Performance evaluation method for flange seal based on bolt axial stress

Abstract

The present application relates to the technical field of petrochemical equipment sealing, and particularly relates to a flange sealing performance evaluation method based on bolt axial stress, comprising the following steps: establishing a nonlinear stress relationship between a gasket and bolt axial force; establishing a relationship between bolt axial stress and gasket contact surface sealing contact stress under different working pressures; constructing a three-dimensional model of a real sealing surface; establishing a three-dimensional model of the sealing surface contact process; obtaining the change of the leakage gap between the contact surfaces under different contact stress stresses, establishing a relationship model of the contact interface leakage gap and the sealing contact stress; establishing a quantitative evaluation model between the sealing contact stress, the leakage gap and the leakage rate; obtaining the contact stress under the actual working condition, calculating whether the corresponding leakage rate meets the tightness judgment standard to judge the flange sealing state. The present application aims to solve the technical problem that the metal sealing performance of the in-service flange connection cannot be quantified and online evaluated in the prior art.

Description

Technical Field

[0001] The present invention relates to the technical field of petrochemical equipment sealing, and particularly relates to a performance evaluation method for flange sealing based on bolt axial stress. Background Art

[0002] In industrial fields such as petrochemical and energy power, bolted flange connections are one of the most critical static sealing forms in equipment such as pressure vessels, pipelines, and valves. Their sealing performance directly relates to production safety, environmental protection, and economic benefits. Especially for harsh working conditions such as high temperature and high pressure, hydrogen-containing, toxic, and harmful environments, octagonal metal sealing gaskets are widely used. Their sealing mechanism relies on the bolt pre-tightening force to generate sufficient contact stress between the metal gasket and the flange sealing surface to compensate for the separating force generated by the internal pressure, and to cause plastic deformation of the microscopic unevenness on the gasket surface, thereby blocking the leakage channels.

[0003] In the prior art, the pressure test method is the most traditional and common sealing verification means in the industrial field. Its specific operation process is as follows: After the flange connection is installed or repaired, a test medium (usually water - hydrostatic test, or air / nitrogen - pneumatic test) is injected into it, and the pressure is gradually increased to the specified test pressure and maintained for a period of time. During this period, the operator judges whether there is macroscopic leakage in the connection by observing the drop of the pressure gauge reading, checking whether there is medium leakage outside the connection, or applying a foaming agent (for gas) to observe whether bubbles are generated. If the pressure is stable and there is no visible leakage during the pressure holding period, the seal is judged to be qualified. The ex post verification and off-line nature of this method can only be carried out after the equipment is installed or during shutdown and maintenance, which belongs to a kind of "ex post" and "off-line" verification. It cannot perform any form of on-line monitoring and early warning on the sealing state of the equipment under dynamic working conditions such as normal operation, temperature and pressure cycling and fluctuating. The pressure test method is qualitative rather than quantitative, and can only give a qualitative conclusion of "leakage" or "no leakage", and cannot provide any quantitative data such as the magnitude and distribution uniformity of the contact stress at the sealing interface, so it cannot evaluate the safety margin of the sealing performance. The sensitivity of the pressure test method is limited. For extremely微量的 "leakage", it is difficult to detect by the hydrostatic test, and although the pneumatic test has slightly higher sensitivity, it is still far lower than precision methods such as helium detection, and cannot meet the requirements of high sealing grades. The pressure test method has safety risks and costs. Especially when performing a high-pressure pneumatic test, the stored energy is huge, and once it fails, it may trigger serious safety accidents such as explosions. At the same time, the test itself requires shutdown, preparation of media and equipment, consuming a large amount of time and economic costs.

[0004] It should be noted that the text "极其微量的" in the original is not clear, and I translated it as "extremely微量的", you may need to clarify this part for a more accurate translation. Also, the repeated "<00000XX>" tags are kept as they are without further translation as per the requirements.Helium gas chromatography-mass spectrometry (HMS) leak detection is a high-precision qualitative leak detection technique. First, helium gas is introduced into the flange cavity being tested as a tracer gas. Then, a helium mass spectrometer is used to scan the periphery of the sealing interface using a suction gun. This instrument has extremely high sensitivity to helium molecules. Once a leak is detected, the helium gas will be captured by the leak detector through the leak channel, and the instrument will immediately alarm and display the leak rate reading. Helium gas chromatography-mass spectrometry leak detection has extremely high requirements for offline operation. To achieve high-sensitivity detection, a sealed test environment or vacuuming of the system is usually required, which is impossible in most in-service industrial plants, necessitating shutdown and a series of complex preparatory work. Furthermore, it has poor anti-interference capabilities; strong mechanical vibrations, electromagnetic noise, and temperature fluctuations in the field can severely affect the stability and signal-to-noise ratio of the sensor signal, especially for micro-strain measurements, easily leading to measurement errors or even failure. Moreover, installation requirements are stringent, and maintenance is difficult. Sensor installation usually requires modifications such as machining mounting holes and surface treatment of bolts or bases, and good mechanical coupling must be ensured. In harsh industrial environments, sensors and their circuitry are susceptible to damage, posing challenges to their reliability and long-term durability.

[0005] Due to the structural characteristics of flange connections, the contact state of the internal metal gasket sealing surface is difficult to measure directly. Therefore, in industrial practice, indirect methods such as controlling bolt tightening torque or conducting pressure tests are commonly used to assess seal reliability. However, these methods cannot accurately reflect the true contact stress distribution at the sealing interface, thus making it difficult to achieve a precise assessment of metal seal performance. Although some studies have pointed out that bolt stress monitoring can be a potential approach to assessing seal condition, current domestic and international research mostly treats bolt stress analysis and seal contact state assessment as two relatively independent processes, failing to establish an effective quantitative conversion relationship between the two. In terms of theoretical modeling of sealing performance, existing models mostly rely on the surface characteristic parameters of the gasket and its contact mechanical behavior to predict leakage rates. However, these key parameters are often not directly obtainable under equipment service conditions, leading to difficulties in practical engineering applications and hindering the acquisition of accurate assessment results.

[0006] To ensure the long-term sealing safety of in-service flange connection structures, it is urgent to start from the essence of the contact mechanism of metal seals, systematically study the bolt stress evolution law, gasket contact state response and surface morphology damage under complex loads, and then develop a set of effective sealing performance evaluation methods to significantly reduce the probability of leakage accidents.

[0007] In summary, existing technologies are all indirect, qualitative, or offline methods, unable to quantify and evaluate the metal sealing performance of in-service flange connections online. Engineering practice urgently needs a method that can scientifically and accurately assess the actual contact state of the sealing interface and predict leakage rates based on easily measurable parameters, in order to achieve a shift from "preventive maintenance" to "predictive maintenance." Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide a performance evaluation method for flange seals based on bolt axial stress. This method aims to solve the technical problem that the metal sealing performance of in-service flange connections cannot be quantified and evaluated online in the prior art.

[0009] To achieve the above objectives, this invention proposes a performance evaluation method for flange seals based on bolt axial stress, comprising the following steps:

[0010] S1. Establish the nonlinear force relationship between the axial forces of the gasket and the bolt;

[0011] S2. Establish the relationship between the axial stress of the bolt and the sealing contact stress of the gasket contact surface under different working pressures;

[0012] S3. Construct a three-dimensional model of the actual sealing surface;

[0013] S4. Based on step S3, establish a three-dimensional model of the sealing surface contact process;

[0014] S5. Based on S4, obtain the change in leakage gap between contact surfaces under different contact stresses, and establish a model of the relationship between leakage gap at the contact interface and sealing contact stress.

[0015] S6. Based on S5, establish a quantitative evaluation model for the relationship between sealing contact stress, leakage gap, and leakage rate;

[0016] S7. Based on S2, obtain the contact stress under actual working conditions, and then based on S6, calculate whether the corresponding leakage rate meets the tightness judgment standard to determine the flange sealing status.

[0017] Preferably, the flange, bolts, and gaskets are all made of metal. The method for establishing the nonlinear force relationship between the axial forces of the gasket and the bolts is as follows:

[0018] S1.1 Obtaining rebound force: After the bolt is pre-tightened, due to radial compression, the inner conical surface tightly adheres to the sealing groove and generates a rebound force on the main sealing surface.

[0019] S1.2 Obtaining radial self-tightening force: Under operating conditions, due to the effect of internal pressure, the gasket will undergo further radial self-tightening and will be subject to the radial self-tightening force of internal pressure.

[0020] S1.3 Obtain the friction angle. The increase in internal pressure causes relative sliding between the gasket sealing surface and the flange sealing groove, resulting in the contact surface being subjected to frictional force along the tangential direction of the contact surface.

[0021] S1.4 describes the nonlinear influence relationship between the bolt axial force and the gasket main seal contact force under operating conditions based on the rebound force, clamping force, radial self-tightening force, friction angle, and contact force of the main sealing surface, and is used for finite element simulation calculation.

[0022] As a preferred option, the resilience F R The calculation method is as follows

[0023] ;

[0024] Where E is the elastic modulus of the gasket material, f is the cross-sectional area of ​​the gasket, and g is the inward compression gap of the gasket from its free state after assembly to its pre-tightened state; D G The average diameter;

[0025] Radial self-tightening force F G The calculation method is as follows

[0026] ;

[0027] Among them, D G Where is the average diameter of the gasket, P is the internal pressure borne by the flange, and H1 is the height of the pressure-bearing surface of the gasket.

[0028] Under the influence of internal pressure, the bolt axial force F p One part is used to resist the axial force Q generated by the internal pressure, and the rest provides clamping force F for the flange. c Calculate

[0029] ;

[0030] The axial force generated by internal pressure can be expressed as:

[0031] ;

[0032] Contact force of the main sealing surface The clamping force F between the upper and lower connecting parts c Gasket rebound force F R Radial self-tightening force F caused by internal pressure G The resultant force in the normal direction of the contact surface, coupled with the relative sliding between the gasket sealing surface and the flange sealing groove due to the increased internal pressure, causes the contact surface to be subjected to a frictional force along the tangential direction of the contact surface, with a friction angle of θ. ; It can be represented as .

[0033] As a preferred method, the relationship between the bolt axial stress and the sealing contact stress of the gasket contact surface under different working pressures is as follows:

[0034] S2.1 Considering elastic-plastic deformation, material properties and dimensional structure, establish a three-dimensional finite element analysis model of the blowout preventer flange connection that can accurately reflect the mechanical properties of the system;

[0035] S2.2 Set contact conditions for the lower end face of the nut and its adjacent flange end face, as well as the inner and outer sealing interfaces of the metal gasket, and apply loads and constraints step by step;

[0036] S2.3 Under different internal pressures, select the bolt central section and the outer sealing interface of the sealing gasket, and solve the bolt axial stress and the average contact stress of the sealing interface under different internal pressures by area integration, and analyze the changes.

[0037] S2.4 Establish numerical relationship curves between the average contact pressure of the axial stress gasket sealing surface of the bolt group under different working internal pressures.

[0038] As a preferred option, the specific implementation steps of S2.2 are as follows:

[0039] S2.2.1 Apply basic preload stress to all bolts to smoothly establish contact.

[0040] S2.2.2 Apply bolt preload through parametric scanning, with the preload stress fixed step size gradually increasing to achieve the required preload.

[0041] S2.2.3 Apply internal pressure and axial force on the upper end face of the flange step by step through parametric scanning.

[0042] As a preferred method, the steps for constructing a 3D model of the actual sealing surface are as follows:

[0043] S3.1 Surface feature data acquisition: A roughness profiler is used to perform multi-region and multi-directional profile scanning on the sealing surface of the metal gasket to obtain data reflecting the micro-geometric features of the sealing surface.

[0044] S3.2 Point cloud data preprocessing: Based on the principal component analysis normal vector estimation algorithm, the collected raw point cloud data selects the nearest neighbor of each data point in the point cloud, constructs the local covariance matrix and performs eigenvalue decomposition, determines the eigenvector corresponding to the smallest eigenvalue as the normal vector of that point, establishes the normal vector field of the point cloud, and performs data alignment and outlier processing in sequence.

[0045] S3.3 Feature extraction and surface reconstruction generate high-quality point cloud data;

[0046] S3.4, 3D rough surface modeling: The point cloud data in S3.3 is mapped to a continuous surface. An extremely fine mesh generation strategy is adopted to ensure accurate analysis of micro-rough features. Gasket material parameters, boundary conditions and contact properties are defined in the solid mechanics physical field. By solving the surface height field equation, the surface morphology with realistic rough features is output, and finally a high-fidelity 3D rough surface model is output.

[0047] As a preferred method, the steps for feature extraction and surface reconstruction are as follows:

[0048] S3.3.1 Based on the normal vector field, the average curvature and Gaussian curvature of each point are estimated by the local quadratic surface fitting method;

[0049] S3.3.2 By constructing a multi-scale spatial representation of point clouds, Gaussian curvature extrema are detected at different scales as candidate feature points. An adaptive threshold based on curvature change is used to screen stable feature points to ensure the robustness of features to noise and density changes.

[0050] S3.3.3 Construct a comprehensive surface descriptor for each feature point. By statistically analyzing the histogram of the normal vector distribution, curvature distribution characteristics, and height variation parameters of the neighboring points, construct a feature description vector with high discriminative power to fully describe the geometric properties of the feature point and its neighborhood.

[0051] S3.3.4. Non-parametric surface reconstruction is performed using Gaussian process regression. The Matern 5 / 2 kernel function is selected as the covariance function. The length scale parameter and noise variance parameter of the kernel function are optimized by maximum likelihood estimation. The posterior distribution of the Gaussian process is solved by the conjugate gradient method. While maintaining the original geometric features, noise filtering and data smoothing are achieved, generating high-quality point cloud data.

[0052] As a preferred method, in the process of establishing a three-dimensional model of the sealing surface contact process based on step S3, a displacement function that changes with time is defined and applied to the upper surface of the rigid body to gradually move downward to simulate the extrusion contact process and adjust the magnitude of the contact stress. Plastic nodes are added to the contact surface and a hardening function model for the plastic stage is defined. After completing the boundary conditions and load settings, parametric analysis is added in the solution step to capture the stress change and deformation of the rough surface during the contact process with equal time steps.

[0053] As a preferred method, the method for establishing a model of the relationship between the leakage gap at the contact interface and the sealing contact stress based on the change of the leakage gap between the contact surfaces under different contact stresses obtained by S4 includes the following steps:

[0054] S4.1 Establish the relationship between the different deformations and leakage gaps between sealing surfaces when a rough surface is subjected to different contact stresses;

[0055] S4.2 Establish the relationship between deformation and plastic deformation and linear elastic deformation in metal-to-metal contact;

[0056] S4.3 Establish the hardening function of the rough surface in the plastic stage;

[0057] S4.4. Taking the rough sealing contact surface of the metal gasket as the research object, based on the relationships and functions in S4.1-S4.3, the equivalent contact stress and deformation of the rough surface during the contact process are obtained through simulation.

[0058] S4.5. Based on S4.4, establish an exponential numerical relationship model between the leakage gap at the contact interface and the sealing contact stress, and calculate the coefficients related to the surface morphology and compressibility of the material.

[0059] As a preferred method, the steps for establishing a quantitative evaluation model based on S5 to determine the relationship between sealing contact stress, leakage gap, and leakage rate are as follows:

[0060] S5.1 The behavior of leakage of sealing fluid through the tiny gap of the metal sealing interface can be equivalent to laminar flow between two fixed infinitely long parallel plates, thus using Poiseuille's cubic formula for flow between parallel plates.

[0061] S5.2. After the actual rough sealing surfaces come into contact, gaps inevitably exist. A model of the relationship between leakage gap and sealing contact stress is established by introducing roughness flow factor and gap height flow factor for correction.

[0062] S5.3, Based on S5.2, uses the contact pressure of the sealing gasket based on the axial stress of the bolt to obtain a quantitative evaluation model of the final sealing contact stress, leakage gap and leakage rate.

[0063] Compared with existing technologies, the beneficial effects of the flange seal performance evaluation method based on bolt axial stress provided by this invention are as follows: This invention constructs a complete quantitative prediction system, sequentially linking four key parameters—bolt axial force, gasket equivalent contact stress, sealing interface leakage gap, and system leakage rate—to achieve accurate evaluation of sealing performance. First, using nonlinear finite element analysis, a logarithmic function model is established between the bolt axial force and the gasket sealing surface equivalent contact stress, solving the problem of quantitative conversion from bolt load to gasket stress. Then, based on contact mechanics simulation of a real rough surface, an exponential decay relationship model is established between the equivalent contact stress and the average leakage gap, linking macroscopic mechanical parameters with microscopic leakage channels. Finally, the above model is coupled to the parallel plate gap flow theory to calculate the leakage rate under specific working conditions, which is then compared with the internationally recognized PVRC tightness level, thereby achieving a scientific and quantitative determination of the flange connection sealing safety status.

[0064] The features and advantages of the present invention will be described in detail through embodiments and in conjunction with the accompanying drawings. Attached Figure Description

[0065] Figure 1 This is a flowchart of the flange metal sealing performance inversion method based on bolt axial stress according to an embodiment of the present invention.

[0066] Figure 2 This is a schematic diagram of the force applied to the sealing contact surface according to an embodiment of the present invention.

[0067] Figure 3 This is a three-dimensional finite element analysis model of the blowout preventer flange connection, where a is the front view of the blowout preventer flange connection and b is the top view of the blowout preventer flange connection.

[0068] Figure 4 It is the numerical relationship between the axial stress of the bolt and the sealing contact stress under an internal pressure of 30MPa.

[0069] Figure 5 This is the numerical relationship between the axial stress of the bolt and the contact stress of the seal under an internal pressure of 50 MPa.

[0070] Figure 6 It is the numerical relationship between the axial stress of the bolt and the contact stress of the seal under an internal pressure of 70MPa.

[0071] Figure 7 This is the numerical relationship between the axial stress of the bolt and the contact stress of the seal under an internal pressure of 105 MPa.

[0072] Figure 8 It is a process for modeling real metal sealing surfaces.

[0073] Figure 9 It is the contour height point cloud data during the generation process of a three-dimensional rough sealing surface.

[0074] Figure 10 It is the actual rough three-dimensional sealing surface generated during the three-dimensional rough sealing surface generation process.

[0075] Figure 11 It is a metal-to-metal sealed contact model.

[0076] Figure 12 It represents the change in leakage gap under different contact stresses. Detailed Implementation

[0077] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. However, it should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of the invention. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concept of the invention.

[0078] In the description of this invention, it should be noted that when an element is referred to as being "fixed to" or "set on" another element, it can be directly on or indirectly on the other element. When an element is referred to as being "connected to" another element, it can be directly connected to or indirectly connected to the other element.

[0079] In the description of this invention, it should be noted that the terms "center," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified. "Several" means one or more, unless otherwise explicitly specified.

[0080] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0081] The present invention provides a method for evaluating the performance of flange metal seals based on bolt axial stress. Based on finite element simulation and contact mechanics numerical simulation, the system constructs a theoretical model that quantifies and predicts the system leakage rate by considering the bolt axial force, gasket contact stress, leakage gap at the sealing interface, and finally the leakage rate. This method is suitable for online assessment and predictive maintenance of the sealing safety status of metal-sealed bolted flange connections in the petrochemical industry.

[0082] This invention constructs a complete quantitative prediction system, sequentially correlating four key parameters: bolt axial force, gasket equivalent contact stress, sealing interface leakage gap, and system leakage rate, thereby achieving accurate assessment of sealing performance. First, using nonlinear finite element analysis, a logarithmic function model is established between the bolt axial force and the gasket sealing surface equivalent contact stress, solving the quantitative conversion problem from bolt load to gasket stress. Then, based on contact mechanics simulation of a realistic rough surface, an exponential decay relationship model is established between the equivalent contact stress and the average leakage gap, correlating macroscopic mechanical parameters with microscopic leakage channels. Finally, the above model is coupled to the parallel plate gap flow theory to calculate the leakage rate under specific working conditions, which is then compared with the internationally recognized PVRC tightness level, thus achieving a scientific and quantitative determination of the sealing safety status of flange connections.

[0083] This invention enables online quantitative evaluation of the sealing performance of in-service flange connections without disassembling equipment or conducting complex field tests. This significantly improves the scientific rigor, accuracy, and foresight of the evaluation process, providing a theoretical tool and decision-making basis for predictive maintenance of high-risk equipment in the petrochemical industry. The following examples illustrate this in detail.

[0084] See Figure 1 The flowchart illustrates an embodiment of the present invention that provides a method for evaluating the metal sealing performance of a flange based on bolt axial stress, wherein the flange, bolts, and gaskets are all made of metal, and includes the following steps:

[0085] S1, such as Figure 2 As shown, a nonlinear force relationship between the metal washer and the axial force of the bolt is established. The specific steps for establishing this relationship are as follows:

[0086] S1.1 Obtaining rebound force: After the bolt is pre-tightened, due to radial compression, the inner conical surface tightly adheres to the sealing groove and generates a rebound force on the main sealing surface.

[0087] After the bolts are pre-tightened, due to radial compression, the inner conical surface presses tightly against the sealing groove, generating a rebound force F on the main sealing surface. R for:

[0088]

[0089] Where E is the elastic modulus of the gasket material, f is the cross-sectional area of ​​the gasket, and g is the inward compression gap of the gasket from its free state after assembly to after pre-tightening. D G The average diameter is denoted as .

[0090] S1.2 Obtaining radial self-tightening force: Under operating conditions, due to the effect of internal pressure, the gasket will undergo further radial self-tightening and will be subject to radial self-tightening force from the internal pressure.

[0091] Under operating conditions, due to the internal pressure P, the gasket will further undergo radial self-tightening. The radial self-tightening force of the internal pressure can be expressed as:

[0092]

[0093] In the formula, D G Let P be the average diameter of the gasket, P be the internal pressure borne by the flange, and H1 be the height of the pressure-bearing surface of the gasket. Under the influence of the internal pressure, the bolt axial force F... p One part is used to resist the axial force Q generated by the internal pressure, and the rest provides clamping force F for the flange. c ,Right now:

[0094]

[0095] The axial force generated by internal pressure can be expressed as:

[0096]

[0097] S1.3 Obtain the friction angle. The increase in internal pressure causes relative sliding between the gasket sealing surface and the flange sealing groove, resulting in the friction angle of the contact surface being subjected to frictional force along the tangential direction of the contact surface.

[0098] S1.4, based on springback force and clamping force

[0099] The description of radial self-tightening force, friction angle, and contact force of the main sealing surface describes the nonlinear influence relationship between the bolt axial force and the gasket main seal contact force under operating conditions, and is used for finite element simulation calculation.

[0100] Contact force of the main sealing surface The clamping force F between the upper and lower connecting parts cGasket resilience Radial self-tightening force caused by internal pressure The resultant force in the normal direction of the contact surface, coupled with the relative sliding between the gasket sealing surface and the flange sealing groove due to the increased internal pressure, causes the contact surface to be subjected to a frictional force along the tangential direction of the contact surface, with a friction angle of θ. .therefore, It can be represented as:

[0101]

[0102] This formula can describe the nonlinear relationship between the bolt axial force and the gasket main seal contact force under operating conditions, and can be used for finite element simulation calculations.

[0103] S2. Establish the relationship between bolt axial stress and gasket contact stress under different working pressures, and establish an inversion model of metal seal contact state based on bolt axial stress.

[0104] The specific process is as follows:

[0105] S2.1. Considering elastic-plastic deformation, material properties, and dimensional structure, establish a three-dimensional finite element analysis model of the blowout preventer flange connection that can accurately reflect the mechanical properties of the system, such as... Figure 3 As shown.

[0106] S2.2. Establish contact conditions for the lower end face of the nut, its adjacent flange end face, and the inner and outer sealing interfaces of the metal gasket, and apply loads and constraints in stages. First, apply a preload stress of 1 MPa (basic preload) to all bolts to smoothly establish contact. Second, apply bolt preload stress through parametric scanning, gradually increasing the preload stress in 50 MPa (fixed step size) steps to achieve the required preload. Third, apply the internal pressure P and the axial force on the upper end face of the flange step by step through parametric scanning.

[0107] S2.3 When the internal pressure reaches 30, 50, 70, and 105 MPa (different internal pressures), select the central section of the bolt and the outer sealing interface of the gasket, and solve the axial stress of the bolt and the average contact stress of the sealing interface under different internal pressures by area integration, and analyze the changes.

[0108] S2.4 Establish numerical relationship curves between axial stress of bolt assembly and average contact pressure of gasket sealing surface under different working internal pressures, such as... Figures 4-7 The two exhibit a logarithmic positive correlation, which can be accurately fitted using a three-parameter logarithmic model. Where parameters a and c are coefficients determined by the operating internal pressure and the initial preload of the bolt, and b is a coefficient determined by the bolt material and specifications. The fitting results and the values ​​of parameters a, b, and c under different internal pressures are shown in the table below.

[0109] Internal pressure conditions a b c <![CDATA[R 2 ]]> 30 -1945.67 -406.97 66.93 0.984 50 -1614.68 -359.19 7.28 0.986 70 -1514.86 -345.51 -30.72 0.995 105 -1486.61 -348.95 -57.45 0.996

[0110] This fitting model describes the relationship between the contact stress on the outer main sealing surface of the gasket and the axial stress of the bolt group under standard installation conditions. Compared with the linear relationship between bolt stress and gasket contact stress derived from ASME and GB-150 standards, this model considers the structural design of the well control flange connection, gasket size, material properties, and elasto-plastic deformation, making the calculation results more consistent with reality.

[0111] S3. Construction of a Real Metal Sealing Surface

[0112] The process of constructing a three-dimensional model of the sealing surface based on the typical surface morphology of the ground metal seal is as follows: Figure 8 As shown, the specific steps are as follows:

[0113] S3.1 Surface Feature Data Acquisition: A high-precision roughness profilometer was used to perform multi-region, multi-directional profile scanning on the sealing surface of the BX type metal gasket, which meets the requirements of GB / T 22513-2013 standard. By setting appropriate sampling and evaluation lengths, an initial height sequence of the surface profile, including parameters such as peak and valley heights, profile arithmetic mean deviation Ra, and profile root mean square deviation Rq, was obtained to ensure that the data comprehensively reflects the microscopic geometric features of the sealing surface.

[0114] S3.2 Point Cloud Data Preprocessing: The collected raw point cloud data is imported into the PYTHON programming environment for preprocessing. A normal vector estimation algorithm based on principal component analysis is used to select the k-nearest neighbor for each data point in the point cloud, construct a local covariance matrix, and perform eigenvalue decomposition. The eigenvector corresponding to the smallest eigenvalue is determined as the normal vector of that point, establishing the normal vector field of the point cloud. Data alignment and outlier processing are then performed sequentially.

[0115] S3.3 Feature Extraction and Surface Reconstruction: 1) Based on the normal vector field, the average curvature and Gaussian curvature of each point are estimated through a local quadratic surface fitting method. The moving least squares method is used to fit a quadratic surface in the neighborhood of each point. The Gaussian curvature K and average curvature H are calculated analytically using surface coefficients to identify key geometric features such as convexities, depressions, and saddle points on the surface. 2) Improve the traditional SIFT algorithm to adapt to 3D point cloud data processing: By constructing a multi-scale spatial representation of the point cloud, Gaussian curvature extrema are detected at different scales as candidate feature points. An adaptive threshold based on curvature changes is used to screen stable feature points, ensuring the robustness of features to noise and density changes. 3) Construct a comprehensive surface descriptor for each feature point. By statistically analyzing the histogram of normal vector distribution, curvature distribution characteristics, and height variation parameters of neighboring points, a highly discriminative feature descriptor vector is constructed to fully describe the geometric attributes of the feature point and its neighborhood. 4) Use Gaussian process regression for non-parametric surface reconstruction. The Matern 5 / 2 kernel function was chosen as the covariance function, and its length scaling parameter and noise variance parameter were optimized using maximum likelihood estimation. The conjugate gradient method was used to solve for the posterior distribution of the Gaussian process, achieving noise filtering and data smoothing while preserving the original geometric features, thus generating high-quality point cloud data, such as... Figure 9 As shown.

[0116] S3.4, 3D Rough Surface Modeling: A 3D model is created in COMSOL Multiphysics software. The processed point cloud data (processed in S3.3) is mapped to a continuous surface using an interpolation function. An extremely fine mesh generation strategy is adopted, setting the maximum element size to no more than 0.5μm to ensure accurate analysis of micro-roughness features. Gasket material parameters, boundary conditions, and contact properties are defined in the solid mechanics physics field. By solving the surface height field equation, a surface morphology with realistic roughness features is output, ultimately generating a model as shown below. Figure 10 The high-fidelity three-dimensional rough surface model shown provides an accurate geometric basis for subsequent sealing contact analysis.

[0117] S4. Based on step S3, establish a three-dimensional model of the sealing surface contact process to better analyze the metal-sealed contact process.

[0118] Continue constructing the metal-to-metal contact model. Since the material hardness of the flange sealing groove is much greater than that of the sealing gasket, it can be equivalently represented as a smooth, rigid plane, using an equivalent root mean square roughness. The parameter replaces the original root mean square roughness of the profile, where and These are the root mean square roughness parameters of the gasket and the sealing groove, respectively. Further geometric operations are then used to establish... Figure 11 The three-dimensional sealing surface contact model is shown.

[0119] Define a displacement function that varies with time and apply it to the upper surface of the rigid body, allowing it to gradually move downwards to simulate the compression contact process and adjust the magnitude of the contact stress. Add plastic nodes to the contact surface and define a hardening function model for the plastic stage. After completing the boundary conditions and load settings, add a parametric analysis to the solution step, capturing the stress changes and deformation of the rough surface during the contact process with a step size of 0.1 s.

[0120] S5. Based on S4, obtain the change in leakage gap between contact surfaces under different contact stresses, establish a relationship model between the leakage gap at the contact interface and the sealing contact stress, and define the gap parameters during the contact process. The specific method includes the following steps:

[0121] S5.1 Establish the relationship between the different deformation amounts and leakage gaps between sealing surfaces when a rough surface is subjected to different contact stresses.

[0122] Rough surfaces will produce different deformations δ when subjected to contact stresses of varying magnitudes, which is the leakage gap h between the sealing surfaces. c Things will change, h c The expression is:

[0123]

[0124] Where h0 is the initial gap height, which can be obtained based on surface morphology parameters. This represents the amount of deformation during the contact process.

[0125] S5.2 Establish the relationship between deformation and plastic deformation and linear elastic deformation in metal-to-metal contact.

[0126] In metal-to-metal contact, the deformation δ comes from two parts: plastic deformation and linear elastic deformation.

[0127]

[0128] in, For effective plastic strain, Where E is the effective stress and E is the elastic modulus.

[0129] S5.3 Establish the hardening function of rough surfaces in the plastic stage.

[0130] The deformation of a rough surface during the plastic stage is determined by the hardening function. calculate:

[0131]

[0132] here The stress-strain function of the material. This is the yield stress.

[0133] S5.4. Taking the rough sealing contact surface of the metal gasket as the research object, based on the relationships and functions in S5.1-S5.3, the equivalent contact stress and deformation of the rough surface during the contact process are obtained through simulation.

[0134] Taking the rough sealing contact surface of the metal gasket as the research object, and combining the above formula, the equivalent contact stress and deformation of the rough surface during the contact process are obtained through simulation. Since the thickness of the model matrix is ​​small, the matrix deformation can be ignored when calculating the deformation δ through integration, and δ can be equivalent to the normal deformation of the rough surface. The initial sealing height h0 can be replaced by the equivalent roughness of the sealing interface, thus obtaining... Figure 12 The deformation of the contact surface under equivalent contact stress for different rough surfaces is shown below:

[0135] The Roth model indicates that the equivalent contact stress P c With leakage gap h c The relationship between them exhibits an exponential effect, consistent with this trend. However, the result, calculated based on a real sealing surface model, is more accurate than the Roth model, which treats the leakage path cross-section as an "isosceles triangle." Therefore, the leakage gap h at the contact interface can be further established. c Contact stress P with the seal c Numerical relationship model between them:

[0136]

[0137] In the formula, , and It is a coefficient related to the surface morphology and compressive properties of the material. According to h c -P c The curve fitting results are as follows: =0.84, =212.4, 0.721.

[0138] S6. Based on S5, establish a quantitative evaluation model for the relationship between sealing contact stress, leakage gap, and leakage rate, and establish a leakage rate evaluation model for rough sealing surfaces. The specific methods are as follows:

[0139] S6.1 The leakage behavior of a sealing fluid through a tiny gap at the metal-sealed interface can be equated to laminar flow between two fixed, infinitely long parallel plates. Poiseuille's cubic formula describing flow between parallel plates is:

[0140]

[0141] in, The pressure difference between the inlet and outlet is Pa; hc The height of the leakage gap; ρ is the dynamic viscosity of the fluid, Pa·s; b is the width of the sealing contact surface, mm. Fluid density, mg / mm 3 Q m The mass leakage rate is expressed in mg / (mm·s).

[0142] S6.2. After the actual rough sealing surfaces come into contact, gaps inevitably exist. A model of the relationship between leakage gap and sealing contact stress is established by introducing roughness flow factor and gap height flow factor for correction.

[0143] Since Poiseuille's cubic formula applies to smooth surfaces, and due to the existence of surface roughness, gaps inevitably exist after contact between truly rough sealing surfaces, a roughness flow factor needs to be introduced. and gap height flow factor Make corrections:

[0144] ,

[0145]

[0146] in, To achieve the same gap height as an ideal smooth interface, the mass leakage rate of a rough interface. This represents the leakage rate at the rough interface after applying sealing contact stress. The product of these two factors is the pressure-flow factor. :

[0147]

[0148] Research has shown that the pressure-flow factor is related to the film thickness ratio h. c The empirical quantization formula for the function / Rq is:

[0149]

[0150] By introducing a correction factor and substituting it into the improved leakage gap-seal contact stress relationship model, we can obtain:

[0151]

[0152] in, Fluid density, mg / mm 3 .

[0153] S6.3, Based on S6.2, a quantitative evaluation model is obtained for the final sealing contact stress, leakage gap and leakage rate by using the contact pressure of the sealing gasket based on the axial stress of the bolt inversion model of the metal seal contact state.

[0154] Contact pressure P of the sealing gasket c The contact state of the metal seal can be characterized using an inversion model based on the axial stress of the bolt, resulting in:

[0155]

[0156] S7. Based on S2, obtain the contact stress under actual working conditions, and then based on S6, calculate whether the corresponding leakage rate meets the tightness judgment standard to determine the flange sealing status.

[0157] After obtaining the flange leakage rate under the corresponding working conditions based on the above formula, it can be compared with the current PVRC tightness standard. If its leakage rate meets the established tightness level requirements, it can be judged that the flange seal is safe; otherwise, it is necessary to pre-tighten the bolts or replace the steel ring in time.

[0158] The following functions were achieved through the above-described implementation scheme.

[0159] First, the inversion and quantitative evaluation of the contact state of the sealing interface. This invention establishes a numerical correlation model between bolt axial force, gasket equivalent contact stress, and leakage gap at the sealing interface. This transforms the evaluation of sealing performance from relying on traditional pressure testing and helium leak detection methods to quantitatively predicting and analyzing the leakage rate through bolt axial stress. This enables the analysis and characterization of key mechanical states of the sealing interface, including contact stress distribution and the geometric characteristics of leakage channels, thereby changing the long-standing reliance on engineering experience and qualitative testing methods in this field.

[0160] Secondly, it provides scientific and quantitative guidance for on-site bolt preload standards. Addressing the challenges of large preload dispersion and the susceptibility to under-preload (leakage) or over-preload (gasket crushing, bolt yielding) caused by torque control in engineering projects, the model established in this invention can inversely solve for the range of bolt axial forces required to meet the target sealing level. This provides a theoretical basis for the formulation of preload specifications, construction monitoring, and acceptance.

[0161] Third, it replaces high-cost sealing tests, achieving controllable risks throughout the entire lifecycle. For harsh operating conditions such as high pressure and high-risk media, traditional water pressure or air pressure tests are costly and risky. This invention, by establishing a refined digital model that reflects material nonlinearity, contact nonlinearity, and surface morphology, can more safely and economically complete the sealing performance evaluation under extreme operating conditions. It can eliminate risks in the design stage and conduct online safety assessments of changes in operating conditions during the operation and maintenance stage, achieving sealing safety protection covering the entire lifecycle of the equipment.

[0162] This invention presents a comprehensive framework for a quantitative evaluation method of flange metal sealing performance based on bolt axial stress. Through systematic numerical simulation and theoretical modeling, it establishes a complete predictive technical route for "bolt axial force, gasket equivalent contact stress, sealing interface leakage gap, and system leakage rate." Specifically, this framework couples nonlinear finite element analysis, rough surface contact mechanics simulation, and fluid leakage models for joint calculation, thereby realizing a methodological system for quantitatively evaluating sealing status and leakage rate based on measurable or calculable bolt axial stress.

[0163] By establishing a metal seal inversion model, namely a quantitative conversion model between bolt axial force and gasket equivalent contact stress, and its construction method, a refined three-dimensional nonlinear finite element model of the bolt, flange, and gasket system was built. Through simulation calculations, multiple sets of corresponding data of bolt axial force and gasket sealing surface equivalent average contact stress were obtained. Based on this data, a logarithmic function relationship model between the two was established, realizing the direct and quantitative inversion from bolt load to key mechanical state parameters of the sealing interface.

[0164] Furthermore, an improved theoretical model of leakage gap based on a real rough sealing surface and its construction method, built upon the classic Roth model, is developed. First, a statistically representative digital model of the real rough sealing surface is generated. Then, through elastoplastic contact mechanics simulation, different equivalent contact stresses P are obtained. c The average leakage gap h at the sealing interface under action c Finally, P was established based on simulation data fitting. c with h c This model establishes an exponential decay function relationship between macroscopic contact stress and microscopic leakage channels. It overcomes the dependence of traditional models on microscopic morphological parameters that are difficult to measure.

[0165] Finally, the process for generating a high-fidelity digital model of a rough sealing surface used in this method is described. The technical steps include: point cloud data acquired using a standard profilometer; feature analysis and data simplification through PCA normal vector estimation, Gaussian curvature calculation, and an improved SIFT feature extraction algorithm; and finally, surface reconstruction and smoothing using Gaussian process regression to generate a rough surface model suitable for subsequent contact mechanics simulations and reflecting the actual machining morphology.

[0166] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A performance evaluation method for flange seals based on bolt axial stress, characterized in that, Includes the following steps: S1. Establish the nonlinear force relationship between the axial forces of the gasket and the bolt; S2. Establish the relationship between the axial stress of the bolt and the sealing contact stress of the gasket contact surface under different working pressures; S3. Construct a three-dimensional model of the actual sealing surface; S4. Based on step S3, establish a three-dimensional model of the sealing surface contact process; S5. Based on S4, obtain the change in leakage gap between contact surfaces under different contact stresses, and establish a model of the relationship between leakage gap at the contact interface and sealing contact stress. S6. Based on S5, establish a quantitative evaluation model for the relationship between sealing contact stress, leakage gap, and leakage rate; S7. Based on S2, obtain the contact stress under actual working conditions, and then based on S6, calculate whether the corresponding leakage rate meets the tightness judgment standard to determine the flange sealing status.

2. The performance evaluation method for flange seals based on bolt axial stress as described in claim 1, characterized in that, The flange, bolts, and gaskets are all made of metal. The steps for establishing the nonlinear force relationship between the axial forces of the gaskets and bolts are as follows: S1.1 Obtaining rebound force: After the bolt is pre-tightened, due to radial compression, the inner conical surface tightly adheres to the sealing groove and generates a rebound force on the main sealing surface. S1.2 Obtaining radial self-tightening force: Under operating conditions, due to the effect of internal pressure, the gasket will undergo further radial self-tightening and will be subject to the radial self-tightening force of internal pressure. S1.3 Obtain the friction angle. The increase in internal pressure causes relative sliding between the gasket sealing surface and the flange sealing groove, resulting in the contact surface being subjected to frictional force along the tangential direction of the contact surface. S1.4 describes the nonlinear influence relationship between the bolt axial force and the gasket main seal contact force under operating conditions based on the rebound force, clamping force, radial self-tightening force, friction angle, and contact force of the main sealing surface, and is used for finite element simulation calculation.

3. The performance evaluation method for flange seals based on bolt axial stress as described in claim 2, characterized in that, Rebound force F R The calculation method is as follows ； Where E is the elastic modulus of the gasket material, f is the cross-sectional area of ​​the gasket, and g is the inward compression gap of the gasket from its free state after assembly to its pre-tightened state; D G The average diameter of the gasket; Radial self-tightening force F G The calculation method is as follows ； Among them, D G Where is the average diameter of the gasket, P is the internal pressure borne by the flange, and H1 is the height of the pressure-bearing surface of the gasket. Under the influence of internal pressure, the bolt axial force F p One part is used to resist the axial force Q generated by the internal pressure, and the rest provides clamping force F for the flange. c Calculate ； The axial force generated by internal pressure can be expressed as: ； Contact force of the main sealing surface The clamping force F between the upper and lower connecting parts c Gasket rebound force F R Radial self-tightening force F caused by internal pressure G The resultant force in the normal direction of the contact surface, coupled with the relative sliding between the gasket sealing surface and the flange sealing groove due to the increased internal pressure, causes the contact surface to be subjected to a frictional force along the tangential direction of the contact surface, with a friction angle of θ. ; It can be represented as .

4. The performance evaluation method for flange seals based on bolt axial stress as described in claim 1, characterized in that, The method for establishing the relationship between the bolt axial stress and the sealing contact stress of the gasket contact surface under different working pressures is as follows: S2.1 Considering elastic-plastic deformation, material properties and dimensional structure, establish a three-dimensional finite element analysis model of the blowout preventer flange connection that can accurately reflect the mechanical properties of the system; S2.2 Set contact conditions for the lower end face of the nut and its adjacent flange end face, as well as the inner and outer sealing interfaces of the metal gasket, and apply loads and constraints step by step; S2.3 Under different internal pressures, select the bolt central section and the outer sealing interface of the sealing gasket, and solve the bolt axial stress and the average contact stress of the sealing interface under different internal pressures by area integration, and analyze the changes. S2.4 Establish numerical relationship curves between the average contact pressure of the axial stress gasket sealing surface of the bolt group under different working internal pressures.

5. The performance evaluation method for flange seals based on bolt axial stress as described in claim 4, characterized in that, The specific implementation steps of S2.2 are as follows: S2.2.1 Apply basic preload stress to all bolts to smoothly establish contact. S2.2.2 Apply bolt preload through parametric scanning, with the preload stress fixed step size gradually increasing to achieve the required preload. S2.2.3 Apply internal pressure and axial force on the upper end face of the flange step by step through parametric scanning.

6. The performance evaluation method for flange seals based on bolt axial stress as described in claim 1, characterized in that, The steps to construct a 3D model of a realistic sealing surface are as follows: S3.1 Surface feature data acquisition: A roughness profiler is used to perform multi-region and multi-directional profile scanning on the sealing surface of the metal gasket to obtain data reflecting the micro-geometric features of the sealing surface. S3.2 Point cloud data preprocessing: Based on the principal component analysis normal vector estimation algorithm, the collected raw point cloud data selects the nearest neighbor of each data point in the point cloud, constructs the local covariance matrix and performs eigenvalue decomposition, determines the eigenvector corresponding to the smallest eigenvalue as the normal vector of that point, establishes the normal vector field of the point cloud, and performs data alignment and outlier processing in sequence. S3.3 Feature extraction and surface reconstruction generate high-quality point cloud data; S3.4, 3D rough surface modeling: The point cloud data in S3.3 is mapped to a continuous surface. An extremely fine mesh generation strategy is adopted to ensure accurate analysis of micro-rough features. Gasket material parameters, boundary conditions and contact properties are defined in the solid mechanics physical field. By solving the surface height field equation, the surface morphology with realistic rough features is output, and finally a high-fidelity 3D rough surface model is output.

7. The performance evaluation method for flange seals based on bolt axial stress as described in claim 6, characterized in that, The steps of feature extraction and surface reconstruction are as follows: S3.3.1 Based on the normal vector field, the average curvature and Gaussian curvature of each point are estimated by the local quadratic surface fitting method; S3.3.2 By constructing a multi-scale spatial representation of point clouds, Gaussian curvature extrema are detected at different scales as candidate feature points. An adaptive threshold based on curvature change is used to screen stable feature points to ensure the robustness of features to noise and density changes. S3.3.3 Construct a comprehensive surface descriptor for each feature point. By statistically analyzing the histogram of the normal vector distribution, curvature distribution characteristics, and height variation parameters of the neighboring points, construct a feature description vector with high discriminative power to fully describe the geometric properties of the feature point and its neighborhood. S3.3.

4. Non-parametric surface reconstruction is performed using Gaussian process regression. The Matern 5 / 2 kernel function is selected as the covariance function. The length scale parameter and noise variance parameter of the kernel function are optimized by maximum likelihood estimation. The posterior distribution of the Gaussian process is solved by the conjugate gradient method. While maintaining the original geometric features, noise filtering and data smoothing are achieved, generating high-quality point cloud data.

8. The performance evaluation method for flange seals based on bolt axial stress as described in claim 1, characterized in that, In the process of establishing a three-dimensional model of the sealing surface contact process based on step S3, a displacement function that varies with time is defined and applied to the upper surface of the rigid body to gradually move downward to simulate the extrusion contact process and adjust the magnitude of the contact stress. Plastic nodes are added to the contact surface and a hardening function model for the plastic stage is defined. After completing the boundary conditions and load settings, parametric analysis is added in the solution step to capture the stress change and deformation of the rough surface during the contact process with equal time steps.

9. The performance evaluation method for flange seals based on bolt axial stress as described in claim 1, characterized in that, The method for establishing a model of the relationship between the leakage gap at the contact interface and the sealing contact stress based on the change of leakage gap between contact surfaces under different contact stresses using S4 includes the following steps: S5.1 Establish the relationship between the different deformations and leakage gaps between sealing surfaces when a rough surface is subjected to different contact stresses; S5.2 Establish the relationship between deformation and plastic deformation and linear elastic deformation in metal-to-metal contact; S5.3 Establish the hardening function of rough surfaces in the plastic stage; S5.

4. Taking the rough sealing contact surface of the metal gasket as the research object, based on the relationships and functions in S5.1-S5.3, the equivalent contact stress and deformation of the rough surface during the contact process are obtained through simulation. S5.

5. Based on S5.4, establish an exponential numerical relationship model between the leakage gap at the contact interface and the sealing contact stress, and calculate the coefficients related to the surface morphology and compressibility of the material.

10. The performance evaluation method for flange seals based on bolt axial stress as described in claim 1, characterized in that, The steps for establishing a quantitative evaluation model based on S5 to assess the relationship between sealing contact stress, leakage gap, and leakage rate are as follows: S6.1 The behavior of leakage of sealing fluid through the tiny gap of the metal sealing interface can be equivalent to laminar flow between two fixed infinitely long parallel plates, thus using Poiseuille's cubic formula for flow between parallel plates. S6.

2. After the actual rough sealing surfaces come into contact, gaps inevitably exist. A model of the relationship between leakage gap and sealing contact stress is established by introducing roughness flow factor and gap height flow factor for correction. S6.3, Based on S6.2, a quantitative evaluation model is obtained for the final sealing contact stress, leakage gap and leakage rate by using the contact pressure of the sealing gasket based on the axial stress of the bolt inversion model of the metal seal contact state.

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