| In the process of piping fluid, the control valve regulates the flow and the pressure of a medium to control fluid pulse. Along with the development of the industry, new demands have been presented on the performance of the control valves, such as greater flow, higher pressure, etc. Usually, the security and the reliability of control valve are ensured by preventive maintenance, which can result in a "maintenance-surplus" problem. The maintenance surplus could result in some adverse effects, such as larger maintenance costs, reduced reliability and increased failure rate.On the other hand, the control valves are in great demand in the domestic industry. But the fault diagnosis research of the control valve is still insufficient in China. Correspondingly, there are few studies of gas pressure valve, especially fault diagnosis of the reports about large valve. The most advanced technology of high-performance control valve is monopolized by overseas manufactory, and many control valves used in the key portion are of high price, as much as thousands of, or even millions of Chinese yuan. Therefore, much work has to be done on fault diagnosis method of the large control valve, which includes theoretical research and practical application.The aim of this paper is to study the control valve, which regulates combination gas pressure, and to determine its fault detection and diagnosis method, and then to ensure the reliability of the control valve. In this way, the maintenance-surplus problem could be resolved, and scientific basis and technological support are provided in order to realize the localization of the application for the large control valve with high performance.Fault diagnosis method is mainly divided into:analytic model-based method, signal processing method, and knowledge-based method. Combined with the characteristics of the three methods, the system of control valve is analyzed. First, due to the particularity of the application situation, it is difficult to extract failure characteristics when the accident occurs in normal operation condition. In addition, the method using direct signal detection cannot discriminate against the reasons of abnormal signals for the flow pulsation. Second, the acquisitions of knowledge used for fault diagnosis become the bottleneck of realizing fault diagnosis when developing new products. Third, medium flow in control valves follows the conservation laws of physics, so the mathematical description of the system, by which the math models of control valve are established, can be determined in the view of fluid dynamics.Therefore, it is feasible to realize fault detection and diagnosis of the control valves using the residual signals. The residual results from the redundancy, which is generated from the mathematic mold and real system. The paper works on the research of system theory and experiment for model based fault diagnosis method of the control valve, which regulates mixed gas flow. The work provids the important theoretical basis and technical support for the application of fault detection and diagnosis method.As to the model-based method, the mathematical model is the key problem. The method of "Grey box" theory is used to establish a mathematic model of the control valves system. Using "Grey box" theory means to combine theoretical modeling method with identify modeling method. In other words, one part of the mechanism which is known uses the theoretical modeling method, and the other part of the mechanism which is unknown uses the identify modeling method. According to system identification standards, the mathematic model should be gained. Input and output data are the foundation of system identification.Due to the particularity of working environment for large control valve, it is difficult to achieve data by the method of prototype test in the laboratory environment. It is effective to overcome the limitations of laboratory conditions to research flow field of the control valve by the method of computational fluid dynamics. According to the present approximation for variable input/output data, it is viable to establish mathematic models of the control valve system by method of the least square identification.Through analyzing the component's data of the blended gas related data, the rules and method are given in the paper, by which the characteristics of the combined gas in the flow field of the control valve could be discriminated. It is proven effective in practice to use the method of automatic rapid3-D modeling for Flow Field to improve modeling efficiency. The method of block grid is useful to get a better quality of grid mesh. The control equations of flow field about the control valve are established, and then the methods of discretization and numerical calculation are determined. The tests, including normal condition, leakage and block fault states have been done based on the above work.According to the. results of the tests, the relationship between input and output variables has been gained by a numerical analysis in order to establish the type of system identification model. The transient model is based on pressure, and the opening is given by the method of system identification in the paper. In additional, the control model of sampling time for outlet pressure is gained by analyzing the pressure loss and opening change similarly. Comparing the results of normal and faults state, it has been found that there are the differences between detect value and test value for outlet pressure in different states. The differences present a trend, by which the fault detects and' diagnosis for control valves can be realized.The tests in the laboratory have been done in order to prove the validity of the transient model and the reliability of CFD method. The main contents are choosing the equipment and pipes in experimental platform, and ensuring the size of structure and installation. While according to the size, it is necessary to establish the3D models and mesh models for the control valves in different openings. Through the results of the CFD test for the control valve in small size, the mathematic model for small-size valve could be gained. The theory and method of fault detect and diagnoses are submitted, which is through the model-based method for the large control valve. The emphases are the principle of residual design and design of a fault indicator. According to detailed steps of the fault detection and diagnosis, it is needed to analyze the function of hardware and software in order to select the development platform of software and ensure the data flow when a fault detection and diagnosis is going on. The guidelines of development and application are given therewith; of which are the fault detection and diagnosis of control valve with the large flow based on a transient model.This research is based upon work supported by National High-tech Research Development Program of China under Grant No.2008AA04Z130and by Specialized Research Fund for the Doctoral Program of Higher Education under Grant No.20110131110042.
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