Abstract: The valve is the control device of the fluid pipeline. Its basic function is to connect or block the circulation of the pipeline medium, change the moving direction of the medium, regulate the pressure and flow of the medium, and maintain the normal operation of the pipeline equipment. With the continuous development of modern industry, the demand for valves is increasing. Because of the large amount of valves used, valve leakage caused by various reasons not only has a serious impact on the environment, but also causes serious waste of raw materials and products, and sometimes causes serious safety accidents.
Key words: valve design; basic theory 1 design thinking factor valve design is a design activity that considers the factors of process, device, performance and cost. The process and equipment are guaranteed, the function is the purpose, the performance and the cost are the optimization goals. Only by considering various factors can we design a more satisfactory design. In order to facilitate the completion of the process, a standardized design can be implemented for the similar structural features of the product, which facilitates the borrowing of the tool holder. The designed device elements, that is, the multiple parts of the design solution can be successfully assembled, and the parts to be assembled must have the device surface (hexagonal or external hexagon, etc.), and the parts structure that is easy to assemble is used as much as possible (for example, the assembly does not need to distinguish the assembly direction symmetry) Structure, etc.). For parts of similar size, try to make common parts for convenient management; make differences for parts that cannot be shared, and prevent mixing. The functional elements of the design are the basic functions that the product is developed, such as cut-off, voltage regulation, prevention of backflow, shunting or overflow relief. In addition, the valve products should be as simple as possible to facilitate the disassembly and protection of the device. For regulating valves, it should also be designed to prevent misoperation.
The performance factor of the design is the safety and reliability of the product, and it will not cause damage to the user or the use environment. It has certain strength requirements, such as mechanical properties (torque, bending moment), pressure resistance (low pressure, high pressure), Fatigue strength (hot and cold water circulation, anti-pulsation, life), environmental simulation applicability, etc. The cost factor of the design, that is, to realize the technical function and meet the predetermined technical performance requirements, optimize the structure as much as possible, realize the product structure characteristics with as few parts as possible, realize the parts processing and production with the simplest processing technology, and reduce the project cost and product. Price has a positive effect on society (avoiding environmental pollution, maintaining ecological balance, and promoting productivity development).
2 Valve design 2.1 Housing wall thickness design In the design process of the pressure bearing shell of the valve, the designer must fully consider the maximum pressure value that the valve can withstand during the actual operation, and design the maximum value as the resistance of the housing. Pressure rating to ensure the scientific and rational design of the housing.
2.2 Valve body and bonnet sealing structure design 2.2.1 Forced sealing forced sealing structure, its design principle is: tighten the flange bolt, generate a pressure on the sealing gasket, make the gasket compressive deformation, and then seal the structure The gap in the middle surface is filled to provide a sealing effect.
2.2.2 Wood Seal Wood Seal Structure, which is mainly composed of the following parts: valve body, floating valve cover, sealing ring, gasket, four open ring, support ring, and pinned bolt. Before the boosting operation is performed, the operator is required to pre-tension the bolt to move the floating valve cover upward, thereby ensuring a sealing force between the valve cover and the elastic wedge pad. When the seal is pressed, the bonnet will have an upward trend. At this time, the seal specific pressure between the bonnet and the wedge seal will increase as the pressure increases, and the desired sealing effect is finally achieved. Where the valve body is in contact with the sealing ring, in order to improve the hardness of the sealing structure, a hardfacing alloy can be used to achieve the corresponding purpose.
2.3 Sealing design In the design process of the valve seat, the sealing surface of the soft sealing valve is generally made of rubber, PTFE (or RPTFE), nylon or PEEK (polyetheretherketone) as the sealing material, and the design sealing ratio of the valve should be guaranteed. The pressure is not less than the sealing specific pressure of the material. For hard-sealed valves, the sealing surface needs to be surfacing or spray-welded (STL) to ensure the strength and performance of the sealing surface. In the design process of the filler part, the designer preferably uses a flexible graphite clamp stainless steel wire to prepare the filler to properly strengthen the filler. In some special working conditions, the dynamic load structure loaded by the disc spring should be used.
2.4 Design of the connecting bolt First, the designer needs to ensure that the bolt will not be threaded and dead under the working environment; secondly, due to the particularity of the working environment, it is better to choose the coarse thread when designing the bolt, and It is also necessary to increase the clearance of the thread; thirdly, in order to avoid the phenomenon that the bolt is relatively loose when the bolt is actually working, the designer needs to ensure that the remaining preload force exceeds the actual demand value during the design of the specific scheme. This ensures that the connection is stable and, if necessary, the tightening torque of the bolt.
3 Several key technologies in valve design 3.1 Thermal expansion In the process of designing valves, especially the design of high temperature valves, thermal expansion is an important physical phenomenon of materials. The amount of expansion directly determines whether the valve can work effectively. Therefore, designers need to take into account the amount of thermal expansion of the material and the differential thermal load of the component. At the same time, because there are differences in the heat dissipation conditions of different parts of the valve, the amount of thermal expansion is also different. This requires the specific analysis of the designer's specific situation, and in the case of changes in the working clearance of the valve parts, to ensure that there will be no jams, scratches, etc. between the parts, and minimize damage to the parts caused by high temperature.
3.2 Heat exchange In the process of the designer's actual design, it is necessary to specify that the joint between the valve seat and the support member is welded or threaded to ensure its tightness. In addition, large-diameter valves need to be surfacing the valve seat, so that the valve parts that are in contact with the high-temperature medium are prevented from being affected by the alternating stress and excessive fatigue occurs. At the same time, the influence of thermal alternation on the structure of the valve seat and the evaluation of the effect should be taken into account, thereby reducing the influence of heat exchange on the valve design and prolonging the service life of the valve.
In short, when designing the valve, the design input requirements should be fully defined, the product model should be determined, various factors should be considered in the product design, and the product structure should be optimized through risk analysis to reduce or reduce the risk of failure and improve the quality of the valve product.
references:
[1] Zhang Xiao, Yu Xiaoming, Zhu Xiangzheng et al. Integrated design and analysis of ball valve sphere and valve stem [J]. Machinery Manufacturing, 2014(01).
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