145.What are the characters of Control valve and give example of their application?

The typical flow characteristics of a control valve are Linear, Equal percentage, Quick opening type.          The characteristic of valve depends of loop sense, differential pressure across the control valve etc.                  Fuel oil flow control - Controller is Linear characteristic. Error at various valve opening, from 0 to 100%, demands same flow change.The DP across the control valve is high at loads and decrease with respect to load. Hence the CV at low loads shall be small compared to at high loads. A Equal percentage valve will provide such CV. Fuel Oil Temperature Control - Controller is non linear characteristic. Error is temperature. Same error at small valve opening require less flow. At higher valve open demands large flow. The mass of steam required for 50% fuel can only change the temperature by 50% at 100% fuel flow. DP across the valve is constant. Hence an Equal % valve can perform better. However the valves characteristics can be matched with the process using adapting gain in DCS using valve position demand.