A pressure protection system and method for a large-drop dense-phase carbon dioxide pipeline

Abstract

The present invention discloses a pressure protection system and method for a large-drop dense-phase carbon dioxide pipeline, the system comprising a trunk transmission system, an overpressure protection and relief system, and an instrument monitoring system. Based on objective conditions such as carbon dioxide phase characteristics, the shut-off operating conditions of low-point shut-off valves, and the hydrostatic head of large-drop pipelines, the present invention addresses various overpressure scenarios that may arise when a large-drop dense-phase carbon dioxide pipeline is shut off at a low point, including pressure increases caused by upstream inertia and filling water hammer, pressure increases caused by thermal expansion of the shut-off medium, and pressure increases caused by downstream bridging water hammer at the valve. From the perspectives of overpressure condition identification, overpressure condition control, and efficient pressure relief, the present invention sets up a trunk transmission system, an overpressure protection and relief system, a pipeline planned relief system, and an instrument monitoring system, focusing on solving problems such as upstream water hammer overpressure, downstream bridging water hammer overpressure, trunk thermal expansion overpressure, and planned relief during pipeline maintenance when the low-point shut-off valve is shut off.

Description

Technical Field

[0001] The present invention belongs to the technical field of carbon dioxide transportation, and in particular relates to a pressure protection system and method for a large-drop dense-phase carbon dioxide pipeline. 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 (CO2) is a preferred approach for effectively reducing carbon emissions. Long-distance pipeline transportation is a key method for supporting CO2 capture and utilization. CO2 has a critical temperature of approximately 31.1°C and a critical pressure of approximately 7.38 MPa. For long-distance pipeline transportation, supercritical or dense phase transport, with pressures above the critical pressure, is generally considered the preferred method for improving transport efficiency. Due to the heat management characteristics of long-distance pipeline transportation, dense phase transport is currently the best option.

[0003] Carbon dioxide's properties indicate that dense-phase transport requires pressures above its saturated vapor pressure to maintain phase stability. While possessing properties similar to those of liquid fluids, its higher saturated vapor pressure results in greater thermal expansion and lower compressibility than gaseous media. Consequently, dense-phase carbon dioxide pipelines face significant challenges, including transient pressure increases upstream of the shut-off valve during water hammer and a sudden increase in the volume of the medium within the pipeline due to environmental influences during pipeline shutdowns. Furthermore, in areas with significant elevation differences, pipelines with large vertical drops face the added challenge of high hydrostatic pressures at low points. Accidental closure of a shut-off valve at a low point can compound this pressure, leading to even higher transient pressures. Furthermore, as a saturated fluid, its phase stability is affected by operating pressure. Transient low pressures can occur downstream of the shut-off valve, potentially causing partial vaporization of the medium in this area. Similarly, due to the hydrostatic pressure at this low point, this vaporized space can be susceptible to rapid reliquefaction of the vaporized medium due to backflow from downstream media, leading to a water hammer. It's worth noting that after a leak, dense-phase CO2 pipelines experience a prolonged pressure gradient, placing high demands on pipeline toughness and crack arrest. Therefore, it's essential to mitigate the risk of overpressure and ensure safe pipeline operation. Furthermore, dense-phase CO2 experiences a significant temperature drop during adiabatic release, and at lower temperatures, it forms "solid dry ice," potentially impacting the release process, requiring careful consideration during safe release.

[0004] Compared to conventional oil pipelines, dense-phase CO2 pipelines offer more significant technical advantages for low-point water hammer control and a wider range of overpressure scenarios. Currently, no specific research on this issue has been found in domestic or international literature, and few reference practices or experiences are available. Therefore, detailed research on pressure protection systems and methods for large-drop dense-phase CO2 pipelines is needed to ensure pipeline system safety and provide reference and lessons for subsequent large-scale CO2 pipeline transportation projects. Summary of the Invention

[0005] The purpose of the present invention is to provide a large-drop dense-phase carbon dioxide pipeline pressure protection system and method that can safely and efficiently implement dense-phase carbon dioxide pipeline transportation in order to solve the above-mentioned problems.

[0006] Based on objective conditions such as the phase characteristics of carbon dioxide, the shut-off condition of the low-point shut-off valve and the hydrostatic pressure head of the large-drop pipeline, the present invention aims at various overpressure scenarios such as the pressure increase caused by the inertia and filling water hammer upstream of the valve, the pressure increase caused by the thermal expansion of the shut-off medium and the pressure increase caused by the bridging water hammer downstream of the valve when the large-drop dense-phase carbon dioxide pipeline is shut off at a low point. From the perspectives of overpressure condition identification, overpressure condition control and efficient pressure relief, the present invention sets up a trunk transmission system, an overpressure protection and relief system, a pipeline planned relief system, an instrument monitoring system, etc.

[0007] The trunk transmission system consists of the pipeline itself and conventional pipe fittings, providing a normal flow path for dense-phase carbon dioxide pipelines. The overpressure protection and relief system, located upstream and downstream of the low-point shut-off valve, provides overpressure protection against inertia and filling water hammer pressure buildup in the upstream pipeline, thermal expansion pressure buildup in the shut-off valve, and bridging water hammer pressure buildup in the downstream pipeline. The pipeline planned relief system, located upstream and downstream of the low-point shut-off valve, complements the overpressure protection and relief system to meet requirements for trunk line maintenance and relief. The instrument monitoring system, incorporated into this system, provides real-time monitoring of temperature and pressure parameters at key points in the system. This ensures pressure protection for large-drop dense-phase carbon dioxide pipelines and ensures safe system operation.

[0008] The technical solution adopted by the present invention is: a large drop dense phase carbon dioxide pipeline pressure protection system, characterized by: comprising a trunk transmission system, an overpressure protection and relief system and an instrument monitoring system;

[0009] The trunk line transmission system is used to provide a normal transmission flow path for the dense phase carbon dioxide pipeline, including an upstream trunk line pipeline, a downstream trunk line pipeline, and a low-point trunk line shut-off valve arranged between the upstream and downstream trunk line pipelines;

[0010] The overpressure protection and relief system is arranged upstream and downstream of the low-point trunk shut-off valve, and includes an upstream overpressure safety valve, a downstream overpressure safety valve, a bypass pressure balancing safety valve and a multi-stage regulating relief unit. The bypass pressure balancing safety valve is arranged on the bypass of the low-point trunk shut-off valve. When the low-point trunk shut-off valve is closed and the trunk transmission pipeline is overpressured, the bypass pressure balancing safety valve connects the upstream trunk pipeline and the downstream trunk pipeline to form a first-level priority pressure relief protection. The upstream overpressure safety valve and the downstream overpressure safety valve are respectively connected to the upstream trunk pipeline and the downstream trunk pipeline to form a second-level pressure relief protection. The outlet ends of the upstream overpressure safety valve and the downstream overpressure safety valve are connected to the multi-stage regulating relief unit;

[0011] The instrument monitoring system includes a pressure transmitter and a temperature transmitter, which are used to monitor the pressure and temperature of corresponding points of the trunk transmission system and the overpressure protection and relief system in real time.

[0012] The large-drop dense-phase carbon dioxide pipeline pressure protection system described in the present invention has a multi-stage regulating and releasing unit for performing multi-stage pressure regulation and temperature regulation on the overpressure relief medium, and includes a pressure buffer tank, an air bath heat exchanger, a secondary pressure safety valve, and a venting riser connected in sequence. The pressure buffer tank is arranged downstream of the upstream overpressure safety valve and the downstream overpressure safety valve, and the pressure buffer tank is pre-filled with carbon dioxide at a certain pressure.

[0013] In the large-drop dense-phase carbon dioxide pipeline pressure protection system of the present invention, the air-bath heat exchanger is connected to the vent riser via a bursting disc, and the bursting disc is connected in parallel with the secondary pressure safety valve.

[0014] The large-drop dense-phase carbon dioxide pipeline pressure protection system described in the present invention, wherein the trunk transmission system also includes a bypass pipeline, which is connected to the upstream trunk pipeline and the downstream trunk pipeline through an upstream bypass shut-off valve and a downstream bypass shut-off valve respectively, forming a bypass medium flow channel required for planned venting and overpressure protection of the upstream and downstream trunk pipelines, and a bypass connection first shut-off valve and a bypass connection second shut-off valve are provided on the bypass pipeline.

[0015] In the large-drop dense-phase carbon dioxide pipeline pressure protection system described in the present invention, the upstream overpressure safety valve is arranged downstream of the upstream bypass shut-off valve to discharge the overpressure medium of the upstream trunk line, and the downstream overpressure safety valve is arranged downstream of the downstream bypass shut-off valve to discharge the overpressure medium of the downstream trunk line.

[0016] The large-drop dense-phase carbon dioxide pipeline pressure protection system described in the present invention has a bypass pipeline connected to a pipeline planned discharge system, which includes a primary regulating valve and a secondary regulating valve. The primary regulating valve is arranged downstream of the bypass-connected first shut-off valve and the bypass-connected second shut-off valve, and the secondary regulating valve is arranged downstream of the air bath heat exchanger. The primary regulating valve and the secondary regulating valve are only opened when the trunk line is undergoing planned discharge.

[0017] The large-drop dense-phase carbon dioxide pipeline pressure protection system described in the present invention comprises an upstream branch line pressure transmitter and an upstream branch line temperature transmitter arranged on the upstream trunk pipeline, a downstream branch line pressure transmitter and a downstream branch line temperature transmitter arranged on the downstream trunk pipeline, a first overpressure relief temperature transmitter arranged upstream of the pressure buffer tank, a first overpressure relief pressure transmitter arranged downstream of the pressure buffer tank, and a second temperature transmitter arranged downstream of the air bath heat exchanger.

[0018] A method for protecting the pressure of a large-drop dense-phase carbon dioxide pipeline, characterized by comprising the following steps:

[0019] Step 1: When the pipeline is operating normally, keep the upstream trunk pipeline, low-point trunk shut-off valve, downstream trunk pipeline, upstream bypass shut-off valve, and downstream bypass shut-off valve open. Keep the upstream overpressure safety valve, downstream overpressure safety valve, bypass pressure balancing safety valve, pressure buffer tank, air bath heat exchanger, secondary pressure safety valve, bursting disc, and vent riser online. Close the bypass connecting the first shut-off valve and the bypass connecting the second shut-off valve, close the primary regulating valve and the secondary regulating valve. At the same time, maintain the carbon dioxide filled in the pressure buffer tank at a certain pressure, and perform pressure detection through the first overpressure relief pressure transmitter. When the pressure is lower than the set value, promptly replenish the pressure of the pressure buffer tank;

[0020] Set the opening pressure of the bypass pressure balancing safety valve to a certain multiple of the main pipeline design pressure, set the opening pressure of the upstream overpressure safety valve and the downstream overpressure safety valve to a multiple of the main pipeline design pressure greater than the multiple set by the bypass pressure balancing safety valve, and set the opening pressure of the secondary pressure safety valve to be greater than the gas pressure in the pressure buffer tank;

[0021] Step 2: When the low-point main line block valve is accidentally closed, when the upstream pressure exceeds the set pressure of the bypass pressure balancing safety valve, the bypass pressure balancing safety valve automatically opens, and the overpressure medium in the upstream main line pipeline will quickly flow into the downstream main line pipeline through the bypass pressure balancing safety valve;

[0022] When the upstream pressure continues to rise and exceeds the set pressure of the upstream overpressure safety valve, the upstream overpressure medium will flow into the pressure buffer tank through the upstream overpressure safety valve. After the pressure in the pressure buffer tank exceeds the set pressure of the secondary pressure safety valve, the secondary pressure safety valve opens and discharges the medium to the vent riser. At the same time, when the downstream trunk pipeline pressure exceeds the set pressure of the downstream overpressure safety valve, the downstream overpressure medium will flow into the pressure buffer tank through the downstream overpressure safety valve.

[0023] Step 3: When the pipeline stops transporting, if the temperature and pressure of the medium in the upstream trunk pipeline or the downstream trunk pipeline rise, pressure protection is performed by performing the pressure relief operation of the upstream overpressure safety valve and the downstream overpressure safety valve in step 2;

[0024] Step 4: When the upstream trunk pipeline or the downstream trunk pipeline is planned to be emptied, open the bypass to connect to the first block valve or the bypass to connect to the second block valve, close the outlet block valve of the pressure buffer tank, slowly open the first-level regulating valve and the second-level regulating valve, and keep the pressure downstream of the first-level regulating valve not exceeding the opening pressure of the second-level pressure safety valve during the discharge process, and pay attention to the real-time temperature of the upstream branch temperature transmitter or the downstream branch temperature transmitter to ensure that the temperature is not lower than -15°C; after the discharge is completed, close the bypass to connect to the first block valve, the bypass to connect to the second block valve, the first-level regulating valve and the second-level regulating valve, and reopen the outlet block valve of the pressure buffer tank.

[0025] The large-drop dense-phase carbon dioxide pipeline pressure protection method described in the present invention, in the step one, the pressure of the carbon dioxide filled in the pressure buffer tank is between 1.8 and 2.2 MPa.g. When the pressure is lower than 1.8 MPa.g, the pressure buffer tank is pressurized, wherein the opening pressure of the bypass pressure balancing safety valve is set to 1.0 times the design pressure of the trunk pipeline, the opening pressure of the upstream overpressure safety valve and the downstream overpressure safety valve is set to 1.1 times the design pressure of the trunk pipeline, and the opening pressure of the secondary pressure safety valve is set to 2.4 to 2.6 MPa.g.

[0026] In the large-drop dense-phase carbon dioxide pipeline pressure protection method described in the present invention, in step 2, the pressure buffer tank, the air bath heat exchanger and the secondary pressure safety valve are used to provide pressure and temperature relays for carbon dioxide pressure relief to control the medium pressure and temperature during the relief process. When the secondary pressure safety valve fails, the overpressure medium is discharged through the bursting disc and enters the vent riser to complete the relief. After the relief is completed, the pressure of the pressure buffer tank is reduced to 1.8-2.2 MPa.g.

[0027] Compared with the existing technology, the present invention has the following positive effects: based on objective conditions such as the phase characteristics of carbon dioxide, the shut-off operating conditions of the low-point block valve, and the hydrostatic head of the large-drop pipeline, it addresses various overpressure scenarios that may arise when the large-drop dense-phase carbon dioxide pipeline is shut off at the low point, including pressure increase caused by upstream valve inertia and filling water hammer, pressure increase caused by thermal expansion of the shut-off medium, and pressure increase caused by downstream valve bridging water hammer. From the perspectives of overpressure condition identification, overpressure condition control, and efficient pressure relief, the present invention respectively provides a trunk transmission system, an overpressure protection and relief system, a pipeline planned relief system, and an instrument monitoring system. This system focuses on resolving problems such as upstream water hammer overpressure, downstream bridging water hammer overpressure, trunk line thermal expansion overpressure, and planned relief during pipeline maintenance when the low-point block valve is shut off. This system achieves pressure protection for the large-drop dense-phase carbon dioxide pipeline and ensures safe system operation.

[0028] Specifically:

[0029] (1) Scientific setting

[0030] The present invention aims at the scenarios of unplanned shutdown and maintenance shutdown of the low-point shut-off valve that may occur during the operation of a large-drop supercritical carbon dioxide trunk line, identifies the impact of upstream inertia and overpressure water hammer caused by unplanned shutdown, downstream bridging water hammer caused by unplanned shutdown, and the hydrostatic head of the large-drop pipeline on the pipeline, and proposes innovative supercritical carbon dioxide pipeline pressure protection ideas such as "introducing upstream water hammer overpressure medium into the downstream" and "relay discharge of upstream and downstream overpressure media". An overpressure protection and relief system is set up, including an upstream trunk water hammer relief valve, an upstream and downstream connecting pressure balancing valve, a downstream trunk water hammer relief valve, supporting relief pipelines, supporting valves, etc., which are used to balance the inertia of the upstream pipeline of the low-point shut-off valve and the pressure increase of the filling water hammer, the pressure increase of the thermal expansion of the shut-off valve, and the pressure relief of the low-point shut-off valve. Overpressure protection is carried out for the water hammer pressure increase of the downstream pipeline. Upstream and downstream pressure balance protection is preferably used to connect the upstream and downstream, and the upstream overpressure medium is used to supplement the pressure of the downstream pipeline. On the one hand, it reduces the direct discharge of the medium, and on the other hand, it reduces the impact of the downstream water hammer. A pressure buffer tank, a secondary relief safety valve, an air bath heat exchanger, a secondary pressure relief valve, a bypass bursting disc, a vent riser, etc. are provided to perform multi-stage pressure regulation and temperature regulation on the overpressure relief medium to meet the overpressure relief of the trunk line and prevent the freezing of the relief system and the impact of low temperature on safe relief. At the same time, in response to the objective needs of planned maintenance of the trunk line, a pipeline planned relief system is set up based on the overpressure protection and relief system, and the supercritical carbon dioxide in the trunk pipeline is safely released by using the secondary pressure regulation.

[0031] (2) Good economic efficiency

[0032] During operation, the present invention prioritizes upstream-to-downstream pressure relief. This induction valve, when accidentally closed, causes the overpressure medium caused by upstream inertia and charging water hammer to compensate for the localized low pressure caused by downstream inertial water hammer, rather than directly discharging the entire medium into the relief system. This reduces medium emissions and the carbon emissions associated with project operation. Furthermore, the introduction of the upstream water hammer overpressure medium into the downstream pipeline mitigates the impact of downstream water hammer bridging, also reducing medium emissions. Furthermore, the relief system incorporates pressure buffer tanks, air-bath heat exchangers, and other facilities. By utilizing two-stage relief and interstage natural heat exchange, this reduces the need for relief system piping and enhances the project's cost advantage. Furthermore, the use of interstage natural heat exchange reduces energy consumption and improves environmental efficiency.

[0033] (3) Promoting technological development

[0034] The solution proposed in this invention plays an important role in engineering guidance and reference. The proposed key process system configurations and methods, such as low-point overpressure protection for large-drop pipelines, upstream and downstream medium replenishment, and multi-stage discharge, can take into account pipeline safety, system economy, and environmentally friendly operation, and promote technological progress in this field. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The present invention will be described by way of specific embodiments with reference to the accompanying drawings, in which

[0036] Figure 1 It is a structural schematic diagram of the present invention.

[0037] Markings in the figure: 1 is the upstream main pipeline, 2 is the low point main line block valve, 3 is the downstream main pipeline, 4 is the upstream bypass block valve, 5 is the bypass connection first block valve, 6 is the downstream bypass block valve, 7 is the bypass pipeline, 8 is the bypass connection second block valve, 11 is the upstream overpressure safety valve, 12 is the downstream overpressure safety valve, 13 is the bypass pressure balancing safety valve, 14 is the pressure buffer tank, 15 is the air bath heat exchanger, 16 is the secondary pressure safety valve, 17 is the bursting disc, 18 is the vent riser, 21 is the primary regulating valve, 22 is the secondary regulating valve, 31 is the upstream branch pressure transmitter, 32 is the upstream branch temperature transmitter, 33 is the downstream branch pressure transmitter, 34 is the downstream branch temperature transmitter, 35 is the overpressure relief first temperature transmitter, 36 is the overpressure relief first pressure transmitter, 37 is the second temperature transmitter. DETAILED DESCRIPTION

[0038] 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.

[0039] 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 invention as claimed, 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 creative effort shall fall within the scope of protection of the present invention.

[0040] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] like Figure 1 As shown, a large-drop dense-phase carbon dioxide pipeline pressure protection system includes a trunk transmission system, an overpressure protection and relief system, an instrument monitoring system, and a pipeline planned relief system. By setting up this system, efficient pressure protection can be achieved for large-drop dense-phase carbon dioxide pipelines, thereby ensuring the safe operation of the system.

[0045] Among them, the trunk transmission system is used to provide a normal transmission flow channel for the dense phase carbon dioxide pipeline, including an upstream trunk pipeline 1, a downstream trunk pipeline 3 and a low-point trunk shut-off valve 2 arranged between the upstream and downstream trunk pipelines; the trunk transmission system also includes a bypass pipeline 7, which is connected to the upstream trunk pipeline 1 and the downstream trunk pipeline 3 through the upstream bypass shut-off valve 4 and the downstream bypass shut-off valve 6 respectively, forming a bypass medium flow channel required for planned emptying and overpressure protection of the upstream and downstream trunk pipelines. A bypass connection first shut-off valve 5 and a bypass connection second shut-off valve 8 are provided on the bypass pipeline 7.

[0046] Specifically, in the system shown, the upstream trunk pipeline 1 is made of carbon steel and is used to provide a trunk flow channel for dense-phase carbon dioxide; the low-point trunk shut-off valve 2 is arranged on the trunk line at the low point and is used to isolate the pipeline in sections. It is normally open and closed under conditions such as pipeline leakage and planned pipeline maintenance, but it may also be accidentally closed; the downstream trunk pipeline 3 is made of carbon steel and is used to provide a trunk flow channel for dense-phase carbon dioxide; the upstream bypass shut-off valve 4, the bypass-connected first shut-off valve 5, the downstream bypass shut-off valve 6, the bypass-connected second shut-off valve 8 and the bypass pipeline 7 are all made of carbon steel, providing a flow channel between the upstream trunk and the downstream trunk when the medium is pressure-relieved and vented, among which the upstream bypass shut-off valve 4, the bypass-connected first shut-off valve 5 and the downstream bypass shut-off valve 6 are all manual ball valves, the upstream bypass shut-off valve 4 and the downstream bypass shut-off valve 6 are normally open, and the bypass-connected first shut-off valve 5 and the bypass-connected second shut-off valve are normally closed.

[0047] Among them, the overpressure protection and relief system is arranged upstream and downstream of the low-point main line shut-off valve 2, and is used to perform overpressure protection on the inertia of the upstream pipeline of the low-point shut-off valve and the filling water hammer pressure rise, the thermal expansion pressure rise of the shut-off valve, and the bridging water hammer pressure rise of the downstream pipeline of the low-point shut-off valve. It specifically includes an upstream overpressure safety valve 11, a downstream overpressure safety valve 12, a bypass pressure balancing safety valve 13 and a multi-stage regulating relief unit. The bypass pressure balancing safety valve 13 is arranged on the bypass of the low-point main line shut-off valve 2. When the low-point main line shut-off valve 2 is closed and the main transmission pipeline is overpressured, the bypass pressure balancing safety valve 13 connects the upstream main line pipeline 1 and the downstream main line pipeline 3 to form a first-level priority pressure relief protection. The upstream overpressure safety valve 11 and the downstream overpressure safety valve 12 They are respectively connected to the upstream trunk pipeline 1 and the downstream trunk pipeline 3 to form a secondary pressure relief protection. The outlet ends of the upstream overpressure safety valve 11 and the downstream overpressure safety valve 12 are connected to the multi-stage regulating and releasing unit. The multi-stage regulating and releasing unit is used to perform multi-stage pressure regulation and temperature regulation on the overpressure relief medium. It includes a pressure buffer tank 14, an air bath heat exchanger 15, a secondary pressure safety valve 16 and a venting riser 18 connected in sequence. The pressure buffer tank 14 is arranged downstream of the upstream overpressure safety valve 11 and the downstream overpressure safety valve 12. The pressure buffer tank 14 is pre-filled with carbon dioxide at a certain pressure. The air bath heat exchanger 15 is connected to the venting riser 18 through a bursting disc 17. The bursting disc 17 is connected in parallel with the secondary pressure safety valve 16. This system gives priority to upstream and downstream pressure balance protection, connects the upstream and downstream through a bypass pressure balance safety valve, and uses the upstream overpressure medium to supplement the pressure of the downstream pipeline. On the one hand, it reduces the direct discharge of the medium, and on the other hand, it reduces the impact of downstream bridging water hammer. It also provides a multi-stage regulating and relief unit for multi-stage pressure regulation and temperature regulation of the overpressure relief medium to meet the overpressure relief of the main line.

[0048] Specifically, the upstream overpressure safety valve 11 is located downstream of the upstream bypass shut-off valve 4 to release overpressure from the upstream trunk line. The downstream overpressure safety valve 12 is located downstream of the downstream bypass shut-off valve 6 to release overpressure from the downstream trunk line. The bypass pressure-balancing safety valve 13 is located on the bypass side of the low-point trunk shut-off valve 2 to preferentially discharge overpressure from the upstream trunk line into the downstream trunk line. The opening pressures of the upstream overpressure safety valve 11 and the downstream overpressure safety valve 12 are set at 1.1 times the design pressure of the trunk line, and the opening pressure of the bypass pressure-balancing safety valve 13 is set at 1.0 times the design pressure of the trunk line. The pressure buffer tank 14 is made of carbon steel and is arranged downstream of the upstream overpressure safety valve 11 and the downstream overpressure safety valve 12. It is used to provide pre-filled carbon dioxide, maintain back pressure, and avoid the formation of dry ice and low temperature after excessive pressure drop of carbon dioxide directly discharged from the main line. The pressure of carbon dioxide filled in the pressure buffer tank 14 is between 1.8 and 2.2 MPa.g; the air bath heat exchanger 15 is made of carbon steel and is arranged downstream of the pressure buffer tank 14. It is used to regulate the temperature of the medium discharged from the pressure buffer tank, and the theoretical adjustment outlet temperature is higher than -10°C; the secondary pressure safety valve 16 is arranged downstream of the pressure buffer tank 14, with an opening pressure of 2.4 to 2.6 MPa.g, and is used to discharge the medium in the pressure buffer tank 14 at overpressure. It is made of low-temperature carbon steel; a bursting disc 17 is arranged on the bypass of the secondary pressure safety valve 16 to provide redundant protection for the secondary pressure safety valve 16; the vent riser 18 is arranged downstream of the secondary pressure safety valve 16 to safely release the discharged medium. It is made of low-temperature carbon steel.

[0049] Among them, the bypass pipe 7 is connected to the pipeline planned discharge system, which is used to match the overpressure protection and discharge system to meet the needs of trunk line maintenance and discharge, etc. The pipeline planned discharge system includes a first-level regulating valve 21 and a second-level regulating valve 22. The first-level regulating valve 21 is arranged downstream of the bypass-connected first shut-off valve 5 and the bypass-connected second shut-off valve 8, is manually adjusted, and is made of carbon steel. The second-level regulating valve 22 is arranged downstream of the air bath heat exchanger 15, is manually adjusted, and is made of carbon steel. It cooperates with the first-level regulating valve 21 to open. The first-level regulating valve 21 and the second-level regulating valve 22 are only opened when the trunk line is planned to be discharged.

[0050] Among them, the instrument monitoring system includes a pressure transmitter and a temperature transmitter, which are used to monitor the pressure and temperature of the corresponding points of the trunk transmission system and the overpressure protection and relief system in real time, and through data feedback, timely optimize the setting parameters of the pressure relief system in this system.

[0051] Specifically, the instrument monitoring system includes an upstream branch pressure transmitter 31 and an upstream branch temperature transmitter 32 arranged on the upstream trunk pipeline 1, a downstream branch pressure transmitter 33 and a downstream branch temperature transmitter 34 arranged on the downstream trunk pipeline 3, an overpressure relief first temperature transmitter 35 arranged upstream of the pressure buffer tank 14, an overpressure relief first pressure transmitter 36 arranged downstream of the pressure buffer tank 14, and a second temperature transmitter 37 arranged downstream of the air bath heat exchanger 15.

[0052] The working principle of the present invention is:

[0053] (1) General principle: In the transportation of supercritical carbon dioxide pipelines, the temperature of the pipeline medium basically drops below the critical temperature (about 30°C) as the environment during the flow process is affected. Therefore, the main line transportation is basically a dense phase medium. For dense phase carbon dioxide pipelines in mountainous areas, after the shut-off valves set at the low point are closed, they may cause various overpressure scenarios such as pressure increase caused by inertia and filling water hammer upstream of the valve, pressure increase caused by thermal expansion of the shut-off medium, and pressure increase caused by bridging water hammer downstream of the valve. The scale of its thermal expansion overpressure and downstream bridging water hammer overpressure is higher than that of conventional refined oil and crude oil, mainly due to the larger thermal expansion coefficient and higher saturated vapor pressure of carbon dioxide. Therefore, pressure protection and safe discharge are necessary. At the same time, since the direct adiabatic discharge of dense phase carbon dioxide may result in a low temperature of about -90°C and is very easy to form dry ice, it is necessary to adopt reasonable measures to protect the safety of the dense phase carbon dioxide during the discharge process.

[0054] (2) Water hammer and overpressure protection: A bypass pressure balancing safety valve is first installed in the bypass of the low-point main line shut-off valve. The purpose is to effectively control the inertia and charging water hammer pressure increase that may be generated upstream when the low-point main line shut-off valve is quickly closed, and quickly discharge the upstream overpressure medium to the downstream pipeline to compensate for the local pressure drop in the area near the low-point main line shut-off valve due to the inertial flow of the medium in the downstream pipeline. On the one hand, the upstream is used to supplement the downstream pressure to control the upstream overpressure; on the other hand, the upstream overpressure is reduced to directly discharge the medium to the venting system to reduce direct carbon dioxide emissions; at the same time, by supplementing the pressure downstream, the probability of downstream vaporization is minimized to avoid further bridging water hammer caused by the reverse flow of the downstream pipeline medium. Furthermore, overpressure safety valves are installed upstream and downstream of the low-point main line shut-off valve, namely the upstream overpressure safety valve and the downstream overpressure safety valve, to discharge the medium for the overpressure that cannot be eliminated after the bypass pressure balancing safety valve is activated, thereby ensuring pipeline safety.

[0055] (3) Thermal expansion overpressure protection: The upstream overpressure safety valve and the downstream overpressure safety valve 12 are also set upstream and downstream of the low-point main line shut-off valve to provide overpressure protection for the closed pipeline caused by the medium being heated by the environment after the pipeline is stopped.

[0056] (4) Discharge system: Since pipeline overpressure is an unplanned operating condition, it is different from the manual control mode used in the planned venting of supercritical / dense phase carbon dioxide to discharge pressure in an orderly manner; in order to avoid the low temperature and dry ice problems caused by the first-level adiabatic discharge, a pressure buffer tank, an air bath heat exchanger and a second-level pressure safety valve are introduced into the discharge system. The pressure buffer tank is set with a pre-filling pressure as the back pressure for the first-level overpressure discharge, which can effectively reduce the temperature drop of the first-level discharge, taking into account both low temperature control and dry ice formation control; the air bath heat exchanger is set to exchange heat with the environment for the medium discharged through the second-level pressure safety valve after continuous accumulation in the pressure buffer tank, increase the temperature entering the second-level pressure safety valve, and avoid the risk of low temperature and dry ice near the venting riser downstream of the second-level pressure safety valve; at the same time, a bursting disc is set in the bypass of the second-level pressure safety valve to provide redundant protection for discharge. In the present invention, the pressure buffer tank can be injected with carbon dioxide by the initial external equipment to establish internal pressure, and during operation, carbon dioxide can be introduced from the main line for pressure replenishment.

[0057] (5) Planned venting: Planned venting of pipelines is an essential function for both upstream and downstream of the shut-off valve. In the present invention, a first-stage regulating valve and a second-stage regulating valve are provided as pressure and flow control elements for two-stage planned venting, relying on the pressure relief system. The inter-stage temperature is regulated by an air-bath heat exchanger. This is also to avoid the generation of low temperatures and dry ice in the discharge system during planned venting. At the same time, the temperature detection system of the trunk line is used to monitor the temperature of the upstream and downstream trunk lines in real time during the venting process to avoid the impact of low temperatures on the trunk line materials.

[0058] The present invention also discloses a method for protecting the pressure of a large-drop dense-phase carbon dioxide pipeline, which specifically comprises the following steps:

[0059] Step 1: During normal pipeline operation, keep the upstream trunk pipeline 1, low-point trunk shutoff valve 2, downstream trunk pipeline 3, upstream bypass shutoff valve 4, and downstream bypass shutoff valve 6 open. Keep the upstream overpressure safety valve 11, downstream overpressure safety valve 12, bypass pressure balancing safety valve 13, pressure buffer tank 14, air bath heat exchanger 15, secondary pressure safety valve 16, bursting disc 17, and vent riser 18 online. Close the bypass connection first shutoff valve 5, bypass connection second shutoff valve 8, primary regulating valve 21, and secondary regulating valve 22. At the same time, maintain the carbon dioxide pressure in the pressure buffer tank 14 between 1.8 and 2.2 MPa.g. Perform pressure monitoring using the overpressure relief first pressure transmitter 36. When the pressure in the pressure buffer tank 14 falls below 1.8 MPa.g, promptly repressurize the pressure buffer tank 14. Specifically, the opening pressure of the upstream overpressure safety valve 11 and the downstream overpressure safety valve 12 is set to 1.1 times the design pressure of the main pipeline, the opening pressure of the bypass pressure balancing safety valve 13 is set to 1.0 times the design pressure of the main pipeline, and the opening pressure of the secondary pressure safety valve 16 is set to 2.4~2.6MPa.g.

[0060] Step 2: When the low-point main line shut-off valve 2 is accidentally closed, if upstream inertia and filling water hammer occur in the upstream main line, and the upstream main line pressure increases, the upstream overpressure safety valve 11 and the bypass pressure balancing safety valve 13 in this system may be activated. Specifically, when the upstream pressure exceeds the set pressure of the bypass pressure balancing safety valve 13, the bypass pressure balancing safety valve 13 automatically opens, and the overpressure medium in the upstream main line pipeline 1 will quickly flow into the downstream main line pipeline 3 through the bypass pressure balancing safety valve 13, compensating for the local low pressure in the downstream main line pipeline 3 caused by flow inertia; when the upstream pressure continues to rise and exceeds the set pressure of the upstream overpressure safety valve 11, the upstream overpressure medium will flow into the pressure buffer tank 14 through the upstream overpressure safety valve 11; after the pressure of the pressure buffer tank 14 exceeds the set pressure of the secondary pressure safety valve 16, the secondary pressure safety valve 16 opens and discharges the medium to the vent riser. At the same time, when a water hammer occurs in the downstream trunk line 3 and the pressure exceeds the set pressure of the downstream overpressure safety valve 12, the downstream overpressure medium will flow into the pressure buffer tank 14 through the downstream overpressure safety valve 12. Furthermore, the pressure buffer tank 14, the air bath heat exchanger 15, the secondary pressure safety valve 16, etc. provide a pressure and temperature relay for the carbon dioxide pressure relief. The purpose is to control the pressure and temperature of the medium during the relief process and avoid the direct relief to normal pressure causing the relief system to become cold and form dry ice. When the secondary pressure safety valve 16 fails, the overpressure medium is discharged using the bursting disc 17 and enters the vent riser 18 to complete the relief. After the relief is completed, the pressure of the pressure buffer tank 14 is manually reduced to 1.8-2.2 MPa.g.

[0061] Step 3: When the pipeline stops transporting, the medium in the upstream trunk pipeline 1 or the downstream trunk pipeline 3 may be heated and pressurized due to the temperature of the buried soil. Pressure protection is performed by executing the operation of step 2, except for the operation of the bypass pressure balancing safety valve 13.

[0062] Step 4: When performing a planned venting of the upstream trunk pipeline 1 or the downstream trunk pipeline 3, open the bypass connecting the first block valve 5 and the bypass connecting the second block valve 8, close the outlet block valve of the pressure buffer tank 14, and slowly open the first and second control valves 21 and 22. During the venting process, maintain the pressure downstream of the first control valve 21 between 2.0 and 2.6 MPa.g. Monitor the real-time temperature of the upstream branch line temperature transmitter 32 or the downstream branch line temperature transmitter 34 to ensure that the temperature does not fall below -15°C. After the venting is complete, close the bypass connecting the first block valve 5, the bypass connecting the second block valve 8, the first and second control valves 21 and 22, and reopen the outlet block valve of the pressure buffer tank 14.

[0063] 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. A large drop dense phase carbon dioxide pipeline pressure protection system, characterized by: Including trunk transmission system, overpressure protection and relief system and instrument monitoring system; The trunk line transmission system is used to provide a normal transmission flow path for a dense phase carbon dioxide pipeline, comprising an upstream trunk line pipeline (1), a downstream trunk line pipeline (3), and a low point trunk line shutoff valve (2) arranged between the upstream and downstream trunk line pipelines; The trunk transmission system further includes a bypass pipeline (7), the bypass pipeline (7) being connected to the upstream trunk pipeline (1) and the downstream trunk pipeline (3) through an upstream bypass shutoff valve (4) and a downstream bypass shutoff valve (6), respectively, to form a bypass medium flow channel required for planned emptying and overpressure protection of the upstream and downstream trunk pipelines, and a bypass connection first shutoff valve (5) and a bypass connection second shutoff valve (8) are provided on the bypass pipeline (7); The bypass pipeline (7) is connected to a pipeline planned discharge system, and the pipeline planned discharge system includes a primary regulating valve (21) and a secondary regulating valve (22). The primary regulating valve (21) is arranged downstream of the bypass-connected first shut-off valve (5) and the bypass-connected second shut-off valve (8), and the secondary regulating valve (22) is arranged downstream of the air-bath heat exchanger (15). The primary regulating valve (21) and the secondary regulating valve (22) are opened only when the trunk line performs planned discharge. The overpressure protection and relief system is arranged upstream and downstream of the low-point trunk shut-off valve (2), and includes an upstream overpressure safety valve (11), a downstream overpressure safety valve (12), a bypass pressure balancing safety valve (13) and a multi-stage regulating relief unit. The bypass pressure balancing safety valve (13) is arranged on the bypass of the low-point trunk shut-off valve (2). When the low-point trunk shut-off valve (2) is closed and the trunk transmission pipeline is overpressured, the bypass pressure balancing safety valve (13) connects the upstream trunk pipeline (1) and the downstream trunk pipeline (3) to form a first-level priority pressure relief protection. The upstream overpressure safety valve (11) and the downstream overpressure safety valve (12) are respectively connected to the upstream trunk pipeline (1) and the downstream trunk pipeline (3) to form a second-level pressure relief protection. The outlet ends of the upstream overpressure safety valve (11) and the downstream overpressure safety valve (12) are connected to the multi-stage regulating relief unit. The multi-stage regulating and releasing unit is used for performing multi-stage pressure regulation and temperature regulation on the overpressure releasing medium, and comprises a pressure buffer tank (14), an air bath type heat exchanger (15), a secondary pressure safety valve (16) and a venting riser (18) connected in sequence, wherein the pressure buffer tank (14) is arranged downstream of the upstream overpressure safety valve (11) and the downstream overpressure safety valve (12), and the pressure buffer tank (14) is pre-filled with carbon dioxide at a certain pressure; The instrument monitoring system includes a pressure transmitter and a temperature transmitter, which are used to monitor the pressure and temperature of corresponding points of the trunk transmission system and the overpressure protection and relief system in real time.

2. The large drop dense phase carbon dioxide pipeline pressure protection system according to claim 1 is characterized by: The air bath heat exchanger (15) is connected to a vent riser (18) via a bursting disc (17), and the bursting disc (17) is connected in parallel with a secondary pressure safety valve (16).

3. The large drop dense phase carbon dioxide pipeline pressure protection system according to claim 1 is characterized by: The upstream overpressure safety valve (11) is arranged downstream of the upstream bypass shut-off valve (4) for discharging the overpressure medium of the upstream trunk line, and the downstream overpressure safety valve (12) is arranged downstream of the downstream bypass shut-off valve (6) for discharging the overpressure medium of the downstream trunk line.

4. The large drop dense phase carbon dioxide pipeline pressure protection system according to claim 1 is characterized by: The instrument monitoring system comprises an upstream branch line pressure transmitter (31) and an upstream branch line temperature transmitter (32) arranged on the upstream trunk pipeline (1), a downstream branch line pressure transmitter (33) and a downstream branch line temperature transmitter (34) arranged on the downstream trunk pipeline (3), an overpressure relief first temperature transmitter (35) arranged upstream of the pressure buffer tank (14), an overpressure relief first pressure transmitter (36) arranged downstream of the pressure buffer tank (14), and a second temperature transmitter (37) arranged downstream of the air bath heat exchanger (15).

5. A protection method for a large-drop dense-phase carbon dioxide pipeline pressure protection system according to claim 4, characterized in that: The following steps are involved: Step 1: When the pipeline is operating normally, keep the upstream trunk pipeline, low-point trunk shut-off valve, downstream trunk pipeline, upstream bypass shut-off valve, and downstream bypass shut-off valve open. Keep the upstream overpressure safety valve, downstream overpressure safety valve, bypass pressure balancing safety valve, pressure buffer tank, air bath heat exchanger, secondary pressure safety valve, bursting disc, and vent riser online. Close the bypass connecting the first shut-off valve and the bypass connecting the second shut-off valve, close the primary regulating valve and the secondary regulating valve. At the same time, maintain the carbon dioxide filled in the pressure buffer tank at a certain pressure, and perform pressure detection through the first overpressure relief pressure transmitter. When the pressure is lower than the set value, promptly replenish the pressure of the pressure buffer tank; Set the opening pressure of the bypass pressure balancing safety valve to a certain multiple of the main pipeline design pressure, set the opening pressure of the upstream overpressure safety valve and the downstream overpressure safety valve to a multiple of the main pipeline design pressure greater than the multiple set by the bypass pressure balancing safety valve, and set the opening pressure of the secondary pressure safety valve to be greater than the gas pressure in the pressure buffer tank; Step 2: When the low-point main line block valve is accidentally closed, when the upstream pressure exceeds the set pressure of the bypass pressure balancing safety valve, the bypass pressure balancing safety valve automatically opens, and the overpressure medium in the upstream main line pipeline will quickly flow into the downstream main line pipeline through the bypass pressure balancing safety valve; When the upstream pressure continues to rise and exceeds the set pressure of the upstream overpressure safety valve, the upstream overpressure medium will flow into the pressure buffer tank through the upstream overpressure safety valve. After the pressure in the pressure buffer tank exceeds the set pressure of the secondary pressure safety valve, the secondary pressure safety valve opens and discharges the medium to the vent riser. At the same time, when the downstream trunk pipeline pressure exceeds the set pressure of the downstream overpressure safety valve, the downstream overpressure medium will flow into the pressure buffer tank through the downstream overpressure safety valve. Step 3: When the pipeline stops transporting, if the temperature and pressure of the medium in the upstream trunk pipeline or the downstream trunk pipeline rise, pressure protection is performed by performing the pressure relief operation of the upstream overpressure safety valve and the downstream overpressure safety valve in step 2; Step 4: When the upstream trunk pipeline or the downstream trunk pipeline is planned to be emptied, open the bypass to connect to the first block valve or the bypass to connect to the second block valve, close the outlet block valve of the pressure buffer tank, slowly open the first-level regulating valve and the second-level regulating valve, and keep the pressure downstream of the first-level regulating valve not exceeding the opening pressure of the second-level pressure safety valve during the discharge process, and pay attention to the real-time temperature of the upstream branch temperature transmitter or the downstream branch temperature transmitter to ensure that the temperature is not lower than -15°C; after the discharge is completed, close the bypass to connect to the first block valve, the bypass to connect to the second block valve, the first-level regulating valve and the second-level regulating valve, and reopen the outlet block valve of the pressure buffer tank.

6. The protection method according to claim 5, characterized in that: In step one, the pressure of the carbon dioxide filled in the pressure buffer tank is between 1.8 and 2.2 MPa.g. When the pressure is lower than 1.8 MPa.g, the pressure buffer tank is pressurized. The opening pressure of the bypass pressure balancing safety valve is set to 1.0 times the design pressure of the trunk pipeline, the opening pressure of the upstream overpressure safety valve and the downstream overpressure safety valve is set to 1.1 times the design pressure of the trunk pipeline, and the opening pressure of the secondary pressure safety valve is set to 2.4 to 2.6 MPa.g.

7. The protection method according to claim 5, characterized in that: In step 2, the pressure buffer tank, air bath heat exchanger and secondary pressure safety valve are used to provide pressure and temperature relay for carbon dioxide pressure relief to control the medium pressure and temperature during the relief process. When the secondary pressure safety valve fails, the overpressure medium is discharged through the bursting disc and enters the vent riser to complete the release. After the relief is completed, the pressure of the pressure buffer tank is reduced to 1.8~2.2MPa.g.

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