Finite element simulation method for leakage rate of important service water centrifugal pump packing of nuclear power plant

Abstract

The invention discloses a finite element simulation method for the leakage rate of a packing of an important service water centrifugal pump of a nuclear power plant. The finite element simulation method comprises the following steps: S1, establishing a solid model of a pump shaft and the packing of the centrifugal pump by utilizing three-dimensional fluid simulation software; s2, the characteristic length of the packing material of the centrifugal pump is obtained through conservative calculation; s3, obtaining a packing viscous resistance coefficient in an initial state; s4, simulating the seepage model when the centrifugal pump is in the initial state to obtain a first rule schematic diagram about the seepage model; and S5, simulating the wear model when the centrifugal pump is in the wear state to obtain a second rule schematic diagram about the wear model. The finite element simulation method for the leakage rate of the important service water centrifugal pump packing of the nuclear power plant can be used for researching the change rule of the shaft seal water leakage rate, the shaft seal water incoming flow pressure, the packing structure flow field and the shaft seal water flow distribution of the centrifugal pump in the initial state and the abrasion state, so that the pump shaft seal water pressure, the pump packing structure and the adaptability are optimized.

Description

Technical Field

[0001] The invention relates to the field of important service water systems of nuclear power plants, and in particular to a finite element simulation method for the leakage of packings of important service water centrifugal pumps of nuclear power plants. Background Art

[0002] The shaft seal of the centrifugal pump in the important plant water system of the second generation nuclear power technology is a packing seal, and its structure is mainly composed of packing, packing chamber and gland screws. After the shaft seal water is injected, it flows through the packing from the upper and lower parts through the water dividing ring. The upper part is open to air, and its leakage can be observed; the lower part is injected into the pump cavity to prevent the seawater in the pump cavity from flowing up to the pump shaft seal, and the leakage of the lower part cannot be observed.

[0003] Many nuclear power base units have the problem of excessively high seal water pressure in important plant water pumps. For example, the original pump factory seal water design pressure is 5 bar.g, but the actual operating seal water pressure is 8-12 bar.g. In order to ensure that the upper leakage of the seal water meets the requirements of the pump operation and maintenance manual (30-60 drops per minute), the operation and maintenance personnel further tightened the gland screws, which increased the radial deformation of the packing. The packing is in close contact with the pump sleeve, and continuous operation causes wear of the packing, sleeve and pump cover. At present, there is a lack of methods to study the change law of the seal water leakage and the seal water inflow pressure of the packing under different states (initial state + wear state), as well as the flow field of the packing structure and the seal water flow distribution law. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a finite element simulation method for the leakage of the packing of an important centrifugal pump for plant water in a nuclear power plant.

[0005] The technical solution adopted by the present invention to solve the technical problem is: constructing a finite element simulation method for the leakage of the packing of an important centrifugal pump for plant water in a nuclear power plant, which comprises the following steps:

[0006] S1. According to the design drawings of the centrifugal pump and the actual measurement of the on-site equipment, the basic size parameters of the centrifugal pump are obtained, and the solid model of the pump shaft and packing of the centrifugal pump is established by using the three-dimensional fluid simulation software;

[0007] S2. Calculate the porosity of the packing material of the centrifugal pump according to the parameter information of the packing forming material of the centrifugal pump and in combination with the measured data of the packing raw material, and conservatively calculate the characteristic length of the packing material of the centrifugal pump according to the porosity of the packing material;

[0008] S3, obtaining the actual leakage above the packing of the centrifugal pump, and using three-dimensional fluid simulation software to iterate the packing viscous resistance coefficient when the centrifugal pump is in an initial state, the iterative judgment formula is that the calculated leakage above the packing is consistent with the actual leakage above the packing, and the software result is the packing viscous resistance coefficient in the initial state;

[0009] S4. Based on the Reynolds number formula, the flow rate and flow velocity calculated according to the characteristic length of the packing material of the centrifugal pump and the packing viscous resistance coefficient are used to obtain the Reynolds number of the centrifugal pump in the initial state. If the Reynolds number is less than 10, the porous medium model and Darcy's law are applicable to the flow of the shaft seal water through the packing material. At this time, the first operating parameter of the centrifugal pump is input, and the seepage model of the centrifugal pump in the initial state is simulated to obtain a first regular schematic diagram of the seepage model.

[0010] S5. Simulate the wear model of the centrifugal pump when it is in a wear state, input the second operating parameters, and modify the model structure under the assumption that the packing viscous resistance coefficient remains unchanged. Start with a gap of 0 mm between the packing structure and the wall, and gradually increase the gap for simulation to obtain a second law schematic diagram of the wear model.

[0011] In some embodiments, the packing parameter information of the centrifugal pump includes packing raw material density, packing section size, and packing length;

[0012] The measured data of the packing raw materials include the packing forming density and the measured mass of the packing.

[0013] In some embodiments, the first operating parameter is a first inlet pressure, a first outlet pressure, a first pump shutdown pressure, a first pump start-up pressure, a first outlet flow rate, and a first fluid medium.

[0014] In some embodiments, the second operating parameter is a second inlet pressure, a second outlet pressure, a second pump shutdown pressure, a second pump startup pressure, a wear clearance, and a second fluid medium.

[0015] In some embodiments, the first regularity diagram includes a seal water leakage and seal water inflow pressure curve diagram when the centrifugal pump is in an initial state, a packing structure flow field diagram, and a seal water flow distribution diagram.

[0016] In some embodiments, the second law schematic diagram includes a shaft seal water leakage and shaft seal water inflow pressure curve diagram when the centrifugal pump is in a wear state, a packing structure flow field diagram, a shaft seal water flow distribution diagram and a wear clearance change diagram.

[0017] In some embodiments, in step S5, when the centrifugal pump is in a wear state, the flow channel of the shaft seal water is the packing material and the gap channel between the packing and the wall. The model of flowing through the packing material is applicable to Darcy's law, and the flow state is laminar; the model of flowing through the gap channel between the packing and the wall is applicable to Bernoulli's law and the law of resistance loss, and the flow state is turbulent.

[0018] In some embodiments, in step S5, during the simulation process, when the ratio of the flow rate of the shaft seal water through the packing to the flow rate of the shaft seal water through the wear gap is greater than or equal to a preset value, the fluid passing through the packing is simulated based on the porous medium model and Darcy's law, and the fluid passing through the wear gap is simulated based on the turbulence model.

[0019] In some embodiments, in step S5, during the simulation process, when the ratio of the flow rate of the shaft sealing water through the packing to the flow rate of the shaft sealing water through the wear gap is less than a preset value, the flow of the shaft sealing water through the packing is ignored, and only the fluid passing through the wear gap is simulated based on the turbulence model.

[0020] In some embodiments, in step S3, the fluent software iterates the packing viscous resistance coefficient when the centrifugal pump is in the initial state based on the along-the-line resistance loss calculation formula, the local resistance loss calculation formula, the total resistance loss calculation formula and the leakage calculation formula.

[0021] The implementation of the present invention has the following beneficial effects: the finite element simulation method for the leakage of the packing of the important plant water centrifugal pump of the nuclear power plant establishes a simulation method for the shaft seal structure of the important plant water pump of the nuclear power plant, which can be used to study the change law of the shaft seal water leakage and the shaft seal water inlet pressure, the packing structure flow field, and the shaft seal water flow distribution of the centrifugal pump in the initial state and the wear state, thereby optimizing the pump shaft seal water pressure and the pump packing structure and adaptability, and optimizing the operation and maintenance strategy of the pump packing structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solution of the present invention, the present invention will be further described below in conjunction with the accompanying drawings and embodiments. It should be understood that the following drawings only show certain embodiments of the present invention and should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work. In the drawings:

[0023] Figure 1 It is a flow chart of a finite element simulation method for the leakage of packing of an important centrifugal pump for plant water in a nuclear power plant in some embodiments of the present invention. DETAILED DESCRIPTION

[0024] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the specific embodiments of the present invention are now described in detail with reference to the accompanying drawings. In the following description, it should be understood that the directions or positional relationships indicated by "front", "back", "up", "down", "left", "right", "longitudinal", "horizontal", "vertical", "horizontal", "top", "bottom", "inside", "outside", "head", "tail", etc. are based on the directions or positional relationships shown in the accompanying drawings, are constructed and operated in a specific direction, and are only for the convenience of describing the present technical solution, rather than indicating that the device or element referred to must have a specific direction, and therefore cannot be understood as a limitation to the present invention.

[0025] It should also be noted that, unless otherwise clearly specified and limited, the terms such as "installed", "connected", "connected", "fixed", "set" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral one; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. When an element is referred to as being "on" or "under" another element, the element can be "directly" or "indirectly" located on the other element, or there may be one or more intermediate elements. The terms "first", "second", "third", etc. are only for the convenience of describing the present technical solution, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first", "second", "third", etc. can explicitly or implicitly include one or more of the features. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to the specific circumstances.

[0026] See also Figure 1 , is a finite element simulation method for the leakage of packing of an important centrifugal pump for plant water in a nuclear power plant in some embodiments of the present invention, which comprises the steps of:

[0027] S1. According to the design drawings of the centrifugal pump and the actual measurement of the on-site equipment, the basic size parameters of the centrifugal pump are obtained, and the solid model of the pump shaft and packing of the centrifugal pump is established by using the three-dimensional fluid simulation software;

[0028] S2. Calculate the porosity of the packing material of the centrifugal pump based on the parameter information of the packing forming material of the centrifugal pump and the measured data of the packing raw materials. According to the porosity of the packing material, conservatively calculate the characteristic length of the packing material of the centrifugal pump.

[0029] S3. Obtain the actual leakage above the packing of the centrifugal pump, and use three-dimensional fluid simulation software, such as ANSYS Fluent software, to iterate the packing viscous resistance coefficient when the centrifugal pump is in the initial state. The iterative judgment formula is that the calculated leakage above the packing is consistent with the actual leakage above the packing. At this time, the software result is the packing viscous resistance coefficient in the initial state;

[0030] S4. Based on the Reynolds number formula, the flow rate and flow velocity calculated according to the characteristic length of the packing material of the centrifugal pump and the packing viscous resistance coefficient are used to obtain the Reynolds number of the centrifugal pump in the initial state. If the Reynolds number is less than 10, the porous medium model and Darcy's law apply to the flow of the shaft seal water through the packing material. At this time, the first operating parameter of the centrifugal pump is input, and the seepage model of the centrifugal pump in the initial state is simulated to obtain the first law diagram of the seepage model;

[0031] S5. Simulate the wear model of the centrifugal pump when it is in a wear state, input the second operating parameters, and modify the model structure under the assumption that the packing viscous resistance coefficient remains unchanged. Start with a gap of 0 mm between the packing structure and the wall, and gradually increase the gap for simulation to obtain a second law schematic diagram of the wear model.

[0032] Specifically, in step S1, the outer dimensions (length * width * height) of the centrifugal pump are 1457mm, the motor and the pump are connected by a laminated coupling, and the outer dimensions of the entire pump unit are: 2320mm*4171mm. A 1000mm disassembly space is required above the motor. The above basic dimensional parameters are input into the three-dimensional fluid simulation software to establish a solid model of the pump shaft and the packing of the centrifugal pump. The specific structure of the centrifugal pump packing is three packings on the top, one packing on the bottom, and a water dividing ring in the middle.

[0033] In step S2, the packing parameter information of the centrifugal pump includes the packing raw material density, the packing section size and the packing length. The measured data of the packing raw material include the packing forming density and the measured mass of the packing. In a specific embodiment, the packing forming density is 1.3g / cm3, the packing raw material density is 1.57g / cm3, the packing section is 1.4cm*1.4cm, and the length is 37cm. It can be calculated that the packing volume is 1.4*1.4*37=72.52cm3. Under the condition that all the packing raw materials are packing, the packing mass is 72.52*1.57=113.85g, and the measured mass of the packing is 88g, because 113.85-88=25.85g of the packing raw material is actually air, and the volume of this part is 25.85 / 1.57=16.46cm3, and the porosity of the packing material is 16.46 / 72.52=22.7%. The characteristic length of packing material for centrifugal pumps is less than 1 mm through conservative calculation of the porosity of the packing material. This characteristic length of the packing material can be substituted into the Reynolds number formula.

[0034] In step S3, the actual leakage above the packing is obtained by field measurement, and the Fluent software iterates the packing viscous resistance coefficient when the centrifugal pump is in the initial state based on the calculation formula of the resistance loss along the way, the calculation formula of the local resistance loss, the calculation formula of the total resistance loss and the calculation formula of the leakage.

[0035] The resistance loss along the way is caused by the friction between the fluid layers and between the fluid and the wall when the fluid flows in the pipeline. The calculation formula is:

[0036] h f is the resistance loss along the way, λ is the resistance coefficient along the way, l is the length of the pipeline, d is the inner diameter of the pipeline, v is the fluid velocity, and g is the acceleration of gravity.

[0037] The local resistance loss is caused by the significant changes in the fluid caused by the fluid passing through the local obstacle and the collision between the fluid mass points. The calculation formula is:

[0038] h j is the resistance loss along the way, ξ is the local resistance loss, v is the fluid velocity, and g is the acceleration due to gravity.

[0039] The total resistance loss of the pipeline system is the sum of the resistance loss along the way and the local resistance loss. The fluent software can iterate based on the above formula until the packing viscous resistance coefficient of the centrifugal pump in the initial state is obtained.

[0040] In step S4, the Reynolds number is a dimensionless number that can be used to characterize the flow of the fluid, and its calculation formula is Re=ρvd / μ, where v, ρ, and μ are the flow velocity, density, and viscous resistance coefficient of the packing, respectively, and d is the characteristic length of the packing material. The size of the Reynolds number can be used to describe and determine the movement of the fluid, and is the ratio of the inertial force to the viscous force of the flowing fluid. The characteristic length of the packing material and the viscous resistance coefficient of the packing obtained above can be substituted into it for calculation to obtain the Reynolds number. If the Reynolds number of the centrifugal pump in the initial state is less than 10, the porous medium model and Darcy's law apply to the flow of the shaft seal water through the packing material. The initial state of the pump specifically refers to the state of the pump after the new packing material is replaced.

[0041] Darcy's law describes the linear relationship between the seepage velocity and hydraulic gradient of water in saturated soil, also known as the linear seepage law. The seepage volume Q is proportional to the upstream and downstream head difference (h2-h1) and the cross-sectional area A perpendicular to the water flow direction, and inversely proportional to the seepage length L, that is: Q = K*A*(h2-h1) / L. Since the Reynolds number of the centrifugal pump in the initial state is less than 10, Darcy's law is applicable.

[0042] In step S4, the first operating parameter can be input, and the first operating parameter is the first inlet pressure, the first outlet pressure, the first pump shutdown pressure, the first pump start-up pressure, the first outlet flow rate, and the first fluid medium. The seepage model of the centrifugal pump in the initial state is simulated to obtain a first law schematic diagram of the seepage model, which includes a shaft seal water leakage and shaft seal water inflow pressure curve diagram, a packing structure flow field diagram, and a shaft seal water flow distribution diagram when the centrifugal pump is in the initial state. According to the change law obtained from the shaft seal water leakage and shaft seal water inflow pressure curve diagram, the packing structure flow field diagram, and the shaft seal water flow distribution diagram, the pump shaft seal water pressure and pump packing structure and adaptability of the centrifugal pump can be optimized, and the operation and maintenance strategy of the pump packing structure can be optimized.

[0043] In step S5, when the centrifugal pump is in a worn state, the flow channel of the shaft seal water is the packing material and the gap channel between the packing and the wall. The model of the flow through the packing material is applicable to Darcy's law, and the flow state is a laminar state; the model of the flow through the gap channel between the packing and the wall is applicable to Bernoulli's law and the law of resistance loss, and the flow state is a turbulent state. Laminar flow is a flow state of fluid, which flows in layers. When the fluid flows at a low speed in the tube, it presents laminar flow, and its particles move smoothly in a straight line in a direction parallel to the tube axis. For the laminar motion of viscous fluid, there is no obvious irregular pulsation in the trajectory of the fluid micro-group. There is only momentum exchange caused by molecular thermal motion between adjacent fluid layers. Laminar flow only occurs when the Reynolds number is small, that is, the fluid density ρ, characteristic velocity U and object characteristic length L are very small, or the fluid viscosity μ is very large. When the Reynolds number exceeds a certain critical Reynolds number, the laminar flow begins to transition to irregular turbulence due to disturbance, and the motion resistance increases sharply. The critical Reynolds number mainly depends on the flow form. In addition, turbulence is a flow state of fluid. When the flow velocity is very low, the fluid flows in layers without mixing, which is called laminar flow, also known as steady flow or sheet flow; as the flow velocity increases gradually, the streamlines of the fluid begin to swing in a wave-like manner, and the frequency and amplitude of the swing increase with the increase in flow velocity. This flow condition is called transitional flow; when the flow velocity increases to a very high level, the streamlines are no longer clearly discernible, and there are many small vortices in the flow field, the laminar flow is destroyed, and there is not only sliding but also mixing between adjacent flow layers. At this time, the fluid moves irregularly, and a component velocity is generated perpendicular to the axis of the flow tube. This movement is called turbulence, also known as turbulent flow, disturbed flow or turbulent flow.

[0044] The Bernoulli equation is Among them, p is the pressure at a certain point in the fluid, v is the flow velocity of the fluid at that point, ρ is the fluid density, g is the acceleration due to gravity, h is the height of the point, and C is a constant.

[0045] In addition, the second operating parameters are the second inlet pressure, the second outlet pressure, the second pump shutdown pressure, the second pump start-up pressure, the wear gap, and the second fluid medium. The second law diagram includes a shaft seal water leakage and shaft seal water inflow pressure curve diagram when the centrifugal pump is in a worn state, a packing structure flow field diagram, a shaft seal water flow distribution diagram, and a wear gap change diagram. The change law obtained based on the shaft seal water leakage and shaft seal water inflow pressure curve diagram when the centrifugal pump is in a worn state, the packing structure flow field diagram, the shaft seal water flow distribution diagram, and the wear gap change diagram can optimize the pump shaft seal water pressure and pump packing structure and adaptability of the centrifugal pump, and optimize the operation and maintenance strategy of the pump packing structure.

[0046] Further, in step S5, during the simulation process, when the ratio of the flow rate of the shaft seal water through the packing to the flow rate of the shaft seal water through the wear gap is greater than or equal to the preset value, the fluid passing through the packing is simulated based on the porous medium model and Darcy's law, and the fluid passing through the wear gap is simulated based on the turbulence model. When the ratio of the flow rate of the shaft seal water through the packing to the flow rate of the shaft seal water through the wear gap is less than the preset value, the fluid of the shaft seal water through the packing is ignored, and only the fluid passing through the wear gap is simulated based on the turbulence model. It can be understood that when the wear gap gradually increases, the flow rate of the shaft seal water flowing through the gaps on both sides will gradually be greater than the flow rate of the shaft seal water flowing through the packing. When the ratio of the flow rate of the shaft seal water through the packing to the flow rate of the shaft seal water through the wear gap is less than the preset value, due to the large difference between the two flow rates, the leakage of the shaft seal water flowing through the packing can be ignored, while ensuring accuracy and reducing the amount of simulation calculations. The above preset value can be one percent or one thousandth, which can be set according to actual needs.

[0047] In general, the finite element simulation method for the leakage of the packing of the important plant water centrifugal pump of the nuclear power plant establishes a simulation method for the shaft seal structure of the important plant water pump of the nuclear power plant, which can be used to study the change law of the shaft seal water leakage and the shaft seal water inlet pressure, the packing structure flow field, and the shaft seal water flow distribution of the centrifugal pump in the initial state and the wear state, so as to optimize the pump shaft seal water pressure and the pump packing structure and adaptability, and optimize the operation and maintenance strategy of the pump packing structure.

[0048] It can be understood that the above embodiments only express the preferred implementation modes of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the patent scope of the present invention. It should be pointed out that, for ordinary technicians in this field, without departing from the concept of the present invention, the above technical features can be freely combined, and several deformations and improvements can be made, which all belong to the protection scope of the present invention. Therefore, all equivalent changes and modifications made to the scope of the claims of the present invention should belong to the scope covered by the claims of the present invention.

Claims

1. A finite element simulation method for the leakage of packing of an important centrifugal pump for water supply in a nuclear power plant, characterized in that: Includes steps: S1. According to the design drawings of the centrifugal pump and the actual measurement of the on-site equipment, the basic size parameters of the centrifugal pump are obtained, and the solid model of the pump shaft and packing of the centrifugal pump is established by using the three-dimensional fluid simulation software; S2. Calculate the porosity of the packing material of the centrifugal pump according to the parameter information of the packing forming material of the centrifugal pump and in combination with the measured data of the packing raw material, and conservatively calculate the characteristic length of the packing material of the centrifugal pump according to the porosity of the packing material; S3, obtaining the actual leakage above the packing of the centrifugal pump, and using three-dimensional fluid simulation software to iterate the packing viscous resistance coefficient when the centrifugal pump is in an initial state, the iterative judgment formula is that the calculated leakage above the packing is consistent with the actual leakage above the packing, and the software result is the packing viscous resistance coefficient in the initial state; S4. Based on the Reynolds number formula, the flow rate and flow velocity calculated according to the characteristic length of the packing material of the centrifugal pump and the packing viscous resistance coefficient are used to obtain the Reynolds number of the centrifugal pump in the initial state. If the Reynolds number is less than 10, the porous medium model and Darcy's law are applicable to the flow of the shaft seal water through the packing material. At this time, the first operating parameter of the centrifugal pump is input, and the seepage model of the centrifugal pump in the initial state is simulated to obtain a first regular schematic diagram of the seepage model. S5. Simulate the wear model of the centrifugal pump when it is in a wear state, input the second operating parameters, and modify the model structure under the assumption that the packing viscous resistance coefficient remains unchanged. Start with a gap of 0 mm between the packing structure and the wall, and gradually increase the gap for simulation to obtain a second law schematic diagram of the wear model.

2. The finite element simulation method for the leakage of the packing of the important centrifugal pump for nuclear power plant according to claim 1 is characterized in that: The packing parameter information of the centrifugal pump includes packing raw material density, packing section size, and packing length; The measured data of the packing raw materials include the packing forming density and the measured mass of the packing.

3. The finite element simulation method for the leakage of packing of important centrifugal pumps for nuclear power plants according to claim 1 is characterized in that: The first operating parameters are a first inlet pressure, a first outlet pressure, a first pump shutdown pressure, a first pump start-up pressure, a first outlet flow rate, and a first fluid medium.

4. The finite element simulation method for the leakage of the packing of the important centrifugal pump for nuclear power plant according to claim 1 is characterized in that: The second operating parameters are a second inlet pressure, a second outlet pressure, a second pump shutdown pressure, a second pump startup pressure, a wear gap, and a second fluid medium.

5. The finite element simulation method for leakage of packing of important centrifugal pumps for nuclear power plants according to claim 1 is characterized in that: The first regularity diagram includes a seal water leakage and seal water inflow pressure curve diagram when the centrifugal pump is in an initial state, a packing structure flow field diagram, and a seal water flow distribution diagram.

6. The finite element simulation method for the leakage of packing of an important centrifugal pump for nuclear power plant according to claim 1 is characterized in that: The second law schematic diagram includes a shaft seal water leakage and shaft seal water inflow pressure curve diagram when the centrifugal pump is in a wear state, a packing structure flow field diagram, a shaft seal water flow distribution diagram and a wear clearance change schematic diagram.

7. The finite element simulation method for leakage of packing of important centrifugal pumps for nuclear power plants according to claim 1 is characterized in that: In step S5, when the centrifugal pump is in a worn state, the flow channel of the shaft seal water is the packing material and the gap channel between the packing and the wall. The model of the flow through the packing material is applicable to Darcy's law, and the flow state is laminar; the model of the flow through the gap channel between the packing and the wall is applicable to Bernoulli's law and the law of resistance loss, and the flow state is turbulent.

8. The finite element simulation method for the leakage of the packing of the important centrifugal pump for nuclear power plant according to claim 7 is characterized in that: In step S5, during the simulation process, when the ratio of the flow rate of the shaft seal water through the packing to the flow rate of the shaft seal water through the wear gap is greater than or equal to a preset value, the fluid passing through the packing is simulated based on the porous medium model and Darcy's law, and the fluid passing through the wear gap is simulated based on the turbulence model.

9. The finite element simulation method for leakage of packing of important centrifugal pumps for nuclear power plants according to claim 7, characterized in that: In step S5, during the simulation process, when the ratio of the flow rate of the shaft seal water through the packing to the flow rate of the shaft seal water through the wear gap is less than a preset value, the flow of the shaft seal water through the packing is ignored, and only the fluid passing through the wear gap is simulated based on the turbulence model.

10. The finite element simulation method for leakage of packing of important centrifugal pumps for nuclear power plants according to claim 1, characterized in that: In step S3, the fluent software iterates the packing viscous resistance coefficient when the centrifugal pump is in the initial state based on the along-the-line resistance loss calculation formula, the local resistance loss calculation formula, the total resistance loss calculation formula and the leakage calculation formula.

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