Wellhead valve freezing and blocking risk analysis method and system in CO2 pressure reduction process

Abstract

The invention discloses a wellhead valve freezing and blocking risk analysis method and system in the CO2 pressure reduction process, and belongs to the technical field of numerical simulation modeling. The problems that in the existing dense-phase CO2 release process, research on related physical field distribution rules in a pressure release valve and a pipeline is relatively few, structural optimization cannot be conducted on a wellhead valve, and consequently the freezing and blocking phenomena occur frequently are solved. The method comprises the steps that a 1 / 2 3-D wellhead gate valve model is built, and drainage basin extraction, grid division and solver setting are carried out; performing CO2 physical property setting on the set 1 / 2 3-D wellhead gate valve model, and performing model verification to obtain a CO2 freezing and blocking risk numerical simulation model; cFD simulation calculation is conducted on the CO2 freezing and blocking risk numerical simulation model according to the two opening degrees, the freezing and blocking risk of the valve is analyzed according to the calculation result, and therefore structural optimization is conducted on the valve. The device and the method are suitable for research on the distribution rule of related physical fields in the pressure release valve and the pipeline in the dense-phase CO2 release process.

Description

Technical Field

[0001] The present invention belongs to the technical field of numerical simulation modeling, and particularly relates to a method for analyzing the freezing and blocking risk of the wellhead valve during the CO 2 decompression process. Background Art

[0002] For the transportation of large-scale carbon dioxide, high-temperature and normal-pressure liquid-phase carbon dioxide pipeline transportation, also known as "dense-phase" transportation, is adopted. However, compared with the pipeline transportation of hydrocarbons such as natural gas, the industry experience in transporting carbon dioxide through pipelines is much less, and relevant risks should be fully understood and effectively managed.

[0003] There is a need to control the single-phase state of carbon dioxide during transportation. Near the critical region, small differences in pressure and temperature will cause sharp fluctuations in the state characteristics of carbon dioxide. Therefore, the pressure during transportation needs to be maintained above the critical pressure to avoid two-phase transportation. Currently, the operating pressure during transportation is 8.5 MPa - 12 MPa, which is in the "smooth" region of the carbon dioxide phase diagram, that is, the state of carbon dioxide in this region is stable and will not mutate. However, during the decompression and relief process, the characteristics of carbon dioxide will inevitably change sharply. Due to accidental failures (damage caused by third-party construction or corrosion) or planned maintenance, pipelines often need to be decompressed and relieved. During the decompression process, the escaping gas will cool the pipeline. If the temperature drops too much and is lower than the ductile-brittle transition temperature of the pipeline material, the pipeline material will become brittle, resulting in brittle fracture and serious pipeline damage. The formation of dry ice during the carbon dioxide decompression and relief process also poses a certain risk of freezing and blocking to the pipeline or valve. Therefore, the relief of the carbon dioxide pipeline needs to be controlled to ensure that the pipeline temperature is not lower than the design temperature.

[0004] Currently, research on the safe relief of CO 2 pipelines mainly focuses on experimental and simulation studies on the variation laws of the temperature and pressure inside the pipeline during the relief of small-scale short-distance pipelines, as well as studies on the external spraying and diffusion laws, etc. Most of the pipeline outflow models in the literature adopt the homogeneous equilibrium model (HEM), where the fluid phase state is assumed to be in thermal equilibrium and mechanical equilibrium during the compression process, and in addition, the phase slip and gas-liquid non-equilibrium transition phenomena are ignored.

[0005] Domestic scholars have also conducted some explorations in the direction of numerical simulation of CO 2 pipeline leakage. Yan Wei, Lv Yuling, etc. based on the CO in Shengli Oilfield 2The oil recovery project simulated the pipeline section from Qilu Petrochemical to Chunliang Oilfield for supercritical state and dense phase leakage simulation. The pore model and pipeline model proposed by Levenspie, Crowl, etc. were used respectively. The pore model regards the pipeline as a large container and assumes that its internal pressure remains unchanged, but does not consider the unstable state formed by the action of the stop valve. The constant flow rate and state parameters at the outflow are larger than the actual value, and are only applicable to the case of a very small leakage diameter. The pipeline model is only applicable to the case of a complete rupture of the pipeline. Li Kang established a small-scale supercritical CO 2 Numerical simulation, establishment of near-field high-pressure model, development of prediction algorithm for high-pressure pipeline leakage, explanation of the formation of dry ice and bow shock, but did not compare the accuracy of large-scale experiments, did not consider far-field changes, and did not consider phase changes in the near field. 2 Leaking pipeline, deriving supercritical CO 2 The energy equation and enthalpy formula are used to establish the isentropic blocking flow leakage rate model. The near-field velocity and far-field CO 2 The concentration cloud map shows that the temperature in the Mach disk can reach -80°C, and the near-field shock wave structure is clearer. However, as the model size increases, the accuracy of its fitting decreases. 2 Leakage experiment device and small-scale CO 2 The leakage experiment device uses the real gas PR state equation to construct the gas phase CO 2 The diffusion numerical model was developed and applied to supercritical and dense phase conditions. The study found that the gas phase data fit well, and CO 2 The diffusion range of concentration and temperature continues to expand with the increase of leakage aperture, but the fit is not enough in supercritical and dense phase conditions, and CO 2 The influence of fluid phase change after passing through the leakage port.

[0006] In summary, the above research work mainly focuses on high pressure CO 2 The far-field discharge characteristics of pipelines are experimentally studied, and most of the physical structures studied are pipeline-small hole discharge structures, and the physical models are relatively simple. 2 There are relatively few studies on the distribution laws of related physical fields inside pressure relief valves and pipelines during the release process. Summary of the invention

[0007] The present invention aims at the existing dense phase CO 2 There is relatively little research on the distribution law of related physical fields inside the pressure relief valve and pipeline during the discharge process, which makes it impossible to optimize the structure of the wellhead valve, resulting in frequent freezing and blocking. 2 Risk analysis method for wellhead valve freezing during decompression process.

[0008] To achieve the above object, the present invention provides the following solution:

[0009] The present invention provides a method for analyzing the risk of freezing and blocking of the wellhead valve during the CO 2 pressure reduction process, and the method includes the following steps:

[0010] Step S1: Build a 1 / 23-D wellhead gate valve model, and perform watershed extraction, mesh generation, and solver settings;

[0011] Step S2: Perform CO 2 physical property settings on the set 1 / 23-D wellhead gate valve model, and perform model verification to obtain a numerical simulation model for the risk of freezing and blocking of CO 2 ;

[0012] Step S3: Perform CFD simulation calculations on the numerical simulation model for the risk of freezing and blocking of CO 2 for two opening degrees, analyze the risk of valve freezing and blocking according to the calculation results, and thus optimize the valve structure.

[0013] Furthermore, there is also a preferred embodiment. Specifically, the above step S1 is as follows:

[0014] Step S11: Build a 1 / 23-D wellhead gate valve model;

[0015] Step S12: Use model processing software to perform watershed extraction on the wellhead gate valve model to obtain the main throttling area of the wellhead gate valve;

[0016] Step S13: After watershed extraction, divide the wellhead gate valve model using a hybrid mesh method;

[0017] Step S14: After mesh generation, set the solver parameters and the two-phase flow parameters of CO 2 .

[0018] Furthermore, there is also a preferred embodiment. When performing the above watershed extraction, watersheds with diameters 5 times and 10 times the valve inlet diameter are added at the inlet and outlet of the valve respectively.

[0019] Furthermore, there is also a preferred embodiment. When performing the above mesh generation, regional processing is required, specifically:

[0020] In the area near the pressure relief slit of the wellhead gate valve, locally refine the mesh;

[0021] In the area far from the pressure relief slit, use a sparse mesh generation method.

[0022] Furthermore, there is also a preferred embodiment. The two-phase flow parameters of the above CO 2 are specifically:

[0023] Liquid-phase CO2 The parameters of

[0024] gaseous CO 2 include density, viscosity, thermal conductivity, enthalpy at standard state, and reference temperature.

[0025] Furthermore, there is also a preferred embodiment. For the above CO 2 the physical property settings are specifically as follows:

[0026] The physical properties of CO 2 are divided into three segments: 0.1 - 0.6 MPa, 0.6 - 7.37 MPa, 7.37 - 30 MPa;

[0027] In the stage of 0.1 - 0.6 MPa, the temperature is set according to the fitting formula between the saturation temperature and the saturation pressure;

[0028] In the stage of 0.6 - 7.37 MPa, CO 2 undergoes a phase change, and the temperature is set according to the fitting formula between the saturation temperature and the saturation pressure;

[0029] In the stage of 7.37 - 30 MPa, CO 2 does not undergo a phase change, and the saturation temperature at different pressures is set to 35 °C.

[0030] Furthermore, there is also a preferred embodiment. When performing the CFD simulation calculation for the above two opening degrees, the boundary conditions are set as follows:

[0031] The inlet of the valve is a pressure inlet boundary condition with a pressure of 25 MPa. The outlet of the valve is a pressure outlet. The boundary condition of the symmetry plane is set as Symmetry with a pressure of 0 MPa.

[0032] The CO 2 risk analysis method for the freezing blockage of the wellhead valve during the pressure reduction process of the present invention can be fully implemented by computer software. Therefore, correspondingly, the present invention also provides a CO 2 risk analysis system for the freezing blockage of the wellhead valve during the pressure reduction process. The system includes a storage device, and the storage device is used to perform the following steps:

[0033] Step S1: Build a 1 / 23-D wellhead gate valve model, and perform watershed extraction, mesh generation, and solver setting;

[0034] Step S2: Perform CO 2 physical property settings on the set 1 / 23-D wellhead gate valve model, and perform model verification to obtain a CO 2 numerical simulation model for the freezing blockage risk;

[0035] Step S3: For two opening degrees, perform CO 2Perform CFD simulation calculations on the numerical simulation model of the freezing blockage risk, analyze the freezing blockage risk of the valve according to the calculation results, and thus optimize the structure of the valve.

[0036] The present invention also provides a computer-readable storage medium, on which a computer program is stored. When the computer program is run by a processor, it executes the CO described in any one of the above. 2 Analysis method for the freezing blockage risk of the wellhead valve during the pressure reduction process.

[0037] The present invention also provides a computer device, which includes a memory and a processor. A computer program is stored in the memory. When the processor runs the computer program stored in the memory, the processor executes the CO described in any one of the above. 2 Analysis method for the freezing blockage risk of the wellhead valve during the pressure reduction process.

[0038] The beneficial effects of the present invention are as follows:

[0039] 1. The present invention proposes a method for analyzing the freezing blockage risk of the wellhead valve during the pressure reduction process. Through the 1 / 23-D model of the wellhead gate valve, the CO 2 discharge at different opening degrees is modeled and simulated, including the extraction and simplification of the computational domain, mesh generation, boundary condition setting, solver setting, and CO 2 phase parameter setting and loading, etc.; by finely modeling the flow field of the valve during the pressure reduction and discharge process, determining the area where the freezing blockage risk occurs inside the valve, and optimizing the structure of the wellhead valve according to the freezing blockage analysis results. 2

[0040] Furthermore, in order to ensure the simulation accuracy and convergence, when extracting the flow domain, the flow domains with 5 times and 10 times the valve inlet diameter are respectively added at the inlet and outlet of the valve.

[0041] Furthermore, the present invention adopts a hybrid mesh division method to generate high-quality meshes; at the same time, considering the complex flow, heat transfer, and mass transfer phenomena inside the wellhead gate valve, regional processing is carried out when dividing the meshes. That is: in the area near the pressure relief slit of the wellhead gate valve, local mesh refinement is performed on the meshes; in the area far from the pressure relief slit, a sparse mesh division method is adopted.

[0042] Furthermore, the present invention divides the physical properties of carbon dioxide into three sections to ensure the accuracy of the simulation results and avoid large fluctuations in the physical property parameters of dense-phase carbon dioxide during the phase change process, which may affect the simulation accuracy.

[0043] 2. The method for analyzing the freezing blockage risk of the wellhead valve during the pressure reduction process proposed by the present invention, the accuracy of numerical calculation can reach 95%.​​

[0044] 3. The method for analyzing the risk of freezing and blocking of the wellhead valve during the CO 2 pressure reduction process, through the analysis of the simulation results at two opening degrees, large and small, it is found that there is a risk of freezing and blocking in the flow fields at both opening degrees. By comparing the areas of potential freezing and blocking regions, it can be obtained that the valve has a higher risk of freezing and blocking at the small opening degree, and the proportion of the area of the freezing and blocking region is 0.3249, which can provide an effective reference for the subsequent structural design of the wellhead valve.

[0045] The present invention is applicable to the study of the distribution laws of the physical fields inside the pressure relief valve and the pipeline during the 2 venting process of dense-phase CO. Description of the Drawings

[0046] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the following drawings are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0047] Figure 1 It is the flowchart of the method for analyzing the risk of freezing and blocking of the wellhead valve during the CO 2 pressure reduction process described in the embodiments of the present invention;

[0048] Figure 2 It is the structural diagram of the wellhead valve described in the embodiments of the present invention;

[0049] Figure 3 It is the 1 / 23D domain map of the wellhead gate valve described in the embodiments of the present invention;

[0050] Figure 4 It is the schematic diagram of the mesh division of the wellhead gate valve described in the embodiments of the present invention;

[0051] Figure 5 It is the CO 2 corresponding curve of saturation temperature and saturation pressure described in the embodiments of the present invention;

[0052] Figure 6 It is the simulation calculation flow field model and corresponding parameters described in the embodiments of the present invention;

[0053] Figure 7 It is the CO 2 spraying photo and the positions of the Mach disks at different moments described in the embodiments of the present invention;

[0054] Figure 8 It is the schematic diagram of the generation principle of the Mach disk described in the embodiments of the present invention;

[0055] Figure 9 It is the velocity contour map of the simulation under different working conditions described in the embodiments of the present invention;

[0056] Figure 10 It is the flow curve at 25% small opening described in the embodiments of the present invention;

[0057] Figure 11 It is the pressure contour map of the wellhead gate valve pipeline at 25% opening described in the embodiments of the present invention;

[0058] Figure 12 It is the velocity contour map of the wellhead gate valve pipeline at 25% opening described in the embodiments of the present invention;

[0059] Figure 13 It is the liquid volume fraction contour map of the wellhead gate valve pipeline at 25% opening described in the embodiments of the present invention;

[0060] Figure 14 It is the gas volume fraction contour map of the wellhead gate valve pipeline at 25% opening described in the embodiments of the present invention;

[0061] Figure 15 It is the temperature contour map of the wellhead gate valve pipeline at 25% opening described in the embodiments of the present invention;

[0062] Figure 16 It is the area where the valve may have the risk of freezing blockage described in the embodiments of the present invention;

[0063] Figure 17 It is the flow curve at 75% large opening described in the embodiments of the present invention;

[0064] Figure 18 It is the pressure contour map of the wellhead gate valve pipeline at 75% opening described in the embodiments of the present invention;

[0065] Figure 19 It is the velocity contour map of the wellhead gate valve pipeline at 75% opening described in the embodiments of the present invention;

[0066] Figure 20 It is the liquid volume fraction contour map of the wellhead gate valve pipeline at 75% opening described in the embodiments of the present invention;

[0067] Figure 21 It is the gas volume fraction contour map of the wellhead gate valve pipeline at 75% opening described in the embodiments of the present invention;

[0068] Figure 22 It is the temperature contour map of the wellhead gate valve pipeline at 75% opening described in the embodiments of the present invention. Detailed implementation manners

[0069] The following further elaborates on the specific embodiments of the present invention in conjunction with the accompanying drawings. The following embodiments will assist those skilled in the art in further understanding the present invention, but do not limit the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several changes and improvements can still be made, and these all fall within the protection scope of the present invention.

[0070] Embodiment 1. Refer to Figure 1 To describe this embodiment, this embodiment aims at the relatively less research on the distribution law of the pressure relief valve and the internal physical fields of the pipeline during the venting process of the existing dense-phase CO 2 , which makes it impossible to optimize the structure of the wellhead valve, resulting in frequent freezing blockage phenomena. Therefore, a method for analyzing the freezing blockage risk of the wellhead valve during the CO 2 pressure reduction process is proposed. The method includes the following steps:

[0071] Step S1: Build a 1 / 23-D wellhead gate valve model, and perform watershed extraction, mesh division, and solver setting;

[0072] Step S2: Perform CO 2 physical property settings on the set 1 / 23-D wellhead gate valve model, and conduct model verification to obtain a CO 2 freezing blockage risk numerical simulation model;

[0073] Step S3: Conduct CFD simulation calculations on the CO 2 freezing blockage risk numerical simulation model for two opening degrees, analyze the freezing blockage risk of the valve according to the calculation results, and thus optimize the structure of the valve.

[0074] Embodiment 2. Refer to Figure 2 and Figure 5 To describe this embodiment, this embodiment specifically elaborates on the method for analyzing the freezing blockage risk of the wellhead valve during the CO 2 pressure reduction process described in the above Embodiment 1;

[0075] Step S1: Build a 1 / 23-D wellhead gate valve model, and perform watershed extraction, mesh division, and solver setting;

[0076] Specifically:

[0077] The 1 / 23-D model modeling is specifically as follows:

[0078] The wellhead valve is a plate gate valve for venting CO 2 , and its basic structure model is as shown in Figure 2 , mainly including structural components such as valve stem, valve cover, valve plate, valve seat, and valve body. CO 2During the relief process, by rotating the handwheel of the gate valve, the valve stem and the valve plate are gradually opened under the push of the lead screw. The entire valve opening process lasts about 60 seconds. Subsequently, the valve reaches its maximum opening, and a stable relief process is carried out.

[0079] The basin extraction is specifically as follows:

[0080] Based on the above model of the wellhead gate valve, basin extraction was carried out in the model processing software. The structure of the wellhead gate valve is a plane-symmetrical structure. Therefore, for the simulations at different opening degrees of the wellhead gate valve, a 1 / 2 - 3D model was used for simulation calculations. Considering the actual working environment of the valve, its calculation basin was extracted and simplified as shown in Figure 3. As Figure 3 shown, the most important throttling area of the wellhead gate valve is near the gate plate. During the opening process of the valve, the throttling area of the gate plate region changes from small to large. At this time, liquid CO 2 flows through the valve under the drive of high pressure, and the temperature, pressure, and flow velocity change violently, resulting in a phase change. Therefore, in the implementation method, by calculating the detailed changes in the valve flow field at different opening degrees, the possible freezing and blocking problems during the valve opening and working processes are analyzed.

[0081] Furthermore, to ensure the simulation accuracy and convergence, it is necessary to add basins with diameters 5 times and 10 times the valve inlet diameter at the inlet and outlet of the valve, so as to fully develop the flow fields before and after the valve. The inlet and outlet boundary conditions of the basin are specified according to the specific working conditions.

[0082] The mesh generation is specifically as follows:

[0083] After completing the calculation basin extraction, in this embodiment, the calculation mesh was generated according to the structural characteristics of the wellhead gate valve. Although the simplified three-dimensional cavity model structure of the wellhead gate valve is simple, the internal flow characteristics, phase change heat transfer and mass transfer characteristics of the valve cavity are quite complex. Therefore, a hybrid mesh generation method was used to generate high-quality meshes.

[0084] Furthermore, considering the complex flow, heat transfer and mass transfer phenomena inside the wellhead gate valve, it is necessary to process the mesh generation in different regions when generating the mesh. In the region near the relief slit of the wellhead gate valve, the flow situation is relatively complex, and local mesh refinement is required. While in the region far from the relief slit (which can be considered as most of the region inside the wellhead gate valve cavity), the fluid flow is relatively gentle, so relatively sparse meshes can be generated in this region. Generating meshes in different regions in this way can not only ensure the accuracy of the simulation results of the internal flow field of the pipe, but also appropriately reduce the number of meshes, save computing resources and improve the simulation computing speed.

[0085] Furthermore, for CFD analysis, grid density is one of the key factors to ensure the accuracy and convergence of simulation calculations. To obtain the optimal computational grid model, in this embodiment, the wellhead gate valve was simulated using grids with different density levels, and by comparing the temperatures at the valve plate outlet after simulations with different grids, the influence of grid density on simulation accuracy was explored. The results are shown in Table 1 below.

[0086] Table 1

[0087] Number of meshes Temperature at the outlet (K) Deviation (%) 985,473 215.42 2.06 1,247,897 208.04 1.44 2,673,022 210.01 0.50 5,852,652 211.07 -

[0088] As can be seen from Table 1 above, when the number of grids is greater than 2,673,022, the simulation results are independent of grid density, that is, the calculation results converge for the computational grids. Considering both computational efficiency and cost, in this embodiment, a grid generation scheme with 2,673,022 grids was selected for subsequent simulation calculations. The final grid scheme is as Figure 4 shown.

[0089] The solver settings are specifically as follows:

[0090] After completing the grid division, the solver parameters also need to be set in the CFD simulation calculation, as shown in Table 2 below:

[0091] Table 2

[0092]

[0093] In addition to the basic solver parameter settings, the two-phase flow parameters of CO 2 also need to be set before the calculation, and the specific parameters are shown in Table 3 below.

[0094] Table 3

[0095]

[0096] Step S2: Perform CO 2 physical property settings on the set 1 / 23-D wellhead gate valve model and conduct model verification to obtain the CO 2 frozen blockage risk numerical simulation model;

[0097] The CO 2 physical property settings are specifically as follows:

[0098] During the normal-temperature transportation of the dense-phase carbon dioxide pipeline, the working pressure is maintained at around several tens of megapascals. After being discharged through the wellhead gate valve, the pressure drops suddenly, and the corresponding saturation temperature will decrease rapidly. Therefore, the dense-phase carbon dioxide will undergo a phase change from the liquid phase to the gas phase (similar to the flashing phenomenon). The physical property parameters of the dense-phase carbon dioxide will fluctuate within a large range during the phase change process. Correct carbon dioxide physical property parameters can ensure the accuracy of the simulation results.

[0099] The phase diagram of carbon dioxide and the pressure-temperature relationship are as follows Figure 5 shown. The triple critical point is obtained from Refprop, where T lim＝ = 30.92 °C and P lim = 7.37 MPa. The inlet pressure of the wellhead gate valve is 30 MPa, the temperature is 300 K, and the outlet back pressure is 0.1 MPa. According to the inlet and outlet pressures of the wellhead gate valve, the physical properties of carbon dioxide are divided into three sections: 0.1 - 0.6 MPa, 0.6 - 7.37 MPa, and 7.37 - 30 MPa.

[0100] Among them, in the section of 7.37 - 30 MPa, that is, the inlet pressure of the wellhead gate valve drops from 30 MPa to the critical pressure of carbon dioxide, 7.37 MPa. Within this pressure drop range, carbon dioxide does not undergo a phase change, and the saturation temperature at different pressures is set to a constant value of 35 °C (the temperature in the computational domain at the initial moment of simulation is 25 °C).

[0101] In the section of 0.6 - 7.37 MPa, during the process of decreasing the pressure of carbon dioxide, the saturation temperature corresponding to the pressure will drop suddenly, so a phase change process of carbon dioxide will occur in this section. The Refprop software is used to obtain the physical property parameters of saturated carbon dioxide under the pressures in this section, and the results are shown in Table 4 below.

[0102] Table 4

[0103]

[0104]

[0105] Performing polynomial fitting on the discrete point data shown in Table 4 above, the relationship between the saturation temperature and the corresponding saturation pressure is obtained as

[0106] T = 0.241P 3 - 4.2P 2 + 31.46P - 68.15

[0107] where T is the saturation temperature of carbon dioxide in °C; P is the saturation pressure of carbon dioxide in MPa. The correlation coefficient R of this fitting formula 2 = 0.9991, and the fitting accuracy is relatively high.

[0108] In the pressure section of 0.1 - 0.6 MPa, the fitting formula for the saturation temperature of carbon dioxide in the section of 0.1 MPa - 0.6 MPa no longer uses the fitting formula in the section of 0.6 - 7.37 MPa. Based on the data of the triple point and the 0.1 MPa point, a new fitting is performed, and the re-fitted relationship formula is reloaded into the FLUENT evaporation and condensation model.

[0109] The fitting formula is as follows:

[0110] T = - 87.42P2 +124.96P + 176.53

[0111] where T is the saturation temperature of carbon dioxide in °C; P is the saturation pressure of carbon dioxide in MPa. The correlation coefficient R of this fitting formula 2 = 0.9995.

[0112] Embodiment 3. Refer to Figures 6 to 9 to illustrate this embodiment. This embodiment conducts experimental verification on the numerical simulation model of the CO 2 freezing blockage risk described in the above embodiment; specifically, it includes the following steps:

[0113] First, perform mesh generation and boundary conditions:

[0114] According to the experimental steps, an experiment was conducted with a circular leakage hole having an inner diameter of 5 mm. The experiment was for supercritical CO 2 leakage, and the initial pressure of CO in the container 2 was 7.7 MPa and the temperature was 36 °C. As Figure 6 shown, an axisymmetric computational domain was established to improve the computational efficiency. The settings of the computational model in terms of size parameters fully simulated the physical model in the experiment. The computational domain consists of a nozzle (orifice) and the external environment. According to the orifice used in the experiment, the nozzle is 3 mm long and has an inner diameter of 5 mm. The external environment is in close contact with the nozzle outlet, 1000 mm long and 200 mm wide, which can ensure the full development of the jet. All computational domains are meshed with quadrilateral structured grids.

[0115] Secondly, the definitions of the boundary conditions of this model are as follows:

[0116] (1) Inlet boundary (ab): Pressure inlet, pressure 7.7 MPa, temperature 36 °C.

[0117] (2) Wall boundary (bc): No-slip, adiabatic boundary;

[0118] (3) Outlet boundary (cdef): Pressure boundary, pressure and temperature equal to the atmospheric pressure and temperature;

[0119] (4) Axis of symmetry (af): Axisymmetric boundary condition, the normal velocity and any variable gradient on this boundary are both 0.

[0120] Then, set the calculation parameters:

[0121] Based on the Fluent framework, the finite volume method is used to discretize the governing equations. A transient solver is adopted, and the Coupled algorithm is used for pressure-velocity coupling. The convective terms, turbulent kinetic energy equation, and turbulent diffusion equation are discretized using the second-order upwind scheme. Due to the coupling of high-speed flow and rapid phase change during the leakage process, a dynamic time step of 3e-6 s is required to accurately capture the subtle changes in the velocity, pressure, and temperature fields without divergence. At the same time, the source term of the governing equation is modified through a User-Defined Function (UDF) to achieve the simulation of phase change during the flow process.

[0122] Final result verification:

[0123] Figure 7 is supercritical CO 2 The evolution process of the jet near field during leakage. The initial state of this experiment is 7.7 MPa and 36 °C, and the leakage orifice is a circular hole with a diameter of 5 mm. Due to the strong throttling effect at the orifice, the temperature in the atmospheric space drops sharply to CO 2 below the triple point, and a large number of dry ice particles are formed outside the pipe. The white part of the jet visible in the figure is mainly composed of dry ice. In the actual CO 2 jet, it is a mixture of a large number of micron-sized dry ice particles and gaseous CO 2 . As can be seen from the figure, the shape of the supercritical CO 2 jet is a barrel-shaped structure with a pointed bottom. It is a typical under-expanded jet. In the initial stage of the jet, the dry ice concentration is small, and the Mach disk and the overall shock wave structure can be clearly observed.

[0124] Figure 8 The principle of the generation of the Mach disk is as follows. When the gas is over-expanded, compared with the external atmosphere, the gas pressure in the exhaust is lower, resulting in the exhaust being compressed or squeezed inward. This compression increases the exhaust pressure. However, since the flow rate may be compressed too much, its pressure exceeds the atmospheric pressure. At this time, the air flow expands outward again to reduce the pressure, resulting in the pressure inside the plume being lower than the ambient pressure again. As time goes by, the compression and expansion processes are repeated continuously, and the difference between the gas pressure in the exhaust and the external atmospheric pressure gradually decreases until the exhaust pressure is the same as the ambient atmospheric pressure. Figure 9 The velocity contour obtained by establishing the same physical model according to the experiment of Teng Lin et al. shows that a relatively obvious Mach disk structure appears on the velocity contour, which is almost the same as the position of the Mach disk at 37.3 ms in the experiment. This result indicates that the simulation method adopted in the present invention has good accuracy and precision in calculating the CO 2 release process, which can lay a theoretical foundation for subsequent simulation calculations.

[0125] In addition, by determining CO 2The position of the Mach disk in the near-field jet structure is compared with the position of the Mach disk in the experiment, and the accuracy of the simulation model can be specifically quantified. Figure 7 In it, the experimental result of the distance between the Mach disk and the discharge port at 37.3 ms is 15 mm. Figure 9 The simulation result in it is 14.25 mm. From this, the accuracy of the numerical calculation can be obtained as 95%, which meets the technical requirements.

[0126] Embodiment 4. Refer to Figures 10 to 22 This embodiment will be described. This embodiment specifically describes the analysis of the valve freezing risk result described in the above embodiment. According to the above numerical calculation model, this report conducts simulation calculations for two representative opening degrees during the opening process of the gate valve. By analyzing the freezing risk of the wellhead gate valve at typical opening degrees, the freezing risk of the wellhead valve is evaluated, and the valve structure is optimized based on the simulation results at typical opening degrees. The selected typical opening degrees are the small opening degree of 25% and the large opening degree of 75%, which are specifically as follows:

[0127] First, the simulation results at the small opening degree are as follows:

[0128] The boundary conditions are specifically set as follows:

[0129] A numerical calculation model of the valve at the small opening degree is established. Under normal working conditions, the inlet of the valve is given as a pressure inlet boundary condition, and its pressure magnitude is specified as 25 MPa. The boundary of the valve outlet is given as a pressure outlet, and its pressure magnitude is specified as 0 MPa here. According to the calculation requirements of the 1 / 23-D model, the boundary condition of the symmetry plane is set as Symmetry, and the remaining boundaries are set as Wall and Interface as required.

[0130] Simulation results:

[0131] Figure 10 It is the inlet flow rate detection curve at the small opening degree. It can be seen that as the calculation step increases, the inlet flow rate tends to be stable, that is, the simulation results at this time converge, and the results at this time are used as the simulation results at this opening degree.

[0132] Figure 11 It is the pressure nephogram of the wellhead gate valve pipeline at 25% opening degree. It can be seen that at the inlet of the valve, the entire CO 2 The pressure level is at the highest position. When CO 2 flows through the valve plate area of the valve, the pressure nephogram changes greatly, forming a large pressure gradient. This is because the valve is not fully opened, and the small flow area at the valve plate forms a throttle; similarly, when CO 2 flows out of the valve plate area, a relatively obvious pressure gradient is also formed. Here, CO 2 forms a secondary pressure reduction, and finally CO 2The pressure gradually approaches the atmospheric pressure.

[0133] Figure 12 Figure 4 is the velocity contour map of the wellhead gate valve pipeline at 25% opening. According to Bernoulli's principle, when a fluid flows through a slit, the pressure will decrease while the flow velocity will increase. Therefore, it can be seen from the velocity contour map that when the fluid passes through the valve plate area of the valve, the flow velocity of the fluid increases significantly, which is consistent with the change of the pressure contour map. 2 When passing through the valve plate area of the valve, the flow velocity of the fluid increases significantly, which is consistent with the change of the pressure contour map.

[0134] Figure 13 and Figure 14 Figures 5 and 6 are the volume fraction contour maps of liquid and gaseous CO2 in the wellhead gate valve pipeline at 25% opening, respectively. It can be seen that in the pipeline at the inlet, the volume fraction of liquid CO2 is relatively large, about 10%, and the gas holdup here is 90%. Near the valve plate, due to the huge changes in the temperature and pressure of CO2, CO2 undergoes a violent phase change, and the gas holdup here increases significantly compared with the inlet pipeline, reaching a maximum of 100%. In a short section of the pipeline after CO2 flows out of the valve area, due to the decrease in the temperature of CO2, the above-mentioned potential freezing blockage area is formed. At this time, the gas holdup of CO2 decreases to a certain extent compared with that inside the valve. In the second half of the outlet pipeline, due to the decrease in pressure, CO2 is no longer in the liquid state, and the gas holdup at this time is about 100%. 2 The volume fraction of liquid CO2 is relatively large, about 10%, and the gas holdup here is 90%. Near the valve plate, due to the huge changes in the temperature and pressure of CO2, CO2 undergoes a violent phase change, and the gas holdup here increases significantly compared with the inlet pipeline, reaching a maximum of 100%. In a short section of the pipeline after CO2 flows out of the valve area, due to the decrease in the temperature of CO2, the above-mentioned potential freezing blockage area is formed. At this time, the gas holdup of CO2 decreases to a certain extent compared with that inside the valve. In the second half of the outlet pipeline, due to the decrease in pressure, CO2 is no longer in the liquid state, and the gas holdup at this time is about 100%. 2 temperature and pressure change greatly, CO2 2 undergoes a violent phase change, and the gas holdup here increases significantly compared with the inlet pipeline, reaching a maximum of 100%. In a short section of the pipeline after CO2 flows out of the valve area, due to the decrease in the temperature of CO2, the above-mentioned potential freezing blockage area is formed. At this time, the gas holdup of CO2 decreases to a certain extent compared with that inside the valve. In the second half of the outlet pipeline, due to the decrease in pressure, CO2 is no longer in the liquid state, and the gas holdup at this time is about 100%. 2 After flowing out of the valve area, in a short section of the pipeline, due to the decrease in the temperature of CO2, the above-mentioned potential freezing blockage area is formed. At this time, the gas holdup of CO2 decreases to a certain extent compared with that inside the valve. In the second half of the outlet pipeline, due to the decrease in pressure, CO2 is no longer in the liquid state, and the gas holdup at this time is about 100%. 2 temperature drops, forming the above-mentioned potential freezing blockage area. At this time, the gas holdup of CO2 decreases to a certain extent compared with that inside the valve. In the second half of the outlet pipeline, due to the decrease in pressure, CO2 is no longer in the liquid state, and the gas holdup at this time is about 100%. 2 In the second half of the outlet pipeline, due to the decrease in pressure, CO2 is no longer in the liquid state, and the gas holdup at this time is about 100%. 2 is no longer in the liquid state, and the gas holdup at this time is about 100%.

[0135] Figure 15 Figure 7 is the temperature contour map of the wellhead gate valve pipeline at 25% opening. It can be seen that at the inlet pipeline of the valve, the temperature of CO2 is relatively high, remaining at about 300K. When CO2 flows through the valve plate area of the valve, due to the throttling effect of the valve, the temperature of CO2 near the valve plate area drops to about 270K. When CO2 flows out of the valve plate area of the valve, the temperature of CO2 drops suddenly, and the lowest temperature is formed at this time. According to the standard phase diagram of CO2, the triple point temperature of CO2 is 217K. It can be seen that after CO2 flows out of the valve plate area of the valve, the CO2 in the pipeline is below 217K in some areas. At this time, CO2 will undergo a relatively large phase change under the huge changes in pressure and temperature. At the same time, due to the temperature being lower than the triple point, CO2 has the possibility of freezing, that is, there is a risk of freezing blockage in the wellhead gate valve at this time. 2 The temperature is relatively high, remaining at about 300K. When CO2 flows through the valve plate area of the valve, due to the throttling effect of the valve, the temperature of CO2 near the valve plate area drops to about 270K. When CO2 flows out of the valve plate area of the valve, the temperature of CO2 drops suddenly, and the lowest temperature is formed at this time. According to the standard phase diagram of CO2, the triple point temperature of CO2 is 217K. It can be seen that after CO2 flows out of the valve plate area of the valve, the CO2 in the pipeline is below 217K in some areas. At this time, CO2 will undergo a relatively large phase change under the huge changes in pressure and temperature. At the same time, due to the temperature being lower than the triple point, CO2 has the possibility of freezing, that is, there is a risk of freezing blockage in the wellhead gate valve at this time. 2 flows through the valve plate area of the valve, due to the throttling effect of the valve, CO2 2 has a temperature drop near the valve plate area to about 270K. When CO2 2 flows out of the valve plate area of the valve, CO2 2 temperature drops suddenly, and the lowest temperature is formed at this time. According to the standard phase diagram of CO2, the triple point temperature of CO2 is 217K. It can be seen that after CO2 flows out of the valve plate area of the valve, the CO2 in the pipeline is below 217K in some areas. At this time, CO2 will undergo a relatively large phase change under the huge changes in pressure and temperature. At the same time, due to the temperature being lower than the triple point, CO2 has the possibility of freezing, that is, there is a risk of freezing blockage in the wellhead gate valve at this time. 2 standard phase diagram, CO2 2 triple point temperature is 217K. It can be seen that after CO2 flows out of the valve plate area of the valve, the CO2 in the pipeline is below 217K in some areas. At this time, CO2 will undergo a relatively large phase change under the huge changes in pressure and temperature. At the same time, due to the temperature being lower than the triple point, CO2 has the possibility of freezing, that is, there is a risk of freezing blockage in the wellhead gate valve at this time. 2 flows out of the valve plate area of the valve, the CO2 in the pipeline 2 is below 217K in some areas. At this time, CO2 2 will undergo a relatively large phase change under the huge changes in pressure and temperature. At the same time, due to the temperature being lower than the triple point, CO2 has the possibility of freezing, that is, there is a risk of freezing blockage in the wellhead gate valve at this time. 2 exists the possibility of freezing, that is, there is a risk of freezing blockage in the wellhead gate valve at this time.

[0136] Figure 16 It is the area where the valve may have the risk of freezing blockage. By using the area of the region where the temperature contour on the symmetry plane is lower than the triple point as a parameter to measure the freezing blockage risk of the valve, it can be known that the larger this area is, the greater the freezing blockage risk of the wellhead gate valve during use. After calculation, at an opening of 25%, the area ratio of the region lower than the triple point is 0.3249.

[0137] Then, the simulation results at a large opening are as follows:

[0138] The specific boundary conditions are set as follows:

[0139] Establish a numerical calculation model of the valve at a small opening. Under normal working conditions, set the inlet of the valve as the pressure inlet boundary condition, specify its pressure value as 25 MPa, and set the boundary of the valve outlet as the pressure outlet, and specify its pressure value as 0 MPa here. According to the calculation requirements of the 1 / 23-D model, set the boundary condition of the symmetry plane as Symmetry, and set the remaining boundaries as Wall and Interface as required.

[0140] Simulation results:

[0141] Figure 17 It is the inlet flow rate detection curve at a large opening. It can be seen that as the number of calculation steps increases, the inlet flow rate tends to be stable, that is, the simulation results converge at this time, and the results at this time are used as the simulation results at this opening.

[0142] Figure 18 It is the pressure contour of the wellhead gate valve pipeline at an opening of 75%. It can be seen that at the inlet of the valve, the entire CO 2 is at the highest pressure level. When the CO 2 flows through the valve plate area of the valve, the pressure contour changes greatly, forming a large pressure gradient. This is because the valve is not fully opened, and the small flow area at the valve plate forms a throttling effect. Similarly, when the CO 2 flows out of the valve plate area, a relatively obvious pressure gradient is also formed. Here, the CO 2 forms a secondary pressure reduction, and finally the pressure of the CO 2 gradually approaches the atmospheric pressure.

[0143] Figure 19 It is the velocity contour of the wellhead gate valve pipeline at an opening of 75%. According to Bernoulli's principle, the pressure of the fluid will decrease and the flow velocity will increase when it flows through a slit. Therefore, it can be seen from the velocity contour that when the CO 2 flows through the valve plate area of the valve, the flow velocity of the fluid increases significantly, which is consistent with the change of the pressure contour. Compared with the small opening of 25%, at the large opening of 75%, due to the increase in the flow area, the throttling effect weakens, resulting in a decrease in the pressure difference before and after the valve and a decrease in the overall flow velocity.

[0144] Figure 20 and Figure 21 The liquid and gaseous CO in the wellhead gate valve pipeline at 75% opening are 2 From the volume fraction cloud diagram, it can be seen that in the pipeline at the inlet, liquid CO 2 The volume fraction is relatively large, about 5%, and the gas holdup here is 95%. Near the valve plate, due to the CO 2 The temperature and pressure change dramatically, CO 2 A dramatic phase change occurs, and the gas holdup here is significantly higher than that at the inlet pipeline, and can reach up to 100%. 2 In a short section of the pipeline after the valve area, due to the CO 2 The temperature drops, forming the potential freezing area mentioned above. 2 The gas holdup is lower than that inside the valve. In the second half of the outlet pipeline, due to the decrease in pressure, CO 2 It is no longer in liquid state, and the gas holdup is about 100%. Compared with the small opening of 25%, the fluid release rate is faster and the overall gas holdup is larger at a large opening of 75%.

[0145] Figure 22 The temperature cloud diagram of the wellhead gate valve pipeline at 25% opening shows that at the inlet pipeline of the valve, CO 2 The temperature is relatively high, maintained at around 288K, in CO 2 When flowing through the valve disc area, due to the throttling effect of the valve, CO 2 The temperature near the valve plate area drops to about 270K. 2 When flowing out of the valve disc area, CO 2 The temperature dropped suddenly, and the lowest temperature was formed at this time. Since the throttling effect of the valve with a large opening is weak, the temperature in the overall valve flow field is high, and there is no temperature area below the triple point, that is, the area below the triple point accounts for 0.

[0146] In summary, this embodiment has carried out a refined modeling for the wellhead gate valve at two typical openings during the opening process, and has performed calculations based on the modeling. 2 After passing through the narrow slit of the pressure relief valve, the pressure drops suddenly, and CO 2 The phase changes rapidly from liquid to gas. During this adiabatic expansion phase change process, the working fluid absorbs a large amount of latent heat of phase change and the temperature drops rapidly. This process roughly follows the CO 2It is carried out along the temperature-pressure saturation line. At the same time, the pressure inside the pressure relief valve increases with the increase of the valve opening, which in turn leads to the increase of the overall temperature inside the pressure relief valve with the increase of the valve opening. When the valve opening is small and large, the corresponding lowest temperatures inside the pressure relief valve are 188.19 K and 271.71 K respectively. The working medium temperature at a small valve opening is much lower than that at a large valve opening. Dense-phase CO 2 During the pressure-reducing and discharging process, the escaping gas will cool the pipeline. When the temperature is lower than the ductile-brittle transition temperature of the pipeline, the pipeline material will become brittle, resulting in brittle fracture and serious pipeline damage. Therefore, on the premise of ensuring the structural safety of the pressure relief valve, increasing the valve opening of the pressure relief valve can effectively increase the working medium temperature and reduce the risk of embrittlement of the valve and pipeline. At the same time, increasing the valve opening can further increase the dense-phase CO 2 discharge capacity, which can accelerate the discharge of the medium in the pressure-bearing pipeline and related equipment and meet the pressure-reducing requirements.

[0147] There is a risk of freezing blockage in the flow fields at both openings. By comparing the areas of potential freezing blockage regions, it can be obtained that the valve has a higher freezing blockage risk at a small opening, and the area ratio of the freezing blockage region is 0.3249.

[0148] Embodiment 5: This embodiment specifically describes the control equations used in the CFD simulation analysis and calculation of the above embodiments;

[0149] When performing CFD simulation analysis and calculation, the Realizable k-ε turbulence model is used for simulation calculation.

[0150] The phase change mass transfer equation in the CFD method is:

[0151] In the single-phase flow simulation calculation. Usually, only a set of conservation equations for momentum and continuity need to be solved. To realize the change from a single-phase model to a multi-phase model, additional conservation equations must be introduced. To reasonably describe the inter-phase relationship, different phases are treated as mutually penetrating continuous media. In the jet model, a mixture multi-phase flow model is adopted. The solution of the momentum equation for the mixture in the mixture multi-phase flow model can be obtained by summing the momentum equations of all phases, and its expression is as follows:

[0152]

[0153] μ m =∑α k μ k

[0154]

[0155] In the formula, n is the number of phases of the multi-phase flow; is the body force, N; μm is the mixture viscosity, Pa.s; vdr,k is the drift velocity of the multiphase, m / s.

[0156] The relative velocity (also known as the slip velocity) is defined as the velocity of the second phase (p) relative to the primary phase (q), i.e.:

[0157] v qp = v p - v q

[0158] The relationship between the drift velocity (vdr,p) and the relative velocity (vqp) is expressed by the following equation

[0159]

[0160] The mixture model in Fluent uses an algebraic slip formula. The basic assumption of the algebraic slip mixture model is to specify the algebraic relationship of the relative velocity, and local equilibrium between phases should be achieved on a short spatial length scale. The form of the relative velocity is given by the above equation:

[0161] V qp = τ qp a

[0162] where a is the acceleration of the second-phase particles, m / s 2 ; τqp is the relaxation time of the particles, s.

[0163] According to Manninen's theory, the form of τqp is as follows:

[0164]

[0165] where dp is the diameter of the second-phase particles (or droplets or bubbles), m; fdrag is the drag coefficient, and according to the research of Schiller and Naumann, its expression is as follows:

[0166]

[0167] The form of the acceleration a is:

[0168]

[0169] The volume fraction equation of the second phase p can be obtained from the continuity equation of the second phase p as follows:

[0170]

[0171] To achieve the simulation calculation of CO 2 it is necessary to load the mass and energy source terms in the control equations to achieve phase change. When the pressure in the control volume is less than the saturation pressure at this temperature, CO 2Changing from liquid to gas, controlling the reduction of the liquid mass in the body and the increase of the gas. Referring to the model of water phase change, the mass source term is obtained by the following formula:

[0172] S 1 =-S r =kα l ρ l (P - P S ) / P S

[0173] S 1 =-S v =kα v ρ v (P - P S ) / P S

[0174] After obtaining the mass source term, the energy source term caused by the phase change can be obtained by the following formula:

[0175] S b =HS l

[0176] Among them, H is the latent heat of vaporization at the specified temperature, obtained from the enthalpy difference between the gas phase and the liquid phase at this temperature. k is the phase change rate coefficient of the flow in the wellhead gate valve cavity during the release of dense-phase CO2.

[0177] The above is only the implementation mode of the present invention, and it does not limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the scope of the claims of the present invention.

Claims

1. A risk analysis method for wellhead valve freezing during CO2 decompression, characterized in that: The method is: S1: Build a 1 / 23-D wellhead gate valve model and perform watershed extraction, meshing, and solver settings; S2: Set the physical properties of CO2 for the set 1 / 23-D wellhead gate valve model, and verify the model to obtain the numerical simulation model of CO2 freezing risk; S3: Perform CFD simulation calculations on the numerical simulation model of CO2 freezing risk for two openings, analyze the valve freezing risk based on the calculation results, and optimize the valve structure.

2. The method for analyzing the risk of wellhead valve freezing during CO2 decompression process according to claim 1 is characterized in that: S1 is specifically: S11: Build a 1 / 23-D wellhead gate valve model; S12: Use model processing software to extract the watershed of the wellhead gate valve model and obtain the main throttling area of ​​the wellhead gate valve; S13: After the watershed is extracted, the wellhead gate valve model is divided using a hybrid grid method; S14: Set the solver parameters and CO2 two-phase flow parameters after meshing.

3. The method for analyzing the risk of freezing and blocking of wellhead valves in a CO2 decompression process according to claim 2 is characterized in that: When extracting the watershed, the watershed of 5 and 10 times the valve inlet diameter is added at the inlet and outlet of the valve, respectively.

4. The method for analyzing the risk of freezing and blocking of wellhead valves in a CO2 decompression process according to claim 2 is characterized in that: When dividing the network, it is necessary to divide it into different areas, specifically: In the area near the pressure relief slit of the wellhead gate valve, the grid is locally encrypted; In the area far away from the pressure relief slit, a sparse grid division method is used.

5. The method for analyzing the risk of wellhead valve freezing during CO2 decompression process according to claim 2 is characterized in that: The specific two-phase flow parameters of CO2 are: The parameters of liquid CO2 include density, viscosity, and thermal conductivity; The parameters of gas phase CO2 include density, viscosity, thermal conductivity, standard state enthalpy, and reference temperature.

6. The method for analyzing the risk of wellhead valve freezing during CO2 decompression process according to claim 1 is characterized in that: The specific CO2 property settings are: The physical properties of CO2 are divided into three sections: 0.1-0.6MPa, 0.6-7.37MPa, and 7.37-30MPa; In the 0.1-0.6MPa stage, the temperature is set according to the fitting formula between saturation temperature and saturation pressure. In the 0.6-7.37MPa stage, CO2 undergoes phase change, and the temperature is set according to the fitting formula between saturation temperature and saturation pressure; In the 7.37-30MPa stage, CO2 does not undergo phase change, and the saturation temperature at different pressures is set to 35°C.

7. The method for analyzing the risk of wellhead valve freezing during CO2 decompression process according to claim 1 is characterized in that: When performing two-opening CFD simulation, the boundary conditions are set as: The valve inlet is the pressure inlet boundary condition with a pressure of 25 MPa. The valve outlet boundary is the pressure outlet with a pressure of 0 MPa. The boundary condition of the symmetry surface is set to Symmetry. 8.CO2 decompression process wellhead valve freezing risk analysis system, characterized by: The system comprises a storage device, and the storage device is used to execute the method of claim 1.

9. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores a computer program, which, when executed by a processor, executes the method for analyzing the freezing risk of wellhead valves in a CO2 decompression process according to any one of claims 1 to 7.

10. A computer device, characterized in that: The device includes a memory and a processor, wherein a computer program is stored in the memory. When the processor runs the computer program stored in the memory, the processor executes the CO2 decompression process wellhead valve freezing risk analysis method according to any one of claims 1 to 7.

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