What are control valve characteristics and why are they important?

Valve characteristics describe how flow changes with valve opening: Linear - equal increments of travel give equal flow changes, Equal percentage - equal increments give equal percentage flow changes (most common), Quick-opening - large flow change at small opening. Characteristics affect loop gain variation over operating range. Installed characteristic differs from inherent due to system pressure drop variation. Equal percentage is preferred because it provides more linear installed characteristic when valve dP varies with flow, giving consistent loop gain.