Failure Mechanism And Key Technology Of Emulsion Poppet Valve Under Condition Of High-Pressure Large-Impact-Flow

As the critical control element of hydraulic support, large-flow pilot-operated check valve has the basic work performance of high pressure, large flow and high velocity. Thus, the characteristics such as flow characteristics, mechanical properties, impact characteristics, flow capacity and failure mechanism under high pressure and large impact are of core problems for the design of large-flow pilot-operated check valve. Based on hereinbefore problems, the aim of this study is to reveal the failure mechanism of poppet valve under high pressure and large impact flow, and then improve the reliability and working life of large-flow pilot-operated check valve. Primarily, the large-flow pilot-operated check valve is designed, and CFD method is applied to the flow field analysis and structure optimization of poppet valve. Then, the mechanical properties of poppets are studied under the comprehensive application of one-way FSI method, CFD analysis and FEA structural analysis. Simultaneously, impact properties, fluid resistance characteristics and power spectrum are studied by theoretical analysis and experimental research. Finally, the performance of designed structure is verified through comprehensive test. This study mainly include the following sevral aspects:The theory of high pressure impact flow field is elaborated, then the load characteristic and performance requirements of large-flow pilot-operated check valve are analyzed, and large-flow pilot-operated check valve is designed. Furthermore, the structure optimization of large-flow pilot-operated check valve is implemented based on three factors: pilot poppet half cone angle, valve seat structure and seal form, and flow area.The geometry model for large-flow pilot-operated check valve is established and Fluent software is used to simulate the flow field of pilot poppet in open process. And conclusions can be obtained such as: with the increase of open degree of pilot poppet, the flow increases, the flow tend to be steady, and loacal energy lose decreases. The fluid pressure is decreasing obviously when flowing through the poppet area, and the phenomenon of cavitation is appeared for the flow area becomes smaller suddenly when flow through poppet area. Damage caused by cavitation is mainly occurred at the head of the pilot poppet mainly for the flow pressure will raises again when the bubbles in fluid move to the this position.Gas-liquid two-phase flow analysis for each optimized structure is conducted. Primarily, pressure contours, gas-phase volume fraction and liquid-phase volume fraction are obtained by cavitation model. Then, one-way FSI analysis is implemented to pilot and secondary poppet respectively. Conclsuions can be obtained as: 45° and 60° half cone angle will be relataively better for pilot poppet, seat I or II with conic mechanical seal will be better, and larger flow area is relatively superior. During the whole opening process of poppet, the maximum equivalent stress of pilot-poppet appears at head-end, while the maximum equivalent stress of secondary-poppet appears at the tail-end. The maximum equivalent stress values of pilot and secondary poppet are decreasing apparently with the incease of opening degree, which are both within the allowable strength of 45 steel. When opening degree is constant, the maximum equivalent stress of pilot poppet will be smaller with larger half cone angle, and damage probality will be smaller so as to improve the poppet work life. From the aspect of reducing stress and the possibility of cavitation, larger half cone angle should be chosen for pilot poppet. In addition, the most dangerous working condition is the moment poppet is just opening.AMESim software is applied to the establishment of simulation model for pilot-operated check valve. Dynamic characteristics of pilot-operated check valve under impact load is studied, and the causes of unload pressure impact are analyzed. Conclusions can be obtained as: the oscillations of preesure, flow and poppet displacement is severely at the stage of impact unloading. The main cause of pressure impact is equilibrium condition of forces on main valve and control piston is changed that lead to the close of secondary poppet at the stage of pilot-operated check valve reverse opening. With no variation of other factors, dynamic characteristics of pilot-operated check valve can be significantly improved by increasing pilot poppet half cone angle, the pilot-operated check valve will open more rapidly and thus response speed is improved. Futhernore, pressure shock also can be decreased and the stability in opening process can be increased.According to working characteristics of large-flow pilot-operated check valve, the rapid loading experiment system is established, with accumulator as power and pressure cylinder as pressurization element. Then, the impact experiments, fluid-resistance experiment and power spectrum analysis for large-flow pilot-operated check valves are conducted. And conclusions can be obtained as: according to the impact characteristic experiments for different structure, dynamic performance of pilot poppet with 45° half cone angle is relatively better compared with others, and seat I with buffer slot in the front will be more suitable, in addition, pressure vibration will be smaller, unloading will be more rapid and flow rise gradient is moderate. According to the different pressure flow characteristic experiment, in the case of same different pressure, parameters of pilot poppet with 45° half cone angle, valve seat III and flow-area 31.2 mm2 can reach a relatively larger flow at the flow rising stage. The concept of cavitation index is introduced according to the combination analysis of power spectral density and morphology, the analysis of differential pressure ratio indicate that probability and degree of cavitation depend on pressure rather than flow. Bubble breaking leads to high frequency oscillation characteristics stress pulse, causing high-frequency alternating load and impulse-type local plastic deformation on the seal surface. Cavitation erosion pits appear on the seal surface under the repeatedly effect of stress pulse, and finally lead to cavitation damage.For the problem of large-flow pilot-operated check valve performance and sealing life, impact experiment and power spectrum analysis are implemented for further study based on original fundamental. Meanwhile, comprehensive matching experiment is conducted on the optimized poppet, so as to obtain the optimal structure from the aspects such as superior dynamic characteristics, resistance characteristics, and life. Conclusions can be obtained such as: the completely shut down of main poppet could be avoided by increasing control pressure, furthermore, response speed will be improved, pressure impact of unloading will be reduced, the stability in the process of opening is increased and thus lead to a significantly dynamic characteristics improvement of the large-flow pilot-operated check valve. The cavitation index shows a trend of slow decline with the increase of control pressure, it illustrates that the change of control pressure has little influence on cavitation for a certain structure. Thus, the control pressure should be more than 50% of impact pressure when the large-flow pilot-operated check valve is reverse opening. In the aspect of material characteristics, material with excessive high hardness has disadvantages for the improvement of cavitation resistance. Through the optimum match of each component in the poppet, the high-pressure large-flow pilot-operated check valve with good comprehensive performance is designed, the cavitation index is reduced, and working reliability and life expectancy are enhanced.