Numerical simulation of solid contact/impact and gas flow in a high pressure safety valve
McCall, James N
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This work consists of finite element modelling of the sudden opening of a high pressure safety valve due to over-pressure in the tank it is protecting. Tank pressures on the order of 200 atmospheres drive the motion of a valve spindle which opens a flow path for the tank gas to escape to ambient. A mechanical stop incorporated into the interior of the valve body limits the length of travel of the spindle. When driven by such large pressure differences, the spindle impacts the stop and looses most of its kinetic energy through plastic deformation of both the spindle and the valve body. The small amount of energy stored elastically in these pieces and can cause a rebound of the spindle and re-impaction but with no further plastic deformation. In some instances the change in shape caused by the plastic deformation prevents the valve spring from returning the spindle to its closed position when the over-pressure fault subsides. An important question addressed in this study is whether a minor geometric modification to a common spindle design can prevent such valve failure. For the solid mechanics behavior, the time scales associated with the elastic wave dynamics are much smaller than those associated with global spindle dynamics and plastic deformations. Hence, they are not resolved in numerical modelling of the transient response of the system. Modal analysis of these vibratory motions is, however, considered in order to provide some understanding of the short-time solid-phase dynamics. As such, natural frequencies, mode shapes, and wave speeds are determined by both numerical and approximate analytical techniques and compared. Plastic deformation is determined in a quasi-static representation of the transient impact process. A sequence of static loadings is computed until a failure criterion is experienced. The 3D compressible flow of the tank gas as it escapes through the complex passageway in the valve is analyzed using a finite volume based computational fluid mechanics code. Simply modified spindle geometries involving chamfered perimeters of the contact surface are analyzed using the complete solid and fluid mechanics numerical models. The ability of chamfers alone to resolve the valve non-closure problem is assessed.