperature at other low-point locations, reduce the liquid accumulation at the low point, and increase the medium temperature at the low-point location during the discharge process. When the pipeline medium pressure drops to around 1.3MPa, shut down the low-point displacement system. During the discharge process, keep an eye on the temperature transmitter values at relevant low-altitude points to ensure that the values are above -15°C. Otherwise, lower or close the vent regulating valve. Finally, open the bypass vent control valve again and cooperate with the vent control valve to release the remaining medium in the pipeline. During the release process, continue to pay attention to the temperature transmitter value at the relevant low-altitude points to ensure that the value is higher than -15°C, otherwise lower or close the vent control valve.

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