Valve Strength and Seismic Inspection

Valve is an important fluid control device in the industry, involving all walks of life, especially petroleum and petrochemical, liquid transportation and other industries. The emergence of CAD parameterization technology and CAE technology in computer-aided design broke through the disadvantages of traditional calculation methods and greatly promoted valve design and research. Based on the brief introduction of CAD parameterization technology and CAE technology of computer-aided design, this paper takes nuclear power valves as an example and compares and discusses the methods of stress analysis and static seismic inspection of nuclear power valves according to their special requirements such as stress, strength and seismic resistance.
 
Present situation and existing problems of valves

1. Valve characteristics and types
Valves are widely used and used in large quantities. In a group of complete large-scale mechanical devices, the investment of valve equipment accounts for 2%-6%. There are many kinds of valves with great functions, and the performance and quality of valve equipment have an important influence on the normal operation of the whole conveying device or working condition system. Valves can be roughly divided into three categories according to their functions and uses, as shown in Table 1.
 
Table 1 Functions and uses of valves:
 
Cut-off valve: the check valve is used to prevent the transported fluid from flowing back into the reactor or storage container; Prevent suck back caused by a stop or reverse rotation of pump equipment, etc.
The function of the safety valve is that when the equipment or transmission pipeline reaches or even exceeds the required pressure value, it is necessary for the safety valve to discharge pressure in time for safety protection.
The function of the regulating valve is to regulate the flow rate and pressure of the fluid in the device.
Examples: gate valve, closed-circuit valve, needle valve, etc.
 
Vacuum valve: mainly used to control the flow direction of the gas medium in the device system, connect or cut off, control the flow rate and flow rate, etc.
Examples: vacuum ball valve, vacuum flapper valve, vacuum charging valve, etc.
 
Special-purpose valves: valves that have specific purposes or play a specific role in specific occasions to improve efficiency and safety. For example, nuclear power valves can only be used after being verified under different working conditions to ensure nuclear safety.
Examples: blowdown valve, exhaust valve, pigging valve, vent valve and filter.
 
2. Present situation and development trend of foreign valves
Before 1950, many foreign countries had set up valve design units and production industries. In addition, some associations and academic organizations are formed to conduct valve technical exchange, design, research and analysis. Such as the research institutes of the former Soviet Union, the valve industry alliance established by the United States and Britain, and the valve industry association of Japan, etc. These organizations and technology development departments are mainly completed through two modes. Technical innovation and research and development of valve. One of them is the centralized model, in which the Central Research Institute completes the compilation of specifications, the standardized design and production of valves, as well as the testing of new products and new processes. The other mode is to study and analyze the new products and markets of the enterprise, starting from actual production and demand, and aiming at improving performance, safety, service life, high parameters and bad working conditions.
 
In addition, the research department also has a testing center, which can test the performance of new products at any time and put forward the improvement scheme of materials and manufacturing process. All these have promoted the development of the valve industry.
 
3. Current situation and development trend of domestic valves
In recent years, China has established a relatively perfect production and operation system in the field of valves, and finite element analysis and other analytical methods have improved the design capability of valves. However, due to the limitations of production conditions such as production technology and welding technology, some special valves, such as large-diameter valves and nuclear power valves, still, have production problems, and failed to achieve domestic production.
 
At present, there is a certain gap between the industrial structure and product structure of domestic valves and those of developed countries. The valve production in developed countries has the advantages of advanced technology, various types of valves and high production efficiency, among which the most significant one is the low repetition rate of products produced by their enterprises. On the contrary, in China, the most important production equipment and processing technology are similar, and most of the production enterprises only have the ability to produce, process or copy common types and models of valves.
 
It can be seen that the overall production efficiency and technical level of China's valve industry still lags behind developed countries. In order to improve the overall production efficiency and technical level of the valve industry, attention must be paid to the research and development of new technologies and the design of product diversity, mainly in the basic theory research, increasing capital investment, introducing advanced computing technology, perfecting the valve test and inspection standards and focusing on the research of high-parameter and high-performance special valves.
 
(1) Based on the research work of valve basic theory, summarize experience and analysis results, and carry out theoretical innovation and theoretical verification.
 
(2) Integrate the resources of the domestic valve industry, use special funds, increase the investment of talent funds, and avoid unnecessary waste of funds.
 
(3) Focus on the application research of foreign advanced technology and high technology in valve structure design and strength calculation, introduce advanced structure calculation software and manufacturing equipment, and speed up domestic production.
 
(4) Focus on special application valves with high parameters and high performance.
 
(5) Improve valve testing and inspection standards, make data statistics and report analysis on valves with the different system working conditions, evaluate and improve materials and processes, and improve performance and quality.
 
Computer-aided Design CAD Parameterization Technology and CAE Technology

1. CAD parameterization technology
The traditional method of geometric modeling needs to determine the geometric relationship of the model at the early stage of design, and it can only be redesigned if it needs to be modified in the subsequent operation. The fault-tolerant rate is very low, which obviously increases the workload. However, when parametric technology is used to deal with 3D product models with similar or identical structures, the size of design graphics, their cooperation and tolerance can be changed by modifying the parameters, so that the designed products can be automatically generated again, which significantly improves the design efficiency. Parameterized technology can be divided into the stages of technology enlightenment, creation, development and application (Table 2).
 
Table 2 Development process of parameterized technology:

Technological enlightenment stage: Sutherland put forward the concept and viewpoint of the parametric system from 1960 to 1970.
Technical creation stage: From 1970 to 1980, Hilliard put forward a new viewpoint: the coordination of dimensions and tolerances is regarded as a constraint on features.
Technical development stage: feature-based modeling technology from 1980 to 1990: applying artificial intelligence methods such as geometric reasoning and grid to parametric design.
Meter and solid modeling
Technical application stage: From 1990 to the present, Lee Jae Yeol, Xiao-Shanga put forward geometric reasoning method and constraint propagation method.
 
2. CAE technology
The CAE concept of computer-aided engineering was put forward in 1943. It began to be used and calculated in the 1970s, and gradually became commercialized in the 1980s. CAE mainly uses the increasingly powerful computing function of computers to analyze and solve the geometric model or optimize the model and its performance. It can be used for static structural analysis and dynamic analysis, studying linear and nonlinear problems, and analyzing structure (solid), fluid, electromagnetism, etc. In recent 30 years, the finite element analysis method has replaced the formula provided by traditional basic theories such as mechanics of materials, theoretical mechanics and elasticity to calculate. In order to solve complex engineering analysis and calculation problems, the main software is ANSYS, NASTRAN, ABAQUS and ADINA, etc.
 
3. The influence of parametric technology and CAE technology on the domestic valve industry
The domestic valve industry has applied parameterization technology and CAE technology to the calculation and analysis of parts and equipment and their products (Table 3), and there are some relatively perfect system devices for valve design parameterization and CAE, but these systems also have some shortcomings. For example, some systems can only realize simple parametric drawing, which is not suitable for extensive use and large-scale promotion. There are also some valve parametric design systems. Although they reflect the design of individual valves, they cannot complete the design of serialized products. There are also some systems, although the system modules are relatively mature, they lack parametric assembly, and can't automatically check the designed valves for interference.
 
Table 3 Application of parameterized technology and CAE technology in the domestic valve industry:

150 LB, 300LB series gate valves:
Advantages: The parametric design system VCADS is simple, efficient, convenient and quick.
Shortcomings: The scope of application is small and it is not universal.
 
Calculate and analyze individual parts and the whole set of equipment;
Advantages: The design system includes parametric design of products, stress check, strength analysis, technical document generation and other functions.
Shortcomings: lack of parametric assembly module, and no consideration of CAE for checking.
 
Mainly ball valves and butterfly valves:
Advantages: parametric drawing and CAE analysis are realized.
Shortcomings: Parametric drawing is not perfect, and parametric assembly is not considered.
 
The above is only aimed at the common problems existing in domestic valve design parameterization and CAE technology, and for some valves with special requirements, due to the more complicated operating conditions and higher requirements for stress and strength of valves, how to apply valve design parameterization and CAE technology to special valves, such as nuclear power valves and large-caliber steam valves, etc., to reduce dependence on imported valves and realize domestic independent innovation and localization is the most important thing.
 
Mechanical Analysis and Seismic Inspection Method of the Nuclear Power Valves

1. Force analysis of nuclear power valve
At present, the design of domestic nuclear valves is mainly based on American ASME, French RCC-M and other regulations. The standard specifies the qualification requirements and criteria for nuclear-grade I valves, nuclear grade II and III valves. Many load conditions, such as internal pressure, dead weight, pipeline reaction force, earthquake load, temperature effect, etc., are considered separately, and according to different combinations of the above loads, they are divided into five working conditions, such as design reference working condition, operation working condition, emergency working condition, accident working condition and water pressure test, as shown in Table 4. The working conditions of the valve in a nuclear power plant are complex, and the load to be input in each working condition is different, and the qualification standard should be adjusted accordingly. This requires that the film stress, bending stress, primary stress, secondary stress, peak stress, thermal stress and total stress of the valve pressure-bearing parts should be calculated separately for each working condition, which is difficult.
 
Table 4 Environmental load corresponding to each working condition and discrimination basis:

Normal design conditions:
Load: design pressure, design temperature and mechanical load.
Eligible standard: prevent: excessive deformation, plastic instability, elastic and elastic-plastic instability.
Class: ASME: Class A RCC-M: Class O
 
Abnormal working conditions:
Load: transient pressure, thermal load, nozzle load, etc.
Qualified standard: prevention: progressive deformation and fatigue.
Correspondence: ASME: Class B, RCC-M: Class B.
 
Dangerous (urgent) working conditions:
Load: transient pressure, thermal load, nozzle load, etc.
Qualified standard: Prevent excessive deformation, plastic instability, elastic and elastoplastic instability, but the safety margin is small.
Correspondence: ASME: Class C, RCC-M: Class C.
 
Accident condition:
Load: transient pressure, thermal load, nozzle load (including LOCA), SSE
Eligibility standard: prevent: elastic and elastic-plastic instability (equivalent to the loss of pressure boundary integrity), but do not rule out excessive deformation.
Correspondence: ASME: Class D, RCC-M: Class D
 
Test conditions:
Load: test pressure
Eligible standard: prevent: excessive deformation, plastic instability, elastic and elastic-plastic instability.
Class: ASME: Test, RCC-M: Test
 
In addition to the complex working conditions, there are different methods for calculating the strength of nuclear valves at present, the main methods are the computer-aided finite element analysis (CAE) of ASME code in the United States and the RCC-M code in France. There are some differences between ASME Code and RCC-M Code in the corresponding calculation formulas for different nuclear safety levels. At present, there is no complete specification applicable to the actual situation of nuclear power valve engineering in China. This section will find out its characteristics and differences by comparing several methods.
Different. It is roughly the same as the part of the ASME RCC-M code about the calculation method of the strength of the first-class nuclear valve, but it is also different from the whole calculation idea. For example, the criteria for defining large valves and small valves are different. In ASME, the nominal diameter size is used as the criterion, while in RCC-M, the inner diameter size of valves is used as the criterion. This means that in the process of valve strength calculation, some calculation data and judgment conditions are different. There are many similar regulations, and we can compare and analyze the strength calculation methods of the first-class valve in the two codes around the determination of minimum wall thickness and stress evaluation.
 
(1) Determination method of minimum wall thickness
The method for determining the minimum wall thickness of nuclear first-class valves in the ASME RCC-M code is the same, but the referenced data are different, as shown in Table 5.
 
Table 5 Provisions of minimum wall thickness in ASME and RCC-M:

Valve pressure class:
ASME: Standard class valves, special class valves and limited class valves, with different data respectively.
RCC-M: There is no classification of valve pressure class.
 
Valve material:
ASME: different materials of valves are specified according to the pressure class.
RCC-M: There is only one table corresponding to the same material when determining the pressure grade of the valve.
 
Minimum wall thickness value:
ASME: At the same pressure, the minimum values are similar but different.
 
Wall thickness error value:
ASME: with dimensional parameter table
RCC-M: It is obtained by the linear interpolation method.
 
In ASME and RCC-M codes, although the minimum wall thickness values are similar at the same pressure, there are great differences in the pressure class, valve materials, wall thickness error values, etc. when determining the minimum wall thickness, which means that there will be differences in the strength calculation process. ASME codes are more detailed in confirming the minimum wall thickness values. The method for determining the minimum wall thickness of nuclear secondary valves in the ASME RCC-M code is roughly the same. The pressure grade specified in the RCC-M code for the minimum wall thickness of the valve body is listed with seven values like the ASME code. However, the error of the minimum wall thickness obtained by the linear interpolation method is relatively large because the detailed calculation process is not given.
 
(2) Stress assessment method
Firstly, the similarities and differences between ASME and RCC-M codes on stress assessment methods of nuclear grade I valves are analyzed.
1. All codes stipulate that the sum of film stress and bending stress should be evaluated once, but the ASME code does not specify this part in detail.
2. In the RCC-M code, the primary stress and secondary stress of nuclear first-class valves are divided according to different situations, but there is no subdivision in the ASME code, and the calculation formulas of the maximum value of primary plus secondary stress are different.
3. Cyclic load should also be considered in ASME Code as a part of fatigue analysis. Its treatment method is roughly the same as that in RCC-M Code except for system start-up and stops conditions, and its calculation form is the same, but the coefficient before parameters is disproportionate.
 
Secondly, the similarities and differences of stress assessment methods for nuclear secondary valves in ASME and RCC-M codes are shown in Table 6.
 
Table 6 Comparison of evaluation values of nuclear secondary valves:

Use load class a:
Q: Stress limit ASME：1. 0 N
Stress limit RCC-m: 1. 0 n
W+M: stress limit ASME: 1. 5n
Stress limit RCC-m: 1. 5 N
 
Use load class b:
Q: Stress limit ASME：1.1N
Stress limit RCC-m: 1.1n
W+M: stress limit ASME：1.65N
Stress limit RCC-m: 1.65 N
 
Service load class c:
Q: Stress limit ASME：1.5N
Stress limit RCC-m: 1.5n
W+M: stress limit ASME：1.8N
Stress limit RCC-m: 2.25 N
 
Service load class d:
Q: Stress limit ASME：2.0N
Stress limit RCC-m: 2.0n
W+M: stress limit ASME：2.4N
Stress limit RCC-m: 3.0n
 
Note: Q is the overall primary film stress; W is the primary film stress; M is bending stress.
Force; N is the allowable stress of the material
 
From Table 6, it can be seen that in ASME and RCC-M codes, the corresponding stress limits for stress assessment of Class A and Class B valves are the same, but the stress limits for Class C and Class D are different, and the difference is quite large.
In view of the strength check of nuclear secondary valves, if the stress assessment is complicated under special circumstances, then the ASME code can also use pressure to assess whether it is qualified, that is, the requirements in ASME code are flexible, but there is no corresponding alternative in RCC-M code.
 
On the whole, the RCC-M code has stricter requirements for stress assessment of nuclear valves, while the ASME code has more conservative requirements.
 
2. Static seismic inspection of nuclear power valves

(1) inspection method
The static method and dynamic test method are mainly used in the seismic test of nuclear power valves. In the duration of 20 ~ 30 s, the input excitation synthesized by different accelerations in a wide frequency range of 0 ~ 33 Hz is input to replace the earthquake for seismic inspection. The excitation has a duration of 20 ~ 30 s. Because the force of this excitation has dynamic characteristics, the static method can not fully display this dynamic process, which is the limitation of the static method. However, due to a large number of nuclear power valves, many positions and different heights, it is difficult to input earthquake load. If it can be determined that the fundamental frequency of the equipment is greater than 33 Hz, the static method can be used for the seismic test, which undoubtedly saves a lot of time and improves efficiency. Therefore, determining the fundamental frequency of the equipment becomes an important issue.
 
At present, there are mainly calculation methods and test methods to determine the fundamental frequency of equipment. The calculation method mainly uses the powerful processing and operation ability of the computer, uses the finite element analysis method to check the earthquake resistance of the valve, and uses the program to carry out modal analysis to determine the fundamental frequency.
 
(2) Test method
 
Tapping method. The tapping method is to tap the typical parts, such as the extension mechanism, with a small hammer as input, and take its frequency response characteristics. But it also has its disadvantages. When the natural frequencies are dense, this method is not easy to measure accurately.
 
Frequency sweep method. The frequency sweep method is to use sinusoidal sweep wave to input from the table according to 1→50→1 Hz along with the three orthogonal directions of the valve, and the maximum input acceleration is less than or equal to 0. 2 g and the acceleration sensor is arranged at the typical parts of the valve for measurement. If the output response acceleration value is equal to or greater than 3 times or more of the input acceleration value, it is proved that the frequency at this time is the natural frequency. If the output response acceleration value does not exceed 5% of the input acceleration, it can be considered that the valve has no natural frequency in this frequency domain.
 
White noise method. The white noise method is used to replace the sinusoidal sweep input of the sweep method, and the input amplitude is constant and the duration is prolonged. After the experimental results are measured by the seismic pickup, the frequency domain is obtained by Fourier transform, and then the natural frequency is identified. Generally speaking, for valves with DN > 200 mm, most of them can be calculated by static method, but for valves, with DN≤100 mm, static method should not be adopted, but dynamic test method should be adopted. On the one hand, it is the need of its own dynamic characteristics; on the other hand, it is not difficult to conduct dynamic tests with the shaking table, which can meet the installation and load conditions of the testbed, that is, it does not exceed the installation and load limits of the testbed.
 
Prospect

(1) Further speed up the application and understanding of CAD parametric technology and CAE technology to valve design, speed up the improvement of the parametric system and parametric assembly technology, and improve the valve design ability.
(2) The strength calculation method of nuclear grade valves can be carried out according to the general framework of RCC-M code and ASME code, and the specific details and problems need to be dealt with according to their engineering experience and actual needs. At the same time, the materials used, welding and forging process, error control range, corrosion allowance and so on are different, which need to be completed through a lot of tests and investigations.
(3) For the seismic analysis of valves, the approximation of the calculation model and the error in the test should be considered, and both the test measurement method and the calculation method should be used for calculation, and the results should be compared and analyzed to find the similarities and errors, so as to provide ideas for better design schemes.