Risk analysis method and system for CO2 decompression wellhead valve freezing

Abstract

The method and system for analyzing the risk of CO2 pressure relief wellhead valve freezing blockage belong to the technical field of numerical simulation modeling. The existing research on the distribution law of the internal related physical field of the pressure relief valve and pipeline during the dense phase CO2 release process is relatively less, which cannot optimize the structure of the wellhead valve, resulting in frequent freezing blockage. Method: A 1 / 2 3-D wellhead gate valve model is built, and the flow basin is extracted, the grid is divided, and the solver is set. The CO2 physical properties of the set 1 / 2 3-D wellhead gate valve model are set, and the model is verified to obtain a CO2 freezing risk numerical simulation model. The CO2 freezing risk numerical simulation model is calculated by CFD simulation for two kinds of opening degrees, and the valve freezing risk is analyzed according to the calculation results, so as to optimize the structure of the valve. The present application is suitable for the research on the distribution law of the internal related physical field of the pressure relief valve and pipeline during the dense phase CO2 release process.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of numerical simulation modeling, and particularly relates to a CO2 decompression process wellhead valve freezing risk analysis method. BACKGROUND

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

[0003] There is a single-phase control requirement for carbon dioxide in the transportation process. Near the critical region, a small difference in pressure and temperature will cause the state characteristics of carbon dioxide to appear sharp floating. Therefore, the pressure of the transportation process needs to be kept higher than the critical pressure to avoid two-phase transportation. The current operating pressure of the transportation process is 8.5-12 MPa, which is in the "smooth" region of the carbon dioxide phase diagram, that is, the state characteristics of carbon dioxide in this region are stable and will not change abruptly. However, during the decompression release process, carbon dioxide will inevitably undergo sharp changes in characteristics. Due to unexpected failures (third-party construction damage or corrosion) or planned maintenance, the pipeline often needs to be depressurized and released. During the decompression process, the escaping gas will cool the pipeline. If the temperature decreases too much, below the ductile-brittle transition temperature, the pipe material will become brittle, causing brittle fracture and serious pipe damage. The formation of dry ice during the carbon dioxide decompression release process also poses a certain freezing risk to the pipeline or valve. Therefore, the release of the carbon dioxide pipeline needs to be controlled to ensure that the pipeline temperature is not lower than the design temperature.

[0004] Currently, the research on CO2 pipeline safety release mainly focuses on the experimental and simulation research on the temperature and pressure change law in the pipe during small short-distance pipeline release, as well as the pipe outside injection and diffusion law research. Most of the pipeline outflow models in the literature use homogeneous equilibrium model (HEM), which assumes that the fluid phase state in the compression process is in thermal and mechanical equilibrium, and also ignores phase slip and gas-liquid non-equilibrium transition phenomena.

[0005] Domestic scholars have also explored the numerical simulation of CO2 pipeline leakage. Yan Wei, Lv Yuling, etc. simulated the supercritical state and dense phase leakage of the pipeline section from Qilu Petrochemical to Chunliang Oil Production Plant according to the CO2 flooding project of Shengli Oilfield. 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 the internal pressure remains unchanged, but it does not consider the unstable state formed by the action of the stop valve, the constant flow rate and state parameters at the outlet are larger than the actual value, and it is only applicable to the case where the leakage diameter is very small. The pipeline model is only applicable to the case where the pipeline is completely broken. Li Kang established a small-scale supercritical CO2 numerical simulation, established a near-field high-pressure model, and developed a prediction algorithm for high-pressure pipeline leakage, which explained the formation of dry ice and bow shock, but did not compare the accuracy of large-scale experiments and did not consider the far-field changes. Liu Feng, Xun Ruina, etc. established a small CO2 leakage pipeline, derived the energy equation and enthalpy formula of supercritical CO2, established an isentropic choked flow leakage rate model, used a leakage rate prediction model with high accuracy, and found that the temperature in the Mach disc can reach-80℃, the near-field shock wave structure is observed clearly, but the fitting accuracy decreases with the increase of the model size. Zheng Yangguang, etc. based on the industrial-scale CO2 leakage experimental device and small-scale CO2 leakage experimental device, used the real gas P-R state equation, constructed a gas-phase CO2 diffusion numerical model, and applied the model to supercritical and dense phase working conditions. The study found that the data fitting is good in gas phase working condition, and the diffusion range of CO2 concentration and temperature expands with the increase of leakage diameter, but the fitting is not good in supercritical and dense phase working conditions, and the influence of CO2 phase change after leaking through the leakage hole is not considered.

[0006] In summary, the above research work mainly focuses on the experimental study of the far-field release characteristics of high-pressure CO2 pipeline, and most of the research physical structures are pipeline-small hole release structures, and the physical model is relatively simple. The research on the distribution law of the related physical field in the pressure relief valve and pipeline during the dense phase CO2 release process is relatively less. SUMMARY

[0007] The present application aims at the problem that the research on the distribution law of the related physical field in the pressure relief valve and pipeline during the dense phase CO2 release process is relatively less, the wellhead valve cannot be optimized in structure, and the freezing and plugging phenomenon occurs frequently, and proposes a CO2 pressure reduction process wellhead valve freezing and plugging risk analysis method.

[0008] To achieve the above purpose, the present application provides the following scheme:

[0009] The present application provides a CO2 pressure reduction process wellhead valve freezing and plugging risk analysis method, which comprises the following steps:

[0010] Step S1: build a 1 / 2 3-D wellhead gate valve model, and perform flow field extraction, mesh division, and solver setting;

[0011] Step S2: set the CO2 physical properties of the set 1 / 2 3-D wellhead gate valve model, and perform model verification to obtain a CO2 freeze-out risk numerical simulation model;

[0012] Step S3: perform CFD simulation calculation on the CO2 freeze-out risk numerical simulation model for two opening degrees, analyze the valve freeze-out risk according to the calculation results, and optimize the structure of the valve.

[0013] Further, there is a preferred embodiment, and the above step S1 is specifically:

[0014] Step S11: build a 1 / 2 3-D wellhead gate valve model;

[0015] Step S12: use model processing software to extract the flow field of the wellhead gate valve model to obtain the main throttling area of the wellhead gate valve;

[0016] Step S13: divide the wellhead gate valve model using a hybrid mesh method after flow field extraction;

[0017] Step S14: set the solver parameters and CO2 two-phase flow parameters after mesh division.

[0018] Further, there is a preferred embodiment, and when the flow field is extracted, a flow field of 5 times and 10 times the inlet diameter of the valve is added at the inlet and outlet of the valve, respectively.

[0019] Further, there is a preferred embodiment, and when the network is divided, regional processing is required, specifically:

[0020] In the area near the pressure relief slit of the wellhead gate valve, the grid is locally encrypted;

[0021] In the area away from the pressure relief slit, a sparse mesh division method is used.

[0022] Further, there is a preferred embodiment, and the CO2 two-phase flow parameters are specifically:

[0023] The parameters of liquid CO2 include density, viscosity, and thermal conductivity;

[0024] The parameters of gaseous CO2 include density, viscosity, thermal conductivity, standard state enthalpy, and reference temperature.

[0025] Further, there is a preferred embodiment, and the CO2 physical property setting is specifically:

[0026] The CO2 physical properties are divided into three stages: 0.1-0.6 MPa, 0.6-7.37 MPa, and 7.37-30 MPa.

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

[0028] In the 0.6-7.37 MPa stage, CO2 undergoes phase transition, and the temperature is set according to the fitting formula between the saturation temperature and the saturation pressure;

[0029] In the 7.37-30 MPa stage, CO2 does not undergo phase transition, and the saturation temperature at different pressures is set to 35 DEG C.

[0030] Further, in the above two opening CFD simulation calculation, the boundary condition is set as:

[0031] The valve inlet is a pressure inlet boundary condition, the pressure is 25 MPa, the boundary of the valve outlet is a pressure outlet, and the boundary condition of the symmetry surface is Symmetry.

[0032] The CO2 decompression process wellhead valve freezing risk analysis method described in the application can be realized by computer software, therefore, the application also provides a CO2 decompression process wellhead valve freezing risk analysis system, the system comprises a storage device, the storage device is used to execute the following steps:

[0033] Step S1: building a 1 / 23-D wellhead gate valve model, and performing flow basin extraction, grid division and solver setting;

[0034] Step S2: setting the CO2 physical properties of the set 1 / 23-D wellhead gate valve model, and performing model verification to obtain a CO2 freezing risk numerical simulation model;

[0035] Step S3: performing CFD simulation calculation on the CO2 freezing risk numerical simulation model for two kinds of opening, analyzing the valve freezing risk according to the calculation result, and optimizing the structure of the valve.

[0036] The application also provides a computer readable storage medium, the computer readable storage medium stores a computer program, and the computer program is run by a processor to execute the CO2 decompression process wellhead valve freezing risk analysis method of any one of the above.

[0037] The application also provides a computer device, which comprises a memory and a processor, and the memory stores a computer program, and when the processor runs the computer program stored in the memory, the processor executes the CO2 decompression process wellhead valve freezing risk analysis method of any one of the above.

[0038] The beneficial effects of the present application are:

[0039] 1. The present application provides a CO2 depressurization process wellhead valve freezing risk analysis method, which is simulated and calculated by modeling the CO2 release of the 1 / 23-D model of the wellhead gate valve at different opening degrees, including the extraction and simplification of the calculation domain, mesh division, boundary condition setting, solver setting, CO2 phase parameter setting and loading, etc. The flow field of the valve during the depressurization release process is modeled in detail to determine the area where the valve internal freezing risk occurs, and the wellhead valve is optimized in structure according to the freezing analysis result.

[0040] Further, in order to ensure the simulation accuracy and convergence, when the flow domain is extracted, the flow domain of 5 times and 10 times the valve inlet diameter is added at the inlet and outlet of the valve respectively.

[0041] Further, the present application adopts a mixed grid division method to generate high-quality grids; at the same time, considering the complex flow and heat and mass transfer phenomena inside the wellhead gate valve, the grid is divided in a regional manner. That is, in the area near the depressurization slit of the wellhead gate valve, the grid is locally encrypted; in the area far from the depressurization slit, sparse grid division is adopted.

[0042] Further, the present application divides the carbon dioxide properties into three sections to ensure the accuracy of the simulation results and avoid large fluctuations in the dense phase carbon dioxide property parameters during the phase change process, which affects the simulation accuracy.

[0043] 2. The CO2 depressurization process wellhead valve freezing risk analysis method provided by the present application has a numerical calculation accuracy of 95%.

[0044] 3. The CO2 depressurization process wellhead valve freezing risk analysis method provided by the present application analyzes the simulation results at large and small opening degrees, and finds that the flow field at both opening degrees has a risk of freezing. By comparing the area of the potential freezing area, it can be found that the valve has a higher freezing risk at small opening degree, and the freezing area accounts for 0.3249, which can provide effective reference for subsequent wellhead valve structure design.

[0045] The present application is applicable to the research on the distribution law of related physical fields in the depressurization valve and pipeline during the dense phase CO2 release process. BRIEF DESCRIPTION OF DRAWINGS

[0046] In order to more clearly illustrate the technical solutions in the specific embodiments of the present application or the prior art, the drawings required to be used in the description of the specific embodiments or the prior art will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative effort on the basis of these drawings.

[0047] Figure 1 is a flow chart of the CO2 pressure reduction wellhead valve freezing risk analysis method described in the embodiments of the present application;

[0048] Figure 2 is a wellhead valve structure diagram described in the embodiments of the present application;

[0049] Figure 3 is a wellhead gate valve 1 / 23D watershed diagram described in the embodiments of the present application;

[0050] Figure 4 is a grid division diagram of the wellhead gate valve described in the embodiments of the present application;

[0051] Figure 5 is a CO2 saturation temperature and saturation pressure corresponding curve diagram described in the embodiments of the present application;

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

[0053] Figure 7 is a CO2 injection photo and corresponding Mach disk position at different times described in the embodiments of the present application;

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

[0055] Figure 9 is a velocity nephogram under different working conditions described in the embodiments of the present application;

[0056] Figure 10 is a flow curve under 25% small opening described in the embodiments of the present application;

[0057] Figure 11 is a pressure nephogram of the wellhead gate valve pipeline under 25% opening described in the embodiments of the present application;

[0058] Figure 12 is a velocity nephogram of the wellhead gate valve pipeline under 25% opening described in the embodiments of the present application;

[0059] Figure 13 is a liquid volume fraction nephogram of the wellhead gate valve pipeline under 25% opening described in the embodiments of the present application;

[0060] Figure 14 is a gaseous volume fraction cloud chart of the wellhead gate valve pipeline at 25% opening degree according to the embodiment of the present application;

[0061] Figure 15 is a temperature cloud chart of the wellhead gate valve pipeline at 25% opening degree according to the embodiment of the present application;

[0062] Figure 16 is a risk area of the valve where freezing and blocking may occur according to the embodiment of the present application;

[0063] Figure 17 is a flow curve at 75% opening degree according to the embodiment of the present application;

[0064] Figure 18 is a pressure cloud chart of the wellhead gate valve pipeline at 75% opening degree according to the embodiment of the present application;

[0065] Figure 19 is a velocity cloud chart of the wellhead gate valve pipeline at 75% opening degree according to the embodiment of the present application;

[0066] Figure 20 is a liquid volume fraction cloud chart of the wellhead gate valve pipeline at 75% opening degree according to the embodiment of the present application;

[0067] Figure 21 is a gaseous volume fraction cloud chart of the wellhead gate valve pipeline at 75% opening degree according to the embodiment of the present application;

[0068] Figure 22 is a temperature cloud chart of the wellhead gate valve pipeline at 75% opening degree according to the embodiment of the present application. DETAILED DESCRIPTION

[0069] The specific embodiments of the present application will be further described in detail below with reference to the accompanying drawings. The following embodiments will help those skilled in the art to further understand the present application, but do not limit the present application in any form. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present application, and these changes and improvements are within the scope of protection of the present application.

[0070] Embodiment one, see Figure 1 The present embodiment is described, and the present embodiment is directed to the fact that there is relatively less research on the distribution of internal physical fields of the pressure relief valve and the pipeline during the existing dense phase CO2 relief process, and the structure of the wellhead valve cannot be optimized, thereby leading to frequent freezing and blocking phenomenon. Therefore, a CO2 pressure relief process wellhead valve freezing and blocking risk analysis method is proposed, and the method comprises the following steps:

[0071] Step S1: build a 1 / 23-D wellhead gate valve model, and perform flow basin extraction, grid division and solver setting;

[0072] Step S2: CO2 physical property settings are made for the set 1 / 2 3-D wellhead gate valve model, and model verification is performed to obtain a CO2 freeze-out risk numerical simulation model;

[0073] Step S3: CFD simulation calculation is performed on the CO2 freeze-out risk numerical simulation model for two kinds of opening degrees, and valve freeze-out risk is analyzed according to the calculation results, so as to optimize the structure of the valve.

[0074] Embodiment Two, see Figure 2 and Figure 5 This embodiment describes the CO2 decompression process wellhead valve freeze-out risk analysis method described in Embodiment One;

[0075] Step S1: Build a 1 / 2 3-D wellhead gate valve model, and perform flow field extraction, mesh division, and solver setting;

[0076] Specifically:

[0077] The 1 / 2 3-D model is specifically:

[0078] The wellhead valve is a plate gate valve used for CO2 discharge, and its basic structure model is as shown in Figure 2 , mainly including valve stem, valve cover, valve plate, valve seat, valve body and other structures. During the CO2 discharge 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, and the entire opening process lasts about 60 seconds, and then the valve reaches the maximum opening degree and performs a stable discharge process.

[0079] The flow field extraction is specifically:

[0080] Based on the above-mentioned model of the wellhead gate valve, the flow field extraction is performed in the model processing software. The structure of the wellhead gate valve is a plane symmetric structure, therefore, the simulation under different opening degrees of the wellhead gate valve is simulated by using the 1 / 2 3-D model, and the calculation flow field is extracted and simplified as shown in 3, considering the actual working environment of the valve. As shown in Figure 3 , the most important throttling area of the wellhead gate valve is near the gate plate, and during the opening process of the valve, the throttling area of the gate plate changes from small to large, at this time, the liquid CO2 flows through the valve under the drive of high pressure, and the temperature, pressure and flow rate change dramatically, and phase change occurs, therefore, the embodiment analyzes the freeze-out problem that may occur during the opening and working process of the valve by calculating the detailed changes of the flow field of the valve under different opening degrees.

[0081] Further, in order to ensure the simulation accuracy and convergence, a flow field of 5 times and 10 times the valve inlet diameter needs to be added at the inlet and outlet of the valve to make the flow field before and after the valve fully developed, wherein the outlet and inlet boundary conditions of the flow field are specified according to specific working conditions.

[0082] The grid division is specifically as follows:

[0083] After the calculation of the flow field extraction is completed, the grid division is performed according to the structural characteristics of the wellhead gate valve in the embodiment. The simplified three-dimensional wellhead gate valve cavity model structure is simple, but the flow characteristics, phase change heat transfer and mass transfer characteristics in the valve cavity are quite complex. Therefore, a mixed grid division method is adopted to generate high-quality grids.

[0084] Further, considering the complex flow and heat transfer and mass transfer phenomena in the wellhead gate valve, the grid needs to be processed in different regions when the grid is divided. In the region near the pressure relief slit of the wellhead gate valve, the flow condition is relatively complex, and the grid needs to be locally encrypted. In the region far away from the pressure relief slit (which can be considered as most of the region in the wellhead gate valve cavity), the fluid flow is relatively smooth, so relatively sparse grids can be divided in this region. Such regional grid division can not only ensure the accuracy of the simulation results of the pipe flow field, but also appropriately reduce the number of grids, save computing resources and improve the simulation calculation speed.

[0085] Further, for CFD analysis, the grid density is one of the key factors to ensure the simulation calculation accuracy and convergence. In order to obtain the optimal calculation grid model, the wellhead gate valve is simulated by using grids of different density levels, and the influence of grid density on simulation accuracy is explored by comparing the temperature at the outlet of the valve plate after simulation of different grids, and the results are shown in Table 1.

[0086] Table 1

[0087] Number of grids Temperature at exit (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] It can be found from Table 1 that when the number of grids is greater than 2,673,022, the simulation result is independent of the grid density, that is, the calculation is converged to the calculation grid. Considering the calculation efficiency and cost, the grid generation scheme with the number of grids of 2,673,022 is selected for subsequent simulation calculation in the embodiment, and the final grid scheme is shown in Table 1. Figure 4

[0089] The solver setting is specifically as follows:

[0090] After the grid division is completed, the solver parameters need to be set in the CFD simulation calculation, which is specifically shown in Table 2.

[0091] Table 2 ​

[0092]

[0093] In addition to the basic solver parameter settings, two-phase flow parameters of CO2 need to be set before calculation, and the specific parameters are shown in Table 3.

[0094] Table 3

[0095]

[0096] Step S2: CO2 property setting is performed on the set 1 / 23-D wellhead gate valve model, and model verification is performed to obtain a CO2 freeze-out risk numerical simulation model;

[0097] The CO2 property setting is specifically as follows:

[0098] The working pressure of the dense-phase carbon dioxide pipeline at normal temperature is maintained at about tens of megapascals, and the pressure drops sharply after being discharged through the wellhead gate valve, and the corresponding saturation temperature will rapidly decrease, so the dense-phase carbon dioxide will change from liquid phase to gas phase (similar to flash evaporation phenomenon). The dense-phase carbon dioxide property parameters will fluctuate in a large range during the phase change process, and correct carbon dioxide property parameters can ensure the accuracy of the simulation results.

[0099] The phase diagram and pressure-temperature relationship of carbon dioxide are as shown in Figure 5 The three-phase critical point is obtained by Refprop, T lim＝ 30.92℃, 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 carbon dioxide properties 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 7.37-30 MPa section, the carbon dioxide pressure decreases from the inlet pressure 30 MPa of the wellhead gate valve to the critical pressure 7.37 MPa of carbon dioxide. In this pressure drop range, the carbon dioxide does not change phase, and the saturation temperature under different pressures is set to a constant value of 35℃ (the temperature in the calculation domain at the initial moment of simulation is 25℃).

[0101] In the 0.6-7.37 MPa section, the saturation temperature under the corresponding pressure will drop sharply during the decrease of the carbon dioxide pressure, so the phase change process of carbon dioxide will occur in this section. The saturation carbon dioxide property parameters under the pressure in this section are obtained by using the Refprop software, and the results are shown in Table 4.

[0102] Table 4

[0103]

[0104]

[0105] The discrete point data shown in Table 4 is subjected to polynomial fitting to obtain a relationship between the saturation temperature and the corresponding saturation pressure,

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

[0107] In the formula, T is the carbon dioxide saturation temperature ℃; P is the carbon dioxide saturation pressure MPa. The correlation coefficient R of the fitting formula is 0.9991, and the fitting accuracy is high. 2

[0108] And in the 0.1-0.6 MPa pressure section, the carbon dioxide saturation temperature in the 0.1-0.6 MPa section is no longer used in the 0.6-7.37 MPa fitting formula, and the three-phase point and 0.1 MPa point data are re-fitted. The relationship after fitting is reloaded into the FLUENT evaporation and condensation model.

[0109] The fitting formula is as follows:

[0110] T = -87.42 P 2 + 124.96 P + 176.53

[0111] In the formula, T is the carbon dioxide saturation temperature ℃; P is the carbon dioxide saturation pressure MPa. The correlation coefficient R of the fitting formula is 0.9995. 2

[0112] Embodiment three, see Figure 6 to Figure 9 This embodiment is to verify the CO2 freeze-out risk numerical simulation model described in the above embodiment; specifically including the following steps:

[0113] First, the grid division and boundary conditions are performed:

[0114] According to the experimental steps, a circular leakage hole with an inner diameter of 5 mm is used for the experiment. The experiment is supercritical CO2 leakage, and the initial pressure of CO2 in the container is 7.7 MPa and 36℃. As shown in Figure 6 The axisymmetric calculation domain is established to improve the calculation efficiency. The calculation model is completely simulated in terms of the size parameters of the physical model in the experiment. The calculation domain is composed of a nozzle (hole) and an external environment. According to the orifice used in the experiment, the nozzle is 3 mm long and 5 mm in inner diameter. The external environment is next to the nozzle outlet, 1000 mm long and 200 mm wide, which can ensure the full development of the jet. All the calculation domains are divided into quadrilateral structure grids.

[0115] Secondly, the boundary conditions of the model are defined as follows:​​

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

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

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

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

[0120] Then the parameter settings are calculated:

[0121] Based on the Fluent framework, the control equations are discretized by finite volume method. The transient solver is adopted, and the pressure-velocity coupling adopts Coupled algorithm. The second-order upwind format is adopted for the discretization of convection term, turbulent kinetic energy equation and turbulent diffusion equation. Due to the coupling of high-speed flow and rapid phase change in the leakage process, a dynamic time step of 3e-6s is adopted to accurately capture the subtle changes of velocity, pressure and temperature field, and not to diverge. At the same time, the source term of the control equation is modified by user-defined function (UDF) to realize the phase change simulation in the flow process.

[0122] Finally, the results are verified:

[0123] Figure 7 is the evolution process of supercritical CO2 jet near field. The initial state of the experiment is 7.7 MPa, 36℃, and the leakage port is a circular hole with a diameter of 5mm. Due to the strong throttling effect of the orifice, the temperature in the atmospheric space is sharply reduced below the CO2 triple point, and a large amount 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 CO2 jet, it is a mixture of a large number of micron-sized dry ice particles and gaseous CO2. As can be seen from the figure, the shape of the supercritical CO2 jet is a barrel-shaped structure with a sharp 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 structure can be clearly observed.

[0124] Figure 8The principle for the Mach disk to be generated is that when the gas is over-expanded, the gas pressure in the exhaust is lower compared to the external atmosphere, causing the exhaust to be compressed or squeezed inward. This compression increases the exhaust pressure. However, the flow can be compressed too much so that its pressure exceeds the atmospheric pressure. At this time, the gas flow expands outward again to reduce the pressure, causing the pressure inside the plume to be lower than the ambient pressure again. Over time, the compression and expansion processes are repeated, 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 For the velocity cloud diagram obtained by establishing the same physical model according to the experiment of Tenglin et al., it can be seen that a relatively obvious Mach disk structure appears on the velocity cloud diagram, which is almost the same as the Mach disk position at 37.3 ms in the experiment. This result shows that the simulation method adopted by the present application has good accuracy and precision in calculating the release process of CO2, which can lay a theoretical foundation for subsequent simulation calculation.

[0125] In addition, by determining the position of the Mach disk in the CO2 near-field jet structure and comparing it with the position of the Mach disk in the experiment, the precision of the simulation model can be quantified. Figure 7 In the experiment, the distance between the Mach disk at 37.3 ms and the release port is 15 mm, Figure 9 and the simulation result in the present embodiment is 14.25 mm. Therefore, the precision of the numerical calculation is 95%, which meets the technical requirements.

[0126] Embodiment Four, see Figure 10 to Figure 22 This embodiment is a specific description of the analysis of the valve freeze-up risk results described in the above embodiments. According to the numerical calculation model described above, this report simulates two representative opening degrees during the opening process of the gate valve, analyzes the freeze-up risk of the wellhead gate valve at the typical opening degrees to evaluate the freeze-up risk of the wellhead valve, and optimizes the structure of the valve according to the simulation results at the typical opening degrees. The selected typical opening degrees are 25% small opening degree and 75% large opening degree, which are as follows:

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

[0128] The specific boundary conditions are as follows:

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

[0130] Simulation results:

[0131] Figure 10 For the inlet flow detection curve at small opening, it can be seen that the inlet flow tends to be stable with the increase of calculation steps, that is, the simulation result converges at this time, and the result at this time is taken as the simulation result at this opening.

[0132] Figure 11 For the pressure cloud chart of the wellhead gate valve pipeline at 25% opening, it can be seen that at the inlet of the valve, the entire CO2 pressure level is at the highest position, and when CO2 flows through the valve plate area of the valve, the pressure cloud chart changes greatly, forming a large pressure gradient, which is due to the fact that the valve has not been fully opened, the small flow-through area at the valve plate forms throttling; similarly, when CO2 flows out of the valve plate area, a more obvious pressure gradient is also formed, here CO2 forms secondary pressure reduction, and finally the pressure of CO2 gradually approaches atmospheric pressure.

[0133] Figure 12 For the velocity cloud chart of the wellhead gate valve pipeline at 25% opening, according to Bernoulli's principle, the pressure of the fluid will decrease and the flow rate will increase when it flows through the slit, so from the velocity cloud chart it can be seen that when CO2 passes through the valve plate area of the valve, the flow rate of the fluid increases significantly, which is consistent with the change of the pressure cloud chart.

[0134] Figure 13 With Figure 14 The volume fraction cloud charts of liquid and gaseous CO2 in the wellhead gate valve pipeline at 25% opening can be seen that in the pipeline at the inlet, the volume fraction of liquid CO2 is large, about 10%, and the gas holdup is 90%. Near the valve plate, due to the great change of temperature and pressure of CO2, CO2 undergoes violent phase change, and the gas holdup here increases significantly compared with the inlet pipeline, and can reach 100%. In a small section of pipeline after CO2 flows out of the valve area, due to the decrease of CO2 temperature, the above-mentioned potential frozen blockage area is formed, and at this time the gas holdup of CO2 decreases compared with that in the valve. In the second half of the outlet pipeline, due to the decrease of pressure, CO2 is no longer in liquid state, and the gas holdup at this time is about 100%.

[0135] Figure 15For the temperature cloud chart of the wellhead gate valve pipeline at 25% opening, it can be seen that the CO2 temperature is relatively high at the inlet pipeline of the valve, maintaining at about 300K. When CO2 flows through the valve plate area, the temperature of CO2 near the valve plate area decreases to about 270K due to the throttling effect of the valve. When CO2 flows out of the valve plate area, the temperature of CO2 drops sharply, and the lowest temperature is formed. 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, the CO2 in the pipeline is below 217K in some areas. At this time, CO2 will undergo a large phase change due to the large change in pressure and temperature, and CO2 may freeze due to the low temperature below the triple point, i.e. the wellhead gate valve has the risk of freezing.

[0136] Figure 16 For the area of the temperature cloud chart below the triple point on the symmetry plane, it can be seen that the larger the area, the greater the risk of freezing of the wellhead gate valve during use. After calculation, the area below the triple point at 25% opening accounts for 0.3249.

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

[0138] The specific boundary conditions are as follows:

[0139] A numerical calculation model of the valve at small opening is established. Under normal working conditions, the inlet of the valve is given a pressure inlet boundary condition, and the specified pressure is 25MPa. The outlet of the valve is given a pressure outlet boundary condition, and the specified pressure is 0MPa. According to the calculation requirements of the 1 / 23-D model, the boundary condition of the symmetry plane is set to Symmetry, and the remaining boundaries are set to Wall and Interface as required.

[0140] Simulation results:

[0141] Figure 17 For the inlet flow detection curve at large opening, it can be seen that with the increase of calculation steps, the inlet flow tends to be stable, i.e. the simulation results at this time converge, and the results at this time are taken as the simulation results at this opening.

[0142] Figure 18For the pressure cloud of the wellhead gate valve pipeline at 75% opening, it can be seen that at the inlet of the valve, the entire CO2 pressure level is at the highest position, and when CO2 flows through the valve plate area, the pressure cloud changes greatly, forming a large pressure gradient, which is due to the fact that the valve has not been fully opened, the small flow area of the valve plate forms throttling; Similarly, when CO2 flows out of the valve plate area, a more obvious pressure gradient is also formed, and CO2 forms a secondary pressure reduction, and finally the pressure of CO2 gradually approaches atmospheric pressure.

[0143] Figure 19 For the velocity cloud of the wellhead gate valve pipeline at 75% opening, according to Bernoulli's principle, the pressure of the fluid will decrease and the flow rate will increase when it flows through the slit, so from the velocity cloud it can be seen that when CO2 passes through the valve plate area, the flow rate of the fluid increases significantly, which is consistent with the change of the pressure cloud. Compared with 25% small opening, at 75% large opening, due to the increase of the flow area, the throttling effect is weakened, the pressure difference before and after the valve is reduced, and the overall flow rate is also reduced.

[0144] Figure 20 With Figure 21 The volume fraction clouds of liquid and gaseous CO2 in the wellhead gate valve pipeline at 75% opening can be seen that in the inlet pipeline, the volume fraction of liquid CO2 is large, about 5%, and the gas holdup is 95%. Near the valve plate, due to the large change in temperature and pressure of CO2, CO2 undergoes a dramatic phase change, and the gas holdup is significantly higher than that in the inlet pipeline, reaching 100% at most. In a small section of the pipeline after CO2 flows out of the valve area, due to the decrease in CO2 temperature, a potential frozen plugging area is formed, and the gas holdup of CO2 is lower than that inside the valve. In the second half of the outlet pipeline, due to the decrease in pressure, CO2 is no longer in a liquid state, and the gas holdup is about 100%. Compared with 25% small opening, at 75% large opening, the fluid is released faster and the overall gas holdup is larger.

[0145] Figure 22 The temperature cloud of the wellhead gate valve pipeline at 25% opening can be seen that in the inlet pipeline of the valve, the temperature of CO2 is high, about 288K, and when CO2 flows through the valve plate area, due to the throttling effect of the valve, the temperature of CO2 near the valve plate area decreases to about 270K. When CO2 flows out of the valve plate area, the temperature of CO2 drops sharply, forming the lowest temperature. Due to the weak throttling effect of the large opening valve, the temperature in the overall valve flow field is high, and there is no temperature below the triple point, i.e. the area ratio below the triple point is 0.

[0146] In summary, the embodiment is aimed at the opening process of the wellhead gate valve at two typical opening degrees, and the calculation is carried out on the basis of modeling. After the dense phase CO2 passes through the narrow gap of the pressure relief valve, the pressure drops suddenly, and the phase state of CO2 changes from liquid to gas rapidly. During this adiabatic expansion phase change process, the working medium absorbs a large amount of latent heat of phase change, and the temperature drops rapidly. This process is roughly along the CO2 temperature-pressure saturation line. At the same time, the pressure in the pressure relief valve increases with the increase of the valve opening, and then the overall temperature in the pressure relief valve rises with the increase of the valve opening. When the valve opening is small and large, the minimum temperature in the pressure relief valve corresponding to the valve opening is 188.19 K and 271.71 K respectively, and the working medium temperature under small valve opening is much smaller than that under large valve opening. During the pressure relief process of dense phase CO2, the escaped gas will cool the pipeline, and when the temperature 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. Therefore, under the premise of ensuring the safety of the structure of the pressure relief valve, increasing the valve opening of the pressure relief valve can effectively improve the working medium temperature and reduce the risk of valve and pipeline embrittlement. At the same time, increasing the valve opening can further increase the discharge amount of dense phase CO2, which can accelerate the discharge of the medium in the pressure pipeline and related equipment, and meet the pressure reduction demand.

[0147] The flow field under two opening degrees has the risk of freezing and blocking. By comparing the area of the potential frozen and blocked area, it can be seen that the valve under small opening degree has a higher risk of freezing and blocking, and the area ratio of the frozen and blocked area is 0.3249.

[0148] Embodiment five, this embodiment is a specific description of the control equation used in the CFD simulation analysis and calculation of the above-mentioned embodiments;

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

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

[0151] When simulating and calculating single-phase flow, usually only a set of momentum and continuity conservation equations need to be solved. In order to realize the change from single-phase model to multi-phase model, additional conservation equations must be introduced. In order to reasonably describe the relationship between different phases, different phases are treated as mutually penetrating continuous media. In the jet model, a hybrid multiphase flow model is adopted. The solution of the momentum equation for the mixture (Momentum equation for the mixture) 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] where n is the number of phases of the multiphase flow; is the volume force, N; μm is the viscosity of the mixture, Pa.s; vdr,p is the drift velocity of the multiphase, m / s.

[0156] The relative velocity (also referred to as slip velocity) is defined as the velocity of the second phase (p) relative to the velocity of 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 given by

[0159]

[0160] The algebraic slip mixture model in Fluent uses an algebraic slip formulation. The basic assumption of the algebraic slip mixture model is that the local balance between phases is to 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 particle, m / s 2 ; τqp is the relaxation time of the particle, s.

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

[0164]

[0165] where dp is the diameter of the second phase particle (or droplet or bubble), m; fdrag is the drag force coefficient, which according to the Schiller and Naumann study, has the form:

[0166]

[0167] The form of the acceleration a is:

[0168]

[0169] The volume fraction equation for the second phase p is obtained from the continuity equation for the second phase p as follows:

[0170]

[0171] In order to realize the simulation calculation of CO2, it is necessary to load the mass and energy source term in the control equation to realize the phase change. When the pressure in the control body is less than the saturation pressure at the temperature, the CO2 changes from liquid to gas, and the liquid mass in the control body decreases and the gas mass increases. Referring to the model of water phase change, the mass source term is obtained by the following formula:

[0172] S1 = -S r = k a l p l (P - P S ) / P S

[0173] S1 = -S v = k a v p v (P - P S ) / P S

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

[0175] S b = H S l

[0176] Wherein, H is the latent heat of vaporization at the specified temperature, which is obtained by the enthalpy difference between the gas and liquid at the temperature. k is the phase change rate coefficient of the flow in the wellhead gate valve cavity when the dense phase CO2 is released.

[0177] The above only describes the embodiments of the present application and is not limited to the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the scope of the claims of the present application.

Claims

1. A risk analysis method for CO2 depressurization process wellhead valve freezing, characterized in that, The method comprises the following steps: S1: build a 1 / 2 3-D wellhead gate valve model, and perform watershed extraction, grid division, and solver setting; S1 specifically comprises the following steps: S11: build a 1 / 2 3-D wellhead gate valve model; S12: perform watershed extraction on the wellhead gate valve model by using a model processing software to obtain a main throttling area of the wellhead gate valve; S13: after the watershed extraction, perform division on the wellhead gate valve model by using a hybrid grid method; S14: after the grid division, set solver parameters and two-phase flow parameters of CO2; During the watershed extraction, 5 times and 10 times of the inlet diameter of the valve are added to the inlet and outlet of the valve, respectively; During the grid division, regional processing needs to be performed, specifically as follows: In a region close to the pressure relief slit of the wellhead gate valve, local encryption processing is performed on the grid; In a region far from the pressure relief slit, sparse grid division is performed; S2: perform CO2 property setting on the set 1 / 2 3-D wellhead gate valve model, and perform model verification to obtain a CO2 freeze-out risk numerical simulation model; The CO2 property setting specifically comprises the following steps: The CO2 property is divided into three stages: 0.1-0.6 MPa, 0.6-7.37 MPa, and 7.37-30 MPa; During the 0.1-0.6 MPa stage, the temperature is set according to a fitting formula between the saturation temperature and the saturation pressure During the 0.6-7.37 MPa stage, CO2 undergoes phase change, and the temperature is set according to a fitting formula between the saturation temperature and the saturation pressure; During the 7.37-30 MPa stage, CO2 does not undergo phase change, and the saturation temperature under different pressures is set to 35℃; S3: perform CFD simulation calculation on the CO2 freeze-out risk numerical simulation model for two opening degrees, analyze the freeze-out risk of the valve according to the calculation results, and thus optimize the structure of the valve.

2. The CO2 reduced pressure process wellhead valve freeze-out risk analysis method of claim 1, wherein, The two-phase flow parameters of CO2 specifically comprise the following parameters: The parameters of liquid-phase CO2 include density, viscosity, and thermal conductivity; The parameters of gas-phase CO2 include density, viscosity, thermal conductivity, standard state enthalpy, and reference temperature.

3. The CO2 reduced pressure process wellhead valve freeze-out risk analysis method of claim 1, wherein, During the CFD simulation calculation for the two opening degrees, the boundary conditions are set as follows: The inlet of the valve is a pressure inlet boundary condition, and the pressure is 25 MPa; the outlet of the valve is a pressure outlet, and the pressure is 0 MPa; and the boundary condition of the symmetry surface is Symmetry.

4. A CO2 pressure reduction process wellhead valve freeze-up risk analysis system, characterized by, The system comprises a storage device, and the storage device is used to perform the method in claim 1.

5. A computer readable storage medium, characterized in that, The computer readable storage medium stores a computer program, and the computer program is run by a processor to perform the CO2 pressure reduction process wellhead valve freeze-out risk analysis method in any one of claims 1-3.

6. A computer device, comprising: The device comprises a memory and a processor, and the memory stores a computer program, and when the processor runs the computer program stored in the memory, the processor performs the CO2 pressure reduction process wellhead valve freeze-out risk analysis method in any one of claims 1-3.

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