Single-Stage vs Multi-Stage Orifice Plates — Which Is Right for Your System When fluid moves through a restriction in a piping system, it accelerates through the narrowed passage and then expands on the downstream side. That expansion is where the engineering challenge lives. As the fluid decelerates after passing the vena contracta — the point of minimum cross-sectional area and maximum velocity — kinetic energy converts back to pressure. If that recovery happens too quickly