Explain the usage of the adjustable port valves? By adjusting the port size, we can change the valve capacity, To restrict the flow (when fully open) to a very low level, do we need a restrict orifice, or the adjustable port valve can do it itself?
The port opening size is changing for all control valves compared to on/off isolation valves.  Also, valves are available with port size selections.  Many reduced port (bore) ball valves have a port one line size smaller than the piping.  Reduced port sizes for control valves permit trim selection applied to specific process condition ranges.  However the valve internals must be replaced to change the port size. They also have the Varimax rotary and the Wee Willie with tinier capacities than the VariPak.  All three have adjuatable capacities by moving pivot points in the linkage. The down side of that is when you set them to the low-end of adjustable range, the trim is just not opening much.  With the plug close to the seat, erosion and wire-drawing are always phenomena that might occur.
Groth has/had a tank-blanketing regulator with a rotary-positionable cylinder around the trim so the user could dial in the capacity he needs for the application. A window in the cylinder could blank off part of the waterway so that the regulator's capacity was adjustable.  
Most adjustable Cv valves are as JLSeagull mentioned: pop out the trim, snd insert something of a different size orifice..

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