Analaysis of Dynamic Gas Forces on Reciprocating Compressor Valves

This paper describes a method by which the Kerr [1] mathematical model of a reciprocating compressor having rigid valve plates may be adapted to investigate the dynamic response of flexible valves of certain geometries. The method is restricted to flexible valves which can be modelled by spring-mass systems. Also, the section of the valve plate which is directly over the valve port must remain substantially parallel with the valve seat during motion since the effect of plate tilt on the gas force is unknown at present. The curve of gas force versus time for the moving valve plate is derived by the Kerr model from measured values of gas force applied to' model valves, in a specially designed test rig which produces curves of gas force versus displacement for a range of pressure differences. A Fourier analysis is carried out with a view to determining whether or not any valve natural frequency is likely to resonate with a harmonic component. THE RIG FOR MEASURING GAS FORCES The rig shown diagrammatically in Fig. is designed to measure gas f'orces exerted normal to the bottom surf'ace of the valve plate. The gas f'orce data fed into the computerised compressor model is derived from the gas forces measured in this rig. The rig is capable of measuring the variation of' the gas force with time as the valve plate is rapidly pulled from its seat to release the pressurised gas in the plenum chamber to atmosphere. Clearly it is also capable of measuring the gas force caused by the steady flow of gas through the valve at fixed increments of plate displacement. The differences between the gas forces resulting from transient motion of the valve plate and those which result from the steady flow of gas through the valve at a series of fixed increments of valve lift are discussed elswbere [2] and the influence of these differenes on certain aspects of compressor behaviour have also been investigated [ 3]. ~OLD ON JICTION OF= ELI'5 FOFlCE MEA5URING RIG.