Flow Forces and the Tilting of Spring Loaded Valve Plates - Part I

Up to the present,so far as the author is aware, it has been considered selfevident that spring loaded valve plates remain parallel to the seat during valve lift when nominally symmetrical conditions o~ flow and spring force apply. In this paper it will be shown, that this in general is not the case. In the author's opinion the problem of instability in valve plate motion has not been studied because of insufficient knowledge of forces resulting from flow in valve channels. A complete theory for flow forces is complex but a simplified treatment makes clear the fundamentals of the phenomenon. Forces acting on a valve plate during opening and closing are discussed. Flow forces resulting from deflection of the gas flow coupled with spring forces govern valve dynamics, except within small region near seat and guard. Flow forces increase considerably (by some 25%) with increasing lift. This is shown for the case of a simple slot with 90° deflection of the flow by potential flow solution, which gives a close approximation to the real flow. If increase in flow force with lift exceeds the increase in spring force, valve plate motion becomes unstable and degenerates to tilted motion.Conditions for stability are given in t erma of valve parameters and discussed in detail. INTRODUCTION Seat parallel motion of the valve plate is very important for valve life time. In the opinion Of the author,failure of valve plates i~ connected closely with tilted motion and consequent impact.A hypothesis of the cause of these failures is presen-· ted elsewhere in these Proceedings. Before looking closer at stability we have 185 to discuss the forces actins on a valve plate. These forces are flow forces and spring force,fig.1. Flow force on valve plate arises solely as a consequence of deflection in the gas flow, except small regions near seat and guard. We shall call this force the impulsive force F .• Near the gu~d(when opening) there is an 1 additiona..ljl'low effect causing a"arueezing force" Fsqu• This effect is norm ly important only for distances less than 0.2mm between plate and ~ard(in the absence of valve plate tilt) [1]. The squeezing force is especially important for high pressure compressors. It does not occur when steady state flow force measurements are perfor-med. When the valve plate is relatively near to the seat,reattachment of flow to seat wall occurs and causes pressure recovery and hence increases impulsive force F .• Accor~ing to (2] reattachmen-t up to y/te-b)~o.s 1s to be expected. In computer calculations of valve dynamics a viscous damping force, proportional to ~late velocity often is introduced. There 1s little physical basis in the flow process for postulating such a force.The above mentioned squeezing force becomes only important in the vicinity of the gu~d. Mechanical friction associated with gu1~es or in the bending arms of the spr1ngs may cause some damping1 the magnitude of which is difficult to estimate. We may conclude that the impulsive force governs motion in the main part of valve lift together with the spring force. THE IMPULSIVE FORCE For a basic investigation of the stability phenomenon it is helpful to begin with a simple situation, accessible to theoretical treatment. We start with flow through a parallel entrance slot of infinite FIGURE 1 Forces acting an valve plate length,deflected by a valve plate normal to the slot, fig.2. The plate is assumed wide enou~ to ensure deflection of effectively 90 (this means e.g. e;e:;.1.5b, which corresponds to real conditions).Quantitiee such as impulsive force, spring force, valve plate mass etc. ar~ related to unit length of slot and given the suffix "1". For this flow problem the theory of jets of an ideal fluid allows a very good approach to real fluid flow. Real flow has a separation line along the seat edge and forms a wake of approximately constant pressure, which corresponds to the boundary condition of ideal jet flow. The jet is concentrating from b to d. Kinetic energy of the leaving jet(velocity w ) frwt is lost. The pressure loss ~ p ( =p~essilre difference acrose the valve) is therefore