Calculated impact valve velocities can be twice that of measured impact valve velocities depending on the gas composition and compressor design. An inability to accurately model impact velocities can lead to dramatic consequences in valve design. Current methods for calculating impact velocities assume that cylinder pressure is homogeneous throughout the compression cycle. In this research, the pressure waves that are developed in the cylinder of a reciprocating compressor are mapped It is demonstrated how these pressure fluctuations reduce the differential pressure across the valve and reduce sealing element impact velocities so low that in some cases actually prevent the valve from opening. The effect of dynamic flow inside the cylinder is most significant for large-bore, short-stroke compressors, high-speed compressors and for heavy gas applications.