The velocity is one of the more important considerations in sizing a control valve. For long life on liquid applications the velocity at the exit of the valve body should be less than 10 m/s. This compares with generally accepted line velocities of about 3 m/s, which explains why control valves often are smaller than the line size.
On gases and vapours the velocity at the exit of the valve body should be less than 0.33 Mach (1/3rd of sonic) for noise control valves and less than 0,5 Mach where noise is not a consideration.