Design Reliability Analysis and Quality Control of Nuclear Pressure Reducing Valve

In order to improve the localization level of nuclear power and solve the “neck” problem of nuclear pressure reducing valve in CAP1400 project, the design reliability analysis and quality control requirements of domestic prototype of nuclear pressure reducing valve was studied. Through the analysis and application of the technical requirements of the pressure reducing valve, the control was strengthened from the reliability requirements of seismic analysis, identification test and quality control points, which provided a strong technical support for the localization of nuclear grade pressure reducing valve. The seismic analysis required that the stress, resonance frequency and deformation analysis on the dangerous section of the main parts of nuclear pressure reducing valve met the requirements of regulations and standards. The reliability control was carried out from three aspects: the selection of test medium, the stability control of pressure behind the valve and the requirements of failure operation performance. Quality point control taked quality control measures from three aspects: tooling design, standard application and test bench management. All measurements ensured that the design quality of nuclear pressure reducing valve meets the requirements. The successful development of the domestic prototype of nuclear grade pressure reducing valve shows the effectiveness of the design reliability analysis and quality control method, which provides a reference for the localization of equipment in the future.


Introduction
The nuclear grade pressure reducing valve in the AP1000 project adopted imported equipment.In order to avoid international constraints on nuclear grade equipment, furtherly improve the localization level of nuclear power equipment, and solve the "bottleneck" problem of important nuclear power equipment, China had accelerated the development of domestic nuclear grade pressure reducing valves [1][2][3].The design and qualification of the first domestically produced prototype of a nuclear grade pressure reducing valve faced many problems [4][5][6].In order to improve the design and manufacturing quality of the nuclear grade pressure reducing valve prototype [7][8], this article proposes reliability control requirements and measures for the design and manufacturing quality of the pressure reducing valve from three dimensions: seismic analysis, qualification testing, and quality control points, and implements strict quality control during the design and manufacturing process.At present, many design and manufacturing quality control measures in the R&D stage of domestic equipment have achieved significant results [9][10][11][12][13].Under the control of design quality reliability, the successful development of the pressure reducing valve prototype has made a significant breakthrough in the localization of pressure reducing valves.

Seismic Analysis Reliability Control
China's nuclear power units require equivalent static loads to be applied simultaneously along three mutually perpendicular axis directions (X, Y, Z) at the center of the valve assembly to evaluate the seismic performance of the valve, and to evaluate the combined torque and load on each hazardous section of the valve assembly.During the seismic analysis process, it should be ensured that the resonance frequency of the three mutually perpendicular axis directions is greater than 33Hz.The mass and volume of the pressure reducing valve prototype are not large, and there is no driving mechanism.After seismic analysis and calculation, under Class A, B, C, and D operating conditions, the stress on the dangerous section of the main components meets the requirements, and the resonance frequency in all three directions is greater than 33Hz.
In the process of seismic analysis, it is necessary to consider conducting operability analysis of the pressure reducing valve under A-level, B-level, C-level, and D-level operating conditions, that is, conducting deformation analysis of the moving components of the pressure reducing valve.After analysis, the deformation of the moving parts of the pressure reducing valve prototype is the largest under D-level working conditions, with a deformation amount of 0.009mm.According to the dimensions and shape tolerance requirements of the pressure reducing valve component drawings, the minimum gap between the moving parts and the stationary parts is 0.1mm, which is much greater than the deformation of the moving parts under D-level working conditions.Therefore, when ensuring the normal machining size and assembly accuracy of components, the operability requirements of the pressure reducing valve can be guaranteed.
Through calculation, the resonance frequency of the pressure reducing valve prototype in the X, Y, and Z directions is greater than 33Hz.The deformation accuracy of the moving components under the most severe working conditions meets the requirements, and the reliability control of seismic analysis meets the development requirements.

Reliability Control of Qualification Testing
There are generally three methods for design verification: design review, other calculation methods, and qualification tests.Due to the fact that the pressure reducing valve is a domestically produced project, in addition to using preliminary design calculations, the first prototype also needs to undergo design qualification tests.
Based on the review of the qualification test outline and combined with the test process, this article conducts reliability control on the following three aspects of test requirements.

Selection of Identification Test Media
According to the requirements of the working environment and specifications of the pressure reducing valve, the circulating medium of the nuclear grade pressure reducing valve is compressed air.Due to the limitations of the air pressure capacity of the appraisal testing unit, the pressure reducing valve shell pressure resistance strength test, sealing test, flow load characteristic test, pressure load characteristic test, rated flow coefficient test, cold cycle operation test, and other test media are considered to be selected according to the standard requirements.
If conducting a test medium conversion, it is necessary to ensure that the valve Kv value is consistent.Due to the pressure accuracy deviation of the pressure reducing valve prototype mainly reflected in the opening of the valve core, the test medium conversion can be carried out while ensuring that the Kv value of the valve after compressed air and water medium conversion is consistent, that is, the opening change of the valve is consistent.The specific transformation ideas are as follows: If the medium is compressed air, the inlet pressure is 1.8MPa, the outlet pressure is 0.83MPa, and the flow rate is 119Nm 3 /h, the calculated Kv value is 0.4; If the medium is water, the inlet pressure is 1.8MPa, the outlet pressure is 0.83MPa, and the flow rate is 0.28Nm 3 /h, the calculated Kv value is also 0.4.
On the premise of maintaining the same Kv value of the valve during medium conversion, the pressure reducing valve shell pressure resistance strength test, sealing test, flow load characteristic test, pressure load characteristic test, rated flow coefficient test, cold cycle operation test, and other test media are changed from compressed gas to water, and the test results meet the requirements.

Stability Control of Pressure behind the Valve
The nuclear grade pressure reducing valve is a normally open valve, ensuring stable pressure behind the valve is crucial for the stable operation of the system.The design specification for the pressure reducing valve specifies that the pressure after the valve is 0.83MPa, with a variation of ± 5% (41KPa), which is 0.79MPa -0.87MPa.During the pressure regulating test of the pressure reducing valve prototype, the pressure in front of the valve continuously changes, while the pressure behind the valve remains basically stable at 0.83MPa.However, the technical specifications issued by the upstream design institute do not clearly specify the pressure change value after the valve during the operability assessment during seismic conditions, and the stability of the pressure after the pressure reducing valve under seismic conditions is also an important aspect of whether the prototype can pass the verification smoothly.Through communication with the design institute, during the entire seismic test process, the pressure reducing valve prototype was loaded at 1.38MPa in front of the valve, with compressed air as the loading medium.By recording and tracking the pressure changes after the valve, it was found that during the seismic test process, the pressure changes after the valve ranged from 0.78MPa to 0.89MPa, with a variation of [-6%,+7%].After the experiment, analysis was conducted and it was found that compared to the pressure regulating test, the valve front pressure pipeline (see Figure 1) used in the seismic test is thinner than the valve front pressure pipeline (see Figure 2) used in the pressure regulating test, and the vibration of the seismic test bench is greater, which may cause fluctuations in the valve rear pressure.

Performance Requirements for Failure Operation
The inlet pressure fluctuation range of the pressure reducing valve prototype is large, while the outlet pressure accuracy requirement is high, only ± 5%.During the operation of the valve, the inlet pressure changes from the highest 27.58MPa to 1.38MPa.The large imbalance force generated by the inlet pressure on the valve core can disrupt the established force balance of the pressure reducing valve core.If only a first stage pressure reducing valve is designed, it may cause a large deviation in the outlet pressure, which may not meet the requirements.
The technical specifications of the pressure reducing valve prototype require that it should be prevented from fully closing in a failed state, and the prototype is designed as a two-stage pressure reduction.If the first or second stage of the valve fails, the remaining operable stage should also ensure that the outlet pressure is limited to below 2.07MPa.The first stage pressure reducing outlet pressure of the pressure reducing valve prototype is designed to be 1.4MPa, and the second stage pressure reducing outlet pressure is 0.83MPa.Each stage valve core is designed as a self balancing structure, so that the force on the valve core is almost unaffected by changes in inlet pressure, in order to improve the stability of outlet pressure and meet the requirements of the technical specification.
By controlling the reliability of three aspects of identification testing, the various identification tests of the nuclear grade pressure reducing valve prototype were successfully carried out, and the test results met the development requirements.

Quality Point Reliability Control
According to the reliability control requirements for seismic analysis and qualification testing mentioned above, in the design and manufacturing process of pressure reducing valves, the H-point is used to control the following quality control points: assembly, all qualification testing items.Based on the requirements of identification testing, the control measures for quality control points are as follows:

Strengthen Tooling Design
Sealing test is a test to verify the amount of sealing leakage, and the prerequisite for ensuring the smooth progress of the test is that the tooling does not leak.During the first sealing test of the prototype, about 5 minutes after the start of the test, there was a leak in the valve front tooling, causing the system to release pressure and the test was aborted.The testers successfully passed the second sealing test by adjusting the valve front tooling.After the experiment is completed, the design of the test fixture needs to be taken as a corrective measure and included in the improvement items of the experiment.Therefore, strengthening the design and improvement of tooling, reducing the impact of tooling on testing, is a necessary condition to ensure that the prototype successfully passes all identification testing projects.

Strengthen Standard Training and Application
As a domestically produced prototype of CAP1400, the pressure reducing valve is facing the application and transformation of standards.All qualification test items should not only meet the requirements of the technical specifications, but also meet the relevant domestic and foreign standards involved.Especially for the acceptance limits of identification tests, the requirements of domestic and foreign standards and specifications should be fully referenced.The flow characteristic test of the pressure reducing valve prototype design was conducted.The evaluation criteria for the early test outline were "negative deviation value of its outlet pressure".
During the test, it was found that there was a positive and negative difference in the test pressure.After verification, the standard used for the test did not require a negative deviation value, only a deviation of no more than 20% was required.By taking corrective measures, quality control personnel have found that designers did not fully digest and absorb the standard requirements when preparing the qualification test outline in the early stage.Therefore, the qualification test outline should indicate the basis for standard specifications.While strengthening the training of design on standards, the enthusiasm of the personnel responsible for preparing and approving the qualification outline should be fully utilized, and the application and feedback of standards should be strengthened.

Test Bench Management
In addition to standard support, appraisal test projects also require a good test bench as a carrier.The prototype factory test bench is relatively simple and can be assembled whenever needed; However, flow characteristic tests, seismic tests, etc. require regular management and maintenance of the test bench.Taking the flow load characteristics as an example (see Figure 3), explain the quality reliability control requirements of the test bench during the pressure reducing valve test process.Firstly, the design quality and reliability of the test bench itself need to meet basic requirements.For example, the diameter of the pressure reducing valve prototype is DN25, and the pipeline diameter of the test bench used is not large.During the operation of the test bench, pipeline vibration causes instrument vibration, and the instrument cannot directly read data.Therefore, it is necessary to manually stabilize the instrument reading.This situation not only prolongs the test time, but also affects the normal service life of the instrument; Secondly, the selection of instruments should also meet the requirements.The selection of measuring ranges for instruments (such as pressure gauges before and after valves) and the validity of instrument identification certificates should be verified during the design and test preparation stages; Finally, it is necessary to strengthen the recording of test bench information.During the testing process, in addition to the information required for testing, the information of the bench during the testing process should also be sorted out to provide reference for the maintenance and upkeep of the bench.
By taking measures to control the quality of the above three aspects, the reliability of the nuclear grade pressure reducing valve in terms of human, machine, material, method, and environment has been improved, ensuring that the prototype passes the appraisal smoothly.

Conclusions
Through the reliability analysis and control of seismic analysis, identification testing, and quality control points in three dimensions, on the basis of meeting the technical specifications and standard specifications, the design and manufacturing of the domestically produced prototype of the pressure reducing valve have achieved complete success, laying a solid foundation for the future domestic application of the pressure reducing valve.

Figure 3 .
Figure 3. Schematic diagram of flow load characteristic test device [1] Design and construction rules for mechanical components of PWR nuclear islands: RCC-M Edistion 2002+2002 addenda [M].