Control valves vena contracta

The noise sources in control valves include mechanical vibration (usually below 100 dBA) hydrodynamic noise caused by liquid turbulence, cavitation, or flashing (usually below 110 dBA) and aerodynamic noise (can reach 150 dBA). In control valve design, aerodynamic noise can be a major problem. Aerodynamic noise generation, in general, is a function of mass flow rate and the pressure ratio (p /pf) across the valve. The point at which sonic speed is reached in the valve vena contracta is a function of the valve design. 

The pressure profile in a controi valve is shown in Figure 8 [2], Pressure inside the valve will drop from its inlet pressure (pi) to the lowest pressure (pvc) due to the narrow path inside the valve body. Then, the valve pressure will be recovered to its outlet pressure (p2). pvc is control valve pressure at its vena contracta point, the smallest cross sectional area in control valve body. Vena contracta is usually located downstream of valve plug. Fliud velocity at valve vena contracta point is the highest, since at this point, the largest portion of pressure energy is converted to kinetic energy. 

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