The pipeline in coal chemical industry and polysilicon production system has characters of high pressure and alternative change. The valve opens frequently, running multiple circular opening and closing for a long time. IF the full-stroke fast switching time of valve is rapid, it’s easy to cause the failure of valves
. There is a severe test to each performance of the valve. Yield strength design principles measured by conventional static load can not as the only strength index. It is necessary to consider the relevant fatigue design principles. This article take the valve stem as an example to discuss the design method of fatigue analysis.
Under the action of alternating stress, even if the component stress below the yield strength, with the long-term effect after repeated action, can also break suddenly. Even the relative better plastic material, there is no obvious plastic deformation before fracture. This phenomenon is called fatigue failure.
The metal structure will not change because of the alternating stress. From the micro structure analysis, under the alternating stress that is large enough, the weakest crystal or the crystal which is in the most unfavorable position forms slip band along the direction of maximum shear stress. Slip band cracking lead to micro cracks crack. Component shape mutation (rounded corners, incision, groove) , cut marks on the surface, internal material inhomogeneity and defective parts, also cause micro crack because of the large stress concentration. Dispersed micro crack after agglomeration, forms the macroscopic crack. The formed macroscopic crack gradually expands under the action of alternating stress, continuing or stopping from time to time with stress level of high or low. This is the process of crack extension. Even if the process is slow and is discontinuous, as the crack propagates, components surface will gradually weakened. When it is weakened to a certain limitation, the parts will be fracture suddenly. Many parts damage was caused by fatigue failure. When the component stress does not exceed a certain limitation, bear cycles can be unlimited. That is, structural components can experience an infinite loop and won’t appear fatigue. This alternating stress limit value is the fatigue limit or endurance limit.
3. Structural Improvement
In order to improve the fatigue strength of the stem, the structure of the valve stem should be improved.
(1) The design completely avoids conventional key institutions: holes, gaps and shoulder. The connection of stem end moving to adopt smooth rectangle structure. Shoulder transition parts adopt as far as possible big rounded transition. Set load reducing groove and relief groove processing transition. These improvements effectively reduce the stress concentration and improve the fatigue strength of the valve stem.
(2) Improve the surface quality of the valve stem. The maximum stress of valve stem occurs in the surface. Fatigue crack also often appears in the surface layer. Put an end to the marks and scratches of surface in the process of machining. Prevent the stress concentration. Decrease fatigue limitation. Use high intensity precipitation hardening stainless steel. Coat the surface with high hardness carbide before grinding. Hardness can reach up to 60 HRC. Using special equipment further, mechanical rolling can form a high quality of surface processing. The surface roughness is Ra0. 4 microns. It will also make the surface form the compressive pre-stress. Weaken the stress in the work that is likely to cause crack. Ensure that give full play to the performance of high strength, which can improve the fatigue limitation of the component.
(3) Set two support bearing on both ends of the valve stem. Reduce radial wobble of term. Mitigate the unbalanced load. Effectively increase the stem operation stability. Ensure the valve stem to run for a long period of time.
(4) Stepped stem: Set big head end on the valve body cavity and make small head outside body. Once the valve stem damaged, dangerous section is located outside of the body, providing security for the system.
With the progress and development of science and technology, frequent switch valve will be more widely used, people should pay more attention to the fatigue strength design.