Boosters and Control Valve Assemblies

<...what to do? kindly tell me but dont sat that deepends upon process condition or criteria of the plant or that perticular loop...>

OK, fine. What is to be stated, then, if it does depend upon process conditions?

<...boosters are needed in fast acting loops...>

If something happens in less than 1 sec, then, in a process plant, it isn't real.

<...Is there any practical way to delete or resolve CAVITATION AND FLASHING IN control valve....>

Yes. Make sure the process conditions are such that liquid doesn't boil as it comes through the valve.

<...The graph against (d2/D2) vs %of pressure loss gives the answer of net pressure loss of Square edge and conical, but how to find the net pressure loss of quadrant edge orifice.?...>

In the case of a control valve, concentrate on the parameter being controlled, i.e. the measured process variable as applied to the controller comparator, and not the pressure loss at the valve, which is usually a secondary consideration. If there is no pressure drop at the valve, then the valve isn't doing anything useful!

<...IS booster is required in steel sector and is positioner essential for pneumatic control valves....>

Not necessarily. It depends on how fast the valve needs to respond to changing process conditions (back to the first response comment above....).