Metal To Metal Seal And Set Pressure Variation Of Pressure Relief Valve

The metal seal structure design of the valve mainly relies on the experience.The lack of accurate calculation method of leakage for metal seal restricted the good performance and reliability of metal seal.This lack was attributed to the superficial understanding of the hydromechanics at the microsacle,the interaction between fluid and the wall,and the difficulty in bridging the microsacle phenomina and mascroscale performance of metal seal.To address this issue,in this study,a calculation modle on the leakage of metal seal was established based on the numerical simulation on the hydrodynamics at the microscale at the metal sealing interface using finite element method(FEM)and Lattice Boltsman Method(LBM).The accuracy of the modle was verified by the experimental results.The interaction between the fluid and the wall at the microscale at the interface of the metal seal was investigated using classical density functional theory(DFT).We firstly revealed the cause of the difference in set pressures of safety valves using compressed air and steam as the medium.The above research findings and results were applied in the design and fabrication of the nuclear main steam safety valves,promoting the seal performance of the vales.The main research contents and innovative results are as following:(1)Microsacle hydrodynamics at meatal seal interfaceThe simulation on the contacting evolution of the two rough metal surfaces was simulated using FEM and the surface rebuilding method associated with atomic force microscope(AFM)and mid-point random displacement method.The leakage channel was extract by bubbling algorithm.It was found that the effect of roughness on the contact pressure as a function of contact area ratio is negligible and the percolate threshold is independent from roughness(0.33).The simulation on the hydrodynamics at the leaking channel of the seal interface was carried out using lattice Boltzmann method with two relax time(LBM-TRT).It was found that the increase in the ratio(?)of Ra to the the average height of the leaking channel(H)decreased the leakage flow rate when the flow was at the slip boundaries state.In contrast,for the flow at the transition state,the rise in ? possibly increased the leakage flow rate.The torture characteristic of streamline of the leaking channel at the seal interface was investigated using FEM simulation.It was found that the tortuosity could approximate the value of 1,which justified the simplification of 3D simulation of the leakage at the interface to the 2D one.The calculation model on the leakage of metal seal was established based on the simulation results.The good accuracy of the model was verified by the deviation of 12%between the calculated leaking flow rate and the experimental one.(2)Metal seal design and degradation of PRVsThe calculation modle on the leaking flow rate for PRVs was proposed considering the metal seal structure features of PRVs.The damages of the metal seal surfaces and the following sealing degradation by erosion and the impact load during re-closing process were investigated basd on the 3D transient simulation using computation fluid dynamics(CFD).The results showed that the erosion resulted in an obvious damage on the seal surface with the increase in roughness by 4.8 times,leading to a rise in leakage by 151 times.In comparison,the effect of the impact load on the seal performance could be ignored though the plastic deformation at the exterior seal zone achieved 5%which reduced the seal width of 5%.Therefore,for metal seal,enhancement of the anti-erosion ability of metal seal material is an important measure to guarantee seal reliability.(3)Interaction between fluid and wall at the seal interface effect on set pressure of pressure relief valveThe set pressure in pressure relief valves(PRVs)varying with the type of sealed medium has been a puzzling problem in the field of chemical machinery and equipment for many years.Here two individual types of medium gases(i.e.,saturated steam and air)sealed in different PRVs were investigated,and upon a multiscale model of the apertures in PRVs,the additional adsorption pressures were evaluated by using classical DFT,Our calculation shows that the adsorption force of steam was always higher than that of air,resulting in a lower set pressure disregarding the use of different PRVs.The theoretical results were compared with the experimental measurements,displaying qualitatively good agreement,which supports our surmise.The adsorption force rose with the increase in the hydrophilicity,thus decreasing the set pressure of PRV.The above methods and conclusions were applied the design and fabrication of nuclear main steam PRVs such as CAP1400,ACP1000,K2K3 and C3C4,achieving good economic and social benefits.